Swift observes MAXI J1820+070 in transition from the soft to the hard-intermediate state

ATel #12064; S. E. Motta, J. Bright, R. Fender (University of Oxford)
on 26 Sep 2018; 15:52 UT
Credential Certification: Sara Elisa Motta (sara.motta@physics.ox.ac.uk)

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The transient black hole candidate ASASSN-18ey/MAXI J1820+070 (ATel #11820) was discovered in March 2019 by the MAXI mission when it entered the outburst phase (ATel #11399, #11400). After an extended hard-state, the source rapidly transitioned to the soft state on MJD 58300 (ATel #11820).

Following the report of a rapid increase in the hard X-ray of MAXI J1820+070, observed by MAXI/GSC and reported by the MAXI team (MAXI alerts and ATel #12057), we triggered the AMI radio telescopes (ATel #12061) and Swift. We report on the first of a series of observations, taken on September 25th (T01:45:57), and on two observations taken earlier, on September 17th (17T21:23:57) and 22nd (T03:34:57), respectively. We extracted energy spectra in the 0.3-10 keV band from Swift/XRT data, and we fitted them with a absorbed powerlaw + diskbb model. The resulting best fits show a clear evolution of the spectral shape of the source.

On September 17th the energy spectrum is well described by a multicolour disk blackbody with inner disk temperature (0.603 +/- 0.004)keV and a weak powerlaw tail (normalisation 0.3 +/- 0.1) with slope 1.7 +/- 0.3. On September 22nd the energy spectrum shows a disk blackbody with temperature consistent with the above, but a harder (and slightly more prominent, normalisation 0.7 +/- 0.2) powerlaw tail, with slope 1.6 +/- 0.2. On September 25th the multicolour disk blackbody showed a lower temperature of (0.481 +/- 0.008)keV and a stronger (normalisation 2.9 +/- 0.4) hard tail, with slope 1.92 +/- 0.08. These parameters are consistent with those reported in ATel #12057.

The above shows that MAXI J1820+070 was still in the soft state on September 17th. However, we can refine the time of the transition to the hard-intermediate state, which likely occurred on (or shortly before) September 22nd, consistent with what reported by the MAXI team (ATel #12057), who found a spectral change between September 21st and 23rd. We note that while the source has clearly left the soft state, it might moving back and forth along the hard intermediate, as the apparent non-monotonic evolution of the hard-tail slope seems to indicate.

Swift will continue monitoring this source and we encourage multi-wavelength followup observations. We thank the Swift team for the prompt scheduling of these observations (despite the inconvenient night trigger).