ATCA detection of an extremely bright radio flare from Swift J151857.0-572147

ATel #16518; F. Carotenuto (Oxford), T. D. Russell (INAF-IASF Palermo), on behalf of the JACPOT collaboration
on 9 Mar 2024; 14:53 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Francesco Carotenuto (francesco.carotenuto@cea.fr)

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The radio counterpart of the recently discovered X-ray transient Swift J151857.0-572147 (GCN #35849, #35835, ATel #16500, #16506, #16516) was first detected with MeerKAT (ATel #16503) at a flux density of 10.2 mJy at 1.3 GHz on 2024-o3-04 (MJD 60373.1).

As a follow-up, we observed Swift J151857.0-572147 with the Australia Telescope Compact Array (ATCA) on 2024-03-09 between 10:35:10 UT and 11:06:20 UT (MJD 60378.45). Data were recorded simultaneously at central frequencies of 5.5 GHz and 9 GHz, with 2 GHz of bandwidth at each central frequency. We used PKS 0823-500 for bandpass and flux calibration, while B1511-55 was used for phase calibration. Data were flagged, calibrated, and imaged following standard procedures with the Common Astronomy Software Application (CASA, v5.1.2; CASA Team et al. 2022). We imaged using a Briggs weighting scheme with a robust parameter of 0.

An initial analysis of the data reveals that the source is undergoing a bright radio flare. Fitting for a point source in the image plane, we measure preliminary flux densities of:
1.70 +/- 0.17 Jy at 5.5 GHz and
1.55 +/- 0.16 Jy at 9 GHz,
assuming a conservative 10% uncertainty on the flux densities due to the use of PKS 0823-500 for flux calibration. This corresponds to a radio spectral index α = -0.2 +/- 0.3, where S_ν ∝ ν^α. We note that the observed radio flaring is consistent with a galactic black hole X-ray binary launching relativistic discrete ejecta at the transition between the hard and the soft X-ray state.

Further radio observations are planned and multiwavelength support is strongly encouraged.

We thank Jamie Stevens and the ATCA staff for scheduling the observation. We acknowledge the Gomeroi people as the traditional owners of the ATCA observatory site.