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Swift/XRT localization of SLX 1746-331

ATel #1237; J. A. Kennea (PSU), D. Steeghs (Warwick/CfA), M. A.P. Torres (CfA), J. Homan (MIT) and J. M. Miller (Michigan)
on 12 Oct 2007; 21:18 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Subjects: X-ray, Black Hole, Transient

Referred to by ATel #: 1252, 1633, 3098

We report on a target of opportunity observation of SLX 1746-331 taken by Swift/XRT on Oct 12th, 2007 at 15:13UT  for 2ks. SLX 1746-331 is a black hole candidate and recurrent transient source which has recently been reported to have returned to an active state (Markwardt & Swank 2007, ATEL #1235). Photon Counting mode data was taken to ensure that an accurate localization of this transient could be found. Analysis of XRT data reveals a very bright point source at the following coordinates:

RA(J2000) = 17h 49m 49.0s,
Dec(J2000) = -33d 12m 14.9s,

with an estimated uncertainty of 4 arcseconds radius (90% confidence). This position lies 29 arcseconds away from the initial position of this source reported by Skinner et al (1990, MNRAS, 243, 72), inside of their reported 35 arcsecond error radius and lies 7.2 arcseconds from the ROSAT survey source 1RXS J174948.4−331215, which has an 8 arcsecond error circle. We therefore conclude that the bright source we detect is SLX 1746-331 in outburst and, as suggested by Motch et al (1996, AASS, 132, 341), 1RXS J174948.4−331215 is SLX 1746-331.

We note that the XRT error circle contains two 2MASS point sources, 2MASS 17494887-3312152 and 2MASS 17494929-3312123 2.5 and 3.4 arcseconds from the XRT position respectively, however the crowded nature of this field means that infrared/optical identification of the source will require follow-up observations.

Due to the source brightness, the PC mode data is highly piled up making spectral analysis difficult. However by fitting the PSF wings we find the following spectral parameters utilizing a thermal bremsstrahlung model: kT = 3.4 +/- 0.2 keV, NH = (1.0 +/- 0.1) x 1022 cm-2. We note that this spectral fit is considerably harder than the kT=1.5 keV temperature reported by Skinner et al. (1990). A bremsstrahlung model is favored for these data compared to a power-law model (reduced Chi^2 = 1.0 and 1.7 for the thermal and power-law models respectively with 76 degrees of freedom). A disk blackbody model is also a good fit to the spectrum with Tin = 1.26 +/- 0.04 keV and NH = (0.71 +/- 0.04) x 1022 cm-2 (reduced Chi^2 = 0.95 for 76 degrees of freedom), supporting the idea that this source is most likely a black hole transient. We note that with the disk blackbody model, the NH value is approximately double the expected Galactic absorption of 3.9 x 1021 cm-2. Fixing the absorption to the expected galactic value results in a poor  spectral fit.

The XRT measured flux of this source is 5 x 10-9 erg/s/cm2 (1-10 keV), uncorrected for absorption, although we note that this value most likely has a large uncertainty due to the high degree of pile up in the XRT data.