Discovery of X-ray eclipses from the transient source CXOGC J174540.0-290031 with XMM-Newton

We present the XMM-Newton observations obtained during four revolutions in Spring and Summer 2004, of CXOGC J174540.0-290031, a moderately bright transient X-ray source, located at only 2.9 $^{\prime\prime}$ from Sgr A*. We report the discovery of sharp and deep X-ray eclipses, with a period of $27\,961\pm5$ s and a duration of about $1100\pm100$ s, observed during the two consecutive XMM-Newton revolutions from August 31 to September 2. No deep eclipses were present during the two consecutive XMM-Newton revolutions from March 28 to April 1, 2004. The spectra during all four observations are described well with an absorbed power law continuum. While our fits on the power law index over the four observations yielded values that are consistent with $\Gamma$=1.6-2.0, there appeared to be a significant increase in the column density during the Summer 2004 observations, i.e. the period during which the eclipses are detected. The intrinsic luminosity in the 2-10 keV energy range was almost constant with $1.8{-}2.3 \times 10^{34}~({d_{8~\rm kpc}})^{2}$ erg s-1 over the four observations. In the framework of eclipsing semidetached binary systems, we show that the eclipse period constrains the mass of the assumed main-sequence secondary star to less than 1.0 $M_\odot$. Therefore, we deduce that CXOGC J174540.0-290031 is a low-mass X-ray binary (LMXB). Moreover the eclipse duration constrains the mass of the compact object to less than about 60 $M_\odot$, which is consistent with a stellar mass black hole or a neutron star. The absence of deep X-ray eclipses during the Spring 2004 observations could be explained if the centroid of the X-ray emitting region moves from a position on the orbital plane to a point above the compact object, possibly coincident with the base of the jet which was detected in radio at this epoch. According to our study, CXOGC J174540.0-290031 is a LMXB, and is more likely to have a black hole than a neutron star as its primary, which would entail an inclination angle greater than 75$^{\circ}$; i.e. the binary system and the accretion disk are seen close to edge-on.