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 (firstname.lastname@example.org)
Subjects: Radio, Infra-Red, X-ray, Request for Observations, Black Hole, Transient
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