ASKAP observations of the black-hole X-ray candidate MAXI J1810-222

ATel #16136; Akash Anumarlapudi (UWM), David Kaplan (UWM), Gregory Sivakoff (U Alberta), Dougal Dobie (Swinburne U.), Laura Driessen (CSIRO), and Tara Murphy (U. Sydney) on behalf of the VAST collaboration.
on 17 Jul 2023; 15:09 UT
Credential Certification: David L. Kaplan (kaplan@uwm.edu)

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Following the report of a record radio brightening of the black-hole X-ray binary (XRB) candidate MAXI J1810-222 (ATel #12254), we examined data taken for the Variable And Slow Transients Survey (VAST; Murphy et al. 2021) and Rapid ASKAP Continuum Survey (RACS; McConnell et al. 2020) at 887.5 MHz with the Australian SKA Pathfinder (ASKAP). This source was suggested to exhibit a transition from a hard X-ray state to soft-X-ray state accompanied with flaring at radio wavelengths (MeerKAT - 1.28GHz, ATel #16129). Here, we report the radio detection of this source in VAST/RACS data prior to and during the MeerKAT flaring.

The field of MAXI J1810-222 was observed multiple times as a part of RACS/VAST pilot survey prior to December 2022. Since then, it has been observed with a cadence of roughly every two weeks as a part of the VAST survey. We report a clear detection of this system on 2023-01-21 with a flux density of 2.3+/-0.3 mJy. Subsequent observations resulted in non-detections (< 1.25 mJy at 5-sigma confidence) until 2023-06-22, when the source was detected at 3.3+/-0.3 mJy, which declined to 1.3+/-0.3 mJy on 2023-07-04.

The 2023-06-22 detection is the highest flux density (Fnu) and the highest recorded radio luminosity (nuLnu) recorded for this source to date. The latest ASKAP measurements temporally bracket the MeerKAT radio flaring and precede the Swift detection of the source being in a soft accretion state in ATel #16129. While the lower frequency ASKAP data show a decreasing flux, the MeerKAT data shows an increasing flux.

XRB jets may consist of one or both of: a compact steady jet that is normally spectrally flat (alpha~0, where Fnu~nu^alpha) and is thought to quench during the transition from the hard to soft accretion state (and reappear during the soft to hard accretion state); and/or a pair (or pairs) of optically thin ejecta that are spectrally steep (alpha~-0.6), are thought to be launched during the hard to soft state transition, and fade as they adiabatically expand (unless they interact with the nearby medium). Given this context, we cannot distinguish between these origins for the 2023-06-22 VAST data, but believe that the 2023-06-24 MeerKAT data was dominated by emission from jet ejecta. The 2023-06-30 MeerKAT and 2023-07-04 VAST data may either arise from a pair of jet ejecta that underwent a somewhat unexpected brightening or a second pair of jet ejecta. We note that the latter is consistent with the source having gone back and forth across the hard to the soft state several times in its past (e.g., Russell et al. 2022, MNRAS, 513, 6196; Del Santo et al. 2023, MNRAS, 523, L15); however, this would require a more rapid set of accretion state transitions than have been yet observed for this source. Rapid transitions may also be supported by the single VAST detection on 2023-01-21.

This work uses data obtained from Inyarrimanha Ilgari Bundara / the Murchison Radio-astronomy Observatory. We acknowledge the Wajarri Yamaji People as the Traditional Owners and native title holders of the Observatory site. 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.

VAST lightcurve