Astronomers See Flares Coming from the Milky Way’s Supermassive Black Hole

There’s plenty of action at the center of the galaxy, where a supermassive black hole (SMBH) known as Sagittarius A* (Sgr A*) literally holds the galaxy together. Part of that action is the creation of gigantic flares from Sgr A*, which can give off energy equivalent to 10 times the Sun’s annual energy output. However, scientists have been missing a key feature of these flares for decades – what they look like in the mid-infrared range. But now, a team led by researchers at Harvard’s Center for Astrophysics and the Max Planck Institute for Radio Astronomy has published a paper that details what a flare looks like in those frequencies for the first time.

Astronomers have been observing Sgr A* since the 1990s and have known about the flares, which were initially seen as variances in the SMBH’s brightness. It has been observed with all manner of telescopes, including the Chandra X-ray observatory and, perhaps most famously, the Event Horizon Telescope, which was responsible for the famous first image of M87*, another black hole at the center of the Messier galaxy. EHT also released an image from Sgr A* itself in May of 2022.

So far, those observations have been in visible light through infrared and from far infrared up through X-rays. There has always been a gap in the middle of the infrared range. Several factors explain this gap.

First, Sgr A* is relatively weak in the mid-infrared range compared to other ranges, so it doesn’t stand out as much against the background noise of the universe. Second, much of the mid-infrared emissions get obscured by the dust cloud surrounding the SMBH at the galaxy’s center, blocking it from detectors at Earth 28,000 light years away. Third, there were technological limitations to infrared sensors themselves. There were ground-based telescopes that could have detected the signal, but the Earth’s atmosphere blocked even more of it.

That required scientists to wait for the long-delayed James Webb Space Telescope (JWST). When it finally launched in late 2021, it was only a matter of time before they would get observational time to watch Sgr A* and hopefully observe a flare with the most powerful infrared detector ever launched into orbit. 

JWST did indeed get observational time with Sgr A* and saw a flare, representing the first-ever recording of a flare in the mid-infrared range. But the research team didn’t stop there – they were also watching with several other telescopes for confirmation of the JWST signal.

They didn’t find any in the X-ray range with Chandra, though that was probably because the flare wasn’t strong enough to emit a significant amount of X-rays. But they did see a signal from the Sub-Millimeter Array (SMA) in Hawai’i, which detected radio waves following along about 10 minutes behind the detected mid-infrared signal.

That confirmation was necessary because it allowed the experimentalists to provide even more insight about the same flare to the theoreticians. Their job is then to confirm the models and simulations of what causes the flares in the first place. The current theory is that they occur when magnetic field lines in the SMBH’s accretion disk join up and emit massive amounts of radiation in a process known as synchrotron emission. In synchrotron emission, a bunch of charged particles – typically electrons – get pushed down the magnetic field lines like they were part of a massive particle accelerator.

The data from JWST fits nicely into that theory. However, there appear to be additional unanswered questions about whether that feature was specific to Sgr A* or whether it could be observed for other SMBHs such as M87*. For now, that remains to be seen, though given the interest in this particular black hole in this specific wavelength, while this might have been the first study published on the topic, it probably won’t be the last.

Learn More:
CfA – Scientists Make First-Ever Detection of Mid-IR Flares in Sgr A*
von Fellenberg et al – First mid-infrared detection and modeling of a flare from Sgr A*
UT – Echoes of Flares from the Milky Way’s Supermassive Black Hole
UT – A Black Hole Emitted a Flare Away From us, but its Intense Gravity Redirected the Blast Back in our Direction

Lead Image:
This artist’s conception of the mid-IR flare in Sgr A* captures the variability, or changing intensity, of the flare as the black hole’s magnetic field lines approach each other. The byproduct of this magnetic reconnection is synchrotron emission. The emission seen in the flare intensifies as energized electrons travel along the SMBH’s magnetic field lines at close to the speed of light. The labels mark how the flare’s spectral index changes from the beginning to the end of the flare.
Credit: CfA/Mel Weiss

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