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

Abstract

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 −8 erg cm −2 s −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 × 10 35 ( D /9.5 kpc) 2 erg s −1 . Optical and infrared photometry together with the detection of a narrow Hα emission line (full width at half maximum = 292 ± 9 km s −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.