ASKAP VAST radio detections of Swift J151857.0-572147

ATel #16617; Akash Anumarlapudi (UWM), David Kaplan (UWM), Gregory Sivakoff (U Alberta), Dougal Dobie (U. Sydney/OzGrav), Hao Qui (SKAO), Tara Murphy (U. Sydney), Laura Driessen (U. Sydney), and Emil Lenc (CSIRO) on behalf of the VAST collaboration.
on 8 May 2024; 15:04 UT
Credential Certification: Akash Anumarlapudi (aakash@uwm.edu)

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Swift J151857.0-572147 (GCN #35849) was initially discovered on 2024-03-03T06:33:18 UT and classified as a Galactic transient owing to its position within the Galactic plane. Subsequent observations with Swift showed a persistent bright source that did not show any signs of fading (ATel #16500). Follow-up observations at radio wavelengths with MeerKAT (ATel #16503) and ATCA (ATel #16518) and at infra-red wavelengths with REM (ATel #16506) discovered counterparts to this source. Observations from ATCA (ATel #16518) indicated a flat spectrum source indicative of a transition from a hard to soft state and launching of a jet, and the later Swift observations seemed to indicate a spectral energy distribution consistent with this.

We report Australian SKA Pathfinder (ASKAP) observations of Swift J151857.0-572147 taken as part of the Variable And Slow Transients Survey (VAST; Murphy et al. 2021) at 887.5 MHz. The position of Swift J151857.0-572147 has been observed roughly with a two-week cadence since December 2023 with each observation lasting approximately 12 minutes. The source was not detected in the early VAST data predating the discovery, and observations taken after the discovery resulted in a bright persistent source, confirming the transient nature of the source. VAST detections show a highly variable behavior of the radio light curve with the flux density varying by a factor of 6 over the detections (see light curve below). The details of all detections plus the most recent non-detection are given below.

| Start time of observation (UTC) | Peak flux density (mJy) | Error (mJy)
| 2024-02-21T19:38:25 | <2.5 | 0.52
| 2024-03-11T18:40:17 | 46.67 | 0.35
| 2024-03-27T17:25:04 | 7.14 | 0.33
| 2024-04-12T17:56:29 | 17.55 | 0.33
| 2024-04-26T18:15:45 | 10.23 | 0.32

The bright detection on March 11 seems to be consistent with the fading of the initial radio burst observed by ATCA (ATel #16518) and the subsequent variable detections may be arising as the transient jet expands and interacts with the surrounding material.

This work uses data obtained from Inyarrimanha Ilgari Bundara / the Murchison Radio-astronomy Observatory. CSIRO’s ASKAP radio telescope is part of the Australia Telescope National Facility (https://ror.org/05qajvd42). Operation of ASKAP is funded by the Australian Government with support from the National Collaborative Research Infrastructure Strategy. ASKAP uses the resources of the Pawsey Supercomputing Research Centre. Establishment of ASKAP, Inyarrimanha Ilgari Bundara, the CSIRO Murchison Radio-astronomy Observatory and the Pawsey Supercomputing Research Centre are initiatives of the Australian Government, with support from the Government of Western Australia and the Science and Industry Endowment Fund.

ASKAP light curve for this event can be obtained here.