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

TL;DR

This paper reports the discovery of a peculiar X-ray source with eclipse-like behavior, analyzing its timing and spectral properties, and debating whether it is a quiescent low-mass X-ray binary or a peculiar active galactic nucleus.

Contribution

It provides detailed timing and spectral analysis of a unique X-ray source and explores its nature, challenging the initial association with a Seyfert 2 galaxy.

Findings

X-ray flux decreases by a factor of 6.5 within 1 hour

Spectrum is very soft with a photon index of ~2.8

Source's properties are inconsistent with a background AGN

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 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.