Confirmation of IGR J01363+6610 as a Be X-ray binary with very low quiescent X-ray luminosity

TL;DR

This paper confirms IGR J01363+6610 as a Be X-ray binary with an unusually low quiescent X-ray luminosity, providing insights into its physical properties and variability.

Contribution

The study provides the first detection of IGR J01363+6610 in quiescence with detailed spectral analysis, confirming its classification as a Be X-ray binary with very low luminosity.

Findings

Detected variable X-ray source consistent with a Be star counterpart.

Measured very low quiescent X-ray luminosity (~9.1e31 erg/s).

Observed spectral properties indicative of a hard power-law spectrum.

Abstract

The field containing the candidate High Mass X-ray Binary IGR J01363+6610 was observed by XMM-Newton on 2009 July 31 for 28 ks. A Be star was previously suggested as the possible counterpart of the INTEGRAL source, and although Chandra, during a 2007 observation, did not detect an X-ray source at the position of the Be star, we find a variable source (XMMU J013549.5+661243) with an average X-ray flux of 2e-13 ergs/cm2/s (0.2-12 keV, unabsorbed) at this position with XMM-Newton. The spectrum of this source is consistent with a hard power-law with a photon index of 1.4 +/- 0.3 and a column density of 1.5e22 cm^-2 (90% confidence errors). These results, along with our optical investigation of other X-ray sources in the field, makes the association with the Be star very likely, and the 2 kpc distance estimate for the Be star indicates an X-ray luminosity of 9.1e31 ergs/s. This is lower than typical for a Be X-ray binary, and the upper limit on the luminosity was even lower (<1.4e31 ergs/s assuming the same spectral model) during the Chandra observation. We discuss possible implications of the very low quiescent luminosity for the physical properties of IGR J01363+6610.