Swift follow-up of gravitational wave triggers: results from the first aLIGO run and optimisation for the future (1606.05001)
P.A. Evans, J.A. Kennea, D.M. Palmer, M. Bilicki, J.P. Osborne, P.T. O'Brien, N.R. Tanvir, A.Y. Lien, S.D. Barthelmy, D. N. Burrows, S. Campana, S.B. Cenko, V. D'Elia, N. Gehrels, F. E. Marshall, K.L. Page, M.Perri, B. Sbarufatti, M.H. Siegel, G. Tagliaferri, E. Troja
Dec. 20, 2018 astro-ph.IM, astro-ph.HE
During its first observing run, in late 2015, the advanced LIGO facility announced 3 gravitational wave (GW) triggers to electromagnetic follow-up partners. Two of these have since been confirmed as being of astrophysical origin: both are binary black hole mergers at ~500 Mpc; the other trigger was later found not to be astrophysical. In this paper we report on the Swift follow up observations of the second and third triggers, including details of 21 X-ray sources detected; none of which can be associated with the GW event. We also consider the challenges that the next GW observing run will bring as the sensitivity and hence typical distance of GW events will increase. We discuss how to effectively use galaxy catalogues to prioritise areas for follow up, especially in the presence of distance estimates from the GW data. We also consider two galaxy catalogues and suggest that the high completeness at larger distances of the 2MASS Photometric Redshift Catalogue (2MPZ) makes it very well suited to optimise Swift follow-up observations.
Swift and NuSTAR observations of GW170817: detection of a blue kilonova (1710.05437)
P.A. Evans, S.B. Cenko, J.A. Kennea, S.W.K. Emery, N.P.M. Kuin, O. Korobkin, R.T. Wollaeger, C.L. Fryer, K.K. Madsen, F.A. Harrison, Y. Xu, E. Nakar, K. Hotokezaka, A. Lien, S. Campana, S.R. Oates, E. Troja, A.A. Breeveld, F. E. Marshall, S.D. Barthelmy, A. P. Beardmore, D.N. Burrows, G. Cusumano, A. D'Ai, P. D'Avanzo, V. D'Elia, M. de Pasquale, W.P. Even, C.J. Fontes, K. Forster, J. Garcia, P. Giommi, B. Grefenstette, C. Gronwall, D.H. Hartmann, M. Heida, A.L. Hungerford, M.M. Kasliwal, H.A. Krimm, A.J. Levan, D. Malesani, A. Melandri, H. Miyasaka, J.A. Nousek, P.T. O'Brien, J.P. Osborne, C. Pagani, K.L. Page, D.M. Palmer, M. Perri, S. Pike, J.L. Racusin, S. Rosswog, M.H. Siegel, T. Sakamoto, B. Sbarufatti, G. Tagliaferri, N.R. Tanvir, A. Tohuvavohu
With the first direct detection of merging black holes in 2015, the era of gravitational wave (GW) astrophysics began. A complete picture of compact object mergers, however, requires the detection of an electromagnetic (EM) counterpart. We report ultraviolet (UV) and X-ray observations by Swift and the Nuclear Spectroscopic Telescope ARray (NuSTAR) of the EM counterpart of the binary neutron star merger GW170817. The bright, rapidly fading ultraviolet emission indicates a high mass ($\approx0.03$ solar masses) wind-driven outflow with moderate electron fraction ($Y_{e}\approx0.27$). Combined with the X-ray limits, we favor an observer viewing angle of $\approx 30^{\circ}$ away from the orbital rotation axis, which avoids both obscuration from the heaviest elements in the orbital plane and a direct view of any ultra-relativistic, highly collimated ejecta (a gamma-ray burst afterglow).
Colour variations in the GRB 120327A afterglow (1709.08865)
A. Melandri, S. Covino, E. Zaninoni, S. Campana, J. Bolmer, B. E. Cobb, J. Gorosabel, J.-W. Kim, P. Kuin, D. Kuroda, D. Malesani, C. G. Mundell, F. Nappo, B. Sbarufatti, R. J. Smith, I. A. Steele, M. Topinka, A. S. Trotter, F. J. Virgili, M. G. Bernardini, P. D'Avanzo, V. D'Elia, D. Fugazza, G. Ghirlanda, A. Gomboc, J. Greiner, C. Guidorzi, J. B. Haislip, H. Hanayama, L. Hanlon, M. Im, K. M. Ivarsen, J. Japelj, M. Jelinek, N. Kawai, S. Kobayashi, D. Kopac, A. P. LaCluyze, A. Martin-Carrillo, D. Murphy, D. E. Reichart, R. Salvaterra, O. S. Salafia, G. Tagliaferri, S. D. Vergani
Sept. 26, 2017 astro-ph.CO, astro-ph.HE
We present a comprehensive temporal and spectral analysis of the long Swift GRB 120327A afterglow data to investigate the possible causes of the observed early time colour variations. We collected data from various instruments/telescopes in different bands (X-rays, ultra- violet, optical and near-infrared) and determined the shapes of the afterglow early-time light curves. We studied the overall temporal behaviour and the spectral energy distributions from early to late times. The ultra-violet, optical, and near-infrared light curves can be modelled with a single power-law component between 200 and 2e4 s after the burst event. The X-ray light curve shows a canonical steep-shallow-steep behaviour, typical of long gamma-ray bursts. At early times a colour variation is observed in the ultra-violet/optical bands, while at very late times a hint of a re-brightening is visible. The observed early time colour change can be explained as a variation in the intrinsic optical spectral index, rather than an evolution of the optical extinction.
X-rays from the location of the Bactrian Transient ASASSN-15lh (1610.01632)
R. Margutti, B. D. Metzger, R. Chornock, D. Milisavljevic, E. Berger, P. K. Blanchard, C. Guidorzi, G. Migliori, A. Kamble, R. Lunnan, M. Nicholl, D. L. Coppejans, S. Dall'Osso, M.R. Drout, R. Perna, B. Sbarufatti
Oct. 5, 2016 astro-ph.HE
We present the detection of persistent soft X-ray radiation with L_x ~ 10^41-10^42 erg/s at the location of the extremely luminous, double-humped transient ASASSN-15lh as revealed by Chandra and Swift. We interpret this finding in the context of observations from our multiwavelength campaign, which revealed the presence of weak narrow nebular emission features from the host-galaxy nucleus and clear differences with respect to superluminous supernova optical spectra. Significant UV flux variability on short time-scales detected at the time of the re-brightening disfavors the shock interaction scenario as the source of energy powering the long-lived UV emission, while deep radio limits exclude the presence of relativistic jets propagating into a low-density environment. We propose a model where the extreme luminosity and double-peaked temporal structure of ASASSN-15lh is powered by a central source of ionizing radiation that produces a sudden change of the ejecta opacity at later times. As a result, UV radiation can more easily escape, producing the second bump in the light-curve. We discuss different interpretations for the intrinsic nature of the ionizing source. We conclude that, IF the X-ray source is physically associated with the optical-UV transient, ASASSN-15lh most likely represents the tidal disruption of a main-sequence star by the most massive spinning black hole detected to date. In this case, ASASSN-15lh and similar events discovered in the future would constitute the most direct probes of very massive, dormant, spinning, supermassive black holes in galaxies. Future monitoring of the X-rays may allow us to distinguish between the supernova and TDE hypothesis.
Evidence for the magnetar nature of 1E 161348-5055 in RCW 103 (1607.04264)
A. D'Aì, P.A. Evans, D.N. Burrows, N.P.M. Kuin, D.A. Kann, S. Campana, A. Maselli, P. Romano, G. Cusumano, V. La Parola, S.D. Barthelmy, A.P. Beardmore, S.B. Cenko, M. De Pasquale, N. Gehrels, J. Greiner, J.A. Kennea, S.Klose, A. Melandri, J.A. Nousek, J.P. Osborne, D.M. Palmer, B. Sbarufatti, P. Schady, M.H. Siegel, G. Tagliaferri, R. Yates, S. Zane
Sept. 4, 2016 astro-ph.HE
We report on the detection of a bright, short, structured X-ray burst coming from the supernova remnant RCW 103 on 2016 June 22 caught by the Swift/BAT monitor, and on the follow-up campaign made with Swift/XRT, Swift/UVOT and the optical/NIR GROND detector. The characteristics of this flash, such as duration, and spectral shape, are consistent with typical short bursts observed from soft gamma repeaters. The BAT error circle at 68 per cent confidence range encloses the point-like X-ray source at the centre of the nebula, 1E161348-5055. Its nature has been long debated due to a periodicity of 6.67 hr in X-rays, which could indicate either an extremely slow pulsating neutron star, or the orbital period of a very compact X-ray binary system. We found that 20 min before the BAT trigger, the soft X-ray emission of 1E161348-5055 was a factor of ~100 higher than measured 2 yr earlier, indicating that an outburst had already started. By comparing the spectral and timing characteristics of the source in the two years before the outburst and after the BAT event, we find that, besides a change in luminosity and spectral shape, also the 6.67 hr pulsed profile has significantly changed with a clear phase shift with respect to its low-flux profile. The UV/optical/NIR observations did not reveal any counterpart at the position of 1E161348-5055. Based on these findings, we associate the BAT burst with 1E161348-5055, we classify it as a magnetar, and pinpoint the 6.67 hr periodicity as the magnetar spin period.
First simultaneous microlensing observations by two space telescopes: $Spitzer$ & $Swift$ reveal a brown dwarf in event OGLE-2015-BLG-1319 (1606.02292)
Y. Shvartzvald, Z. Li, A. Udalski, A. Gould, T. Sumi, R. A. Street, S. Calchi Novati, M. Hundertmark, V. Bozza, C. Beichman, G. Bryden, S. Carey, J. Drummond, M. Fausnaugh, B. S. Gaudi, C. B. Henderson, T.G. Tan, B. Wibking, R. W. Pogge, J. C. Yee, W. Zhu, Y. Tsapras, E. Bachelet, M. Dominik, D. M. Bramich, A. Cassan, R. Figuera Jaimes, K. Horne, C. Ranc, R. Schmidt, C. Snodgrass, J. Wambsganss, I. A. Steele, J. Menzies, S. Mao, R. Poleski, M. Pawlak, M.K. Szymański, J. Skowron, P. Mróz, S. Kozłowski, Ł. Wyrzykowski, P. Pietrukowicz, I. Soszyński, K. Ulaczyk, F. Abe, Y. Asakura, R. K. Barry, D. P. Bennett, A. Bhattacharya, I.A. Bond, M. Freeman, Y. Hirao, Y. Itow, N. Koshimoto, M.C.A. Li, C.H. Ling, K. Masuda, A. Fukui, Y. Matsubara, Y. Muraki, M. Nagakane, T. Nishioka, K. Ohnishi, H. Oyokawa, N. J. Rattenbury, To. Saito, A. Sharan, D.J. Sullivan, D. Suzuki, P. J. Tristram, A. Yonehara, U. G. Jørgensen, M. J. Burgdorf, S. Ciceri, G. D'Ago, D. F. Evans, T. C. Hinse, N. Kains, E. Kerins, H. Korhonen, L. Mancini, A. Popovas, M. Rabus, S. Rahvar, G. Scarpetta, J. Skottfelt, J. Southworth, N. Peixinho, P. Verma, B. Sbarufatti, J. A. Kennea, N. Gehrels
June 7, 2016 astro-ph.EP
Simultaneous observations of microlensing events from multiple locations allow for the breaking of degeneracies between the physical properties of the lensing system, specifically by exploring different regions of the lens plane and by directly measuring the "microlens parallax". We report the discovery of a 30-55$M_J$ brown dwarf orbiting a K dwarf in microlensing event OGLE-2015-BLG-1319. The system is located at a distance of $\sim$5 kpc toward the Galactic bulge. The event was observed by several ground-based groups as well as by $Spitzer$ and $Swift$, allowing the measurement of the physical properties. However, the event is still subject to an 8-fold degeneracy, in particular the well-known close-wide degeneracy, and thus the projected separation between the two lens components is either $\sim$0.25 AU or $\sim$45 AU. This is the first microlensing event observed by $Swift$, with the UVOT camera. We study the region of microlensing parameter space to which $Swift$ is sensitive, finding that while for this event $Swift$ could not measure the microlens parallax with respect to ground-based observations, it can be important for other events. Specifically, for detecting nearby brown dwarfs and free-floating planets in high magnification events.
Spectral and timing evolution of the bright failed outburst of the transient black hole Swift J174510.8-262411 (1512.02805)
M. Del Santo, T. M. Belloni, J. A. Tomsick, B. Sbarufatti, M. Cadolle Bel, P. Casella, A. Castro-Tirado, S. Corbel, V. Grinberg, J. Homan, E. Kalemci, S. Motta, T. Munoz-Darias, K. Pottschmidt, J. Rodriguez, J. Wilms
Dec. 9, 2015 astro-ph.HE
We studied time variability and spectral evolution of the Galactic black hole transient Swift J174510.8-262411 during the first phase of its outburst. INTEGRAL and Swift observations collected from 2012 September 16 until October 30 have been used. The total squared fractional rms values did not drop below 5% and QPOs, when present, were type-C, indicating that the source never made the transition to the soft-intermediate state. Even though the source was very bright (up to 1 Crab in hard X-rays), it showed a so called failed outburst as it never reached the soft state. XRT and IBIS broad band spectra, well represented by a hybrid thermal/non-thermalComptonisationmodel, showed physical parameters characteristic of the hard and intermediate states. In particular, the derived temperature of the geometrically thin disc black body was about 0.6 keV at maximum.We found a clear decline of the optical depth of the corona electrons (close to values of 0.1), as well as of the total compactness ratio lh/ls. The hard-to-hard/intermediate state spectral transition is mainly driven by the increase in the soft photon flux in the corona, rather than small variations of the electron heating. This, associated with the increasing of the disc temperature, is consistent with a disc moving towards the compact object scenario, i.e. the truncated-disc model. Moreover, this scenario is consistent with the decreasing fractional squared rms and increasing of the noise and QPO frequency. In our final group of observations, we found that the contribution from the non-thermal Comptonisation to the total power supplied to the plasma is 0.59+0.02/-0.05 and that the thermal electrons cool to kTe<26 keV.
The central engine of GRB 130831A and the energy breakdown of a relativistic explosion (1509.09234)
M. De Pasquale, S. R. Oates, J. L. Racusin, D. A. Kann, B. Zhang, A. Pozanenko, A. A. Volnova, A. Trotter, N. Frank, A. Cucchiara, E. Troja, B. Sbarufatti, N. R. Butler, S. Schulze, Z. Cano, M. J. Page, A. J. Castro-Tirado, J. Gorosabel, A. Lien, O. Fox, O. Littlejohns, J. S. Bloom, J. X. Prochaska, J. A. de Diego, J. Gonzalez, M. G. Richer, C. Román-Zúñiga, A. M. Watson, N. Gehrels, H. Moseley, A. Kutyrev, S. Zane, V. Hoette, R. R. Russell, V. Rumyantsev, E. Klunko, O. Burkhonov, A. A. Breeveld, D. E. Reichart, J. B. Haislip
Sept. 30, 2015 astro-ph.HE
Gamma-ray bursts (GRBs) are the most luminous explosions in the universe, yet the nature and physical properties of their energy sources are far from understood. Very important clues, however, can be inferred by studying the afterglows of these events. We present optical and X-ray observations of GRB 130831A obtained by Swift, Chandra, Skynet, RATIR, Maidanak, ISON, NOT, LT and GTC. This burst shows a steep drop in the X-ray light-curve at $\simeq 10^5$ s after the trigger, with a power-law decay index of $\alpha \sim 6$. Such a rare behaviour cannot be explained by the standard forward shock (FS) model and indicates that the emission, up to the fast decay at $10^5$ s, must be of "internal origin", produced by a dissipation process within an ultrarelativistic outflow. We propose that the source of such an outflow, which must produce the X-ray flux for $\simeq 1$ day in the cosmological rest frame, is a newly born magnetar or black hole. After the drop, the faint X-ray afterglow continues with a much shallower decay. The optical emission, on the other hand, shows no break across the X-ray steep decrease, and the late-time decays of both the X-ray and optical are consistent. Using both the X-ray and optical data, we show that the emission after $\simeq 10^5$ s can be explained well by the FS model. We model our data to derive the kinetic energy of the ejecta and thus measure the efficiency of the central engine of a GRB with emission of internal origin visible for a long time. Furthermore, we break down the energy budget of this GRB into the prompt emission, the late internal dissipation, the kinetic energy of the relativistic ejecta, and compare it with the energy of the associated supernova, SN 2013fu.
There is a short gamma-ray burst prompt phase at the beginning of each long one (1408.1608)
G. Calderone, G. Ghirlanda, G. Ghisellini, M.G. Bernardini, S. Campana, S. Covino, D'Avanzo, V. D'Elia, A. Melandri, R. Salvaterra, B. Sbarufatti, G. Tagliaferri
Jan. 21, 2015 astro-ph.HE
We compare the prompt intrinsic spectral properties of a sample of short Gamma--ray Burst (GRB) with the first 0.3 seconds (rest frame) of long GRBs observed by Fermi/GBM. We find that short GRBs and the first part of long GRBs lie on the same E_p--E_iso correlation, that is parallel to the relation for the time averaged spectra of long GRBs. Moreover, they are indistinguishable in the E_p--L_iso plane. This suggests that the emission mechanism is the same for short and for the beginning of long events, and both short and long GRBs are very similar phenomena, occurring on different timescales. If the central engine of a long GRB would stop after ~0.3 * (1+z) seconds the resulting event would be spectrally indistinguishable from a short GRB.
A complete sample of bright Swift short Gamma-Ray Bursts (1405.5131)
P. D'Avanzo, R. Salvaterra, M. G. Bernardini, L. Nava, S. Campana, S. Covino, V. D'Elia, G. Ghirlanda, G. Ghisellini, A. Melandri, B. Sbarufatti, S. D. Vergani, G. Tagliaferri
May 20, 2014 astro-ph.HE
We present a carefully selected sample of short gamma-ray bursts (SGRBs) observed by the Swift satellite up to June 2013. Inspired by the criteria we used to build a similar sample of bright long GRBs (the BAT6 sample), we selected SGRBs with favorable observing conditions for the redshift determination on ground, ending up with a sample of 36 events, almost half of which with a redshift measure. The redshift completeness increases up to about 70% (with an average redshift value of z = 0.85) by restricting to those events that are bright in the 15-150 keV Swift Burst Alert Telescope energy band. Such flux-limited sample minimizes any redshift-related selection effects, and can provide a robust base for the study of the energetics, redshift distribution and environment of the Swift bright population of SGRBs. For all the events of the sample we derived the prompt and afterglow emission in both the observer and (when possible) rest frame and tested the consistency with the correlations valid for long GRBs. The redshift and intrinsic X-ray absorbing column density distributions we obtain are consistent with the scenario of SGRBs originated by the coalescence of compact objects in primordial binaries, with a possible minor contribution (~10%-25%) of binaries formed by dynamical capture (or experiencing large natal kicks). This sample is expected to significantly increase with further years of Swift activity.
Optical and X-ray Rest-frame Light Curves of the BAT6 sample (1403.3245)
A. Melandri, S. Covino, D. Rogantini, R. Salvaterra, B. Sbarufatti, M. G. Bernardini, S. Campana, P. D'Avanzo, V. D'Elia, D. Fugazza, G. Ghirlanda, G. Ghisellini, L. Nava, S. D. Vergani, G. Tagliaferri
March 13, 2014 astro-ph.CO, astro-ph.HE
We present the rest-frame light curves in the optical and X-ray bands of an unbiased and complete sample of Swift long Gamma-Ray Bursts (GRBs), namely the BAT6 sample. The unbiased BAT6 sample (consisting of 58 events) has the highest level of completeness in redshift ($\sim$ 95%), allowing us to compute the rest-frame X-ray and optical light curves for 55 and 47 objects, respectively. We compute the X-ray and optical luminosities accounting for any possible source of absorption (Galactic and intrinsic) that could affect the observed fluxes in these two bands. We compare the behaviour observed in the X-ray and in the optical bands to assess the relative contribution of the emission during the prompt and afterglow phases. We unarguably demonstrate that the GRBs rest-frame optical luminosity distribution is not bimodal, being rather clustered around the mean value Log(L$_{R}$) = 29.9 $\pm$ 0.8 when estimated at a rest frame time of 12 hr. This is in contrast with what found in previous works and confirms that the GRB population has an intrinsic unimodal luminosity distribution. For more than 70% of the events the rest-frame light curves in the X-ray and optical bands have a different evolution, indicating distinct emitting regions and/or mechanisms. The X-ray light curves normalised to the GRB isotropic energy (E$_{\rm iso}$), provide evidence for X-ray emission still powered by the prompt emission until late times ($\sim$ hours after the burst event). On the other hand, the same test performed for the E$_{\rm iso}$-normalised optical light curves shows that the optical emission is a better proxy of the afterglow emission from early to late times.
GRB 130427A: a Nearby Ordinary Monster (1311.5254)
A. Maselli, A. Melandri, L. Nava, C. G. Mundell, N. Kawai, S. Campana, S. Covino, J. R. Cummings, G. Cusumano, P. A. Evans, G. Ghirlanda, G. Ghisellini, C. Guidorzi, S. Kobayashi, P. Kuin, V. La Parola, V. Mangano, S. Oates, T. Sakamoto, M. Serino, F. Virgili, B. -B. Zhang, S. Barthelmy, A. Beardmore, M. G. Bernardini, D. Bersier, D. Burrows, G. Calderone, M. Capalbi, J. Chiang, P. D'Avanzo, V. D'Elia, M. De Pasquale, D. Fugazza, N. Gehrels, A. Gomboc, R. Harrison, H. Hanayama, J. Japelj, J. Kennea, D. Kopac, C. Kouveliotou, D. Kuroda, A. Levan, D. Malesani, F. Marshall, J. Nousek, P. O'Brien, J. P. Osborne, C. Pagani, K. L. Page, M. Page, M. Perri, T. Pritchard, P. Romano, Y. Saito, B. Sbarufatti, R. Salvaterra, I. Steele, N. Tanvir, G. Vianello, B. Wiegand, K. Wiersema, Y. Yatsu, T. Yoshii, G. Tagliaferri
Feb. 4, 2014 astro-ph.HE
Long-duration Gamma-Ray Bursts (GRBs) are an extremely rare outcome of the collapse of massive stars, and are typically found in the distant Universe. Because of its intrinsic luminosity ($L\sim 3 \times 10^{53}$ erg s$^{-1}$) and its relative proximity ($z=0.34$), GRB 130427A was a unique event that reached the highest fluence observed in the gamma-ray band. Here we present a comprehensive multiwavelength view of GRB 130427A with Swift, the 2-m Liverpool and Faulkes telescopes and by other ground-based facilities, highlighting the evolution of the burst emission from the prompt to the afterglow phase. The properties of GRB 130427A are similar to those of the most luminous, high-redshift GRBs, suggesting that a common central engine is responsible for producing GRBs in both the contemporary and the early Universe and over the full range of GRB isotropic energies.
How to switch on and off a Gamma-ray burst through a magnetar (1306.0013)
M. G. Bernardini, S. Campana, G. Ghisellini, P. D'Avanzo, D. Burlon, S. Covino, G. Ghirlanda, A. Melandri, R. Salvaterra, S. D. Vergani, V. D'Elia, D. Fugazza, B. Sbarufatti, G. Tagliaferri
Jan. 14, 2014 astro-ph.CO, astro-ph.HE
One of the most elusive features of Gamma Ray Bursts (GRBs) is the sporadic emission prior to the main prompt event observed in at least $\sim 15\%$ of cases. These precursors have spectral and temporal properties similar to the main prompt emission, and smaller, but comparable, energetics. They are separated from the main event by a quiescent time that may be extremely long and, in some cases, more than one precursor has been observed in the same burst. Precursors are still a puzzle: despite many attempts none of the proposed models can account for all the observed features. Based on the complete sample of bright long GRBs observed by Swift (BAT6), we propose a new scenario for which precursors are explained by assuming that the central GRB engine is a newly born magnetar. In this model the precursor and the prompt emission arise from accretion of matter onto the surface of the magnetar. The accretion process can be halted by the centrifugal drag exerted by the rotating magnetosphere onto the in-falling matter, allowing for multiple precursors and very long quiescent times.
A magnetar powering the ordinary monster GRB 130427A? (1401.1972)
M. G. Bernardini, S. Campana, G. Ghisellini, P. D'Avanzo, G. Calderone, S. Covino, G. Cusumano, G. Ghirlanda, V. La Parola, A. Maselli, A. Melandri R. Salvaterra, D. Burlon, V. D'Elia, D. Fugazza, B. Sbarufatti, S. D. Vergani, G. Tagliaferri
Jan. 9, 2014 astro-ph.HE
We present the analysis of the extraordinarily bright Gamma-Ray Burst (GRB) 130427A under the hypothesis that the GRB central engine is an accretion-powered magnetar. In this framework, initially proposed to explain GRBs with precursor activity, the prompt emission is produced by accretion of matter onto a newly-born magnetar, and the observed power is related to the accretion rate. The emission is eventually halted if the centrifugal forces are able to pause accretion. We show that the X-ray and optical afterglow is well explained as the forward shock emission with a jet break plus a contribution from the spin-down of the magnetar. Our modelling does not require any contribution from the reverse shock, that may still influence the afterglow light curve at radio and mm frequencies, or in the optical at early times. We derive the magnetic field ($B\sim 10^{16}$ G) and the spin period ($P\sim 20$ ms) of the magnetar and obtain an independent estimate of the minimum luminosity for accretion. This minimum luminosity results well below the prompt emission luminosity of GRB 130427A, providing a strong consistency check for the scenario where the entire prompt emission is the result of continuous accretion onto the magnetar. This is in agreement with the relatively long spin period of the magnetar. GRB 130427A was a well monitored GRB showing a very standard behavior and, thus, is a well-suited benchmark to show that an accretion-powered magnetar gives a unique view of the properties of long GRBs.
The ultraluminous GRB 110918A (1311.5734)
D.D. Frederiks, K. Hurley, D.S. Svinkin, V.D. Pal'shin, V. Mangano, S. Oates, R.L. Aptekar, S.V. Golenetskii, E.P. Mazets, Ph.P. Oleynik, A.E. Tsvetkova, M.V. Ulanov, A.V. Kokomov, T.L. Cline, D.N. Burrows, H.A. Krimm, C. Pagani, B. Sbarufatti, M.H. Siegel, I.G. Mitrofanov, D. Golovin, M.L. Litvak, A.B. Sanin, W. Boynton, C. Fellows, K. Harshman, H. Enos, R. Starr, A. von Kienlin, A. Rau, X. Zhang, J. Goldstein
Nov. 22, 2013 astro-ph.HE
GRB 110918A is the brightest long GRB detected by Konus-WIND during its 19 years of continuous observations and the most luminous GRB ever observed since the beginning of the cosmological era in 1997. We report on the final IPN localization of this event and its detailed multiwavelength study with a number of space-based instruments. The prompt emission is characterized by a typical duration, a moderare $E_{peak}$ of the time-integrated spectrum, and strong hard-to-soft evolution. The high observed energy fluence yields, at z=0.984, a huge isotropic-equivalent energy release $E_{iso}=(2.1\pm0.1)\times10^{54}$ erg. The record-breaking energy flux observed at the peak of the short, bright, hard initial pulse results in an unprecedented isotropic-equivalent luminosity $L_{iso}=(4.7\pm0.2)\times10^{54}$erg s$^{-1}$. A tail of the soft gamma-ray emission was detected with temporal and spectral behavior typical of that predicted by the synchrotron forward-shock model. Swift/XRT and Swift/UVOT observed the bright afterglow from 1.2 to 48 days after the burst and revealed no evidence of a jet break. The post-break scenario for the afterglow is preferred from our analysis, with a hard underlying electron spectrum and ISM-like circumburst environment implied. We conclude that, among multiple reasons investigated, the tight collimation of the jet must have been a key ingredient to produce this unusually bright burst. The inferred jet opening angle of 1.7-3.4 deg results in reasonable values of the collimation-corrected radiated energy and the peak luminosity, which, however, are still at the top of their distributions for such tightly collimated events. We estimate a detection horizon for a similar ultraluminous GRB of $z\sim7.5$ for Konus-WIND, and $z\sim12$ for Swift/BAT, which stresses the importance of GRBs as probes of the early Universe.
Spectroscopy of Optically Selected BL Lac Objects and their gamma-ray emission (1310.1837)
A. Sandrinelli, A. Treves, R. Falomo, E. P. Farina, L. Foschini, M. Landoni, B. Sbarufatti
Oct. 7, 2013 astro-ph.CO
We present Very Large Telescope optical spectroscopy of nine BL Lac objects of unknown redshift belonging to the list of optically selected radio loud BL Lacs candidates. We explore their spectroscopic properties and the possible link with gamma ray emission. From the new observations we determine the redshift of four objects from faint emission lines or from absorption features of the host galaxy. In three cases we find narrow intervening absorptions from which a lower limit to the redshift is inferred. For the remaining two featureless sources, lower limits to the redshift are deduced from the very absence of spectral lines. A search for gamma counterpart emission shows that six out of nine are Fermi gamma-ray emitters with two new detections. Our analysis suggests that most of the BL Lac still lacking of redshift information are most probably located at high redshift.
BL Lacertae identifications in a ROSAT-selected sample of Fermi unidentified objects (1310.1916)
N. Masetti, B. Sbarufatti, P. Parisi, E. Jimenez-Bailon, V. Chavushyan, F.P.A. Vogt, V. Sguera, J.B. Stephen, E. Palazzi, L. Bassani, A. Bazzano, M. Fiocchi, G. Galaz, R. Landi, A. Malizia, D. Minniti, L. Morelli, P. Ubertini
The optical spectroscopic followup of 27 sources belonging to a sample of 30 high-energy objects selected by positionally cross correlating the first Fermi/LAT Catalog and the ROSAT All-Sky Survey Bright Source Catalog is presented here. It has been found or confirmed that 25 of them are BL Lacertae objects (BL Lacs), while the remaining two are Galactic cataclysmic variables (CVs). This strongly suggests that the sources in the first group are responsible for the GeV emission detected with Fermi, while the two CVs most likely represent spurious associations. We thus find an 80% a posteriori probability that the sources selected by matching GeV and X-ray catalogs belong to the BL Lac class. We also show suggestions that the BL Lacs selected with this approach are probably high-synchrotron-peaked sources and in turn good candidates for the detection of ultra-high-energy (TeV) photons from them.
Dust extinction for an unbiased sample of GRB afterglows (1303.4743)
S. Covino, A. Melandri, R. Salvaterra, S. Campana, S. D. Vergani, M.G. Bernardini, P. D'Avanzo, V. D'Elia, D. Fugazza, G. Ghirlanda, G. Ghisellini, A. Gomboc, Z.P. Jin, T. Kruehler, D. Malesani, L. Nava, B. Sbarufatti, G. Tagliaferri
In this paper we compute rest-frame extinctions for the afterglows of a sample of gamma-ray bursts complete in redshift. The selection criteria of the sample are based on observational high-energy parameters of the prompt emission and therefore our sample should not be biased against dusty sight-lines. It is therefore expected that our inferences hold for the general population of gamma-ray bursts. Our main result is that the optical/near-infrared extinction of gamma-ray burst afterglows in our sample does not follow a single distribution. 87% of the events are absorbed by less than 2 mag, and 50% suffer from less than 0.3-0.4 mag extinction. The remaining 13% of the afterglows are highly absorbed. The true percentage of gamma-ray burst afterglows showing high absorption could be even higher since a fair fraction of the events without reliable redshift measurement are probably part of this class. These events may be due to highly dusty molecular clouds/star forming regions associated with the gamma-ray burst progenitor or along the afterglow line of sight, and/or to massive dusty host galaxies. No clear evolution in the dust extinction properties is evident within the redshift range of our sample, although the largest extinctions are at z~1.5-2, close to the expected peak of the star formation rate. Those events classified as dark are characterized, on average, by a higher extinction than typical events in the sample. A correlation between optical/near-infrared extinction and hydrogen-equivalent column density based on X-ray studies is shown although the observed NH appears to be well in excess compared to those observed in the Local Group. Dust extinction does not seem to correlate with GRB energetics or luminosity.
The prompt-afterglow connection in Gamma-Ray Bursts: a comprehensive statistical analysis of Swift X-ray light-curves (1203.1059)
R. Margutti, E. Zaninoni, M. G. Bernardini, G. Chincarini, F. Pasotti, C. Guidorzi, L. Angelini, D. N. Burrows, M. Capalbi, P. A. Evans, N. Gehrels, J. Kennea, V. Mangano, A. Moretti, J. Nousek, J. P. Osborne, K. L. Page, M. Perri, J. Racusin, P. Romano, B. Sbarufatti, S. Stafford, M. Stamatikos
Oct. 11, 2012 astro-ph.CO, astro-ph.HE
We present a comprehensive statistical analysis of Swift X-ray light-curves of Gamma-Ray Bursts (GRBs) collecting data from more than 650 GRBs discovered by Swift and other facilities. The unprecedented sample size allows us to constrain the REST FRAME X-ray properties of GRBs from a statistical perspective, with particular reference to intrinsic time scales and the energetics of the different light-curve phases in a common rest-frame 0.3-30 keV energy band. Temporal variability episodes are also studied and their properties constrained. Two fundamental questions drive this effort: i) Does the X-ray emission retain any kind of "memory"of the prompt gamma-ray phase? ii) Where is the dividing line between long and short GRB X-ray properties? We show that short GRBs decay faster, are less luminous and less energetic than long GRBs in the X-rays, but are interestingly characterized by similar intrinsic absorption. We furthermore reveal the existence of a number of statistically significant relations that link the X-ray to prompt gamma-ray parameters in long GRBs; short GRBs are outliers of the majority of these 2-parameter relations. However and more importantly, we report on the existence of a universal 3-parameter scaling that links the X-ray and the gamma-ray energy to the prompt spectral peak energy of BOTH long and short GRBs: E_{X,iso}\propto E_{gamma,iso}^{1.00\pm 0.06}/E_{pk}^{0.60\pm 0.10}.
A complete sample of bright Swift Gamma-Ray Bursts: X-ray afterglow luminosity and its correlation with the prompt emission (1206.2357)
P. D'Avanzo, R. Salvaterra, B. Sbarufatti, L. Nava, A. Melandri, M. G. Bernardini, S. Campana, S. Covino, D. Fugazza, G. Ghirlanda, G. Ghisellini, V. La Parola, M. Perri, S. D. Vergani, G. Tagliaferri
June 11, 2012 astro-ph.CO, astro-ph.HE
We investigate wheter there is any correlation between the X-ray afterglow luminosity and the prompt emission properties of a carefully selected sub-sample of bright Swift long Gamma-Ray Bursts (GRBs) nearly complete in redshift (~90%). Being free of selection effects (except flux limit), this sample provides the possibility to compare the rest frame physical properties of GRB prompt and afterglow emission in an unbiased way. The afterglow X-ray luminosities are computed at four different rest frame times (5 min, 1 hr, 11 hr and 24 hr after trigger) and compared with the prompt emission isotropic energy E_iso, the isotropic peak luminosity L_iso and the rest frame peak energy E_peak. We find that the rest frame afterglow X-ray luminosity do correlate with these prompt emission quantities, but the significance of each correlation decreases over time. This result is in agreement with the idea that the GRB X-ray light curve can be described as the result of a combination of different components whose relative contribution and weight change with time, with the prompt and afterglow emission dominating at early and late time, respectively. In particular, we found evidence that the plateau and the shallow decay phase often observed in GRB X-ray light curves are powered by activity from the central engine. The existence of the L_X-E_iso correlation at late times (t_rf > 11 hr) suggests a similar radiative efficiency among different bursts with on average about 6% of the total kinetic energy powering the prompt emission.
The X-ray absorbing column density of a complete sample of bright Swift Gamma-Ray Bursts (1112.5111)
S. Campana, R. Salvaterra, A. Melandri, S. D. Vergani, S. Covino, P. D'Avanzo, D. Fugazza, G. Ghisellini, B. Sbarufatti, G. Tagliaferri
Feb. 15, 2012 astro-ph.HE
A complete sample of bright Swift Gamma-ray Bursts (GRBs) has been recently selected by Salvaterra et al. (2011). The sample has a high level of completeness in redshift (91%). We derive here the intrinsic absorbing X-ray column densities of these GRBs making use of the Swift X-ray Telescope data. This distribution has a mean value of log(NH/cm-2)=21.7+-0.5. This value is consistent with the distribution of the column densities derived from the total sample of GRBs with redshift. We find a mild increase of the intrinsic column density with redshift. This can be interpreted as due to the contribution of intervening systems along the line of sight. Making use of the spectral index connecting optical and X-ray fluxes at 11 hr (beta_OX), we investigate the relation of the intrinsic column density and the GRB `darkness'. We find that there is a very tight correlation between dark GRBs and high X-ray column densities. This clearly indicates that the dark GRBs are formed in a metal-rich environment where dust must be present.
The Dark Bursts population in a complete sample of bright Swift Long Gamma-Ray Bursts (1112.4480)
A. Melandri, B. Sbarufatti, P. D'Avanzo, R. Salvaterra, S. Campana, S. Covino, S. D. Vergani, L. Nava, G. Ghisellini, G. Ghirlanda, D. Fugazza, V. Mangano, M. Capalbi, G. Tagliaferri
Feb. 14, 2012 hep-ph, astro-ph.HE
We study the properties of the population of optically dark events present in a carefully selected complete sample of bright Swift long gamma-ray bursts. The high level of completeness in redshift of our sample (52 objects out of 58) allow us to establish the existence of a genuine dark population and we are able to estimate the maximum fraction of dark burst events (~30%) expected for the whole class of long gamma-ray burst. The redshift distribution of this population of dark bursts is similar to the one of the whole sample. Interestingly, the rest-frame X-ray luminosity (and the de-absorbed X-ray flux) of the sub-class of dark bursts is slightly higher than the average luminosity of the non-dark events. At the same time the prompt properties do not differ and the optical flux of dark events is at the lower tail of the optical flux distribution, corrected for Galactic absorption. All these properties suggest that dark bursts events generate in much denser environments with respect to normal bright events. We can therefore exclude the high-z and the low-density scenarios and conclude that the major cause of the origin of optically dark events is the dust extinction.
A complete sample of bright Swift Long Gamma-Ray Bursts: Sample presentation, Luminosity Function and evolution (1112.1700)
R. Salvaterra, S. Campana, S. D. Vergani, S. Covino, P. D'Avanzo, D. Fugazza, G. Ghirlanda, G. Ghisellini, A. Melandri, L. Nava, B. Sbarufatti, H. Flores, S. Piranomonte, G. Tagliaferri
Feb. 14, 2012 astro-ph.CO, astro-ph.HE
We present a carefully selected sub-sample of Swift Long Gamma-ray Bursts (GRBs), that is complete in redshift. The sample is constructed by considering only bursts with favorable observing conditions for ground-based follow-up searches, that are bright in the 15-150 keV Swift/BAT band, i.e. with 1-s peak photon fluxes in excess to 2.6 ph s^-1 cm^-2. The sample is composed by 58 bursts, 52 of them with redshift for a completeness level of 90%, while another two have a redshift constraint, reaching a completeness level of 95%. For only three bursts we have no constraint on the redshift. The high level of redshift completeness allows us for the first time to constrain the GRB luminosity function and its evolution with cosmic times in a unbiased way. We find that strong evolution in luminosity (d_l=2.3\pm 0.6) or in density (d_d=1.7\pm 0.5) is required in order to account for the observations. The derived redshift distribution in the two scenarios are consistent with each other, in spite of their different intrinsic redshift distribution. This calls for other indicators to distinguish among different evolution models. Complete samples are at the base of any population studies. In future works we will use this unique sample of Swift bright GRBs to study the properties of the population of long GRBs.
A minor body falling onto a neutron star as an explanation for the unusual gamma-ray burst GRB 101225A (1112.0018)
S. Campana, G. Lodato, P. D'Avanzo, N. Panagia, E. M. Rossi, M. Della Valle, G. Tagliaferri, L. A. Antonelli, S. Covino, G. Ghirlanda, G. Ghisellini, A. Melandri, E. Pian, R. Salvaterra, G. Cusumano, V. D'Elia, D. Fugazza, E. Palazzi, B. Sbarufatti, S. D. Vergani
The tidal disruption of a solar mass star around a supermassive black hole has been extensively studied analytically and numerically. In these events the star develops into an elongated banana-shaped structure. After completing an eccentric orbit, the bound debris fall onto the black hole, forming an accretion disk and emitting radiation. The same process may occur on planetary scales, if a minor body orbits too close to its star. In the Solar System, comets fall directly onto our Sun or onto planets. If the star is a compact object, the minor body can become tidally disrupted. Indeed, one of the first mechanisms invoked to produce strong gamma-ray emission involved accretion of comets onto neutron stars in our Galaxy. Here we report that the peculiarities of the `Christmas' gamma-ray burst (GRB 101225A) can be explained by a tidal disruption event of a minor body around a Galactic isolated neutron star. This result would indicate either that minor bodies can be captured by compact stellar remnants more frequently than it occurs in our Solar System or that minor body formation is relatively easy around millisecond radio pulsars. A peculiar Supernova associated to a GRB may provide an alternative explanation.
Discovery of the Onset of Rapid Accretion by a Dormant Massive Black Hole (1104.4787)
D. N. Burrows, J. A. Kennea, G. Ghisellini, V. Mangano, B. Zhang, K. L. Page, M. Eracleous, P. Romano, T. Sakamoto, A. D. Falcone, J. P. Osborne, S. Campana, A. P. Beardmore, A. A. Breeveld, M. M. Chester, R. Corbet, S. Covino, J. R. Cummings, P. D'Avanzo, V. D'Elia, P. Esposito, P. A. Evans, D. Fugazza, J. M. Gelbord, K. Hiroi, S. T. Holland, K. Y. Huang, M. Im, G. Israel, Y. Jeon, Y.-B. Jeon, N. Kawai, H. A. Krimm, P. Mészáros, H. Negoro, N. Omodei, W.-K. Park, J. S. Perkins, M. Sugizaki, H.-I. Sung, G. Tagliaferri, E. Troja, Y. Ueda, Y. Urata, R. Usui, L. A. Antonelli, S. D. Barthelmy, G. Cusumano, P. Giommi, F. E. Marshall, A. Melandri, M. Perri, J. L. Racusin, B. Sbarufatti, M. H. Siegel, N. Gehrels
April 27, 2011 astro-ph.HE
Massive black holes are believed to reside at the centres of most galaxies. They can be- come detectable by accretion of matter, either continuously from a large gas reservoir or impulsively from the tidal disruption of a passing star, and conversion of the gravitational energy of the infalling matter to light. Continuous accretion drives Active Galactic Nuclei (AGN), which are known to be variable but have never been observed to turn on or off. Tidal disruption of stars by dormant massive black holes has been inferred indirectly but the on- set of a tidal disruption event has never been observed. Here we report the first discovery of the onset of a relativistic accretion-powered jet in the new extragalactic transient, Swift J164449.3+573451. The behaviour of this new source differs from both theoretical models of tidal disruption events and observations of the jet-dominated AGN known as blazars. These differences may stem from transient effects associated with the onset of a powerful jet. Such an event in the massive black hole at the centre of our Milky Way galaxy could strongly ionize the upper atmosphere of the Earth, if beamed towards us.