Swift J073006.8-193709 / ASASSN-20kt: Radio upper limit from MeerKAT

ATel #15412; Andrew K. Hughes, Gregory R. Sivakoff (UAlberta), Rob Fender (University of Oxford), Patrick Woudt (University of Cape Town), James Miller-Jones (Curtin University) on behalf of the ThunderKAT collaboration
on 1 Jun 2022; 19:58 UT
Credential Certification: Gregory R Sivakoff (sivakoff@ualberta.ca)

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Swift J073006.8-193709 was detected by Swift-XRT as a new X-ray transient on 2022 May 30 from 08:13:32 to 08:39:29 UT (ATel #15404). The source is coincident with an optical source found in both the PanSTARRS-1 (PS1) and Gaia Catalogs. The PS1 catalog revealed that the coincident source underwent an optical outburst beginning on 2013 June 24-25, and ending sometime before 2014 Jan 19. Follow-up observations with Swift-XRT and Swift-UVOT on 2022 May 30 and May 31, found that the X-ray source is stable but its UV/Optical counterpart is fading (ATel #15409). Swift J073006.8-193709 has been recently associated with ASASSN-20kt, a cataclysmic variable (CV; ATel #15411).

As part of the ThunderKAT Large Survey Programme (Fender et al. 2017, arXiv:1711.04132), we observed Swift J073006.8-193709 for 15 minutes under the X-ray binary monitoring programme with MeerKAT on 2022 May 31 12:11:32 UT, 1.15 days after the initial X-ray detection. The observation was made at a central frequency of 1.28 GHz with a bandwidth of 856 MHz. We do not detect any radio emission at the position of Swift J073006.8-193709 and report a 3-sigma upper limit of 60uJy. Assuming a flat spectral index (flux density S_ν ∝ ν^0), the upper limit of the radio luminosity is L_R,5 GHz = 3.6e26 (d/1 kpc)^2 erg/s, where d is the distance to the source. This source was targeted on the potential (now ruled out by its association with a CV) that the transient source was an X-ray binary.

The recent Swift confirmation of this outburst reports an absorbed 0.3 - 10 keV X-ray flux of ~9e-13 erg/s/cm^2 from observations on 2022 May 30 and May 31 (ATel #15409). This corresponds to an unabsorbed 1-10 keV X-ray flux of ~9e-13 erg/s/cm^2, with an estimated 20% error (1 sigma) on this flux due to the existing constraints of the source spectrum. At nearly the same time as the radio observations, the source had a 1-10 keV X-ray luminosity of ~1e32 (d/1 kpc)^2 erg/s.

The X-ray luminosity during an outburst is consistent with CVs previously placed on the radio-X-ray luminosity plane (e.g., SS Cyg; Russell et al. 2016, 460, 3270) if the source is at a distance of ~ 1 kpc. At this distance, a radio luminosity below the upper limit presented here is expected given the luminosities of most previously CVs detected in the radio (Coppejans and Knigge, New Astronomy Reviews, 89, 101540).

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 South African Radio Astronomy Observatory (SARAO) for their rapid scheduling of these observations. 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.