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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 354; no. 3; pp. 666 - 674
• Main Authors: Jonker, P. G., Galloway, D. K., McClintock, J. E., Buxton, M., Garcia, M., Murray, S.
• Format: Journal Article
• Language: English
• Published: 23 Ainslie Place , Edinburgh EH3 6AJ , UK . Telephone 226 7232 Fax 226 3803 Blackwell Science Ltd 01.11.2004; Blackwell Science
• Subjects: accretion; accretion discs; accretion, accretion discs; binaries: general; stars: individual: 4U 1636-53; stars: individual: XTE J1709-267; stars: neutron; X-rays: binaries; Neutron stars; X ray observation; Soft X radiation; Atmosphere model; X-rays: binaries; Binary X ray source; Cosmic x-ray sources; Luminosity; stars: individual: 4U 1636-53; Globular clusters; Star models; Optical counterpart; Accretion disks; Precursor; stars: neutron; Transient X ray source; Type I X ray burst; accretion, accretion discs- binaries: general; X-ray binary stars; Power law; X ray emission; stars: individual: XTE J1709-267
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2004.08246.x

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 RXTE 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 × 1033 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.