Studies of Past X-ray Flares from Sgr A*

Sagittarius A* / Galactic Center
Credit: Chandra: NASA/CXC/SAO; IXPE: NASA/MSFC/F. Marin et al;

In a new Nature paper astronomers have reported exciting evidence that the supermassive black hole at the center of our Galaxy, Sagittarius A* (Sgr A* for short), produced an intense flare of X-rays about 200 years ago. Sgr A* is 28,000 light-years from Earth, but even from this considerable distance, if a similar flare occurred today then X-ray telescopes like IXPE and Chandra may be damaged if they looked at Sgr A*.

Currently Sgr A* shows frequent but weak outbursts, and has been referred to as a “sleeping giant” by members of the Event Horizon Telescope Collaboration.

In the new study astronomers learned about Sgr A*’s past outbursts by observing X-rays from clouds of gas around the supermassive black hole. While the primary X-rays from previous outbursts would have reached Earth in the past, X-rays reflected from clouds of gas will take a longer path and can arrive in time to be recorded by telescopes like Chandra and IXPE. This idea goes back decades, with the astronomers referring to a paper published in 1980. In the 1990s, several papers reported evidence for X-ray flares from the center of the Galaxy, including one in 1996 titled “ASCA View of Our Galactic Center: Remains of Past Activities in X-Rays?”.

After Chandra was launched in 1999, it began to add to the evidence for a flare from our Galactic supermassive black hole. First, in 2007 a Chandra press release in 2007 reported that an X-ray source, probably Sgr A*, became about 100,000 times brighter for about two to three years approximately 60 years ago. Then a Chandra press release in 2013 reported evidence for two major outbursts from Sgr A* in the past few hundred years.

The new observations by IXPE gave an important test of these claims. If X-rays from a past Sgr A* flare were indeed reflecting off nearby clouds of gas, then IXPE should observe the X-rays to be polarized, i.e. the X-rays should mainly oscillate in one direction. Furthermore, the inferred direction of the original X-rays should point back to Sgr A*. The new Nature paper led by Frédéric Marin shows that both predictions are correct, making a strong case that Sgr A* was not a sleeping giant about 200 years ago.

— Peter Edmonds