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SAX J1750.8-2900 is returning to quiescence

ATel #1662; M. Linares, N. Degenaar, D. Altamirano, R. Wijnands, M. van der Klis (Amsterdam), C. Markwardt (CRESST/U. Md/NASA GSFC)
on 15 Aug 2008; 18:39 UT
Credential Certification: Manuel Linares (mlinares@science.uva.nl)

Subjects: X-ray, Binary, Neutron Star, Transient

Referred to by ATel #: 1775, 1777

The neutron star transient SAX J1750.8-2900 has been in outburst since March 15th, 2008 (ATel #1425). The X-ray flux started to decay around mid May and fell below the detection threshold of RXTE-ASM and Swift-BAT around August 2nd and 8th, respectively. We report here on a Swift-XRT observation of the source during its transition to the quiescent state.

Swift observed the field on August 14th. The XRT observation was performed almost entirely in pc mode, with a total exposure of ~1.9 kiloseconds. We clearly detect SAX J1750.8-2900 within the reported error circle (ATel #1490) at an average net count rate of ~0.02 c/s in the 0.5-10 keV band. Due to the low number of source counts the spectrum is poorly constrained. However, by fixing the absorbing column density to the reported value of 2.3*10^22 cm^-2 (ATel #1446) we are able to characterize the average spectrum and measure the flux. A power law fit gives a photon index around 3.0 and alternatively a black body fit yields a temperature of kT ~ 0.7 keV. This suggests that the spectrum is soft, although further observations are needed to confirm this. The absorbed(unabsorbed) 2-10 keV flux is 1.4(2.0)*10^-12 erg/s/cm2.

At 6.3 kpc (Kaaret et al 2002, ApJ) this corresponds to a luminosity of ~10^34 erg/s. Comparing with the last detection in the RXTE-PCA Galactic bulge scans on August 8th. at a (2-10 keV) luminosity of 3.2*10^35 erg/s, we conclude that the source luminosity has decayed by about one order of magnitude in one week. This decay is slower than that observed in other neutron star transients (e.g. SAX J1810.8-2609; ATel #1260). More Swift-XRT pointings are planned.

We thank the Swift team for scheduling the observation.