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A bright outburst of IGR J17191-2821 and detection of a type-I X-ray burst

ATel #1065; M. Klein-Wolt (University of Amsterdam), R. Wijnands (University of Amsterdam), J. H. Swank (NASA/GSFC), C. B. Markwardt (CRESST/U. Md./NASA/GSFC)
on 3 May 2007; 18:44 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Marc Klein-Wolt (klein@science.uva.nl)

Subjects: X-ray, Request for Observations, Neutron Star, Transient

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

The transient IGR J17191-2821 (Atels #1021,#1022,#1025) has reached a state of increase activity demonstrating a bright outburst. During RXTE/PCA bulge scan observations taken on April 29 (14:39:50 UTC) and May 2 (16:30:43 UTC) the source was detected at a level of ~30 and ~70 mCrab (2-10 keV), respectively. During a Swift/XRT TOO observation (1 ksec) on May 1 (13:03:20 UTC) we detected the source at a high level of ~1.2E-9 erg/sec/cm^2, corresponding to ~ 46 mCrab (absorbed 2-10 keV flux). The persistent flux can best be fitted with a power law of index 1.75 +/- 0.03 and an interstellar extinction of 5.308 +/- 0.223 E21 cm^-2. Before April 29 IGR J17191-2821 has been largely undetected in the RXTE/PCA bulge scans, despite sporadic detections at ~10 mCrab on March 3 (Atel#1022) and at ~4 mCrab on April 18. The most recent RXTE/PCA bulge scan observations and our Swift/XRT observation clearly suggests a bright outburst that has already lasted for a period of approximately 4 days.

In addition during the end of our Swift/XRT observation a type-I X-ray burst was discovered that lasted for about 50 seconds and during which the count rate increased by a factor ~3. The burst spectrum is best fitted with a black body of temperature 0.99 +/- 0.08 keV (fixing the absorption at the level found for the persistent emission) and we find a unabsorbed bolometric flux of ~4.7E-9 erg/s/cm^2. Assuming that the Type-I X-ray burst reached the critical Eddington bolometric peak luminosity of 3.8E38 erg/s (Kuulkers et al. 2003, A&A, 399, 663) we find an upper limit for the distance of 23 kpc. Given the relative low peak bolometric flux the Eddington luminosity is most likely not reached during the peak, the real distance to IGR J17191-2821 is likely to be smaller. The detection of the type-I X-ray burst has revealed the nature of the compact object in this system to be a neutron star. Furthermore, as type-I X-ray bursts have never yet been found for high mass X-ray binaries the neutron star in IGR J17191-2821 has most likely a low mass companion.

The Swift/XRT observation was performed in the wt-mode, hence an accurate position could not be determined yet. More Swift/XRT observations are scheduled and we hope to obtain an position accurate to a few arcseconds as soon as possible. The behavior of IGR J17191-2821 is unusual showing short periods (in the order of days) of relatively faint episodes, that now seem to have followed by a longer period of stronger activity at an even higher (up to a factor of 7) flux. Follow-up observations at all wavelengths are highly desired to shed more light on this enigmatic system.

We would like to thank the Swift Team for making these observations possible.