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Swift/XRT follow-up and a refined position of the X-ray transient XTE J1728-295

ATel #2824; Y. J. Yang, A. Patruno, N. Degenaar, D. M. Russell, D. Altamirano, R. Wijnands (University of Amsterdam)
on 31 Aug 2010; 22:38 UT
Credential Certification: David M. Russell (D.M.Russell@uva.nl)

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

Referred to by ATel #: 2825, 2827, 2870, 2991, 2997, 3011

Following the report of the newly detected outburst of the transient XTE J1728-295 (possibly IGR J17285-2922 detected by INTEGRAL) in the RXTE PCA bulge scans (ATel #2823), we requested a Swift ToO observation to follow up the source based on the position given by the RXTE PCA best fit results (ATel #2823). The observation was performed on 2010-08-30 14:00:46 (UT) for about 1 ks. We found only one bright source in the Swift XRT field of view. The source is off-center, but within the RXTE position error circle (5-10 arcmin). We obtained a refined position of the source at RA (J2000) = 17:28:38.97 (262.1624), and DEC= -29:21:44.9 (-29.3625) with an estimated error circle of 3.5 arcsec (90% confidence). Previously, Tomsick et al. (2008) reported 6 possible Chandra counterparts within the INTEGRAL error circle (2') for IGR J17285-2922. However, none of the 6 Chandra sources overlap with the Swift 3.5" error circle, indicating that the source was in quiescence during the Chandra observation.

The Swift lightcurve shows no strong variability during the observation. We also analyzed the energy spectrum of the observation. The spectrum is best fitted with an absorbed power-law model. The obtained column density NH is 5.4 +/- 1.2 x 10^21 cm^-2, which is consistent with the galactic value along the line of sight toward the source direction (Dickey & Lockman 1990). The power-law photon index is 2.23 +/- 0.26 which is similar to the photon index reported from previous INTEGRAL results (Barlow et al. 2005). The unabsorbed flux (0.3-10 keV) is 6.1x10^-10 erg cm^-2 s^-1. Assuming a distance of 8 kpc (distance to galactic center), we estimate the X-ray luminosity to be 4.7x10^36 ergs/sec. More observations are needed to reveal the nature of the transient, as well as to verify if the source is indeed the counterpart of IGR J17285-2922. Further optical follow-up observations have been arranged with the Faulkes Telescope.

We thank the Swift team for their prompt arrangement of the observation. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester.