MAXI/GSC detection of a soft-to-hard state transition of MAXI J1631-479

ATel #12421; H. Negoro, M. Nakajima, A. Sakamaki, W. Maruyama, M. Aoki, K. Kobayashi (Nihon U.), T. Mihara, S. Nakahira, F. Yatabe, Y. Takao, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, 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, T. Midooka (JAXA), Y. Ueda, A. Tanimoto, T. Morita, S. Yamada, S. Ogawa (Kyoto U.), Y. Tsuboi, W. Iwakiri, R. Sasaki, H. Kawai, T. Sato (Chuo U.), H. Tsunemi, T. Yoneyama, K. Asakura, S. Ide (Osaka U.), M. Yamauchi, K. Hidaka, S. Iwahori (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), M. Shidatsu (Ehime U.), and Y. Kawakubo (LSU)
on 25 Jan 2019; 14:25 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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On 2019 January 23, MAXI/GSC detected a rapid spectral change of the newly discovered X-ray nova MAXI J1631-479 (Kobayashi et al. ATel #12320; Miyasaka et al. #12340; Russell et al. #12396; Onori et al. #12418). Following a gradual increase of the hard (>10 keV) X-ray flux since January 17 (MJD 58500), the 2-10 keV X-ray flux rapidly decreased by half in two days on January 23-24. At the same time, the hard X-ray flux increased twice. This clearly indicates that the source underwent a soft-to-hard state transition.

The flux at the transition was roughly 700-800 mCrab at 2-20 keV, corresponding to about 7e-8 ergs/cm2/s at 1-60 keV assuming a power-law spectrum with a photon index of 1.7. If the transition luminosity is 1-4% of the Eddington luminosity (e.g., Maccarone, 2003, A&A 409, 697), the source distance is 1.2-2.5 kpc for a 10 solar mass black hole. However, if we take into account a high absorption column density of 2.9e22 atoms/cm2 obtained by NuSTAR (#12340), the source may be more massive and far.

Finally, we note that a hard-to-soft transition was recognized soon after the discovery, MJD 58475-58480. Thus, during the ATCA observation (#12396) the source was already in the soft state, and it is plausible that the nature of the source is a black hole binary (also see the NuSTAR results of #12340).

Multi-wavelength follow-up observations are encouraged.

MAXI data for MAXI J1631-479