The region around Sagittarius A*, the supermassive black hole that represents the dynamical center of our Milky Way Galaxy, harbors a large number of accreting neutron stars and black holes. Between 2005 and 2008, we targeted this region every few months using the X-ray instruments onboard the Chandra and XMM-Newton satellites. The main objective of this monitoring campaign was to study the behavior of transient X-ray binaries. These spend most of their time in a dim quiescent state, during which they often can not be detected, but experience occasional outbursts of bright X-ray emission when the neutron star or black hole pulls off and accretes matter from its companion star.

Our observations covered a region of 1.2 square degree around Sagittarius A* that contains 17 known X-ray transients, 8 of which were active during our campaign. We performed a detailed study of the energy distribution and temporal variations of their X-ray emission. From one of the active neutron stars we detected two thermonuclear explosions, which occurred within a time interval of only 3.8 minutes. Such a short repetition time is only rarely seen and poses a challenge for theoretical models. In addition, we discovered a previously unknown X-ray source, which we tentatively classify as an accreting white dwarf.

Most remarkably, the majority of X-ray transients located near Sagittarius A* are considerably fainter during outburst than is usually seen for accreting neutron stars and black holes. One possible explanation for their sub-luminous character is that these X-ray binaries have very small orbits, in which the compact primary and their companion revolve around each other in less than two hours. Finding such binaries is of particular interest, because they are thought to be strong sources of gravitational waves. The existence of gravitational waves is one of the predictions of Einstein’s theory of General Relativity, which future space-missions hope to prove.

Degenaar, Wijnands, Cackett et al. 2012, A&A 545, 49: A four-year XMM-Newton/Chandra monitoring campaign of the Galactic centre: analysing the X-ray transients

Paper link: ADS

Chandra X-ray image of the center of our Milky Way Galaxy.
Credit: NASA/Wang et al. 2002.