text:
\section{Introduction}
Almost all low-mass black hole X-ray binaries (BHBs) are transient sources, which means that they show outbursts, which typically last from weeks to months. During their outbursts they evolve through different states \citep{2006csxs.book..157M,2010LNP...794...53B}. Using hardness intensity diagram \citep[HID;][]{2001ApJS..132..377H,2005A&A...440..207B,2005Ap&SS.300..107H,2006MNRAS.370..837G,2006csxs.book..157M,2009MNRAS.396.1370F,2010LNP...794...53B,2011BASI...39..409B}, hardness root-mean square (rms) diagram \citep[HRD;][]{2005A&A...440..207B} and rms intensity diagram \citep[RID;][]{2011MNRAS.410..679M} the evolution during the outbursts can be studied. At the beginning and end of an outburst the BHBs are found in the low-hard state (LHS). In this state an rms of several tens of per cent is observed and the emission is dominated by thermal Comptonization in a hot, geometrically thick, optically thin plasma located in the vicinity of the black hole, where softer seed photons coming from an accretion disk are up-Comptonized \citep[see][for reviews]{2007A&ARv..15....1D,2010LNP...794...17G}. Many BHBs show transitions to the high-soft state (HSS), in which the variability is much lower \citep[fractional rms $\sim$1 per cent, e.g.][]{2005A&A...440..207B} and in which the spectrum is clearly dominated by an optically thick, geometrically thin accretion disk \citep{1973A&A....24..337S}.

It is thought that the transitions reflect major changes in the properties of the inner accretion flow \citep[\eg\ ][]{2007A&ARv..15....1D}. A geometrically thin, optically thick accretion disk with an inner truncation radius varying as a function of the accretion rate can explain the different states and transitions between them \citep[\eg\ ][]{1997ApJ...489..865E,2001ApJ...555..483E}. While there is observational evidence that during the HSS the accretion disk extends down to the innermost stable circular orbit \citep[ISCO;][]{2004MNRAS.347..885G,2010ApJ...718L.117S}, the extension of the accretion disk in the LHS is still a matter of ongoing debate. Different observational studies give different estimates of how close the disk comes to the black hole in this state \citep[\eg\ ][]{2006ApJ...653..525M,2014A&A...564A..37P,2010MNRAS.407.2287D,2014MNRAS.437..316K,2015A&A...573A.120P}. The truncated disk model assumes that the disk recedes in the LHS and that the inner parts are filled by a radiatively inefficient, optically thin, advection-dominated accretion flow \citep[\eg\ ][]{1995ApJ...452..710N,1997ApJ...489..865E}.

Not all Galactic BHBs make the transition to the soft state and $\sim40$\% of them remain in the hard state during the entire outburst \citep{2016ApJS..222...15T}. This type of outburst has been dubbed ``failed'' outburst \citep{2009MNRAS.398.1194C,2016MNRAS.460.1946S}. The physical reason why some outbursts remain in the hard state, while other make it to the soft state, is a topic of ongoing research. Failed outbursts tend to have lower peak luminosities ($\la0.11L_{\mathrm{edd}}$) and they seem to occur more commonly in shorter orbital period systems \citep{2016ApJS..222...15T,2013MNRAS.434.2696S}. They might be connected to a premature decrease of the mass accretion rate \citep{2009MNRAS.398.1194C} .

The different states also show different features in power density spectra \citep[PDS;][]{2014SSRv..183...43B}. In  a large number of sources type-C quasi-periodic oscillations (QPOs) \citep[][and references therein]{1999ApJ...514..939W,2011MNRAS.418.2292M} have been observed in the LHS and hard intermediate state \citep[HIMS;][]{2006csxs.book..157M,2010LNP...794...53B}. These oscillations have centroid frequencies ranging from 0.01 to 30 Hz, and their quality factor ($Q=\nu_0/(2\Delta)$, where $\nu_0$ is the centroid frequency, and $\Delta$ is the half width at half maximum) is $\ga10$ \citep[see \eg\ ][]{2005ApJ...629..403C,2010ApJ...714.1065R}. Often they appear with one or two overtones and at times with a sub-harmonic. The PDS always show  band limited noise and the characteristic frequency of the QPO is anti-correlated with the total broad-band fractional rms variability. These oscillations are thought to be caused by Lense-Thirring precession of a radially extended region of the hot inner flow \citep{1998ApJ...492L..59S,2009MNRAS.397L.101I}. The presence of type-B QPOs together with a weaker power-law noise defines the soft intermediate state (SIMS). Type-B QPOs  have centroid frequencies of 0.8 -- 6.4 Hz, $Q>6$, have a 5 -- 10\% fractional rms and appear often together with an overtone and a sub-harmonic. The third type of QPOs are type-A QPOs. They have centroid frequencies of 6.5 -- 8 Hz, are broad ($Q\sim1-3$) and weak (fractional rms $<5$\%).  The three QPO types are well separated as a function of the total integrated fractional rms in the power density spectrum.

In this paper, we present a comprehensive study of the temporal variability properties of the X-ray transient MAXI\,J1820+070\ observed during four outbursts between March 2018 and October 2019. On March 11, 2018 MAXI/GSC \citep{2018ATel11399....1K} detected a bright uncatalogued  hard X-ray transient. About half a day later the source was detected by \textit{Swift}/BAT \citep{2018ATel11403....1K}. Based on follow-up observations in the radio, optical and X-ray band the source has been classified as a black hole X-ray binary candidate \citep{2018ATel11418....1B,2018ATel11420....1B}.  State transitions and the detection of quasi-periodic oscillations (QPOs) have been reported \citep{2018ATel11576....1H,2018ATel11578....1B,2018ATel11820....1H}. \citet{2019Natur.565..198K} found thermal reverberation lags that are shorter than those previously observed from black hole X-ray binaries. While the timescale of the reverberation lags shows some evolution, the shape of the broadened iron K emission line remains rather constant. These findings suggest that there is not much evolution in the truncation radius of the inner disk during the luminous hard state. A detailed analysis of the spectral properties of MAXI\,J1820+070\ during this outburst based on MAXI/GSC and \textit{Swift}/BAT data has been reported \citep{2019ApJ...874..183S}. The results of a spectral study based on \textit{Swift}/XRT and NuSTAR data taken a few days after the detection of the outburst have been presented \citep{2019MNRAS.487.5946B}. X-ray spectra and a light curve based on \textit{XMM-Newton}\ data obtained during outburst rise are presented in \citet{2019arXiv190606519K}. The light curve shows pronounced dipping intervals. In the same study the UV/X-ray cross correlation function was investigated. A detailed optical spectroscopic follow-up study of the optical counterpart ASASSN-18ey detected clear accretion disk wind features in the hard state \citep{2019ApJ...879L...4M}. Here, we investigate the timing properties of MAXI\,J1820+070\ using Neil Gehrels \textit{Swift}\ Observatory/XRT and UVOT and NICER (Neutron star Interior Composition Explorer) data. More details on the data used and on the data analysis are given in Sect.~\ref{Sec:obs}. We present our results in Sect.~\ref{Sec:res} and discuss them in Sect.~\ref{Sec:dis}.

\section[]{Observation and data analysis}
\label{Sec:obs}
\subsection{Neil Gehrels \textit{Swift}\ Observatory}
\label{Sec:obs_sw}
\subsubsection{XRT}
All \textit{Swift}/XRT \citep{2005SSRv..120..165B} monitoring data of MAXI\,J1820+070\ obtained in window timing mode between 2018 March 11th and 2019 September 28th are analysed in this study.
To obtain power density spectra (PDS) in the 0.3 -- 10 keV energy band, we make use of the GHATS package (v.~1.1.1), developed under the IDL environment at INAF-OAB \footnote{\url{http://www.brera.inaf.it/utenti/belloni/GHATS\_Package/Home.html}}. We follow the procedure outlined in \citet{2006MNRAS.367.1113B}, subtracting the contribution due to Poissonian noise \citep{1995ApJ...449..930Z}, normalising the PDS according to \citet{1983ApJ...272..256L} and converting to square fractional rms \citep{1990A&A...227L..33B}. We did not include any corrections of the PDS due to background photons. We determine the contribution due to Poissonian noise by fitting the flat tail of the PDS at the high-frequency end with a constant. This allows us to take into account deviation from the expected value of 2, that are caused by pile-up effects in the \textit{Swift}/XRT data \citep{2013ApJ...766...89K}. To fit the PDS within \textsc{isis} \citep[v.~1.6.2;][]{2000ASPC..216..591H} we use zero-centered Lorentzians for band-limited noise (BLN) components, and Lorentzians for QPOs.

\subsubsection{UVOT}
For all \textit{Swift}/XRT monitoring observations we also analysed the UVOT data \citep{2005SSRv..120...95R}. This data have been pre-processed at the Swift Data Centre \citep{2010MNRAS.406.1687B} with the current HEAsoft version and current calibration files and we use the HEAsoft task \texttt{uvotproduct} to obtain magnitudes from the images in the different bands available for each observation. As source position we used R.A. = 18$^{\rm h}$20$^{\rm m}$21\fs9, dec. = +07$^{\circ}$11$^{\prime}$07\farcs3 \citep{2018ApJ...867L...9T} with an extraction radius of five arc-seconds. A nearby source free region was used to define a background region with a radius of ten arc-seconds.

\subsection{NICER}
NICER \citep{2012SPIE.8443E..13G} observed MAXI\,J1820+070\ between 2018 March 12th and 2018 July 5th, where observations with an exposure of less than 1 ks have been excluded from our study. NICER resumed observing this source on 2018 September 25th and followed it until 2019 October 25th. (All NICER observations taken between 2018 July 6th and 2018 September 24th ares given with an exposure of zero seconds in the HEADAS data base.) We use the pre-processed event files provided by the NICER data center. These files are produced using the current HEAsoft version and current calibration files. We derive Poissonian noise subtracted, Leahy normalised and to square fractional rms converted PDS using HEAsoft (v.~6.23) tasks.

\begin{figure*}
\resizebox{\hsize}{!}{\includegraphics[clip,angle=0]{plot_lc.eps}}
\caption{Light curves and time evolution of the hardness ratio and the fractional rms (from top to bottom) of the four outbursts of MAXI\,J1820+070\ between March 2018 and October 2019 based on Swift/XRT and NICER data. Each data point represents one observation. Observations of the four outbursts are indicated by different colours. The first two outbursts are displayed in the left hand panels, while the third and fourth outbursts are shown in the right hand panels. Please notice the change on the scales of the y-axis for corresponding panels. Observations in which a type-C QPO has been detected are marked by (green) triangles. In case of NICER observations filled  triangles indicate QPOs detected at $\ge3\sigma$, while open triangles indicate QPOs with a detection significance below $3\sigma$. T=0 corresponds to March 11th 2018 00:00:00.000 UTC.}
\label{Fig:LC}
\end{figure*}

\begin{figure*}
\resizebox{0.45\hsize}{!}{\includegraphics[clip,angle=0]{plot_hid.eps}}\vspace{0.5cm}\resizebox{0.45\hsize}{!}{\includegraphics[clip,angle=0]{plot_hid_ni.eps}}
\caption{Hardness-intensity diagram (HID; upper panel) and hardness-rms diagram (HRD; lower panel), derived using \textit{Swift}/XRT data (left panels). In case of NICER data (right panels) the upper two panels are the HID, where the lower one shows a higher HR range. These observations are not shown in the HRD (lower three panels; before the soft state, after the soft state, third and fourth outburst from top to bottom) as their rms values are not constrained, due to their low count rates. Filled and open down-pointing triangles indicate observations with a QPO detected above or below 3$\sigma$, respectively. Open squares indicate observations of the third outburst (after day 330), open up-pointing triangles observations of the fourth outburst (after day 500). The star marks the first observation. Observations of the three outbursts are indicated by different colours, following the colour scheme of Fig.~\ref{Fig:LC}. Each data point represents one observation. }
\label{Fig:HID}
\end{figure*}

\begin{figure}
\resizebox{\hsize}{!}{\includegraphics[clip,angle=0]{plot_rid.eps}}\\
\smallskip
\resizebox{\hsize}{!}{\includegraphics[clip,angle=0]{plot_rid_ni.eps}}\\
\smallskip
\resizebox{\hsize}{!}{\includegraphics[clip,angle=0]{plot_rid_log_ni.eps}}
\caption{rms-intensity diagram, derived using \textit{Swift}/XRT (upper panel) and NICER (lower two panels) data. While the middle panel uses linear axes, logarithmic axes are used in the bottom panel. Each data point represents one observation. Symbols and colours are the same as in Fig.~\ref{Fig:HID}.}
\label{Fig:RID}\
\end{figure}

\section[]{Results}
\label{Sec:res}

\subsection{Diagnostic diagrams}
\label{Sec:diag}
Based on \textit{Swift}/XRT data we determined source count rates in three energy bands using the online data analysis tools provided by the Leicester \textit{Swift}\ data centre\footnote{\url{http://www.swift.ac.uk/user\_objects/}} (which used HEAsoft v.6.22), including single pixel events only: total (0.3 -- 10 keV), soft (0.3 -- 3 keV), and hard (3 -- 10 keV). We also derived NICER count rates in the 0.5 -- 10 keV, 0.5 -- 2 keV and 2 -- 10 keV bands. The \textit{Swift}/XRT and NICER light curves and time evolution of the hardness ratio (HR) and the fractional rms are shown in Fig.~\ref{Fig:LC}. To obtain HRs we divide the count rate observed in the hard band by the one obtained in the soft band. For \textit{Swift}/XRT the fractional rms is determined in the 0.3 -- 10 keV band and in the $4\times10^{-3}$ -- 35.13 Hz frequency range, while for NICER we use the total energy band (0.0 -- 15.01 keV) and the $6.1\times10^{-3}$ -- 50 Hz frequency range. The hardness-intensity diagram (HID) and hardness-rms diagram (HRD) are shown in Fig.~\ref{Fig:HID}, while Fig.~\ref{Fig:RID} is the rms-intensity diagram (RID).

The light curves show a steep rise lasting for about 15 days, before a plateau at a rather constant count rate of about $10^3$ ($10^4$) cts/s for \textit{Swift}/XRT (NICER) is reached. This plateau lasts for about 55 days, before the count rate starts to decrease slowly, as can be seen from the NICER light curve, which provides a better coverage during this part of the outburst. After another 30 days the evolution of the light curves reverses and the count rate starts rising again for about 15 days before a second, slowly decaying plateau at a higher count rate of $3-4\times10^3$ cts/s for \textit{Swift}/XRT is reached. This second plateau lasts for about 73 days. When the count rate drops to values comparable to the highest values observed during outburst rise (around day 10) the decay of the light curve steepens significantly. The NICER and \textit{Swift}/XRT motoring end after 255.8 and 252.2 days, respectively. 78.2 days after the last NICER observation NICER resumed observing MAXI\,J1820+070. About 37 days after the beginning of these observations MAXI\,J1820+070\ shows another outburst that stays below the count rate at which the source was detected originally. The rise is again rather steep and it is directly followed by a decay similar to the one seen at the end of the first plateau. The long-term MAXI/GSC light curve shows that the count rate between the second and third outburst is similar to the pre-detection level. About 64.62 days after the last observation of the third outburst NICER resumed observing MAXI\,J1820+070\  and covered another outburst that reaches a similar count rate and has a similar shape as the third outburst.

The HID shows that the source softens during outburst rise and that the softening continues during the first plateau seen in the light curve until an HR of 0.15 (0.17) for \textit{Swift}/XRT (NICER) is reached. The decrease in count rate takes place at a slightly higher HR of 0.17 (0.18--0.19). The second plateau has softer HRs, where most observations have an HR$<0.1$. The softest HR is obtained at the end of the second plateau. During the steep decay MAXI\,J1820+070\ hardens. Below NICER count rates of 200 cts/s the HR decreases slightly from 0.26 to 0.24 with decreasing count rate, before it hardens significantly at NICER count rates $< 30$ cts/s and softens again at $< 20$ cts/s. In observations taken before the third outburst at a NICER count rate $< 5$ cts/s the source has HR between 0.4 and 0.9.  The HRs observed during the third outburst are a little bit harder than the HRs observed towards the end of the second outburst, but softer than the HR at the beginning of the first outburst. Anyway, the source remained in the hard state during its third outburst. The HRs observed during the fourth outburst lie in a similar range as those observed during the third outburst, so this outburst remained again in the hard state.

From the HRD we see that the softening during outburst rise and first plateau correlates with a decrease in the fractional rms to about 15\% (in \textit{Swift}/XRT data), and that the decrease in count rate takes place at higher rms values around 20\%. In the second plateau the fractional rms dropped significantly to values of about 2--3\%, although error bars are quite large at those low rms values. During the steep drop of the count rate, when the source hardens, we also observe an increase in rms, reaching values of 35\% in the NICER data. The  decrease in count rate below a NICER count rate of 200 cts/s corresponds to a decrease in rms from 35 to 14\%. The softening at $< 20$ cts/s corresponds to an increase in the rms from 10 to 20\%.

The RID shows the hard line at outburst rise, at fractional rms values similar to those seen in GX\,339-4\ \citep{2011MNRAS.410..679M}. During the first plateau the fractional rms drops to about 20\% and then again increases slowly to 30\% while the total rms decreases, leading to a flat loop-like structure in the RID. During the second rise the total rms increases with increasing count rate, while the fractional rms decreases. The points of the second plateau lie in the region of the soft branch \citep{2011MNRAS.410..679M}. During the steep decay seen in the light curve, the increase in fractional rms corresponds to an increase of total rms, until the fractional rms gets close to 20\%, when the total rms starts to decrease and MAXI\,J1820+070\ follows again the hard line.

\subsection{UVOT light curves}
UVOT data are available in the V, U, UVW1, UVW2, and UVM2 bands. Not all bands are covered in each observation. Light curves in the different bands together with the \textit{Swift}/XRT light curve are shown in Fig.~\ref{Fig:lc_uvot}. The beginning of the outburst is well covered in the UVW2 band. In this band, as well as in the other two UV bands, the first outburst appears to be much more peaked than in the X-ray data and the first plateau seen in the X-ray band is not present in the UV data. The coverage of the beginning of the outburst in the V band is spare, while in the U band the detector saturates in many observations when the source is at its brightest stage, which results in a constant magnitude with a large error bar. Comparing the UV bands to the X-ray data, we also notice that the second outburst, which is brighter than the first one in the X-ray data, does not reach the magnitudes of the first outburst in the UV bands. We also notice that during the decay of the second outburst a rebrightening occurs in all UVOT bands around day 214. This feature is most prominent in the V band, and it is not seen in the corresponding X-ray data. The rebrightening takes place when the source evolves along the lower horizontal branch in the HID (Fig.~\ref{Fig:HID}) in the LHS at \textit{Swift}/XRT HRs between 0.14 and 0.22.

Investigating the correlation between fluxes in the near ultraviolet band (UVW1) and the \textit{Swift}/XRT hard X-ray band (see Fig.~\ref{Fig:uvx}) we find a linear correlation of these two quantities at the beginning of the first outburst, before the first plateau in the X-rays. The same correlation is found for data points corresponding to observations taken after the peak of the rebrightening in the UVOT bands. The data points of the third and fourth outburst also follow this correlation rather closely. These findings can be interpreted that it is the same process that creates the UV emission at the beginning of the outburst and after the peak of the rebrightening, while the UV emission in between is due to another process. The rebrightening in the UVOT bands indicates a change in the accretion morphology \eg\ in the structures of the accretion disk or jet.

\begin{figure}
\centering
\resizebox{\hsize}{!}{\includegraphics[clip,angle=0]{plot_lc2.eps}}
\caption{Light curve in the different UVOT bands. For comparison the \textit{Swift}/XRT light curve is shown in the lowest panel. Each data point represents one observation. Observations of the four outbursts are indicated by different colours, following the colour scheme of Fig.~\ref{Fig:LC}. The U band observations in which the detector saturates when the source is at its brightest stage are not shown. The dashed lines indicate the time of the brightest magnitude in the UV bands and of the rebrightening in the UVOT data during the decay of the second outburst. The dash-dotted lines indicate the times of the highest observed X-ray flux and of the X-ray peak of the third and fourth outburst.}
\label{Fig:lc_uvot}
\end{figure}

\begin{figure}
\centering
\resizebox{\hsize}{!}{\includegraphics[clip,angle=0]{plot_xuv.eps}}
\caption{Correlation between the \textit{Swift}/XRT count rate in the hard (3 -- 10 keV) band and the magnitude in the near ultraviolet (UVW1) band. Each data point represents one observation. Observations of the four outbursts are indicated by different colours, following the colour scheme of Fig.~\ref{Fig:LC}. The arrow indicates the observation in which the rebrightening seen in the UVOT bands peaks.}
\label{Fig:uvx}
\end{figure}

\subsection{X-ray timing properties}
\label{Sec:time}
\subsubsection{\textit{Swift}/XRT}
\label{Sec:time_sw}
The PDS of the observations taken until day 22.4 can be well described by two BLN components. From day 23.8 onwards a QPO is present and it can be detected in most observations of the first plateau and during the first decay and at the beginning of the second rise. Observations where the PDS shows a QPO have a \textit{Swift}/XRT count rate between $\sim$400 and 1280 cts/s and a total fractional rms between 15.6 and 25.4\%. The characteristic frequency ranges from 0.08 to 0.86 Hz, which only covers the low frequency end of the range in which QPOs are typically observed in black hole XRBs. For most observations the Q factor is between $\sim$3 and 10. Details on individual observations can be found in Tables~\ref{Tab:pds1} and \ref{Tab:pds2} and examples of PDS are shown in Fig.~\ref{Fig:pds}. The PDS of observations taken during the HSS (total fractional rms $\sim$2\%) are dominated by power law noise. The first three observations where the rms is above 10\% show again QPOs with a characteristic frequency between 0.43 and 0.87 Hz. The observed Q factors are low ($<3$). The PDS of the remaining observations can be described by one or two BLN components and no QPO is detected. The correlation between total fractional rms and characteristic frequency is shown in Fig.~\ref{Fig:fchar_rms}. The PDS of the observations during the third outburst can be well fitted by two BLN components. In the observation taken on day 374.12 a QPO with a characteristic frequency of $\sim 6.19$ Hz and a Q factor of 18.4 is detected at  $2.97\sigma$.

\begin{figure*}
\resizebox{\hsize}{!}{\includegraphics[clip,angle=0]{plot_pds.eps}}
\caption{Examples of PDS for three \textit{Swift}/XRT observations, taken during brightening of the outburst. PDS of corresponding NICER observations are also shown. In addition, two NICER PDS obtained on day 116 and day 215 are shown.}
\label{Fig:pds}
\end{figure*}

\begin{figure}
\resizebox{\hsize}{!}{\includegraphics[clip,angle=0]{plot_fchar_rms_sw.eps}}\\
\resizebox{\hsize}{!}{\includegraphics[clip,angle=0]{plot_fchar_rms_swl.eps}}
\caption{Correlation of the total fractional rms variability with the characteristic QPO frequency, derived from Swift/XRT data. Circles indicate observations taken during the first outburst and the rise of the second outburst, while squares indicate observations taken during the decay of the second outburst (open symbols indicate QPOs with a detection significance below $3\sigma$). For observations in which more than one QPO is detected (only the case during outburst rise) up-pointing triangles, stars, and down-pointing triangles indicate QPOs at higher frequencies (including upper harmonics). In the lower panel the x-axis is logarithmic to display the data points at lower frequencies more clearly. }
\label{Fig:fchar_rms}
\end{figure}

\begin{figure}
\resizebox{\hsize}{!}{\includegraphics[clip,angle=0]{plot_time_fchar.eps}}
\caption{Evolution of the characteristic frequency of the fundamental QPO (red circles) and its upper-harmonic (blue triangles) derived from NICER data until day 116. Open symbols indicate QPOs with a detection significance below $3\sigma$.}
\label{Fig:qpo_evolv}
\end{figure}

\begin{figure}
\resizebox{\hsize}{!}{\includegraphics[clip,angle=0]{plot_fchar_rms_ni.eps}}\\
\resizebox{\hsize}{!}{\includegraphics[clip,angle=0]{plot_fchar_rms_nil.eps}}
\caption{Correlation of the total fractional rms variability with the characteristic QPO frequency, derived from NICER data. Circles indicate observations taken during the first outburst and the rise of the second outburst, while squares indicate observations taken during  the decay of the second outburst (open symbols indicate QPOs with a detection significance below $3\sigma$). For observations in which more than one QPO is detected up-pointing (down-pointing) triangles indicate QPOs at the higher frequency (including upper harmonics) in observations taken during outburst rise (decay). In some observations taken during outburst rise (before day 117) two Lorentizans are need to fit the QPO at higher frequency. If this is the case, green stars indicate this additional feature. Observations taken during the third outburst (after day 330) are indicated using a diamond shape. The black asterisk indicates the QPO at 13.7 Hz found in the observation at the peak of the third outburst. The QPOs and upper harmonics observed during the fourth outburst are indicted by cyan cross points and pink x-shape points, respectively. The inset displays data points at frequencies below 2 Hz more clearly, while in the lower panel the x-axis is logarithmic to display the data points at even lower frequencies ($<0.5$ Hz) more clearly.
}
\label{Fig:fchar_rms_ni}
\end{figure}

\subsubsection{NICER}
Like for the \textit{Swift}/XRT data, the PDS of the NICER observations taken until day 5.2 can be well described by two BLN components. In most of the NICER observations taken after day 10 and until day 116 two QPOs are detected, with the characteristic frequency increasing from 0.03 to 1.57 Hz and from 0.08 to 3.17 Hz, respectively, although in some observations these features are detected at less than three sigma. The Q values lie between 2 and 10 for most observations. In some observations the lower detection significance is related to the fact that two Lorentzians, a broader and a narrow one, are required to obtain a proper fit of the QPO. In other observations a rather sharp, edge like feature is visible, which gives a high Q factor ($>10$) if fitted with a Lorentzian. Between day 35 and 116 most observations show an additional peaked noise component, with a characteristic frequency between 3 and 8 Hz. Details on individual observations can be found in Tables~\ref{Tab:pds1ni} and \ref{Tab:qpo_ni} and examples of PDS are shown in Fig.~\ref{Fig:pds}. From Fig.~\ref{Fig:qpo_evolv}, which shows the evolution of the characteristic frequency up until day 116, it seems that for most observations taken between day 92 and 109 only the QPO at higher frequency is detectable. The data point at lower frequencies on day 26.28 indicates that on this day the sub-harmonic was detected by chance (together with the QPO).

On day 116 two QPOs are detected, with the characteristic frequency increasing from 2 to 4 Hz and from 4 to 8 Hz, respectively. At the beginning of this observation the higher frequency QPO, which has a lower Q factor, is detected at a higher significance than the lower frequency QPO. Then a third QPO with a characteristic frequency around 0.6 -- 0.8 Hz appears. After this third QPO has disappeared again, the detection significance of the other two QPOs differs much less. The evolution of these QPOs is shown in Fig.~\ref{Fig:qpo_evolv} and details are given in Table~\ref{Tab:qpo_ni}.

In the observations taken after 2018 September 25th, QPOs with $>2.9\sigma$ and $Q>2$ are detected on days 199, 215 ,233, 241, 246 and have characteristic frequencies of 0.11, 0.17, 0.02, 0.49, 0.50 Hz, respectively. There are a few more QPOs with high Q values and characteristic frequencies between 0.6 and 0.03 Hz, that are detected around $2\sigma$. These QPOs are also included in Table~\ref{Tab:qpo_ni}.

During the third and fourth outburst (observations taken in 2019) we find some more QPOs with high Q values that are detected around $2\sigma$. Two QPOs are detected with more than 3$\sigma$ on days 375 and 380 and have characteristic frequencies of 0.23 and 3.17 Hz, respectively. At the brightest point of the third outburst at day 372 we find a QPO at a characteristic frequency of 13.75 Hz that is detected at 2.8$\sigma$. These QPOs are also included in Table~\ref{Tab:qpo_ni}.

The correlation between total fractional rms and characteristic frequency is shown in Fig.~\ref{Fig:fchar_rms_ni}. Most QPOs detected at $>3\sigma$ follow an anti correlation between rms and frequency. This anti correlation holds from the lowest frequency of 0.03 Hz to the highest at 8 Hz, and suggests that the QPOs are of type-C. The QPOs observed between day 92 and 109, where only the QPO at higher frequency is detected, seem to follow a steeper anti-correlation than the remaining QPOs. It also seems that the QPOs observed during the third and fourth outburst (most of them only detected $<3\sigma$) follow a flat correlation at $\sim43$\% rms.

We derive covariance spectra on long (5 s; 20 bins) and short (0.01 s; 50 bins) time scales \citep{2009MNRAS.397..666W,2015MNRAS.452.3666S}. The covariance ratios, obtained by dividing  spectra on long time scales by those on short time scales, for a sample of observations, are shown in Fig.~\ref{Fig:cov_ratio}, and show an increase towards lower energies, as observed \eg\ in GX\,339-4\ and Swift\,J1753.5-0125 \citep{2009MNRAS.397..666W,2015MNRAS.452.3666S}. Although the steepness of the increase changes over time it is present throughout the hard state over more than 100 days. We also find that the steepness of the increase is not correlated with the amount of rms variability and that it does not show a monotonic evolution along the outburst (see Fig.~\ref{Fig:cov_ratio}). We also derived covariance spectra and ratios for observations of the third outburst (Fig.~\ref{Fig:cov_ratio2}). Due to the coverage of this outburst by NICER data, we only have one observation before the outburst peak (and several observations after the peak). Considering only covariance ratios above 4 keV and below 0.8 keV, the ratios appear to be much flatter than the correlations observed during the first and second outburst. A comparison of the covariance ratios for the different outbursts in the 0.8 to 4 keV range shows that the covariance ratios of the third outburst show a decrease towards lower energies in this energy range. The covariance ratio of the fourth outburst that are obtained in a similar count rate range as those of the third outburst show the same shape as those of the third outburst. Although earlier phases of this outburst are covered compared to the coverage of the third outburst, we cannot determine the shape of the covariance ratios in these observations due to the low count rate.

\begin{figure*}
\resizebox{\hsize}{!}{\includegraphics[clip,angle=0]{plot_covspecratio2.eps}}
\caption{Covariance ratios of a sample of observations taken during the low-hard state with NICER. The small percentage numbers indicate the fractional rms values of the individual observations. The steepness of the increase towards lower energies neither correlates with the amount of rms variability nor with the outburst evolution.}
\label{Fig:cov_ratio}
\end{figure*}

\begin{figure}
\resizebox{\hsize}{!}{\includegraphics[clip,angle=0]{plot_covspecratio4.eps}}
\caption{Covariance ratios of a sample of observations taken during the third and fourth outburst with NICER. The small percentage numbers indicate the fractional rms values of the individual observations. For comparison the covariance ratios for the observation on day 4 are given.}
\label{Fig:cov_ratio2}
\end{figure}

\section[]{Discussion}
\label{Sec:dis}
We investigate the \textit{Swift}/XRT and NICER monitoring observations of MAXI\,J1820+070\ taken in 2018 and 2019. The HID derived from the \textit{Swift}/XRT data show the typical q-shape or turtle-head pattern observed in many black hole X-ray binary outbursts. The light curve reveals that MAXI\,J1820+070\ underwent four outbursts, with the source remaining in the hard state during its first outburst, while the rise of the second outburst corresponds with the transition to the HSS. These two outbursts are followed by two weaker outbursts during which MAXI\,J1820+070\ remains again in the hard state, so-called ``failed" outbursts. The first two outbursts can be regarded as a double outburst and a similar outburst of GX\,339-4\ has been observed in 2004 \citep{2007ApJ...657..400J,2014MNRAS.442.1767P}. In case of GX\,339-4\ the outburst lasted about twice as long as the double outburst of MAXI\,J1820+070\ reported here. Comparing the UV to the X-ray light curves shows that the first outburst, which is brighter than the second outburst in the UV bands, is much more peaked in the UV bands than in the X-ray band, and that the plateau seen in the X-ray data is not present in the UV data. During the decay of the second outburst a rebrightening is visible in the UVOT data that has no corresponding feature in the X-ray light curves.

For many observations during the first outburst and the rise of the second outburst QPOs have been detected in the \textit{Swift}/XRT and NICER data. For most of these observations the characteristic frequencies of the QPOs detected at about the same time in both data sets agree with each other. Due to the much better signal-to-noise ratio in the NICER PDS, QPOs at lower frequencies and weaker features (like upper harmonics or additional peaked noise components) can be detected in these data, which are not detectable in \textit{Swift}/XRT PDS. For the QPOs observed during outburst rise (up until day 116), the frequency range in which the QPOs are observed, the Q factors and the anti-correlation between total fractional rms and characteristic frequency suggest these QPOs to be type-C QPOs, consistent with the detection of these QPOs during the hard state. The QPOs detected during the first plateau, the first decay and at the beginning of the second rise are detected at rather low characteristic frequencies below 1 Hz. Only in the last two NICER observations taken before the source entered the soft state, QPOs with a characteristic frequency above 1 Hz are detected and the characteristic frequency increases from 1.57 to 3.94 Hz within 24.8 hrs, while the overall total fractional rms decreases from $10.4^{+1.2}_{-1.1}$ to $6.3^{+1.1}_{-1.0}$\%.  In the \textit{Swift}/XRT observations taken about 19 hrs after the last NICER observation the rms has decreased to $2^{+3}_{-2}$\% and MAXI\,J1820+070\ has entered the soft state, in which it stayed for about 81 days. On day 199.4 the rms has increased to $10.9^{+1.5}_{-1.3}$\% and two QPOs with characteristic frequencies of 0.11 and 0.20 Hz are detected. On day 202 and 203 QPOs with characteristic frequencies of about 0.5 and 0.4 Hz, respectively, are detected in both \textit{Swift}/XRT and NICER data. A few more QPOs are detected in the NICER data during the decay of the second outburst and during the two ``failed'' outbursts.

At the brightest point of the third outburst at day 372 we find a QPO at $\sim13.75$ Hz. It is worth noting that a QPO at $\sim21$ Hz has been observed in the initial low-hard state of XTE\,J1752--223 \citep{2010MNRAS.404L..94M}.

We would like to point out that during the first outburst and during most of the times of the second, third and fourth outburst, the characteristic frequency of the QPO remains below 1 Hz, while in other black hole X-ray binaries that normally show single-peak outbursts (including so-called failed outbursts), the characteristic frequency of type-C QPOs increases up to 10 Hz \citep{2013MNRAS.429.2655S}. The observation of type-C QPOs below 1 Hz during large parts of an outburst of a black hole X-ray binary is exceptional. A comparison of the QPOs and their properties observed here to those detected in the 2004 outburst of GX\,339-4, shows that during the 2004 outburst only one QPO with a characteristic frequency below 1 Hz was detected \citep{2011MNRAS.418.2292M}. QPO-like features at rather low characteristic frequencies (0.03 and 0.05 Hz) have also been observed in the initial low-hard state observations of XTE\,J1752--223 \citep{2010MNRAS.404L..94M}, but during its evolution the source showed QPOs at frequencies above 1 Hz in many observations \citep{2010ApJ...723.1817S}. QPOs at frequencies between 0.1 and 1 Hz have also be detected in Cyg\,X--1 \citep{2003A&A...407.1039P}. The QPOs below 1 Hz do not show a correlation with the source flux, as has been observed for example in \citet{2014ApJ...789..100S}. During the first plateau we observe an increase in the characteristic frequency from $\sim 0.04$ to $\sim0.3$ Hz, while the source luminosity is slightly decreasing.

In case of the 2004 double outburst of GX\,339-4\ only in one observation during the decay of the second outburst a QPO was detected. For MAXI\,J1820+070\ we found five QPOs in the NICER data of the decay of the second outburst. So it seems that double outbursts show QPOs only in few observations during  the decay of the second outburst compared to the higher numbers of QPOs observed during the decay of single-peak outbursts of other black hole X-ray binaries \citep{2013MNRAS.429.2655S}. However, one needs to be cautious when comparing the amount of QPOs detected in individual outbursts and in different parts of an outburst, as the number of QPOs found not only depends on the properties of the system, but also on the amount of observations that are available for each part of an outburst.

It is also remarkable that the increase in QPO frequency and the transition to the soft state take place in less than two days. This is an exceptional fast state transition (see Table~16 of \citet{2016ApJS..222...15T} that lists durations of state transitions for transient Galactic BHBs).

The detection of a disk wind in the optical during the hard state \citep{2019ApJ...879L...4M} together with the fast transition from the hard to the soft state, seems to indicate the presence of a standard accretion disk in the hard state of MAXI\,J1820+070. However, the fact that the characteristic frequencies of the QPOs remains below 1 Hz for most of the outburst seems to imply that the source remains in a configuration close to the one typically observed at the beginning of an outburst, when the accretion efficiency is still low and the disk is truncated far away from the black hole. In the Lense-Thirring precession model \citep{2009MNRAS.397L.101I} low QPO frequencies correspond to an accretion disk truncated at several tens of gravitational radius from the central black hole. Hence the observed low QPO frequencies imply an accretion disk truncated far away from the black hole and not much evolution of the truncation radius during the outburst. This is consistent with the results of \citet{2019Natur.565..198K} that the truncation radius of the accretion disk does not show much evolution during the hard state based on X-ray reverberation lags. An explanation why the source remains in a state of low accretion efficiency can be the presence of the disk wind in the hard state that has not been observed in any other black hole X-ray binary before, and that hampered the formation of a stable accretion regime.

We also investigate \textit{Swift}/UVOT light curves. In the UV light curves the first outburst is brighter than the second one, contrary to what has been observed in the X-rays, and it is much more peaked than in the X-rays. The source of UV emission in the hard state of BHBs can be the jet, the cool disk, the hot advection-dominated accretion flow or X-ray reprocessing in the accretion disk \citep{2003A&A...397..645M,2005ApJ...620..905Y, 2007ApJ...666.1129R,2009MNRAS.398.1638M}. As the shape of the light curves differs between the X-ray and UV bands, viscous dissipation in the disk does not seem to be the primarily cause of the UV emission. Jet quenching, which has been suggested as cause of a similar feature observed in \textit{Swift}\ data of the 2010 outburst of GX\,339-4\ \citep{2012MNRAS.427L..11Y}, also does not seem to be a good explanation here, as MAXI\,J1820+070\ remains in the hard state for about another 100 days, before the transition to the soft state takes place.

Furthermore, the \textit{Swift}/UVOT light curves show a rebrightening that peaked about 15 days after the source left the HSS. The X-ray light curves do not show a corresponding feature and keep on decaying. The rebrightening was accompanied by an increase in the X-ray hardness ratio, which indicates a change in the accretion morphology. This change in the accretion morphology is further supported by the finding that a linear correlation between the hard X-ray count rate and the near UV flux similar to the one at the beginning of the outburst is observed after the peak of the rebrightening feature. A similar UV rebrightening with continued decay in the X-rays and an increase in the X-ray hardness ratio was observed in the 2012 outburst of SWIFT\,J1910.2--0546 \citep{2014ApJ...784..122D}. The rebrighetning in the UVOT bands is reminiscent of the secondary nIR/optical maxima seen in other BHBs that have been ascribed to the revival of a jet \citep[e.g.][]{2012ApJ...753...55D,2013ApJ...779...95K}. Alternatively, the rebrightening may be interpreted as the recovery of a hot inner flow \citep{2011MNRAS.414.3330V,2013MNRAS.430.3196V}.

The covariance ratios derived from NICER data of the first and second outburst show an increase towards lower energies. In previous studies, we related the increase in covariance ratio to outbursts making a transition to the high-soft state, while for outbursts that remain in the hard states, we observed a flat or decreasing covariance ratio \citep{2016MNRAS.459.4038S,2016MNRAS.460.1946S}. Thus the increase in the covariance ratios of MAXI\,J1820+070\ indicated that the source should make a transition to the soft state, which in the end took place, after MAXI\,J1820+070\ stayed in the hard state at rather constant luminosity for about 116 days. A similar shape of the covariance ratio has also been found from \textit{XMM-Newton}\ observations taken during the first outburst of the 2004 double-outburst of GX\,339-4\ \citep{2019AN....340..314S}.  The steepness of the increase is not correlated with the amount of rms variability and it does not show a monotonic evolution along the outburst, contrary to what has been observed during the 2015 outburst decay of GX\,339-4\ \citep{2017ApJ...844....8S}. This finding gives further evidence that there is no correlation between the shape of the covariance ratio and the amount of fractional rms variability, as reported in \citet{2019AN....340..314S}.

The overall shape of the covariance ratios of the third and fourth, hard state only outbursts clearly differs from the shape seen in the first two outbursts. Between 0.8 and 4 keV the covariance ratios decay towards lower energies. There seems to be a slight increase to even lower energies and the shape is different from the ones reported in \citet{2016MNRAS.459.4038S} and \citet{2016MNRAS.460.1946S}. This difference may be related to the phase of the outburst during which the observations have been taken. Here, covariance ratios are obtained from NICER observations of MAXI\,J1820+070\ shortly before the peak of its third and fourth outburst and the outburst decays are well covered. Contrary, the observations studied in \citet{2016MNRAS.459.4038S} and \citet{2016MNRAS.460.1946S} have been taken during early outburst rise of the hard state only outbursts. Furthermore, the two hard state only outbursts of MAXI\,J1820+070\ follow a bright double outburst, while the outbursts studied in \citet{2016MNRAS.459.4038S} and \citet{2016MNRAS.460.1946S} are single hard state only outbursts that are separated by much longer time span from any previous outburst. Hence, the accretion geometry from the hard state only outbursts studied here may be different from the one in the outbursts studied in \citet{2016MNRAS.459.4038S} and \citet{2016MNRAS.460.1946S}. More observations of other hard state only outburst will be needed to study the evolution of the covariance ratios during this kind of outburst and to further investigate differences between full and hard state only outbursts.

\acknowledgments
We like to thank the anonymous reviewer for his/her thoughtful comments that have been helpful in improving the presentation of our results.
This project is supported by the Ministry of Science and Technology of
the Republic of China (Taiwan) through grants 105-2119-M-007-028-MY3 and 107-2811-M-007-029.
This research has made use of data obtained through the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA/Goddard Space Flight Center. This research has made use of the General High-energy Aperiodic Timing Software (GHATS) package developed by T.\ M.\ Belloni at INAF Osservatorio Astronomico di Brera. This work makes use of software tools provided by Simon Vaughan.

{\it Facilities:} \facility{\textit{Swift}, NICER}.
\software{HEAsoft, Isis, GHATS}
\clearpage

\bibliographystyle{aasjournal}

redpajama_set_name: RedPajamaArXiv


text:
EVALUATION OPPORTUNITIES
NPC INFORMATION
TOKYO 2020 INFO
Tokyo Paralympics preview: Para swimming day 8
Day eight in the Tokyo Aquatics Centre will offer plenty of Para swimming action and will mark the end of an era for Daniel Dias 18 Aug 2021
China's Jia Ma is the world record holder and the reigning world champion in the women's 100m breaststroke S11 coming to Tokyo as the favourite for the gold medal
ⒸCatherine Ivill/Getty Images
By Filip Ozbolt | For World Para Swimming
The first day of the new month will see 15 medal events in the eight day of competition at the Tokyo Aquatics Centre on 1 September.
Colombia will have high expectations for Wednesday's opening race as Carlos Serrano will be defending his Rio 2016 Paralympic title in the men's 100m breaststroke SB7. He is also also the current world champion and the fastest in the event this season.
RPC's Egor Efrosinin will be Zarate's main rival in Tokyo as the reigning European champion from Madeira 2020 and the third-fastest man this year. He finished second at the London 2019 World Championships behind Serrano.
Australia's world champion Tiffany Thomas-Kane will be facing USA's Para swimming legend Jessica Long for a place on the women's 100m breaststroke SB7 podium. Three out of Long's 23 Paralympic medals came in this event.
She is the second-fastest this year behind New Zealand's Nikita Howarth. Thomas-Kane appears in third in the 2021 rankings.
🔥 U N S T O P P A B L E 🔥
Happy Birthday to one of the most decorated Paralympians of all time, @JessicaLong ! 🥇🇺🇸#ParaSwimming @Paralympics @TeamUSA pic.twitter.com/qGVLMkCssY
— #ParaSwimming (@Para_swimming) March 1, 2021
Men's 100m freestyle S6, men's and women's 400m freestyle S10
Next up will be the men's 100m freestyle S6 in which Italy's Antonio Fantin is the fastest man in 2021. The former water polo player set his season-best time at the European Championships in Madeira where he also broke the world record.
To win his first Paralympic gold Fantin will have to beat Cuban legend Lorenzo Perez Escalona, the defending champion. His Rio 2016 gold was the first for Cuba in Para swimming at the Paralympic Games.
Thijs van Hofweegen from the Netherlands and Ukraine's Oleksandr Komarov are second and third in the world rankings this year. Lima 2019 Parapan American Games champion Nelson Crispin from Colombia has the fourth-fastest time this year.
Two more freestyle events will come up next at the Tokyo Aquatics Centre. First is the men's 400m freestyle S10 final with Ukraine's Maksym Krypak targeting another first place.
He is the defending Paralympic champion and world record holder. Also on Krypak's CV are the London 2019 World Championships and the Madeira 2020 European Championships gold.
Dutch Para swimmer Bas Takken and Italy's Stefano Raimondi have not been successful in their recent attempts to beat Krypak finishing second at the London 2019 Worlds and Madeira 2020 Euros, respectively.
Canada's Aurelie Rivard will be targeting her second consecutive Paralympic gold in the women's 400m freestyle S10. The 25-year-old from Quebec is the world record holder and is likely to face fierce competition from Poland's Oliwia Jablonska and Hungary's Bianka Pap.
Pap is the reigning European champion from Madeira and the fastest woman in the event this year. She is followed by Jablonska who won a silver medal at the Euros. The Polish swimmer won back-to-back world titles in 2017 and 2019.
🌌 -Universe: What about breaking a WR during the Heats? 🤔
🇮🇹 -Antonio Fantin: Why not? 😎
The Italian finished first in the men's 100m freestyle S6 Heat 1 with 1:10.86! ⏱
📊Results: https://t.co/vHKE8X9ptO #Madeira2020 @Paralympics @SDGaction @CIPnotizie @FINPARALIMPICO pic.twitter.com/sZ1ByM6O5q
— #ParaSwimming (@Para_swimming) May 17, 2021
Men's and women's 100m breaststroke SB11
The second set of breaststroke events will follow on day eight with China's world record holder Yang Bozun leading the way in the men's 100m breaststroke SB11. He is a two-time Paralympic champion and the London 2019 world champion.
Japan's Keiichi Kimura is a two-time Paralympic and four-time World Championships medallist who will be in search of his first gold in the event. He has the second-best time of the year behind Dutch swimmer Rogier Dorsman.
The women's 100m breaststroke SB11 is also dominated by a Chinese swimmer. Her name is Jia Ma and she is the reigning world champion and world record holder. The vision-impaired athlete is also the fastest in 2021 setting her time at the Berlin World Cup in June.
Liesette Bruinsma from the Netherlands and Ukraine's Yana Berezhna are second and third in the world rankings this year. Bruinsma is the Rio 2016 Paralympics and London 2019 Worlds silver medallist, while Berezhna holds the European title from Madeira in May.
"I still have a lot of room for improvement. The postponement would give me the time to squarely tackle the sport, and to think about other things too."
Overseas training re-invigorates 🇯🇵 Keiichi Kimura for @Tokyo2020 👉🏻 https://t.co/A3jJxwSlhD#ParaSwimming 🏊🏻 @Paralympics pic.twitter.com/U3ObLGkuGo
— #ParaSwimming (@Para_swimming) September 24, 2020
Men's and women's 200m individual medley SM9
The two individual medley events on 1 September will be the men's and women's 200m SM9. Italy's Federico Morlacchi is the reigning Paralympic and two-time world champion in the male event, although without a gold medal since the Dublin 2018 Euros.
French Para swimmer Ugo Didier triumphed at the Madeira 2020 European Championships, while the RPC's Andrei Kalina took gold at the London 2019 Worlds. Australia's Timothy Hodge has the best time of the year.
New Zealand and Sophie Pascoe will be hoping for more gold in the women's 200m SM9. The world record holder and five-time Paralympic medallist will be facing the likes of newly crowned European champion Zsofia Konkoly from Hungary, alongside Spain's Nuria Marques and Sarai Gascon.
Fantastic effort by 🇪🇸's @o_nuriamarques to take 🥇in the 200m IM SM9 and chase down by @britishswimming Toni Shaw who takes 🥈#Dublin2018 pic.twitter.com/EoTtXTWB68
— #ParaSwimming (@Para_swimming) August 17, 2018
There will be more breaststroke finals in the SB13 class with the men's and women's 100m. This will be a rare case in which Belarus' Ihar Boki comes to a race without being the favourite for gold. Uzbekistan's Firdavsbek Musabekov is the defending Paralympic champion and London 2019 Worlds silver medallist.
Germany's 19-year-old Taliso Engel is the current world champion and took European gold in Madeira in May both times ahead of Boki.
Another German will be going for gold in the women's 100m breaststroke SB13. Elena Krawzow is the fastest in the race this year and the overall World Series winner.
She will be battling for the top of the podium against London 2019 world champion Rebecca Redfern from Great Britain and USA's Coleen Young - silver and bronze from Rio 2016.
Future shines bright 🌟
🥇 17-years-old Taliso Engel 🇩🇪 conquered the men's 100m breaststroke SB13 at the London 2019 World Championships.
"I managed to defeat my two biggest competitors."#ParaSwimming @Paralympics @teamdpara pic.twitter.com/lhDYas7lkT
— #ParaSwimming (@Para_swimming) February 15, 2021
Men's 50m freestyle S5, women's 50m freestyle S8
The men's 50m freestyle S5 is set to be the highlight of day eight with Paralympic legend Daniel Dias making his final appearance in the Games before his retirement. He is a two-time Paralympic champion and five-time World Championships gold medallist in the event.
His main rivals in the Tokyo Aquatics Centre will be Chinese swimmers Weiyi Yuan and Tao Zheng, first and second in the world rankings this year. They have both set their season-best results at the Berlin World Cup two months ago.
Dias himself set the third-fastest time this year at the National Trials in June with Italy's Francesco Bocciardo following in fourth place. Italian Para swimmer is the back-to-back European champion and silver medallist from the Worlds in London in 2019.
Next will be the women's 50m freestyle S8. RPC's Viktoriia Ishchiulova is the reigning European champion and the fastest woman in the event this year.
Italy's Xenia Francesca Palazzo sits in second, while Brazil's Cecilia Araujo holds onto third place in 2021. The morning heats will see Alejandra Aybar become the first Para swimmer from Dominican Republic to compete at the Paralympic Games.
Alejandra Aybar on a journey to make her coach and her country proud! 🇩🇴🏊🏻♀
She was the first person from Dominican Republic to qualify for the Parapan American Games and hopes this will be repeated at @Tokyo2020.
👉 https://t.co/4J3gcSAkWx#ParaSwimming @Paralympics pic.twitter.com/3LVwKo40xW
— #ParaSwimming (@Para_swimming) March 12, 2021
Day eight will conclude with the men's and women's 100m breaststroke SB12. Ukraine's Oleksii Fedyna is the reigning world champion and the world record holder but he lost the European title to Azerbaijan's Vali Israfilov in May.
Uladzimir Izotau from Belarus is another one to watch as the defending Paralympic champion, five-time World Championships, and four-time European Championships medallist.
Brazil's Maria Gomes Santiago is the fastest woman this year in the women's 100m breaststroke SB12. She set her best time at the National Trials in June in Sao Paulo.
Complete schedule and results from the Para swimming competition at Tokyo 2020 will be available on Paralympic.org.
World Series 2021 winners ready for Tokyo
World Swimming Para Sport partner
Official website of World Para Swimming

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text:
The Talon
Amnesty International club hosts human rights talks
By Sitara Sriram|January 15, 2015
On Thursday January 15, Cephus Johnson, a prominent human rights activist and uncle of Oscar Grant, and Zahra Billoo, executive director of the San Francisco Chapter of the American-Islamic relations council, spoke about the issues of police brutality in the Eagle Theater during first and third period.
The event was organized primarily by the Amnesty International Club with support from the Youth Alliance Organization.
"We decided to get Cephus Johnson because his nephew's experience related exactly to what our focus is on this year: police brutality," Amnesty International co-president Anna Martirosyan said. "We also wanted to get a second speaker, and decided to contact Zahra Billoo because we thought that racial discrimination is a part of the issue of police brutality and Muslims experience that racism too. We also thought [having her speak would] be good in light of the attacks in France."
Johnson spoke first about how issues like increased militarization of the police has lead to a rapid growth in fatal shootings of civilians. His nephew, Oscar Grant, was fatally shot in 2009 by an Oakland BART police officer.
"What happened to Oscar, you got to understand is not just an isolated incident," Johnson said. "This is happening all over the country at an unprecedented rate. So we have to really understand the time that we're in, to understand what the movement is about and how to move forward in bringing about a change so we all have a right to life."
Additionally, Johnson stressed that issues like police brutality should matter to everyone, regardless of race.
"This is not a black issue," Johnson said. "This is an American issue. I want you to look at it like this. This is not about me, and it's really not even about you. It's about the babies that you bring into this world as it comes time for you to do that. And if you fail… to stand up and take the leadership of this movement what kind of life will your babies have? Think about that. If you…turn a blind eye to the issues that are happening around us today…[these issues] will come to your home."
In particular, Johnson stated his belief of the importance to have schools like Los Altos High take part in these conversations to enact change for the better.
"[When] Americans see young white students from a school that is not plagued with the issues that are being experienced in communities that are really in tremendous decay, and [instead see] schools like this where students that care take the leadership: that is a game changer," Johnson said.
Following Johnson, Billoo spoke about the war on terror and how racism and police brutality affect the Muslim community in America. Billio also spoke about the differences between how each minority is affected by perceptions of their distance from law enforcement.
"There is often times a disconnect with the African American Community and the Latino Community and their experience with the war on drugs and the law enforcement, and then the idea of being the model minority, of moving to this country to do one thing and do it well," Billio said. "Many American Muslims, many Asian Americans, many South Asians, many Arabs [mistakenly] thought, 'Well the war on drugs doesn't affect me. Because my kids don't do drugs. I'm a good immigrant. Because I'm going to pay my bills and stay away from the police and I'll be fine.'"
She also emphasized that although the US has shifted from a war on drugs to a war on terror, the nature of the racism people of color face at the hands of the law enforcement is still fundamentally the same.
"Now, anyone who looks like those people [actually involved in terrorist attacks] must be a terrorist," Billioo said. " [This is] no different from saying anyone who looks like that drug dealer must be a drug dealer. No different from the idea that if law enforcement says they're bad, then they must be bad."
The event concluded with music and poetry performances by Los Altos High School students. Senior Mehr Singh performed a slam poem, and senior Oscar Martinez performed a rap.
"The goal [of the event was] just to spread more awareness and talk about these issues because a lot of us, especially at Los Altos, feel like it might be really distant from us, though it's really not," Anna said.
Zahra Billoo
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District wellness coordinator says students might not yet see benefits of HAERT program, clarifies its objectives
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HomeNEWSI Am Absolutely Aware Of Nigerians' Sufferings, Says Buhari
I Am Absolutely Aware Of Nigerians' Sufferings, Says Buhari
President Muhammadu Buhari on Wednesday said he is absolutely aware of the suffering and pains Nigerians go through to feed themselves and their families under his administration.
The President who stated this during an interview with Channels Television, however, encouraged the people to see agriculture as an option.
"I'm absolutely aware of it (the suffering of the people)," he said. "But like I said, look at the vast population of Nigerians, only 2.5 percent of the land revenues are being cultivated.
"We realized this rather too late, but we have to go back to the land (agriculture)."
During the interview, President Buhari also spoke about a number of burning issues including security, economy, and politics, among others.
He assessed his administration's performance since he assumed office in 2015, especially in the three key areas of security, improving the economy, and fighting corruption.
The President believes the situation has improved and his administration has continued to improve the economy, despite the nation's dwindling resources.
"For Nigerians to be fair with this administration is to try and find out from the time we won the election in 2015 till now, in the three promises we made – improving security, improving the economy, and trying to fight corruption; securing the country in the North East, if you ask anybody from Borno State, from Yobe State, from Adamawa State, there are a number of local governments – about 18 – that were in the hands of Boko Haram, no local government now is strictly in the hands of Boko Haram or ISWAP. So, in that respect, we have done something," he asserted.
"In the economy, don't forget and I challenge so many people to go and check with the central bank or NNPC. The production from 1999 to 2014 was 2.1 million barrels per day. When we came, somehow the militants were unleashed in the South-South.
"Production went down to half a million barrels per day, and coincidentally, the price again collapsed to about $37 per barrel. But look at what we did within the time frame and the resources available to us relative to the government we inherited."
Watch Signature TV News Highlights - Nigerian military warns governors, others against wearing camouflage uniform
Lagos Govt To Provide Land For Shangisha Landlords To End Magodo Dispute


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Битва при Нехтансмере — битва между англами и пиктами. Произошла в , маленькой деревне, расположенной неподалёку от города Форфар в Ангусе, 20 мая 685 года. Результаты битвы во многом определили последующую историю Англии и Шотландии.

О битве мало достоверной информации, возможно из-за особенностей местности англы были окружены и уничтожены во время неожиданной атаки. Победа пиктов была абсолютной, король Нортумбрии Эгфрит и почти вся его армия погибли. Затем пикты освободили Каледонию от оставшихся англов, которые занимали эти земли около 30 лет.

Значение битвы чрезвычайно высоко по двум причинам:
 Победа пиктов нарушила баланс силы между германскими племенами, оккупировавшими Британию — теперь сила была не на стороне англов, а на стороне саксов, таким образом, предопределив последующие войны между Шотландией и Англией.
 Могущество англов в Британии было навсегда разрушено, приведя к их неспособности завоевать то, что сейчас является Шотландией. Если бы они смогли сделать это, то Шотландия могла бы никогда не существовать, и весь остров был бы в конечном итоге под контролем одного властителя — англов.

После битвы при Нехтансмере скотты и пикты воевали между собой, а также боролись с набегами викингов. В итоге это привело к объединению двух стран Дал Риады и королевства пиктов в одну — Альба. В течение двух столетий они присоединили также Стратклайд и вновь разбили англов, захватив Лотиан и основав государство Шотландия.

Ссылки
 tartans.com — Battle of Nechtansmere
 pictavia.org.uk — Battle Of Dunnichen
 To view the possible location as it is now
 Фотографии монумента
 История пиктов на сайте BBC

Сражения по алфавиту
Сражения Нортумбрии
Сражения пиктов
Сражения в Шотландии
Сражения 685 года
VII век в Англии
VII век в Шотландии

redpajama_set_name: RedPajamaWikipedia


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FAB Education Grants Funds Lux et Veritas at NSU Art Museum
May 2, 2022 Author: admin Category: Blog
According to the museum, "Lux et Veritas" focuses on a generation of artists of color who attended Yale School of Art for graduate studies between 2000 and 2010. The exhibition's title alludes to Yale University's motto, "Lux et Veritas," which translates from Latin to "Light and Truth." In the context of this exhibition, the title references how these artists thought with critical complexity about their work and their movement through institutional structures. Featured artists include Mike Cloud, William Cordova, Njideka Akunyili Crosby, Abigail DeVille, Torkwase Dyson, John Espinosa, Luis Gispert, Rashawn Griffin, Leslie Hewitt, Loren Holland, Titus Kaphar, Jamerry Kim, Eric N. Mack, Wardell Milan, Wangechi Mutu, Mamiko Otsubo, Ronny Qevedo, Mickalene Thomas, Anna Tsouhlarakis, Shoshanna Weinberger and Kehinde Wiley.
As with similar programs, Yale School of Art, in New Haven, Connecticut, had not been historically diverse, which spurred these art students to form affiliations across the departments of painting, graphic design, sculpture, photography and art history. They filled gaps in the school's curriculum and counteracted the lack of diversity among the faculty by inviting artists, curators and writers of color as advisors and guest speakers, developing an interdisciplinary forum, publishing art journals, organizing exhibitions and documenting their experiences in video and photography. The relationships they formed at school evolved into communities that networked and provided essential support and feedback for one another, often passing on these efforts beyond graduate study. Their reevaluation of the Western art canon, and commitment to the method and practice of teaching has contributed to a greater recognition of artists of color, challenged stereotypes and enriched the overall shared spaces of learning and thinking about art and the art praxis.
"Lux et Veritas" provides a public forum in which to address the directions these artists took based on the explorations that began in graduate school and were instilled thereafter in their practice. The exhibition is curated by Bonnie Clearwater, director and chief curator, NSU Art Museum. Mike Cloud (Yale, MFA 2003), William Cordova (Yale, MFA 2004), Leslie Hewitt (Yale, MFA 2004) and Irene V. Small, associate professor, Contemporary Art & Criticism, Princeton University (Yale, Ph.D. 2008) are advisors on the exhibition. Oral histories with the artists who attended the School of Art provided significant insight into their experiences, relationships, and work.
The exhibit is currently on display until October 23. It is presented by S. Donald Sussman and sponsored by Funding Arts Broward.


Editor's Note: First, for a bit of housekeeping. There was a great amount of diversified feedback on this article. I so appreciate the time you kind folks took to write up responses. It shows that people care about what is written here and the ideas conveyed.
When one or two people differ from the rest on the article, it's perhaps a matter of their opinion. But when it's in larger numbers as the last article, it generally falls on the writer not conveying his ideas well – his intent not being clear. Allow me to attempt a bit of that now.
Diversify your sources. Someone may seem a "master" because of a marketing effort, and likely they do some techniques exceptionally well. But balance that by seeing what other people say about a particular process, both on YouTube and in your online and offline communities. Generate conversation and discover new sources. Often times, "masters" aren't immediately found in a Google search. Now, on to the article.
In our woodworking world, "celebrity" woodworkers are often cited, shared, and their techniques rehearsed. Individuals are propped up to be masters of their craft, regardless of their technique. It's often because they are also the ones with the marketing effort.
Cosman and Sellers. Schwarz and Underhill. The list is long and ever increasing. Forums and message boards have their own individual celebrities. And fan clubs will fight vigorously for their particular hero, sometimes simply because that hero was the squeaky wheel which got the grease.
It's not all a sham; some "stars" are rightly there. They not only convey ideas well, but the ideas conveyed utilize good technique. They work with skill, care, and respect, and usually aren't propped up by sponsorships.
And it's not the fault of the "stars" that the people who gravitate towards them tend to be those who are searching for & learning a new skill, and are eager to share that new skill with others.
But I find more often that the real masters are the ones who aren't propping themselves up as such, or are put on a pedestal.
There are those who do, and those who talk do-do.
The real masters will tell you they aren't that, but their work speaks for itself. They're the ones who are making more and talking less.
They're the ones who tend not to focus so much on the quantity of their online posts, but let the quality of their work speak forth.
Real masters I've encountered will happily answer your questions, and often pull you aside privately to give input and feedback. I don't usually care for unsolicited advice, but a real master will present it in such a way that it was almost like you had asked for his opinion.
Perhaps most importantly, real masters are always seeking to learn more themselves, and never call themselves masters.
Be careful that popularity of an individual does not become a litmus for good woodworking technique. But balance that knowing that those who are popular rose there for a reason. Life is a balance and we must be discerning.
As you ingest woodworking information in this media-rich internet world, pay more attention to the quality of work, the techniques used (and cross-reference for what you don't know), and not necessarily the ease of following their YouTube channel. Or blog.
Editor's Note: By naming the individuals in paragraph two, I am not pushing their "master" status one way or another.


§ 50-2 Running at large.
§ 50-3 Dangerous or diseased dogs.
§ 50-4 Walking and exercising dogs.
§ 50-6 Seizure and impoundment.
§ 50-8 Exceptions to regulations.
§ 50-9 Violations and penalties.
Any person having the custody or control of any dog or who permits a dog to remain on his or her property.
It shall be unlawful for the owner of any dog to permit said animal to run at large, at any time, within the Township of Daugherty. For the purpose of this section, any dog not restrained by leash or by other suitable means of control, when said dog is not upon the premises of the owner thereof, shall be considered to be running at large.
Except as authorized by 3 P.S. § 459-502-A et seq., it shall be unlawful to keep at any place within the Township any dangerous or vicious dog or any dog with a contagious or infectious disease, when said dog by reason of its disposition or state of health constitutes a clear and present danger to the health, safety and welfare of any person or persons.
It shall be unlawful for any owner to walk or exercise any dog not upon the premises of the owner thereof unless the said dog is restrained by a leash held by the owner or persons designated by the owner who shall be capable of properly restraining the movements of said dog. No dog shall be walked or exercised other than upon the premises of the owner thereof unless the owner or person accompanying the said dog shall carry and use suitable facilities for gathering up any and all matter excreted by the said dog during the course of said walk or exercise, and said owner or person shall be responsible for promptly removing said material and placing it in the container carried for that purpose, and failure to do so shall constitute a violation of this article.
It shall be unlawful for any person to keep, own, harbor, maintain or possess any dog that barks and/or makes noise repeatedly that disturbs the quiet of the neighborhood and is a continuous annoyance to the residents of that neighborhood.
Any dog that barks, howls, yelps or bays or makes repeated noise that disturbs the quiet of the neighborhood for more than one hour on two or more days during a seven-day period shall be deemed to be in violation of this section.
This section shall not apply to any dog located on a farm while it is being used on that farm for agricultural purposes.
Any dog found running at large in the Township or disturbing the peace as set forth in § 50-5 hereof or infected with a contagious disease is hereby declared to be a public nuisance. Any dog found running at large upon public or private property or upon any street in the Township may be seized or taken by any Township police officer, by any humane society officer, or by any other person as authorized by the Supervisors and impounded in a facility designated by the Township. For any dog bearing a proper license tag or permanent identification, the owner shall be given notice to reclaim the dog within five days after receipt of the notice and shall be allowed to reclaim the dog after paying a penalty to the Township of $15 and all fees or costs of impoundment. If the dog is not reclaimed or the penalty is not paid within the five-day period after receipt of notice as herein provided, the dog may be sold or given to a humane society or association for the prevention of cruelty to animals. For unlicensed dogs, the dog will be held for 48 hours at a licensed kennel, and if is unclaimed after 48 hours, it may be humanely killed or given to a humane society or association for the prevention of cruelty to animals.
It shall be unlawful for any person to permit any dog within his control to deposit on any public property, or upon any private property without consent of the owner of the property, any animal wastes or feces. Permitting the deposit of waste or feces by any such dog shall constitute a public nuisance. Where the owner or person in control of the dog immediately removes all feces deposited by the dog upon private property and in a sanitary manner disposes of the feces, that nuisance as to that owner shall be considered abated.
This article shall not apply to Seeing Eye dogs engaged in guiding persons with defects of eyesight.
Any person violating any provision of this article, upon conviction before a Magisterial District Judge of this county, shall be sentenced to pay a fine not exceeding $300 and the cost of prosecution and/or to undergo imprisonment for a term not to exceed 90 days.
All ordinances or any part of any ordinance insofar as it is inconsistent herewith are hereby repealed, and in particular Ordinance No. 106 is hereby repealed.