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The Phys.org article Cosmic filament probes our galaxy's giant black hole links to A Nonthermal Radio Filament Connected to the Galactic Black Hole? 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, which in turn links to Zhao et al. 1999. 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?

uhoh
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    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 Dec 23 '17 at 19:27
  • @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 Dec 24 '17 at 03:43
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    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 Sep 22 '19 at 01:48
  • @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?) – uhoh Sep 22 '19 at 01:58
  • @BitChaser It looks like I never figured out how to do that, so instead I've added a bounty. – uhoh Mar 26 '21 at 23:57

1 Answers1

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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. 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

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., 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

Iwata et al. 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

Brinkerink et al. 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. and Yusef-Zadah et al. 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

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. and Iwata et al. also talk about the 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, which has an orbital period of 16 years and the fastest known ballistic orbit, exceeding speeds of 1/60 the speed of light.

Connor Garcia
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