Radio and X-ray Observations of the rebrightening Black Hole MAXI J1820+070

ATel #13041; Joe Bright, Sara Motta, Dave Williams (Oxford), Rob Fender (Oxford, UCT), Patrick Woudt (UCT), James Miller-Jones (Curtin) on behalf of the ThunerKAT collaboration
on 21 Aug 2019; 13:33 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Sara Elisa Motta (sara.motta@physics.ox.ac.uk)

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We report radio and X-ray observations of the black hole X-ray binary MAXI J1820+070/ASASSN-2018ey, which underwent a major outburst in 2018 (e.g., Tucker+2018) and is now undergoing its second re-brightening (ATels #13014 and #13025).

We observed the field of MAXI J1820+070 with the MeerKAT radio interferometer and the Arcminute Microkelvin Imager Large Array on the 16th of August 2019 (the time centroids of the observations were MJD 58711.8901 and MJD 58711.7816, respectively). Observations were carried out at 1.28 GHz and 15.5 GHz, respectively, and were separated by approximately three hours. Data flagging, reduction and imaging were performed using standard procedures. MAXI J1820+070 is clearly detected in both images, with a peak flux density of 1.68 +/- 0.05 mJy/beam at 1.28 GHz and 4.4 +/- 0.3 mJy/beam at 15.5 GHz. The RMS noise in each image is 50 uJy and 300 uJy, respectively. The spectral index is ~0.4, suggesting the presence of a compact core jet as is typically found in the hard X-ray state.

We extracted an energy spectrum from the Swift/XRT observation closest to the MeerKAT pointing (MJD 58712, August 17th, 2019). We detect MAXI J1820+070 at a flux of 2.2 (+0.4,-0.3) E-10 erg/cm^2/s in the 0.3 - 10 keV energy band, corresponding to an average count rate of 5 counts/s. Comparing this flux with the latest flux reported ((2.1+/-0.3) E-12 erg/cm^2/s in the same energy band, measured on August 13th, 2019, ATel #13025), we see that MAXI J1820+070 has brightened by approximately a factor of 100 in 7 days. We modelled the energy spectrum with an absorbed power law, and the best fit returns a photon index of 1.5 (+0.4, -0.3), confirming that MAXI J1820+070 is currently in the hard state.

We will continue to monitor MAXI J1820 at radio and X-ray wavelengths.

ThunderKAT will run for 5 years and aims to monitor 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 Mullard Radio Astronomy Observatory and the Swift team for promptly scheduling the AMI and Swift observations, respectively. We thank the staff at the South African Radio Astronomy Observatory (SARAO) for their rapid scheduling of the MeerKAT 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.