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, XMMU J174554.4-285456 , observed in outburst with XMM-Newton on October 3, 2002, and located at 6.3 $^{\prime}$ 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 $1.6\pm0.2$ and absorption column density of $14.1^{+1.6}_{-1.4}\times10^{22}$ 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 $1.0 \times 10^{35}\,(d/{\rm 8\,kpc})^{2}$ 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.