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Swift/XRT Observations of IGR J17091-3624: Continued Activity

ATel #4173; A. L. King (Michigan), J. M. Miller (Michigan), A. C. Fabian (Cambridge), C. S. Reynolds (Maryland), J. Raymond (CfA)
on 13 Jun 2012; 17:27 UT
Credential Certification: Ashley King (ashking@umich.edu)

Subjects: X-ray, Black Hole, Transient

Referred to by ATel #: 4283, 4773

We report on the last four Swift/XRT observations of the X-ray transient IGR J17091-3624, obtained 2012 May 9, May 14, May 30 and June 6. These observations have approximately 1 ks exposures, with the exception of the May 14 exposure, which has a 2.1 ks exposure length. IGR J17091-36224 shows continued high activity, which begins to increase during the final two observations. The XRT count rate for each of the four observations is as follows: 2.04+\-0.05 cts/s, 1.20+\-0.02 cts/s, 3.28 +\- 0.05 cts/s, and 3.28+\-0.06 cts/s, respectively. As there was enough signal-to-noise to fit the continuum spectra in each of these observations, we extracted the spectra and fit a disk black-body component, powerlaw and Galactic absorption. We chose to freeze the Galactic column density at nH=7.6E+21 cm^-2 (Dickey and Lockman 1990), while allowing the other parameters to vary. The fit resulted in a black body temperature of approximately kT=2.3keV for all but the lowest flux observation (which was dominated by a hard powerlaw component), and a powerlaw index of approximately, gamma=4.8. The corresponding unabsorbed fluxes, measured between 0.5-10 keV, are 2.5+\-1.2 E-10 ergs/cm^2/s, 1.4+\-1.0 E-10 ergs/cm^2/s, 3.2+\-1.4 E-10 ergs/cm^2/s and 3.5+\-1.5 E-10 ergs/cm^2/s, respectively. Our highest flux measurement corresponds to a luminosity of 2.7E+36 ergs/s at 8 kpc and 2.6E+37 ergs/s at 25 kpc. We note the continued high and persistent nature of transient IGR J17091-3624, as well as the potentially rising trend in flux. In addition, the latest flux measurement is reminiscent of the renewed activity seen in 2011 Feb 4 (ATel #3148), and is only a factor of 3 away from the high flux activity noted in King et al. (2012, ApJ, 746, L20).