The 2007 outburst of the X-ray binary XTE J1856+053

TL;DR

This study analyzes the 2007 outburst of XTE J1856+053 using X-ray and IR observations, suggesting it is a low-mass black hole binary with a disk temperature indicating a black hole and estimating its mass and orbital period.

Contribution

First detailed multi-wavelength analysis of XTE J1856+053's 2007 outburst, constraining the black hole mass and system properties.

Findings

X-ray spectrum fits a thermal accretion disk model with kT=0.75 keV

Estimated black hole mass between 1.3 and 4.2 solar masses

Orbital period estimated between 3 and 12 hours

Abstract

On 28 February 2007 a new outburst of the previously known transient source XTE J1856+053 was detected with RXTE/ASM. We present here the results of an XMM-Newton (0.5-10.0 keV) Target of Opportunity observation performed on 14 March 2007, aimed at constraining the mass of the compact object in this X-ray binary and determining its main properties. The EPIC-pn camera was used in Timing mode and its spectrum fit together with the RGS data. IR observations with GROND at the 2.2 m telescope in La Silla provide further information on the system. The X-ray light curve shows that both the 1996 and the 2007 outbursts had two peaks. The X-ray spectrum is well fit with a thermal accretion disk model, with kT=0.75+/-0.01 keV and foreground absorption N_H=4.5(+/-0.1)E22 cm**-2. The low disk temperature favours a black-hole as accreting object, with an estimated mass in the range 1.3-4.2 M_sun. From the IR upper limits we argue that XTE J1856+053 is a low mass X-ray binary. We estimate the orbital period of the system to be between 3 and 12 hours.