Optical and X-ray observations of the neutron star soft X-ray transient XTE J1709-267

Abstract

In this paper we report on the discovery of the optical counterpart to the neutron star soft X-ray transient (SXT) XTE J1709-267 at an R-band magnitude of R = 20.5 ± 0.1 and 22.24 ± 0.03, in outburst and quiescence, respectively. We further report the detection of type I X-ray bursts in RKTE data obtained during an outburst of the source in 2002. These bursts show a precursor before the onset of the main burst event, reminiscent of photospheric radius expansion bursts. Sifting through the archival RXTE data for the burster 4U 1636-53, we found a nearly identical burst with precursor in 4U 1636-53. A comparison of this burst to true photospheric radius expansion bursts in 4U 1636-53 leads us to conclude that these bursts-with-precursor do not reach the Eddington limit. Nevertheless, from the burst properties we can derive that the distance to XTE J1709-267 is consistent with the distance of the Globular Cluster NGC 6293. We further report on the analysis of a 22.7 ks observation of XTE J1709-267 obtained with the Chandra satellite when the source was in quiescence. We found that the source has a soft quiescent spectrum which can be fit well by an absorbed black body or neutron star atmosphere model. A power law contributes less than ∼20 per cent to the 0.5-10 keV unabsorbed flux of (1.0 ± 0.3) × 10-13 erg cm-2 s-1. This flux is only slightly lower than the flux measured right after the outburst in 2002. This is in contrast to the recent findings for MXB 1659-29, where the quiescent source flux decreased gradually by a factor of ∼7-9 over a period of 18 months. Finally, we compared the fractional power-law contribution to the unabsorbed 0.5-10 keV luminosity for neutron star SXTs in quiescence for which the distance is well-known. We find that the power-law contribution is low only when the source quiescent luminosity is close to ∼1-2 × 10 33 erg s-1. Both at higher and lower values the power-law contribution to the 0.5-10 keV luminosity increases. We discuss how models for the quiescent X-ray emission can explain these trends.