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Radio emission from IGR J17091-3624 observed with the ATCA

ATel #3167; S. Corbel, J. Rodriguez (Univ. Paris Diderot & CEA Saclay), T. Tzioumis (ATNF/CSIRO), J. Tomsick (UCB/SSL)
on 15 Feb 2011; 16:58 UT
Credential Certification: S. CORBEL (corbel@discovery.saclay.cea.fr)

Subjects: Radio, X-ray, Transient

Referred to by ATel #: 3168, 3179, 3203, 3225, 3230, 3232, 3266, 3299, 3913, 4773

In response to the recent increase in X-ray flux of IGR J17091-3624 reported by Swift/BAT (ATel #3144), followed up by Swift/XRT (ATel #3148) and INTEGRAL(ATel #3159), we have conducted radio observations of the X-ray transient IGR J17091-3624 with the Australia Telescope Compact Array (6A configuration) and the new CABB back-end. Data were obtained on 2011 February 9 at both 5.5 and 9 GHz.

The preliminary analysis of the ATCA data indicates the presence of a single radio source within the X-ray error circle, with a location of RA(J2000) = 17:09:07.61 and Dec(J2000) = -36:24:25.6 (0.1" positional uncertainty), consistent with the optical position reported in ATel #3150. The flux densities of ~1.3 mJy at 5.5 and 9 GHz are consistent with a flat radio spectrum. Such spectra are routinely associated with self absorbed compact jets, and it, therefore, further confirms that IGR J17091-3624 is an accreting black hole in the hard state.

Swift/XRT performed X-ray observations on February 9 and 10. The data were in window timing mode, and we extracted the spectrum from a circular region centred on the source position with a circle of 47 arcsec diameter. By fitting the data with an absorbed power-law model, we obtain the parameters (error listed at the 90% confidence level) listed below that are also consistent with an accreting black hole in the hard state.

February 9: N(H) = 0.72 +/- 0.05 x 10^22 cm-2, Gamma = 1.41 +/- 0.05; Reduced chi-squared = 1.09 for 259 degrees of freedom

February 10 (from quick-look data): N(H) = 0.74 +/- 0.04 x 10^22 cm-2, Gamma = 1.52 +/- 0.04; Reduced chi-squared = 1.15 for 325 degrees of freedom

The average unabsorbed 3-9 keV X-ray flux is 3.2E-10 erg/cm^2 s. Assuming a distance of 8 kpc (still uncertain), this places IGR J17091-3624 in between the standard track in the radio/X-ray correlation diagram (Corbel et al. 2003, A&A, 400, 1007) and the track that could possibly be indicative of a radiatively efficient black hole in the hard state (Coriat et al. 2011, MNRAS, in press, astro-ph/1101.5159). Further observations at all wavelengths are therefore encouraged.