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kHz QPOs during the decay of the outburst of IGR J17191-2821

ATel #1075; 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 16 May 2007; 10:38 UT
Credential Certification: Marc Klein-Wolt (klein@science.uva.nl)

Subjects: X-ray, Neutron Star, Transient

Referred to by ATel #: 1096, 4170

We continued to monitor the outburst of the transient IGR J17191-2821 (ATels #1021,#1022,#1025,#1065,#1068,#1069,#1070) with RXTE/PCA and Swift/XRT observations. After our last Swift/XRT observation on May 07 (18:30 UTC, ATel #1070) were we detected the source at a level of ~53 mCrab, our pointed RXTE/PCA observations reveal the source has shown a small increase in flux to ~64 mCrab on May 08 (00:21 UTC) and after that a steady decay to about 8 mCrab (May 12, 01:53 UTC). More recent RXTE/PCA bulge scan observations (May12, 23:41 UTC) and the latest observation obtained during our Swift/XRT monitoring campaign (May 13, 23:33 UTC) detect the source at even lower levels of ~5 and ~2 mCrab, respectively.

In 12 of our 17 RXTE/PCA observations we detected a kHz QPO in the range between 700 and 900 Hz (detection level at ~9 sigma, single trail in the 2-60 keV energy range). Also, on several occasions when we detected this QPO, we detected an additional kHz QPO at a frequency of ~1030 Hz. Very likely this QPO can be identified with the upper kHz QPO and the one at lower frequencies with the lower kHz QPO. The frequency separation is about 330 Hz and is consistent within 3 sigma with the spin frequency as determined from the oscillations seen during the reported type-I X-ray bursts (ATel # 1068).

The behavior of IGR J17191-2821 during its outburst is characteristic for neutron star atoll sources in outburst, and from the combined energy spectral and temporal information we conclude that in the last few days the source has made a transition from the banana to the island state.

We thank the Swift Team for scheduling the Swift/XRT observations.