XMMU J134736.6+173403: an eclipsing LMXB in quiescence or a peculiar AGN?

Aims. We report the discovery of a peculiar object observed serendipitously with XMM-Newton. We present its timing and spectral properties and investigate its optical counterpart. Methods. The light curve of the X-ray source, its spectrum, and the spectrum of the best optical counterpart are presented and analyzed. Results. The X-ray flux decreases by a factor of 6.5 within 1 h and stays in a low state for at least 10 h, thereby suggesting the presence of an eclipse. The spectrum is very soft, a power law with a slope of Gamma~2.8, and does not change significantly before and after the flux drop. The source is spatially coincident within few arc-seconds with a Seyfert~2 galaxy belonging to a galaxy pair. Conclusions. Although the background AGN seems the best counterpart, neither the temporal nor the spectral properties of the X-ray source are compatible with it. We investigate the possibility of having a foreground low-mass X-ray binary in quiescence, where the companion is not detected in the optical wavelength.