MAXI/GSC detection of a rapid increase in the hard X-ray flux of MAXI J1820+070

ATel #12057; H. Negoro, M. Nakajima, A. Sakamaki, W. Maruyama (Nihon U.), T. Mihara, S. Nakahira, F. Yatabe, Y. Takao, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, T. Hashimoto, A. Yoshida (AGU), N. Kawai, M. Sugizaki, Y. Tachibana, K. Morita (Tokyo Tech), S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai (JAXA), Y. Ueda, A. Tanimoto, T. Morita, S. Yamada (Kyoto U.), Y. Tsuboi, W. Iwakiri, R. Sasaki, H. Kawai, T. Sato (Chuo U.), H. Tsunemi, T. Yoneyama (Osaka U.), M. Yamauchi, K. Hidaka, S. Iwahori (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), and M. Shidatsu (Ehime U.)
on 24 Sep 2018; 15:26 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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We report on a rapid increase in the hard X-ray flux of the black hole candidate MAXI J1820+070 in outburst (Kawamura et al. ATel #11399, Denisenko et al. ATel. #11400). During the reflaring activity starting from around 2018 July 1 (MJD 58300), the source underwent a hard-to-soft transition (Homan et al. ATel #11820). Since then, the source had been in the soft state. MAXI/GSC data from 2018 September 22 (MJD 58383) show a significant increase in hard (>4 keV) X-rays. Swift/BAT data also show that the 15-50 keV X-ray flux slightly increased on September 21, and dramatically changed from 0.01 c/cm^2/sec to 0.04 c/cm^2/sec at the beginning of September 22 (https://swift.gsfc.nasa.gov/results/transients/weak/MAXIJ1820p070/). These suggest the source is undergoing a state transition. The count rates in both MAXI/GSC and Swift/BAT, however, peaked on September 23, and decreased again on September 24.

The GSC spectra obtained on September 20-21 (before the transition) and on September 23 are well described by an absorbed disk blackbody (diskbb) plus power-law model with a fixed column density nH of 1e21 atoms/cm^2 (Shidatsu et al., ApJ, submitted). The inner disk temperature, Tin, did not change significantly from 0.53 +/- 0.01 keV to 0.53 +/- 0.02 keV, but the diskbb normalization decreased from (2.8 +/- 0.4)e4 to (1.7 +/- 0.4)e4. On the other hand, the normalization of the power-law component with a photon index fixed at 2.5 increased by a factor of more than 3 (from 1.3+/-0.2 to 4.3 +/- 0.2). We note that these parameters are provided only to show the change of the overall spectral shape, and we do not intend to show a decrease of the inner disk radius.

The unabsorbed 2-20 keV flux at the transition is (0.9-1.1)e-8 erg/cm^2/sec, corresponding to the luminosity of ~1.2e37 erg/s if the source distance of 3.2 kpc (Homan et al. ATel #11576; Gandhi et al. https://arxiv.org/abs/1804.11349) and the isotropic radiation are assumed. As a result, the bolometric luminosity at the transition is, for instance, below 1% of the Eddington luminosity for a 10 solar mass black hole.

We encourage continuous multi-wavelength followup observations of this bright and interesting X-ray nova.

MAXI data of MAXI J1820+070