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RXTE/PCA observations reveal fast-transient nature of IGR J17191-2821

ATel #1022; J. H. Swank (NASA/GSFC), C. B. Markwardt (CRESST/U. Md./NASA/GSFC), M. Klein-Wolt (Amsterdam), and R. Wijnands (Amsterdam)
on 7 Mar 2007; 17:57 UT
Credential Certification: Jean Swank (swank@milkyway.gsfc.nasa.gov)

Subjects: X-ray, Transient

Referred to by ATel #: 1025, 1065, 1068, 1070, 1075, 1096, 4170

On March 3 at 14:31 UTC, the RXTE/PCA had a scan of the Galactic bulge which detected a new source, at a position which is consistent with it being IGR J17191-2821, at an intensity of 10+-1 mCrab (2-10 keV). INTEGRAL's IBIS/IGRI discovered IGR J17191-2821 (Turler et al. 2007, Atel #1021) in an average over the interval March 2, 13:22 to March 4, 05:21 UTC, which included the time or the RXTE scan. However a follow-up RXTE observation on March 6, 14:45 - 15:26 UTC, found a flux of no more than 0.5 mCrab. This flux can be fit with a power law of photon index 2.4+-0.5 or a bremsstrahlung spectrum with kT=5+-2 keV, with little absorption, and is approximately the estimated flux from that position in the Galactic bulge before the source was identified. Therefore it could be due to unresolved emission; the source may have decayed below this level. No significant oscillations or noise which are characteristic of accreting sources were detected in this flux.

In a bulge scan at March 7, 09:06 UTC, the source was not detected, with a 3 sigma upper limit of 1.2 mCrab, confirming the decay of the transient. The scans since their beginning on February 5, 1999 were reanalyzed for contributions from a source at the position of IGR J17191-2821. No flares brighter than 2 mCrab were seen. These non-detections during the bulge scans spanning 8 years show that this source is relatively infrequently active.

So far we know only that this source was active during a maximum of 1.7 d, with a hard spectrum. The source might be binary with a compact object and a high mass companion. Although a low-mass companion cannot be excluded, it then had a very short outburst and an unusually hard spectrum. A stellar flare from a near-by star or an outburst from a magnetar cannot be ruled out.