Chandra observations of the neutron star soft X-ray transient RX J170930.2 - 263927 returning to quiescence

Abstract

We present our analysis of Chandra observations obtained when the soft X-ray transient RX J170930.2 - 263927 (XTE J1709 - 267) returned to quiescence after an outburst. Using the type I burst peak luminosity found by Cocchi et al. and the value of NH we derived from our spectral fits, the distance to RX J170930.2 - 263927 can be constrained to 2.5-10 kpc. RX J170930.2 - 263927 is probably associated with the low-metallicity globular cluster NGC 6293, which has a tidal radius of 14.2 arcmin, as the projected distance to the centre of the cluster is approximately 25 pc (9-10 arcmin). If the association is correct, RX J170930.2 263927 would be at ∼8.5 kpc. We determined that Loutburst/Lquiescence ≳ 105 for this outburst. If the quiescent luminosity is caused by cooling of the neutron star core then enhanced core cooling processes were at work, implying a neutron star mass of ∼ 1.7-1.8 M⊙. Combining our Chandra observations with archival ROSAT observations we show that the source very probably exhibits periods of sustained low-level accretion. Variable, low-level activity could provide an alternative explanation for some of the sources in the recently proposed category of faint soft X-ray transients. We found excess emission at ∼0.6 keV. If such an excess is a unique feature of ultracompact systems, as was recently proposed, RX J170930.2 - 263927 must have a short orbital period as well. From the constraints on the distance and the non-detection of the optical counterpart with mV < 20.5, we conclude that this system must have a low-mass companion.