ATCA radio detection of the new X-ray transient IGR J17591-2342 suggests a black hole candidate
ATel #11954; T. Russell, N. Degenaar, R. Wijnands and J. van den Eijnden (University of Amsterdam)
on 15 Aug 2018; 08:52 UT
Credential Certification: Thomas Russell (t.d.russell@uva.nl)
Subjects: Radio, X-ray, Black Hole, Transient
Following the recent report of the new X-ray transient IGR J17591-2342 (Atels #11941, #11942, #11946), we conducted DDT radio observations with the Australia Telescope Compact Array (ATCA).
We observed IGR J17591-2342 on 2018 August 14 between 10:38 UT and 16:23 UT (MJD 58344.56 +/- 0.12). The radio observations were taken simultaneously at central frequencies of 5.5 and 9.0 GHz, with 2 GHz of bandwidth at each frequency. PKS 1934-638 and 1817-254 were used for flux and phase calibration, respectively. The data were reduced and imaged following standard procedures in CASA (v4.7; McMullin et al. 2007). Imaging was carried out with a Briggs robust parameter of 0 to both balance sensitivity with resolution, as well as minimise the effects from diffuse emission and other bright sources in the field. With the telescope in a hybrid 1.5D configuration due to a current reconfiguration, these choices provided angular resolutions of 6.8"x1.4" at 5.5 GHz and 3.9"x0.9" at 9 GHz (with a position angle 15 degrees North of East).
We significantly detected the radio counterpart to IGR J17591-2342 at a position coincident with the reported X-ray position (ATel #11942). We measure a 9.0 GHz radio position of:
RA (J2000) =17:59:02.86 +/- 0.04, Dec (J2000) = -23:43:08.3 +/- 0.1,
where the errors are the uncertainties on the fitted position, which are larger than the theoretical statistical error of centroiding (Beam/2*SNR).
Fitting for a point source in the image plane, we measure a flux density of 1.09 +/- 0.02 mJy at 5.5 GHz and 1.14 +/- 0.02 mJy at 9.0 GHz. The results give a radio spectral index, α, of 0.1 +/- 0.3, where Sν ∝ να, suggesting a flat spectrum from a compact radio jet. This spectral index implies a 5-GHz radio luminosity of (4.7 +/- 0.2)E29*(d/8kpc)2 erg/s.
Swift/XRT observed IGR J17591-2342 on the same day as our radio observation (snapshots were taken either side of the radio observation on 2018 August 14 between 00:37:01-00:43:57 UT and 16:27:02-16:33:58 UT; obs ID: 0001084002). We extracted the data using the online XRT pipeline (Evans et al. 2009) and fitted it with an absorbed power-law model. We find a best fit with nH = (4.4 +/- 0.8)E22 cm-2 and a photon index, Gamma, of 1.6 +/- 0.3, with an unabsorbed 0.5-10 keV X-ray flux of (3.4 +/- 1.5)E-10 erg/s/cm2, similar to previously reported results (ATel #11942), albeit at a marginally lower X-ray flux, indicating the source has faded slightly. This flux implies an 0.5-10 keV luminosity of (3 +/- 1)E36*(d/8kpc)2 erg/s.
Comparing the determined radio and X-ray luminosities to those of typical hard state X-ray binaries, our results imply that IGR J17591-2342 is consistent with a black hole low-mass X-ray binary, most likely on the radio bright track.
We thank Jamie Stevens and the ATNF staff for making these ATCA observations possible.
