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 \sim 2 h and belongs to the class of rare , intermediately long type-I X-ray bursts .
From the bolometric peak flux of \sim 3.5 \times 10 ^ { -8 } ~ { } \mathrm { erg~ { } cm } ^ { -2 } ~ { } \mathrm { s } ^ { -1 } , we infer a source distance of D \lesssim 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 \sim 2 \times 10 ^ { 35 } ~ { } ( D / \mathrm { 9.5 ~ { } kpc } ) ^ { 2 } ~ { } \mathrm { erg~ { } s } ^ { -1 } .
Optical and infrared photometry together with the detection of a narrow \mathrm { H \alpha } emission line ( FWHM = 292 \pm 9 ~ { } \mathrm { km~ { } s } ^ { -1 } , EW= -9.0 \pm 0.4 Å ) in the optical spectrum confirms that 1RXH J173523.7–354013 is a neutron star low-mass X-ray binary .
The \mathrm { H \alpha } emission demonstrates that the donor star is hydrogen-rich , which effectively rules out that this system is an ultra-compact X-ray binary .