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

On 2008 May 14, the Burst Alert Telescope onboard 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 ~3.5 × 10^-8ergcm^-2s^-1, 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/X-ray Telescope observations as well as archival XMM-Newton and ROSAT data suggests that the system is persistent at a 0.5-10 keV luminosity of ~2 × 10³⁵ (D/9.5 kpc)²ergs^-1. Optical and infrared photometry together with the detection of a narrow Hα emission line (full width at half maximum = 292 +/- 9kms^-1, equivalent width = -9.0 +/- 0.4 Å) in the optical spectrum confirms that 1RXH J173523.7-354013 is a neutron star low-mass X-ray binary. The Hα emission demonstrates that the donor star is hydrogen rich, which effectively rules out that this system is an ultracompact X-ray binary.