Thursday, September 24, 2015

Sagittarius A*: Milky Way's Black Hole Shows Signs of Increased Chatter

Sagittarius A*
Credit NASA/CXC/MPE/G.Ponti et al; Illustration: NASA/CXC/M.Weiss

Three orbiting X-ray telescopes have been monitoring the supermassive black hole at the center of the Milky Way galaxy for the last decade and a half to observe its behavior, as explained in our latest press release. This long monitoring campaign has revealed some new changes in the patterns of this 4-million-solar-mass black hole known as Sagittarius A* (Sgr A*).

The bottom panel of this graphic is a view of the region around Sgr A* where red, green, and blue represent low, medium, and high-energy X-rays detected by NASA's Chandra X-ray Observatory. Sgr A* itself is not visible in this image, but is embedded in the white dot at the end of the arrow. The other two telescopes involved in the 15 years of X-ray observations were ESA's XMM-Newton and NASA's Swift Gamma Ray Burst Explorer, but their data are not included in this image.

Within the past year, the usually quiet black hole has shown an increased level of X-ray flares over its typical rate. This surge in X-ray flares coincides with the passage close to Sgr A* of a mysterious object called G2. Astronomers have been tracking G2 for years, originally thinking it was an extended cloud of gas and dust. However, after passing close to Sgr 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.

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 Sgr 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. This scenario is depicted in the artist's illustrations found in the upper two panels of the graphic.

While the timing of G2's passage with the surge in X-rays from Sgr A* is intriguing, it is not yet an open-and-shut case. That is because astronomers see other black holes that appear to have behavior similar to the most recent increase of activity from Sgr A*. Therefore, it's possible this increased chatter from Sgr 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.

The analysis included 150 Chandra and XMM-Newton observations pointed at the center of the Milky Way over the last 15 years, extending from September 1999 to November 2014. An increase in the rate and brightness of bright flares from Sgr A* occurred after mid-2014, several months after the closest approach of G2 to the huge black hole. The newest set of Chandra, XMM and Swift observations, obtained between August 30 and October 2014, revealed six bright flares within about three days, while an average of only 0.8 bright flares was expected.

A paper on these findings has been accepted by the Monthly Notices of the Royal Astronomical Society and a preprint is available online. The authors of this study were Gabriele Ponti (Max Planck Institute for Extraterrestrial Physics), Barbara De Marco (Max Planck), Mark Morris (University of California, Los Angeles), Andrea Merloni (Max Planck), Teo Muñoz-Darias (University of La Laguna, Spain), Maica Clavel (CEA Saclay, France), Darryl Haggard (Amherst College), Shuo Zhang (Columbia University), Kirpal Nandra (Max Planck), Stefan Gillassen (Max Planck), Kenji Mori (Columbia), Joseph Nielsen (Massachusetts Institute of Technology), Nanda Rea (University of Amsterdam), Natalie Degenaar (University of Cambridge), Regis Terrier (University of Paris), and Andrea Goldwurm (CEA Saclay).

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.

Fast Facts for Sagittarius A*:

Scale: Image is 5 by 5 arcmin (about 38 light years)
Category: Black Holes, Milky Way Galaxy
Coordinates (J2000): RA 17h 45m 40s | Dec -29° 00' 28.00"
Constellation: Sagittarius
Observation Date: 43 pointings from September 21, 1999 to May 18, 2009
Observation Time: 278 hours (11 days 14 hours).
Obs. ID: 242, 1561, 2943, 2951-2954, 3392, 3393, 3549, 3663, 3665, 4683, 4684, 5360, 5950-5954, 6113, 6363, 6639, 6640-6646, 7554-7759, 9169-9174, 10556
Instrument: ACIS
Also Known As: Galactic Center
References: Ponti, G et al, 2015, MNRAS (accepted); arXiv:1407.2243
Color Code: Energy: Red (2-3.3 keV), Green (3.3-4.7 keV), Blue (4.7-8 keV)
Distance Estimate: About 26,000 light years