Multi-wavelength observations of 1RXH J173523.7-354013: revealing an unusual bursting neutron star

TL;DR

This study reports the discovery and detailed multi-wavelength analysis of an unusual, intermediately long type-I X-ray burst from the neutron star binary 1RXH J173523.7-354013, revealing its nature as a hydrogen-rich low-mass X-ray binary.

Contribution

First detailed multi-wavelength characterization of an intermediately long type-I X-ray burst from this neutron star system, establishing its properties and donor star composition.

Findings

Identified the source as a neutron star low-mass X-ray binary.

Detected an intermediately long type-I X-ray burst lasting ~2 hours.

Confirmed the donor star is hydrogen-rich, ruling out an ultra-compact binary.

Abstract

On 2008 May 14, the Burst Alert Telescope aboard the Swift mission triggered on a type-I X-ray burst from the previously unclassified ROSAT object 1RXH J173523.7-354013, establishing the source as a neutron star X-ray binary. We report on X-ray, optical and near-infrared observations of this system. The X-ray burst had a duration of ~2 h and belongs to the class of rare, intermediately long type-I X-ray bursts. From the bolometric peak flux of ~3.5E-8 erg/cm^2/s, we infer a source distance of D<9.5 kpc. Photometry of the field reveals an optical counterpart that declined from R=15.9 during the X-ray burst to R=18.9 thereafter. Analysis of post-burst Swift/XRT observations, as well as archival XMM-Newton and ROSAT data suggests that the system is persistent at a 0.5-10 keV luminosity of ~2E35 (D/9.5 kpc)^2 erg/s. Optical and infrared photometry together with the detection of a narrow Halpha emission line (FWHM=292+/-9 km/s, EW=-9.0+/-0.4 Angstrom) in the optical spectrum confirms that 1RXH J173523.7-354013 is a neutron star low-mass X-ray binary. The Halpha emission demonstrates that the donor star is hydrogen-rich, which effectively rules out that this system is an ultra-compact X-ray binary.