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Radio emission from IGR J17091-3624 observed with the ATCA

ATel #3246; S. Corbel (Univ. Paris Diderot / CEA Saclay) and T. Tzioumis (ATNF/CSIRO)
on 29 Mar 2011; 15:29 UT
Credential Certification: S. CORBEL (corbel@discovery.saclay.cea.fr)

Subjects: Radio, Infra-Red, X-ray, Request for Observations, Black Hole, Transient

Referred to by ATel #: 3247, 3275

Following the discovery of the new transient IGR J17177-3656 by INTEGRAL (ATel #3223) and the recent improvement of its location by Chandra (ATel #3236), we report the results of radio observations conducted with the Australia Telescope Compact Array (1.5A configuration) and the new CABB back-end. Data were obtained on 2011 March 22 simultaneously at 5.5 and 9 GHz.

The preliminary analysis of the ATCA data indicates the presence of a single radio source within the small Chandra X-ray error circle. This radio source lies outside the Swift error box (ATel #3236, #3236).

The measured flux densities are 0.24 +/- 0.06 mJy at 5500 MHz and 0.20 +/- 0.06 mJy at 9000 MHz, giving a poor constrain on the spectral index (alpha = -0.37 +/- 0.79). The radio spectrum is either consistent with the emission from self absorbed compact jets or optically thin synchrotron from a discrete ejection event.

By combining the data-sets at both frequencies, the location of the radio source is RA(J2000) = 17:17:42.59 and Dec(J2000) = -36:56:04.4 (0.5" positional uncertainty). Given its location in the Chandra error box, it likely corresponds to the radio counterpart of IGR J17177-3656. However, variability at radio frequencies would be needed to clearly establish this association. Assuming it corresponds to the right counterpart, the radio position would also rule out the faint infrared star mentioned in ATel #3241.

A Chandra observation (ATel #3236) was conducted the same day as our ATCA observation. The reported X-ray spectral parameters allow us to compare the radio/X-ray luminosity ratio of IGR J17177-3656 to accreting binary systems. For a radio emission originating from self absorbed compact jets and assuming a distance of 8 kpc (still uncertain), IGR J17177-3656 seems to follow the behavior of H 1743-322 at low flux, when it starts to join the standard correlation of GX 339-4 (for more details, see Coriat et al. 2011, MNRAS, in press, astro-ph/1101.5159), possibly suggesting (but no proof yet) that IGR J17177-3656 could be a black hole candidate in the hard state. Alternatively, the radio emission may also be the residual emission of a discrete ejection event. Further observations at all wavelengths are therefore encouraged.