X-ray Echoes of a Past Outburst of Sgr A*
15 Apr 2008
In a forthcoming paper, the history of the X-ray brightness of Sgr A*, the massive black hole at the heart of our Galaxy, is derived from observations made over an 11-year period by four X-ray observatories. The results are based on X-ray line emission from molecular clouds, considered to be reflecting Sgr A*'s X-ray emission with a delay time of 300 years, and they confirm results obtained by INTEGRAL in 2004.
Sgr A* is a compact radio source located at the dynamical centre of our Galaxy as confirmed by, for example, its apparent motion (solely due to our own motion around the Galactic centre) and the orbits of stars close to the Galactic centre. It is generally believed to be a massive black hole of several million solar masses.
Currently, Sgr A* is observed to be in a quiescent state. Its X-ray luminosity is several orders of magnitude below that of active galactic nuclei with a comparable sized black hole. This is the case even during the known frequent flaring events of Sgr A* when the flux increases by a factor of ~10. Observed events in our Galaxy's central region actually occurred ~26 000 years ago as we are at a distance of about 8 kpc from the Galactic centre.
The recent study by Tatsuya Inui et al.  examined an area around 0.6° galactic longitude in the galactic plane. The authors gathered all available archival data for this area from ESA's XMM-Newton X-ray Observatory, Japan's Suzaku and ASCA X-ray satellites and NASA's Chandra X-ray Observatory.
Image 1. XMM-Newton image in the 6-7 keV band of the Sgr B2 region (marked by solid outline).
Credit: Inui, T. et al 
The region representing the overlap of all the observations from the four different missions contains two extended sources on which X-ray analysis is performed (see image 1). The overlap region is referred to as the Sgr B2 region and the two sources used in this study are: the molecular cloud M0.66-0.02 (also known as the Sgr B2 cloud) and the diffuse X-ray source G0570-0.018. The molecular cloud M0.66-0.02 is known to be at a distance of about 300 light years from Sgr A*.
The study focuses on the 6.4 keV line emission of neutral iron. This emission occurs when neutral iron atoms have an electron knocked out of the K-shell (innermost electron shell) and the resulting gap is subsequently filled by an electron from the L-shell. In the process the characteristic photon energy for this Fe I Kα transition is emitted.
The measurements from all four missions were cross-calibrated using an emission line at 6.67 keV, from highly ionised iron (Fe XXV), as a reference. This line is part of the Galactic centre diffuse X-ray emission and is known to be time independent.
Both objects, M0.66-0.02 and G0570-0.018, were found to have a similar light curve for the 6.4 keV line emission in the period from September 1994 to October 2005 which is covered by the combined observations (image 2). The emission peaked in 2000. By 2005 it had dropped to about 60% of the peak value.
Image 2. X-ray light curve of the Sgr B2 region.
Credit: Inui, T. et al 
Although the time variability of the 6.4 keV line emission in the Sgr B2 region was known and reported in earlier publications, the results from Inui et al. extend the period included for study to more than ten years. It has now placed the interpretation of Sgr A* as the external source for X-ray emission on a firmer footing.
The 6.4 keV line emission from neutral iron requires an external source for removing the K-shell electron. This can be either an energetic photon, or an energetic charged particle (electron). Based on the observed time scale (on the order of ten years) for a global change of the 6.4 keV emission in the two objects, and considering they both have an extent of a few tens of light years, the recent study eliminates charged particles as the possible source. Only processes communicating at light-speed can cause the observed global variation in X-ray emission from the two objects. Inui, T. et al. therefore conclude that an X-ray outburst of Sgr A* is responsible for the variation in the 6.4 keV emission from M0.66-0.02 and G0.570-0.018. Due to the distance between the two objects and Sgr A*, the emission from the former is observed 300 years after the Sgr A* event would have been observed.
The results from T. Inui et al. confirm earlier findings by M.G. Revnivtsev et al. , who used INTEGRAL to observe the hard X-ray emission (20-400 keV) from the Sgr B2 molecular cloud. Revnivtsev et al. showed for the first time that the Sgr B2 molecular cloud is sending us an X-ray echo of violent activity of Sgr A* some 300 years ago.
Inui, T., Koyama, K., Matsumoto, H. & Go Tsuru, T., "Time Variability of the Neutral Iron Lines from the Sgr B2 Region and its Implication of a Past Outburst of Sgr A*", accepted for publication in Publications of the Astronomical Society of Japan, 2008
Revnivtsev, M.G. et al., "Hard X-ray view of the past activity of Sgr A* in a natural Compton mirror", Astronomy & Astrophysics, v.425, p.L49-L52 (2004), doi: 10.1051/0004-6361:200400064
Last Update: 09 Jun 2008