Astronomers using NASA's Chandra X-ray Observatory have caught the largest X-ray flare ever detected from what is believed to be a supermassive black hole at the center the Milky Way, also known as Sagittarius A* (Sgr A*).
The event was 400 times brighter than the usual X-ray output from Sgr A* and it raises questions about the behavior of this giant black hole and its surrounding environment. This “megaflare” was nearly three times brighter than the previous brightest X-ray flare from Sgr A* in early 2012.
The unexpected discovery was made while using Chandra to observe how Sgr A* would react to a nearby cloud of gas known as G2. Gas cloud G2 was estimated to be closest to the black hole in the spring of 2014, 24 billion km (15 billion miles) away. The Chandra flare observed in September 2013 was about a hundred times closer to the black hole, making the event unlikely related to G2.
After Sgr A* settled down, Chandra observed another enormous X-ray flare 200 times brighter than usual on October 20, 2014.
The main portion of this graphic shows the area around Sgr A* in a Chandra image where low, medium, and high-energy X-rays are red, green, and blue respectively. The inset box contains an X-ray movie of the region close to Sgr A* and shows the giant flare, along with much steadier X-ray emission from a nearby magnetar, to the lower left. Image credit: NASA/CXC/Amherst College/D.Haggard et al
Astronomers have two theories about what could be causing these "megaflares" from Sgr A*.
The first idea is that the strong gravity around Sgr A* tore apart an asteroid in its vicinity, heating the debris to X-ray-emitting temperatures before devouring the remains.
Their other proposed explanation involves the strong magnetic fields around the black hole. If the magnetic field lines reconfigured themselves and reconnected, this could also create a large burst of X-rays. Such events are seen regularly on the Sun and the events around Sgr A* appear to have a similar pattern in intensity levels to those.
In addition to the giant flares, the G2 observing campaign with Chandra also collected more data on the magnetar located close to Sgr A*. This magnetar is undergoing a long X-ray outburst, and the Chandra data are allowing astronomers to better understand this unusual object.
Image credit: NASA/CXC/Amherst College/D.Haggard et al