ATCA radio detection of the new outburst from IGR J17091-3624

ATel #15286; Thomas Russell, Melania Del Santo, Antonino D'Ai (INAF/IASF Palermo), Sara Motta (INA-OAB), Alex Tetarenko (Texas Tech), James Miller-Jones (ICRAR/Curtin), Rob Fender (U. Oxford), Alessio Marino (ICE-CSIC), Greg Sivakoff (U. Alberta)
on 18 Mar 2022; 13:12 UT
Credential Certification: Thomas Russell (t.d.russell@uva.nl)

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Following reports of a new outburst from the black hole candidate X-ray binary IGR J17091-3624 (ATels #15282, #15283, #15284), we triggered radio observations with the Australia Telescope Compact Array (ATCA). Observations were taken between 2022-03-17 23:00 and 23:58 UT (MJD 59655.92 +/- 0.02) at central frequencies of 5.5 and 9 GHz (with 2 GHz of bandwidth in each frequency band). The array was in an extended 6 km configuration. We used PKS J1934-638 for flux and bandpass calibration, and PMN 1714-336 for phase calibration. Data were flagged, calibrated, and imaged following standard procedures in the Common Astronomy Software Applications package (CASA, version 5.1.2; McMullin et al. 2007). Imaging used a Briggs robust parameter of 0 to balance sensitivity and resolution. These choices resulted in a beam size of 20.2″ x 1.3″ at a position angle of 46 degrees east of north at 5.5 GHz. At 9 GHz, the beam size was 13.6″ x 0.9″ at a position angle of 46 degrees east of north.

We detected the radio counterpart to IGR J17091-3624 at both frequencies. Fitting for a point source in the image plane, we measured a radio position (at 9 GHz) of:
R.A. (J2000) = 17:09:07.5 +/- 1.0″,
Dec. (J2000) = -36:24:27.1 +/- 1.0″,
where uncertainties have been taken as 10% of the beam size.

We measured flux densities of 3.90 +/- 0.04 mJy at 5.5 GHz and 3.62 +/- 0.05 mJy at 9 GHz. From these measurements, we calculate a radio spectral index, α, of -0.16 +/- 0.05, most consistent with emission from a compact radio jet as expected for a black hole X-ray binary in a hard X-ray state (Fender 2001). Our measured radio flux densities are a factor of ~2 brighter than the brightest reported radio points from IGR J17091-3524′s 2011 outburst (Rodriguez et al. 2011).

We encourage multiwavelength observations of this source. Further radio and X-ray follow-up observations are planned.

We thank Jamie Stevens for the rapid scheduling of the radio observations. We acknowledge the Gomeroi people as the traditional owners of the ATCA observatory site.

References:
Fender 2001, MNRAS, 322, 31
McMullin et al. 2007, ASPC, 376, 127
Rodriguez et al. 2011, A&A, 533, L4