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Swift/XRT follow-up of the X-ray transient XTE J1728-295 during its decaying phase

ATel #2991; Y. J. Yang, N. Degenaar, R. Wijnands, D. M. Russell, A. Patruno, R. Kaur, M. Armas Padilla (University of Amsterdam)
on 31 Oct 2010; 16:05 UT
Credential Certification: David M. Russell (D.M.Russell@uva.nl)

Subjects: X-ray, Black Hole, Neutron Star, Transient

Referred to by ATel #: 2997, 3011

We report a Swift follow-up observation of the very faint X-ray transient source XTE J1728-295 (also IGR J17285-2922, ATel #2825) currently in the decaying phase of its recent outburst (ATel #2823, #2824, #2869). The observation was performed on 2010-10-29 07:07:35 UTC for a duration of 2 ks, and was observed in Photon Counting (PC) mode.

Preliminary results show that there is no pile-up and the average count rate for this observation is about 0.5 c/s. We analyzed the energy spectrum of the observation and compared it with previous Swift ToO results (ATel #2824). The spectrum is best fitted with an absorbed power-law model. The obtained column density NH is 6.2 (+1.4/-1.2) x 10^21 cm^-2, which is slightly higher than previous observation, which was taken during the peak phase but is still consistent within the errors. The power-law photon index is 1.7 +/- 0.2, indicating that the source spectrum is hardening as the source intensity decreases. By using previous NH=5.4e21 cm^-2, the spectrum is even harder (gamma ~ 1.5). The unabsorbed flux (0.3-10 keV) is 4.6x10^-11 erg cm^-2 s^-1. Assuming a distance of 8 kpc (distance to galactic center), we estimate the X-ray luminosity to be 3.5x10^35 ergs/s, which is one order of magnitude lower than the luminosity at peak level (ATel #2824 ). The lightcurve from the observation shows slightly more variability than the previous one, and overall a gradual decay in intensity is observed suggesting that the source may enter quiescence soon.

XTE J1728-295 has been observed several times with ground based telescopes to search for the optical counterpart (ATel #2827, #2870). As the X-ray flux decays, we expect that the optical flux should decrease as well. Since the observing windows of most of the major X-ray satellites for this source are closed or will be closed shortly until Feb. 2011 at the earliest, further optical/infrared observations within the next few days to mid Nov. (RXTE window closing) are strongly encouraged where possible.

We especially thank the Swift team for their prompt scheduling of this observation. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. YJY acknowledges support from FP7 Marie Curie initial training network under grant agreement number ITN 215212 "Black Hole Universe". DMR acknowledges support from a Netherlands Organization for Scientific Research (NWO) Veni Fellowship.