<h2>Nature of the variation in signal strength</h2>
<p>From the paper <em>Unprecedented variability of Sgr A* in NIR</em> mentioned in Bit Chaser's comment, <a href="https://arxiv.org/abs/1908.01777" rel="nofollow noreferrer">Do et al.</a> record enormous variability in the strength of emissions from SGR A* in the NIR (Near InfraRed) band typically described at about 215 THz to 400 THz.  Here is a figure from their paper:</p>
<p><a href="https://i.sstatic.net/RsvgO.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/RsvgO.png" alt="enter image description here" /></a></p>
<p>Here we see enormous variability on the order of tens of minutes.  In the radio spectrum, from <em>Time Variations in the Flux Density of Sgr A* at 230 GHz Detected with ALMA</em> written by <a href="https://arxiv.org/abs/2003.08601" rel="nofollow noreferrer">Iwata et al.</a>, we see much less dramatic variability, without clear periodicity, with events again on the order of tens of minutes.  Here is a similar figure from their paper:</p>
<p><a href="https://i.sstatic.net/qFxsS.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/qFxsS.png" alt="enter image description here" /></a></p>
<p><a href="https://arxiv.org/abs/2003.08601" rel="nofollow noreferrer">Iwata et al.</a> also perform some Fourier analysis of their signal flux variability which reveals some &quot;not-so-obvious&quot; periodicities on the order of a minute or so.</p>
<p><a href="https://i.sstatic.net/yV9Fp.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/yV9Fp.png" alt="enter image description here" /></a></p>
<p><a href="https://arxiv.org/abs/1502.03423" rel="nofollow noreferrer">Brinkerink et al.</a> note that there are also periodic variabilities on longer timescales:</p>
<blockquote>
<p>The emission from Sgr A* between frequencies of 20 GHz and 230 GHz
shows flux density variability of a few tens of percent on hour-long
timescales, up to 100% on month-long timescales, as well as occasional
flaring behavior (Dexter et al., 2013). In radio, Sgr A* has an
inverted spectrum (i.e., rising flux density with increasing
frequency) that peaks at the ’submm bump’, around 350 GHz, beyond
which the spectrum steeply drops in the infrared regime.</p>
</blockquote>
<h2>Proposed Mechanisms for the variation in signal strength</h2>
<p>There are several proposed mechanisms for the signal strength variations.  <a href="https://arxiv.org/abs/1502.03423" rel="nofollow noreferrer">Brinkerink et al.</a> and <a href="https://arxiv.org/abs/1706.02750" rel="nofollow noreferrer">Yusef-Zadah et al.</a> propose a relativistic colliminated outflow from SGR A* that interacts with gas and dust nearby.  Yusef-Zadah et al. have this nice map from their paper: <em>ALMA and VLA observations of emission from the environment of Sgr A</em>*:</p>
<p><a href="https://i.sstatic.net/OvlYd.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/OvlYd.png" alt="enter image description here" /></a></p>
<p>The authors also argue that</p>
<blockquote>
<p>millimeter emission is produced by synchrotron emission from
relativistic electrons in equipartition with a ∼1.5mG magnetic field.
The origin of these is unclear but its coexistence with hot gas
supports scenarios in which the gas is produced by the interaction of
winds either from the fast moving S-stars, the photo-evaporation of
low-mass YSO disks or by a jet-driven outflow from Sgr A*.</p>
</blockquote>
<p><a href="https://arxiv.org/abs/1502.03423" rel="nofollow noreferrer">Brinkerink et al.</a> and <a href="https://arxiv.org/abs/2003.08601" rel="nofollow noreferrer">Iwata et al.</a> also talk about the <a href="https://en.wikipedia.org/wiki/Synchrotron_radiation" rel="nofollow noreferrer">synchrotron radiation</a>, which occurs when charged particles are subject to an acceleration perpendicular to their velocity.  We would expect this for any &quot;windy&quot; star with a close periapsis path to the black hole, passing through the massive magnetic field associated with SGR A*.  An example of this cited in some of the literature is the <a href="https://en.wikipedia.org/wiki/S2_(star)" rel="nofollow noreferrer">S0-2 star</a>, which has an orbital period of 16 years and the fastest known ballistic orbit, exceeding speeds of 1/60 the speed of light.</p>

CC BY-SA 4.0
2021-03-30T18:47:09.243Z
As an interesting aside, we saw similar fluctuations in radio emissions from Jupiter due to synchrotron radiation when Shoemaker-Levy 9 crashed into its surface.  https://science.sciencemag.org/content/268/5219/1879
Connor Garcia
CC BY-SA 4.0
2021-03-31T00:59:49.053Z
It will take a while to digest all of this wonderful answer, *thanks!* For now, I've asked [What does “synchrotron emission from relativistic electrons in equipartition with a ∼1.5 mG magnetic field” mean?](https://astronomy.stackexchange.com/q/43206/7982)
uhoh
CC BY-SA 4.0
2021-03-31T01:09:04.807Z
Your new question is a great question!  We mustn't let our astrophysicist and cosmology experts get complacent!
Connor Garcia
CC BY-SA 4.0
2021-03-30T18:28:14.437Z
2021-03-30T18:28:14.437Z
Connor Garcia
<p>The Phys.org article <a href="https://phys.org/news/2017-12-cosmic-filament-probes-galaxy-giant.html" rel="noreferrer">Cosmic filament probes our galaxy's giant black hole</a> links to <a href="https://arxiv.org/abs/1711.04190" rel="noreferrer">A Nonthermal Radio Filament Connected to the Galactic Black Hole?</a> which discusses (among other things) the importance of achieving high dynamic range to study these filaments in the proximity of the strong emission associated with Sgr A*. It mentions the fluctuation of radio intensity from Sgr A* on time scales as short as five minutes (as well as to instrumental time-varying effects that also require attention and calibration).</p>

<p>The paper refers to <a href="https://www.nature.com/articles/354046a0" rel="noreferrer">Zhao et al. 2016</a>, which in turn links to <a href="https://www.nature.com/articles/354046a0" rel="noreferrer">Zhao et al. 1999</a>. But so far I can't track down either <em>the nature</em> of the short term variations seen in the radio brightness of Sgr A* nor <em>the mechanism</em>.</p>

<p>What is the nature of this variation (amplitude, power spectrum, any periodicity?) and what are the proposed mechanisms?</p>

CC BY-SA 3.0
2017-12-23T19:27:27.973Z
One would have to dig into the literature to find out, but my a priori guess would be that there aren't good explanations. There may be no good answer to this question.
zephyr
CC BY-SA 3.0
2017-12-24T03:43:01.153Z
@zephyr thanks. I suppose it could be at least categorized as either related to the generation mechanism, or to some kind of radio scintillation occurring along the line of sight through the galaxy. I'll try to do some reading also.
uhoh
CC BY-SA 4.0
2019-09-22T01:48:52.493Z
Recent bright flash, May 13 2019, could be basis for a more targeted question, probably still only with speculative answer. https://www.sciencealert.com/our-galaxy-s-supermassive-black-hole-just-mysteriously-got-really-really-bright and arxiv https://arxiv.org/abs/1908.01777 .
Bit Chaser
CC BY-SA 4.0
2019-09-22T01:58:17.227Z
@bitchaser that's a great idea, I'll update this today, thanks! (only slightly related: [How did they make a video of the center of the galaxy, and what is it exactly that's flashing there?](https://astronomy.stackexchange.com/q/28940/7982))
uhoh
CC BY-SA 4.0
2021-03-26T23:57:22.980Z
@BitChaser It looks like I never figured out how to do that, so instead I've added a bounty.
uhoh
<radio-astronomy><sagittarius-a>
Nature and mechanism of the short-term variability in radio strength of Sgr A*?
uhoh
# Nature and mechanism of the short-term variability in radio strength of Sgr A*?

(Asked by: username_0 on 2017-12-21)

The Phys.org article [Cosmic filament probes our galaxy's giant black hole](https://phys.org/news/2017-12-cosmic-filament-probes-galaxy-giant.html) links to [A Nonthermal Radio Filament Connected to the Galactic Black Hole?](https://arxiv.org/abs/1711.04190) which discusses (among other things) the importance of achieving high dynamic range to study these filaments in the proximity of the strong emission associated with Sgr A\*. It mentions the fluctuation of radio intensity from Sgr A\* on time scales as short as five minutes (as well as to instrumental time-varying effects that also require attention and calibration).

The paper refers to [Zhao et al. 2016](https://www.nature.com/articles/354046a0), which in turn links to [Zhao et al. 1999](https://www.nature.com/articles/354046a0). But so far I can't track down either *the nature* of the short term variations seen in the radio brightness of Sgr A\* nor *the mechanism*.

What is the nature of this variation (amplitude, power spectrum, any periodicity?) and what are the proposed mechanisms?

**Tags:** radio-astronomy, sagittarius-a
> **Comments:**
> **username_1**: One would have to dig into the literature to find out, but my a priori guess would be that there aren't good explanations. There may be no good answer to this question.  
> **username_0**: @zephyr thanks. I suppose it could be at least categorized as either related to the generation mechanism, or to some kind of radio scintillation occurring along the line of sight through the galaxy. I'll try to do some reading also.  
> **username_2**: Recent bright flash, May 13 2019, could be basis for a more targeted question, probably still only with speculative answer. https://www.sciencealert.com/our-galaxy-s-supermassive-black-hole-just-mysteriously-got-really-really-bright and arxiv https://arxiv.org/abs/1908.01777 .  
> **username_0**: @bitchaser that's a great idea, I'll update this today, thanks! (only slightly related: [How did they make a video of the center of the galaxy, and what is it exactly that's flashing there?](https://astronomy.stackexchange.com/q/28940/7982))  
> **username_0**: @BitChaser It looks like I never figured out how to do that, so instead I've added a bounty.  

---

## 1 answers

### Answer by username_3 on 2021-03-30. Score: 1

## Nature of the variation in signal strength

From the paper *Unprecedented variability of Sgr A\* in NIR* mentioned in Bit Chaser's comment, [Do et al.](https://arxiv.org/abs/1908.01777) record enormous variability in the strength of emissions from SGR A\* in the NIR (Near InfraRed) band typically described at about 215 THz to 400 THz. Here is a figure from their paper:

[![enter image description here](https://i.sstatic.net/RsvgO.png)](https://i.sstatic.net/RsvgO.png)

Here we see enormous variability on the order of tens of minutes. In the radio spectrum, from *Time Variations in the Flux Density of Sgr A\* at 230 GHz Detected with ALMA* written by [Iwata et al.](https://arxiv.org/abs/2003.08601), we see much less dramatic variability, without clear periodicity, with events again on the order of tens of minutes. Here is a similar figure from their paper:

[![enter image description here](https://i.sstatic.net/qFxsS.png)](https://i.sstatic.net/qFxsS.png)

[Iwata et al.](https://arxiv.org/abs/2003.08601) also perform some Fourier analysis of their signal flux variability which reveals some "not-so-obvious" periodicities on the order of a minute or so.

[![enter image description here](https://i.sstatic.net/yV9Fp.png)](https://i.sstatic.net/yV9Fp.png)

[Brinkerink et al.](https://arxiv.org/abs/1502.03423) note that there are also periodic variabilities on longer timescales:

> The emission from Sgr A\* between frequencies of 20 GHz and 230 GHz
> shows flux density variability of a few tens of percent on hour-long
> timescales, up to 100% on month-long timescales, as well as occasional
> flaring behavior (Dexter et al., 2013). In radio, Sgr A\* has an
> inverted spectrum (i.e., rising flux density with increasing
> frequency) that peaks at the ’submm bump’, around 350 GHz, beyond
> which the spectrum steeply drops in the infrared regime.

## Proposed Mechanisms for the variation in signal strength

There are several proposed mechanisms for the signal strength variations. [Brinkerink et al.](https://arxiv.org/abs/1502.03423) and [Yusef-Zadah et al.](https://arxiv.org/abs/1706.02750) propose a relativistic colliminated outflow from SGR A\* that interacts with gas and dust nearby. Yusef-Zadah et al. have this nice map from their paper: *ALMA and VLA observations of emission from the environment of Sgr A*\*:

[![enter image description here](https://i.sstatic.net/OvlYd.png)](https://i.sstatic.net/OvlYd.png)

The authors also argue that

> millimeter emission is produced by synchrotron emission from
> relativistic electrons in equipartition with a ∼1.5mG magnetic field.
> The origin of these is unclear but its coexistence with hot gas
> supports scenarios in which the gas is produced by the interaction of
> winds either from the fast moving S-stars, the photo-evaporation of
> low-mass YSO disks or by a jet-driven outflow from Sgr A\*.

[Brinkerink et al.](https://arxiv.org/abs/1502.03423) and [Iwata et al.](https://arxiv.org/abs/2003.08601) also talk about the [synchrotron radiation](https://en.wikipedia.org/wiki/Synchrotron_radiation), which occurs when charged particles are subject to an acceleration perpendicular to their velocity. We would expect this for any "windy" star with a close periapsis path to the black hole, passing through the massive magnetic field associated with SGR A\*. An example of this cited in some of the literature is the [S0-2 star](https://en.wikipedia.org/wiki/S2_(star)), which has an orbital period of 16 years and the fastest known ballistic orbit, exceeding speeds of 1/60 the speed of light.

> **Comments:**
> **username_3**: As an interesting aside, we saw similar fluctuations in radio emissions from Jupiter due to synchrotron radiation when Shoemaker-Levy 9 crashed into its surface.  https://science.sciencemag.org/content/268/5219/1879  
> **username_0**: It will take a while to digest all of this wonderful answer, *thanks!* For now, I've asked [What does “synchrotron emission from relativistic electrons in equipartition with a ∼1.5 mG magnetic field” mean?](https://astronomy.stackexchange.com/q/43206/7982)  
> **username_3**: Your new question is a great question!  We mustn't let our astrophysicist and cosmology experts get complacent!  

---

