AMI-LA and Swift confirm the multi-wavelength rebrigtening of MAXI J1820+070

ATel #12577; David Williams, Sara Motta, Joe Bright (Oxford), Rob Fender (Oxford/UCT),James Miller-Jones (Curtin), Dave Green, David Titterington (Cambridge)
on 14 Mar 2019; 14:19 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Sara Elisa Motta (sara.motta@physics.ox.ac.uk)

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Following the optical and X-ray re-brightening of the black hole candidate MAXIJ1820+070 (ASASSN-18ey, ATel #12567, ATel #12573), as part of a long-term observing program on this source we triggered Neil Gehrels Swift Observatory observations in the soft X-rays and the Arcminute Microkelvin Imager Large Array (AMI-LA; Zwart et al. 2008; Hickish et al. 2018) observations in radio at 15.5GHz.

We observed MAXI J1820+070 with AMI-LA on 2019-03-13 for 2.3 hours and 2019-03-14 for 8 hours. For both observations, the custom pipeline REDUCE_DC (e.g. Perrott et al. 2015) was used to calibrate and flag the data, with 3C286 as the absolute flux calibrator and J1824+1044 as the interleaved phase calibrator. In the 2019-03-13 observation, we have a marginal detection of a source at the phase centre, with flux density 0.405mJy/beam, with an image rms noise level of 0.130mJy/beam. In the 2019-03-14 observation, the source is clearly detected at the phase centre with a flux density of 1.55mJy/beam and the rms noise level is 0.030mJy/beam. We therefore confirm that MAXIJ1820+070 has also re-brightened in the radio waveband.

We extracted an energy spectrum in the 0.3-10 keV energy band from the latest Swift/XRT data (taken on 2019-03-13). The spectrum is well-fitted by an absorbed power law, with a photon index of 1.75+/-0.17 and a column density of Nh = (1.2+/-0.5)E-21 cm-2. We measure an unabsorbed X-ray flux of (3.1+/-0.4)E-11 erg cm-2 s-1 in the 1-10 keV band.

MAXI J1820+070 has become significantly brighter - by more than an order of magnitude - in both the X-rays and in radio over the course of one day, and the above fluxes indicate that the source is rising along the radio-loud branch of the radio:x-ray correlation (e.g. Plotkin et al. 2015), following the same track traced during the 2018 outburst. The observed behaviour thus suggests that MAXI J1820+070 is undergoing a significant re-brightening, following a brief quiescence phase. Relatively short-lived re-brigthening phases sometimes follow major outburst of black hole X-ray transients (e.g. Chen et al. 1993,1997; Bailyn & Orosz 1995), and have been observed in a number of sources in the past (see e.g. Swift J1753.5-0127, GRO J0422+32, GRS 1009-45, MAXI J1659-152, GRS 1739-278). At present, however, we cannot completely exclude that the observed re-brightening is instead the initial phase of a new canonical outburst.

We plan to continue monitoring this source with AMI and Swift, as well as part of an ongoing ThunderKAT weekly monitoring programme at 1.4 GHz with the MeerKAT telescope. Multi wavelength coverage of the source is encouraged. We would like to thank both the Swift team and the MRAO staff for carrying out these observations promptly.