A long monitoring campaign of the Milky Way’s black hole, called Sagittarius A*, has revealed some unusual activity. Typically relatively quiet, the 4-million-solar-mass black hole had an increase in X-ray flares in 2014. The timing of this surge coincided with the passage close to Sagittarius A* of a mysterious object called G2.
The upper two panels of this graphic represent a possible explanation for a recent increase in X-ray flares from Sagittarius A* seen by three X-ray telescopes. As part of a long monitoring campaign, astronomers observed the black hole as a mysterious object called G2 appeared to pass close to the black hole. The timing of G2’s passage could suggest that material from G2 caused the surge in X-ray activity, but it is also possible that this unrelated behavior of Sagittarius A*. The bottom panel is a view of the region around the black hole where red, green, and blue represent low, medium, and high-energy X-rays detected by NASA’s Chandra X-ray Observatory. Sagittarius A* itself is not visible in this image, but is embedded in the white dot at the end of the arrow. Image credit: NASA / CXC / MPE / G.Ponti et al. / M.Weiss.
By combining information from monitoring campaign by three X-ray space telescopes – ESA’s XMM-Newton, NASA’s Chandra X-ray Observatory and NASA’s Swift satellite, a team of astronomers was able to trace the activity of Sagittarius A* over the last 15 years (from September 1999 through November 2014).
Sagittarius A* has been producing one bright X-ray flare about every 10 days, according to the team, led by Dr Gabriele Ponti of the Max Planck Institute for Extraterrestrial Physics.
However, within the past year, there has been a 10-fold increase in the rate of flares from the black hole, at about one every day. This increase happened soon after the close approach to the black hole by G2.
Astronomers have been tracking G2 for years, originally thinking it was an extended cloud of gas and dust.
However, after passing close to Sagittarius A* in late 2013 its appearance did not change much, apart from being slightly stretched by the gravity of the black hole.
This led to new theories that G2 was not a gas cloud, but instead a star or pair of stars within an extended dusty cocoon.
“For several years, we’ve been tracking the X-ray emission from Sagittarius A*. This includes also the close passage of this dusty object. A year or so ago, we thought it had absolutely no effect on Sagittarius A*, but our new data raise the possibility that that might not be the case,” said Dr Ponti, who is the lead author of a paper accepted for publication by the Monthly Notices of the Royal Astronomical Society (arXiv.org preprint).
If the G2 explanation does explain the recent rise in X-ray flares, it would be the first sign of excess material falling onto the black hole because of the cloud’s close passage.
Some gas would likely have been stripped off the cloud, and captured by the gravity of Sagittarius A*. It then could have started interacting with hot material flowing towards the black hole, resulting in an enhanced feeding rate and the production of X-ray flares.
While the timing of G2’s passage with the surge in X-rays from the black hole is intriguing, it is not yet an open-and-shut case.
That is because scientists see other black holes that appear to have behavior similar to the most recent increase of activity from the Milky Way’s black hole.
Therefore, it’s possible this increased chatter from Sagittarius A* may be a common trait among supermassive black holes and unrelated to G2.
Instead, it could represent, for example, a change in the strength of winds from nearby massive stars that are feeding the black hole.
“It’s too soon to say for sure, but we will be keeping X-ray eyes on Sagittarius A* in the coming months. Hopefully, new observations will tell us whether G2 is responsible for the changed behavior or if the new flaring is just part of how the black hole behaves,” said study co-author Dr Barbara De Marco of the Max Planck Institute for Extraterrestrial Physics.
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G. Ponti et al. 2015. Fifteen years of XMM-Newton and Chandra monitoring of Sgr A*: Evidence for a recent increase in the bright flaring rate. Mon. Not. R. Astron. Soc., accepted for publication; arXiv: 1507.02690
