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 similar to 2 h and belongs to the class of rare, intermediately long type-I X-ray bursts. From the bolometric peak flux of similar to 3.5 x 10-8 erg cm-2 s-1, we infer a source distance of D less than or similar to 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 similar to 2 x 1035 (D/9.5 kpc)2 erg s-1. Optical and infrared photometry together with the detection of a narrow H alpha emission line (full width at half maximum = 292 +/- 9 km s-1, equivalent width = -9.0 +/- 0.4 A) in the optical spectrum confirms that 1RXH J173523.7-354013 is a neutron star low-mass X-ray binary. The H alpha emission demonstrates that the donor star is hydrogen rich, which effectively rules out that this system is an ultracompact X-ray binary.