SIMBAD references

Investigating the spectral and temporal characteristics of the X-rays coming from Sagittarius A* (Sgr A*) is essential to our development of a more complete understanding of the emission mechanisms in this supermassive black hole located at the center of our Galaxy. Several X-ray flares with varying durations and spectral features have already been observed from this object. Here we present the results of two long XMM-Newton observations of the Galactic nucleus carried out in 2004, for a total exposure time of nearly 500 ks. During these observations we detected two flares from Sgr A* with peak 2-10 keV luminosities about 40 times (L_X∼9x10³⁴ ergs/s) above the quiescent luminosity: one on 2004 March 31 and another on 2004 August 31. The total duration of the first flare was about 4 ks. The second, simultaneously detected in the near-IR, lasted about 10 ks. The combined fit to the EPIC spectra yields photon indices of about 1.5 and 1.9 for the first and second flares, respectively. This hard photon index strongly suggests the presence of an important population of nonthermal electrons during the event and supports the view that the majority of flaring events tend to be hard and not very luminous.