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RXTE PCA Detection of a New Outburst of XTE J1728-295 (probably IGR J17285-2922)

ATEL # 2823; Craig B. Markwardt (NASA/GSFC), Jean H. Swank (NASA/GSFC)
on 31 Aug 2010; 4:38 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: Craig B. Markwardt (

Subjects: X-ray, Gamma Ray, Request for Observations, Binaries, Black Holes, Transients
Referred to by ATEL #: 2824, 2825, 2827, 2869, 2870, 2991, 2997, 3011

We report the detection of a new outburst of a source designated XTE J1728-295 in the RXTE PCA scans, which is probably the same as IGR J17285-2922. This source was first detected in August-October 2003 with PCA scans of the galactic center region, and is speculated to be a black hole candidate (Barlow et al. 2005, A&A, 437, L27).

In PCA scans on 2010-08-28 near 09:35 UTC, the source rose to a flux of 6.5 mCrab (2-10 keV). Observations in the previous ~2 weeks set upper limits of ~1 mCrab (95%). The detection on 2010-08-28 signals a new outburst of the source. The peak flux in the previous outburst was also about 6-7 mCrab.

The best fit RXTE PCA position is based on the PCA scans themselves, which yielded a position of R.A. = 262.157, Decl. = -29.520 (J2000), with a position error of approximately 5-10 arcmin. This position is consistent with a transient detected by INTEGRAL, IGR J17285-2922, which was detected by a Galactic Center Deep Exposure during a similar time window (Sep/Oct 2003; Barlow et al. 2005). INTEGRAL did not detect another nearby source during that time, so XTE J1728-295 and IGR J17285-2922 are very likely to be the same source.

We do not yet have PCA pointed observations of the source during this outburst. In the previous outburst, the PCA spectrum of the source during pointed observations on 2003-08-11 and 2003-08-22 were consistent with a steep power law model with photon index 3.6-3.8 and no detectable interstellar absorption, and 2-10 keV flux of ~1e-10 erg/s/cm2. A line was detectable at ~6.7 keV and equivalent width ~1000 eV, which may be due to contamination in the PCA field of view. The source later became detectable by INTEGRAL (approx. 2003-09-25 to 2003-10-10) with a much harder photon index of ~2.1 (Barlow et al. 2005), but still consistent with PCA scan count rates.

No strong variability such as pulsations was detected in PCA pointed observations in August 2003. Similar to Barlow et al. (2005), we speculate that this source is a black hole binary system, which exhibits the commonly known soft and hard states at different times. Further observations at other wavelengths are encouraged.

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