MeerKAT radio detection of the 2022 outburst from 1A 1744-361 / XTE J1748-361

ATel #15410; Andrew K. Hughes, Gregory R. Sivakoff (UAlberta), Rob Fender (University of Oxford), Patrick Woudt (University of Cape Town), James Miller-Jones (Curtin University), Jakob van den Eijnden (University of Oxford), on behalf of the ThunderKAT collaboration
on 1 Jun 2022; 16:26 UT
Credential Certification: Gregory R Sivakoff (sivakoff@ualberta.ca)

Tweet

Following the MAXI detection of an X-ray outburst consistent with the position of the neutron star X-ray binary 1A 1744-361 / XTE J1748-361 (ATel #15277), we targeted 1A 1744-361 with MeerKAT as part of the ThunderKAT Large Survey Programme (Fender et al. 2017, arXiv:1711.04132). We observed the field of 1A 1744-361 on 2022 May 31 for 15 minutes, starting at 23:15 UT (MJD 59730.97). The observation was performed with 60 antennas, at a central frequency of 1.28 GHz (L-band) and with a total bandwidth of 860 MHz.

Our preliminary radio analysis of the SARAO-provided quick look image detects an unresolved 1.20 +/- 0.03 mJy radio source at
RA(J2000) = 17:48:13.07 +/- 0.06, and
Dec(J2000) = -36 07 57.5 +/- 0.7.
Here, the 1 sigma astrometric errors are set by standard systematic limits. This position is consistent with X-ray (ATels #5301, #15408), optical (ATel #267), and radio (ATel #210) detections of XTE J1748-361, which is also assumed to be the source 1A 1744-361. We note that the previous radio detection at this position during the 2003 outburst had a flux density at 4.86 GHz of 0.57 +/- 0.07 mJy, approximately half as bright as MeerKAT detects. Assuming a flat spectral index (flux density S_ν ∝ ν^0), the most recent 5 GHz radio luminosity of the source is L_R,5 GHz = 4.6e29 (d/8 kpc)^2 erg/s, where d is the distance to the source. We estimate a 10% systematic error on this luminosity due to flux calibration limitations.

The recent Swift confirmation of this outburst reports an unabsorbed 0.3-10 keV X-ray flux of ~5e-9 erg/s/cm^2 from observations on 2022 June 1starting at 00:55 UT (ATel #15408). This corresponds to an unabsorbed 1-10 keV X-ray flux of ~3e-9 erg/s/cm^2, with a 20% error on this flux due to imprecise knowledge of the source spectrum. At nearly the same time as the radio observations, the source had a 1-10 keV X-ray luminosity of ~2e37 (d/8 kpc)^2 erg/s.

At an assumed distance of 8 kpc, this would be among the radio brightest hard state neutron star X-ray binaries detected to date, although accreting millisecond X-ray pulsars have reached similar radio luminosities at about a tenth of the X-ray luminosity. We note that this source is more consistent with the radio-X-ray luminosity relations of radio quieter black hole X-ray binaries than typical hard state neutron star X-ray binaries.

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 staff at 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.