MeerKAT radio detection of the transient Galactic black hole candidate Swift J1842.5-1124

ATel #13802; Xian Zhang, Wenfei Yu (Shanghai Astronomical Observatory), Sara Motta (University of Oxford), Rob Fender (University of Oxford), Patrick Woudt (University of Cape Town), James Miller-Jones (Curtin University), Gregory Sivakoff (University of Alberta), on behalf of the ThunderKAT collaboration
on 12 Jun 2020; 15:34 UT
Credential Certification: Wenfei Yu (wenfei@shao.ac.cn)

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Swift J1842.5-1124 is a black hole candidate which was first detected in its 2008 outburst (ATel #1610). Since then, there have been multi-wavelength detections, including X-ray (ATel #1706, #1716), ultraviolet/optical (ATel #1716) and near-infrared (ATel #1720), but not in radio band. On June 1 (MJD 59001), we carried out a MeerKAT observation of the source soon after the source was found in a new outburst by MAXI/GSC (ATel #13762), and put a 3 sigma upper limit of ~80 microJy/beam when the source was likely in the soft state (ATel #13784).

On June 8, we conducted another MeerKAT radio observation of this source for about 15 minutes of exposure (from 04:05:32.9 to 04:20:28.5 UTC; MJD = 59008.171 to 59008.181) as part of the ThunderKAT Large Survey Programme (Fender et al. 2017, arXiv:1711.04132). The data was taken in L band, covering a total bandwidth of 856MHz with 32768 channels centered at 1.284GHz. In the data reduction, J1939-6342 was used as the bandpass and flux density calibrator and J1833-2103 was used as the phase calibrator. We measured a peak flux density of Swift J1842.5-1124 of 208.1 +/- 27 microJy/beam at 1.284GHz. This is the first radio detection of the black hole candidate up to date.

We also performed an X-ray spectral fit using data from the Swift/XRT observation of Swift J1842.5-1124 taken on June 10 (MJD 58010.848). The XRT spectrum is a hard spectrum with a photon index Gamma = 1.71 with a total unabsorbed flux of ~1.18e-10 erg/cm^2/s in the 0.3-10 keV energy band. We thus conclude the source has likely returned to the X-ray hard state, based on the hard spectrum we measured and the detection in the radio band. We will continue to monitor this source in radio. We encourage multi-wavelength follow-up observations of the source.

ThunderKAT will run for 5 years and targets X-ray binaries, Cataclysmic Variables, Supernovae and Gamma-Ray Bursts. As part of this programme we perform weekly monitoring observations of all bright, active, southern hemisphere X-ray binaries in the radio band. For further information on this programme please contact Rob Fender and/or Patrick Woudt.

We thank the staff at the South African Radio Astronomy Observatory (SARAO) for their rapid scheduling of this observation. The MeerKAT telescope is operated by SARAO, which is a facility of the National Research Foundation, an agency of the Department of Science and Technology. We also thank the UK Swift Science Data Centre for building Swift/XRT products of the observation.