MeerKAT and Swift/XRT observations of the transient Galactic black hole candidate Swift J1842.5-1124

ATel #13784; 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 5 Jun 2020; 14:03 UT
Credential Certification: Wenfei Yu (wenfei@shao.ac.cn)

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On May 23, the MAXI/GSC nova alert system triggered a possible new outburst from the Galactic black hole candidate Swift J1842.5-1124 with a significant detection of 13 +/- 3 mCrab in the 2-10 keV band. Then, on May 25 the source was detected at 17 +/- 3 mCrab in the same band (ATel #13762).

Following the X-ray detections of Swift J1842.5-1124, we performed a radio observation on 2020 June 1 for the on-source time of about 15 minutes (from 02:58:24.2 to 03:13:19.8 UTC; MJD = 59001.12389 to 59001.13426) with the MeerKAT array, as part of the ThunderKAT Large Survey Programme (Fender et al. 2017, arXiv:1711.04132). The observation was centered at 1.284 GHz and made use of a total bandwidth of 856 MHz, during which J1939-6342 was used to set the amplitude scale and J1833-2103 was used to solve for the complex gains. We did not detect Swift J1842.5-1124, but we place a 3 sigma upper limit of ~80 microJy/beam.

A Swift/XRT observation of Swift J1842.5-1124 taken on 2020 May 31 at 2020-05 at 11:54:04 to 12:07:55 UTC (MJD = 59000.49588 to 59000.50550) shows a spectrum that can be well-fitted with a disk blackbody plus a power-law model modified by interstellar absorption with an equivalent column density of [3.6 +/- 0.1]*10^21/cm^2. Our best fit returns a total unabsorbed flux of ~1.66e-09 ergs/cm^2/s in the 0.3-10 keV energy band, a photon index Gamma = 1.7 +/- 0.3, and disk temperature Tdisk = [0.49 +/- 0.01] keV.

We therefore conclude that Swift J1842.5-1124 has likely reached the soft state, which is consistent with the upper limit to radio jet emission from our MeerKAT observation. The X-ray monitoring performed with MAXI suggests that the source is currently decaying during this short-duration outburst.

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.