Swift follow-up observations of the fast-transient IGR J17191-2821

ATel #1025; M. Klein-Wolt (University of Amsterdam), R. Wijnands (University of Amsterdam), C. B. Markwardt (CRESST/U. Md./NASA/GSFC), J. H. Swank (NASA/GSFC)
on 9 Mar 2007; 17:57 UT
Credential Certification: Marc Klein-Wolt (klein@science.uva.nl)

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Following the discovery of the fast-transient IGR J17191-2821 by INTEGRAL (Turler et al. 2007, ATEL #1021) and RXTE (Swank et al. 2007, ATEL #1022), we obtained two sequences of in total 7 short Swift/XRT observations of the source on March 8. The sequences are taken in Photon Counting mode, were performed on March 8 at 02:37 and 10:44 UTC and do not show a source within the INTEGRAL error circle (2.5 arcmin, Turler et al. 2007, Atel 1021) with an upper limit of 0.0021 cnts/s. If we assume an galactic absorption of 3.4e21 cm^-2 and a photon index of 1.8 this corresponds to an 2-10 keV unabsorbed flux of 8.7*10E-14 erg/s/cm^2 or 0.004 mCrab. In the first Swift sequence we do detect a source at distance of about 3.5 arcminutes from the best (INTEGRAL) position of IGR J17191-2821 at the following position: (RA, DEC) = (259.8114, -28.3005) (J2000), with an error of 9.1 arcsec. In total we get a count rate of 0.0087 cnts/s, which corresponds to an 2-10 keV unabsorbed flux of 3.6*10E-13 erg/s/cm^2 or 0.02 mCrab (assuming the same Nh and photon index as before). In the second Swift sequence this source is not visible any more. Although the formal INTEGRAL error circle on the position of IGR J17191-2821 would suggest that the faint source we detected with Swift is unrelated to IGR J17191-2821 (its position is 1 arc-minute outside the INTEGRAL error circle for the position of IGR J17191-2821), we consider it still a small possibility that both sources are one and the same. For example, it has happened (Kuulkers et al. 2007, ATEL #1008) that the true position of transient sources can lay significantly outside the reported error circle. Irrespectively whether or not we detected the transient during our Swift observations, it is clear that over the span of 5 days (from March 3 to March 8) IGR J17191-2821 has decreased by a factor 500 or more.