Volatile flickering of the Milky Way’s supermassive black hole

At the heart of our Milky Way galaxy lurks a supermassive black hole. It is called Sagittarius A*, or Sgr A* for short, and contains as much mass as 4 million Suns together. Our Milky Way is not unique in this respect: it is thought that every galaxy in our Universe harbors a supermassive black hole in its center. As they are notorious for, black holes suck up material from their surroundings, but they also blast large amounts of gas and energy into space. By doing so, the monstrous black holes in the centers of galaxies play an important role in how galaxies evolve over cosmic timescales and also how large-scale structures formed in our Universe. Studying the table manners of supermassive black holes is therefore a very active area of research.

The material that is irrevocably dragged towards a black hole lights up and provides us with a measure of how (much) the black hole is eating. Our very own supermassive black hole does not appear to have a large appetite as we see relatively little light coming from its surroundings compared to other supermassive black holes. However, roughly once a day we see a flare of (X-ray) emission coming from the position of Sgr A*. Despite that this flickering has long been known, we do not know yet what is causing it. Possibly, these are instances that the supermassive black hole is taking a gollup of material for instance a comet or some other small astrophysical object. Another possibility is that magnetic fields that must be treading the material swirling around Sgr A* are playing up.

Over the past two decades there have been many different efforts to understand the flickering behavior of Sgr A*, for instance by studying the energy of flares, to map out the distribution of flare properties, or attempting to constrain how often they occur. A challenge for the latter is that the flares are relatively sporadic. It therefore requires a lot of staring with telescopes to collect a sufficiently large number of flares to be able to draw firm conclusions on their recurrence rate. Most telescopes do not allow for such studies, either because it is technically not possible to take long stares at a single point in the sky, because their detectors do not have sufficient angular resolution to separate Sgr A* from the many nearby stars, or because the competition for telescope time is so high that long stares can usually not be granted.

The Neil Gehrels Swift observatory, shortly called Swift, is a satellited that was launched by NASA in 2005 and was specifically designed to be able to rapidly point to different positions in the sky. Because of this rapid slewing capability, it is no trouble for Swift to very frequently visit a certain position in the sky, hence effectively build up a long stare at that position. Taking advantage of this exceptional capability, Swift has been taking brief (about 20-min long) snapshots of the center of our Milky Way galaxy with its onboard X-ray telescope nearly every day since 2006. Having accumulated over 2000 X-ray pictures of Sgr A* by now, these Swift data provide a truly unique opportunity to study its flickering behavior over a 15 year baseline.

In the summer of 2019, Alexis Andes, an undergraduate student in El Salvador at the time, came to Amsterdam as part of the ASPIRE program. This program provides research opportunities for students that cannot easily study astrophysics in their home country. Alexis’ research project was to perform a statistical analysis of Swift’s X-ray data of Sgr A* to understand if its flickering behavior is constant over time. Excitingly, this turned out not to be the case! Our study revealed that there are sequences of years in which our supermassive black hole is frequently flickering but also stretches of years in which it is much more quiet. It was not known before that Sgr A* changes its behavior in this way.

Our results do not provide a conclusive answer on the cause of the flickering just jet. However, we noted that years of enhanced flickering seemed to occur after orbiting stars closely passed Sgr A*, such as the object G2 did a few years ago. Possibly, this can stir up the gas swirling around the supermassive black hole, causing it to be more restless for some period of time. While this is highly speculative at present, we can test this idea with continued Swift monitoring of Sgr A*: we predict that the flickering will become more quiet again within 1-2 years as the gas flow settles again after G2’s passage. So stay tuned!

Press release (English): NOVA

Andes, van den Eijnden, Degenaar, Evans, Chatterjee, Reynolds, Miller, Kennea, Wijnands, Markoff, Altamirano, Heinke, Bahramian, Ponti, Haggard 2022, MNRAS 510, 285: A Swift study of long-term changes in the X-ray flaring properties of Sagittarius A*
Paper link: ADS


Three-color accumulated Swift X-ray Telescope Image of the Galactic center (2006-2014).