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ATEL # 2156; H. A. Krimm (CRESST/GSFC/USRA), J. A. Kennea (PSU), S. D. Barthelmy (GSFC), W. Baumgartner (CRESST/GSFC/UMBC), J. Cummings (CRESST/GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC), C. B. Markwardt (CRESST/GSFC/UMD), D. Palmer (LANL), A. Parsons (GSFC), T. Sakamoto (CRESTT/GSFC/UMBC), G. Skinner (CRESST/GSFC/UMD), J. Tueller (GSFC), T. Ukwatta (GSFC/GWU)
on 10 Aug 2009; 22:39 UT
Password Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)
Subjects: X-ray, Binaries, Transients
Observations with the Swift Burst Alert Telescope (BAT) and X-Ray Telescope (XRT) have revealed an outburst of a gamma-ray source which is likely IGR J17586-2129, an object first reported in the Third INTEGRAL/IBIS/ISGRI catalog (Bird et al 2007, ApJS 170, 175). The source was first detected in the BAT 15-50 keV band at a daily average rate of 0.0022 ± 0.0013 counts/sec/cm2 (10 mcrab) on 2009 June 27 (MJD 55009). It continued at roughly the same brightness for the next month until 2009 July 28 (MJD 54040) when its rate increased to 0.0055 ± 0.0012 counts/sec/cm2 (25 mcrab), a level it has retained since, until a possible small decline in brightness beginning on 2009 August 10. This is significantly brighter than the discovery flux of 0.6 ± 0.1 mCrab (Bird et al, 2007).
A Swift target of opportunity observation was carried out on 2009 August 7, for a total duration of 650 seconds beginning at 14:50:26 UTC. A bright source was found at the location:
RA (J2000) = 17 58 34.56 (269.64398 deg)
Dec (J2000) = -21 23 20.82 (-21.38912 deg)
The estimated error is 4.0 arc sec (radius, 90% confidence).
Based on this position we have given the source the name Swift J175834.6-212331. We note that the XRT position is 3.8 arc minutes from the position of IGR J17586-2129 reported in Bird et al (2007). This puts it outside the INTEGRAL 90% confidence level error circle of 3.3 arc minutes. The best Swift/BAT position is RA = 269.635, dec = -21.376 (estimated error 3.0 arc min, radius 90% confidence), coincident with both the XRT and INTEGRAL positions, but closer to the XRT position. Given these positional discrepancies and its location near the galactic center, we cannot rule out the possibility that the BAT/XRT source is not the same as the INTEGRAL source.
Examination of the DSS images reveals a bright star within the XRT error radius. The star is 2MASS 17583455-2123215 and is located at coordinates, RA=269.643997, dec=-21.389313. The star has reported magnitudes J=11.3, H=9.5, K=8.4 (2MASS). We propose that this star is the counterpart of the X-ray transient. This source is at galactic coordinates L=7.986241, B=1.326585. The brightness of the optical source suggests that this system is a high-mass X-Ray binary. However, based on its location near the galactic center, Revnivtsev et al (2008, A&A 491, 209) have suggested that IGR J17586-2129 is a low-mass X-ray binary.
The XRT light curve is flat at 0.29 ± 0.05 ct/s (0.3-10 keV). The source spectrum can be fit to a power law with photon index 1.14 (+1.00, -0.86), and nH (intrinsic) = 1.11 (+0.63, -0.46) x 1023 cm-2 and nH (galactic) = 7.6 X 1021 cm-2. The unabsorbed flux (0.3-10 keV) is 1.0 x 10-10 erg cm-2 s-1. The source is too faint in the BAT for spectral analysis. The XRT analysis used the tools found at http://www.swift.ac.uk/user_objects/
Swift/BAT hard X-ray transient monitor analysis for IGR J17586-2129