Further radio detections of the AMXP MAXI J1816-195 from MeerKAT and ATCA

ATel #15484; Joe Bright (Oxford), Tom Russell (INAF/IASF Palermo), Evangelia Tremou (NRAO), Rob Fender (Oxford, UCT), Patrick Woudt (UCT), James Miller-Jones (Curtin), Melania Del Santo (INAF/IASF Palermo), Alessio Marino (ICE-CISC)
on 29 Jun 2022; 11:40 UT
Credential Certification: Joe Bright (joe.bright@physics.ox.ac.uk)

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On 2022 June 07 MAXI/GCS detected an uncatalogued X-ray transient source with no known counterpart within the error region (ATel #15418). The source was labelled MAXI J1816-195, with observations by Swift/XRT improving the localisation and confirming that the source was a new X-ray transient (ATel #15467). On 2022 June 08 MAXI J1816-195 was classified as an accreting millisecond pulsar based on observations of coherent pulsations observed with the NICER instrument aboard the ISS (ATel #15425).

We observed the field of MAXI J1816-195 with the MeerKAT radio telescope as part of the ThunderKAT programme on 2022 June 09 (UTC 23:16:28 - UTC 23:31:16) and 2022 June 11 (UTC 23:33:58 - UTC 23:48:46), and with the Australia Telescope Compact Array (ATCA) on 2022 June 12 (UTC 09:52:10 - UTC 20:26.30). MeerKAT observations were taken at 1.28 GHz and were calibrated using the OxKAT pipeline (Heywood 2020) with PKS 1934-638 as the flux and bandpass calibrator and J1833-2103 as the complex gain calibrator. We detected a source with MeerKAT at the updated Swift position (ATel #15467) during our first observation, and measure a flux density of 4.2 mJy +/- 0.4 mJy where we include a 10% flux scale uncertainty. The source is not detected in our second MeerKAT epoch, and we place a 3-sigma upper limit of 70 uJy/beam at the target location. ATCA observations were taken with the array in the compact H214 configuration with data recorded simultaneously at 5.5 and 9 GHz. We used PKS 1934-638 as the primary flux and bandpass calibrator, and PKS 1830-210 as the complex gain calibrator. Data were reduced using standard techniques within CASA v5.1.0 (McMullin et al. 2007) using robust weighting with a Briggs parameter of 0. We detect a source at the updated SWIFT position at 5.5 GHz with flux density 135 uJy +/- 35 uJy and place a 3-sigma upper limit of 75 uJy/beam at 9 GHz. Our MeerKAT observations, and a comparison between our ATCA 5.5 GHz detection and the 6 GHz detection reported by the VLA (ATel #15481), demonstrate significant source evolution. See also upper limits placed by the Yamaguchi Interferometer (ATel #15437). Further observations are planned.

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 scheduling and carrying out these observations. The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation. We thank Jamie Stevens for rapid scheduling of the ATCA observations. We also acknowledge the Gomeroi people as the traditional owners of the ATCA observatory site.

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