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

¹ XMM-Newton Science Operations Centre, ESAC, ESA, PO Box 50727, 28080 Madrid, Spain e-mail: scarpano@sciops.esa.int
² Herschel Science Centre, ESAC, ESA, PO Box 50727, 28080 Madrid, Spain
³ Department of Physics and Astronomy, University of Leicester, Leicester LE1 7 RH, UK
⁴ Isaac Newton Group of Telescopes, Apartado de Correos 321, 38700 Sta. Cruz de La Palma, Spain; Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain

Received: 8 November 2007
Accepted: 8 January 2008

Abstract

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 , 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.