Discovery of a bright X-ray transient in the Galactic Center with XMM-Newton

We report the discovery of a bright X-ray transient object, J174554.4-285456, observed in outburst with XMM-Newton on October 3, 2002, and located at 6.3' from Sgr A*, the supermassive black hole at the Galactic center. This object exhibits a very large X-ray luminosity variability of a factor of about 1300 between two X-ray observations separated by four months. The X-ray spectrum is best fitted by a power-law with a photon index of 1.6±0.2 and absorption column density of 14.1^+1.6_-1.4×10²² cm^-2. This large absorption suggests this source is located at the distance of the Galactic center, i.e., 8 kpc. The 2-10 keV luminosity is about 1.0 × 10³⁵ (d/8 kpc)² erg s^-1. A pulsation period of about 172 s is hinted by the timing analysis. The X-ray properties strongly suggest a binary system with either a black hole or a neutron star for the compact object.