Discovery of the radio counterpart of Swift J151857.0-572147 with MeerKAT

ATel #16503; F. J. Cowie (Oxford), F. Carotenuto (Oxford), R. P. Fender, (Oxford/Cape Town), I. Heywood (Oxford/Rhodes), A. K. Hughes (University of Alberta), and G. R. Sivakoff (University of Alberta), for the X-KAT collaboration
on 4 Mar 2024; 21:27 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Gregory R Sivakoff (sivakoff@ualberta.ca)

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ATel #16500 reported the discovery of a new X-ray transient first thought to be a GRB (GCN #35849, 35835) before being reclassified as a Galactic transient due to its consistent X-ray brightness and localisation within the Galactic plane.

Following the X-ray trigger, we initiated follow-up radio observations with the MeerKAT radio telescope as part of the X-KAT programme. Swift J151857.0-572147 was observed for 15 minutes between 02:13:13.3 and 02:28:08.9 on March 4th, 2024 (MJD 60373.1). Observations were carried out at a central frequency of 1.28 GHz (L-band), with a total bandwidth of 856 MHz. J1939-6342 was used for flux and bandpass calibration, and J1424-4913 was used for phase calibration.

We detect a radio source with a flux density of approximately 10.20+/-0.04 mJy (statistical error) within the X-ray uncertainty region of Swift J151857.0-572147 (ATel #16500), and improve the localisation to RA(J2000) = 15:18:57.5 Dec(J2000) = -57:21:48.7 with an uncertainty of ~1". The detection is highly significant with a background RMS in the image of ~30 microJy. The spectrum of the source is inverted with a preliminary spectral index of α ~+0.5, where flux density as a function of frequency (f_ν) is proportional to ν^α. This radio flux density, inverted index, and the reported X-ray photon index of +1.78 (ATel #16500) are consistent with an X-ray binary in the hard state, suggesting that Swift J151857.0-572147 is either a black hole or (radio-bright) neutron star X-ray binary.

Swift J151857.0-572147 is close on the sky (~0.3 degrees separation) to the well known neutron star X-ray binary Circinus X-1, also studied as part of the X-KAT program. Therefore, we use previous X-KAT data from the same field to place upper limits on the flux density of Swift J151857.0-572147 at previous epochs. The next most recent observation of the field was on February 2nd 2024 (MJD 60342) at a central frequency of 1.28 GHz. No significant emission was found at the location of Swift J151857.0-572147, and we estimate a 3 sigma upper limit of ~125 microJy after primary beam correction. Deep upper limits are obtained from a 4 hour observation at a central frequency of 2.62 GHz (S2-band) taken on August 7th 2023 (MJD 60163). No emission is found and we estimate a 3 sigma upper limit of ~30 microJy after primary beam correction. These limits mean that this radio source has brightened by a factor of at least 80 since February 2nd 2024.

We will continue monitoring Swift J151857.0-572147 with the MeerKAT radio telescope and the Neil Gehrels Swift Observatory. Multi-wavelength observations are encouraged to study the rise of this outburst.

X-KAT is a large MeerKAT open-time programme to observe X-ray binaries in the radio band, performing weekly monitoring of bright, active systems, with capacity for higher cadence observations, and in coordination with large X-ray and optical monitoring programmes. For further information on this programme contact Rob Fender. The Swift follow-up of X-ray binaries is largely performed as part of the SwiftKAT program, which provides quasi simultaneous X-ray coverage of the X-KAT targets.