Rapidly fading the black hole X-ray nova MAXI J1535-571 from MAXI/GSC observations

ATel #11568; H. Negoro (Nihon U.), T. Mihara, S. Nakahira, F. Yatabe, Y. Takao, M. Matsuoka (RIKEN), N. Kawai, M. Sugizaki, Y. Tachibana, K. Morita (Tokyo Tech), T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, T. Hashimoto, A. Yoshida (AGU), M. Nakajima, A. Sakamaki, W. Maruyama (Nihon U.), 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.), M. Shidatsu (Ehime U.) report on behalf of the MAXI team:
on 24 Apr 2018; 14:29 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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Recent MAXI/GSC observations revealed unexpectedly rapid decay of the X-ray flux of the black hole candidate MAXI J1535-571 discovered on 2017 September 2, MJD 57998 (ATel #10699, #10700). The source has been in the soft (thermal dominated) state since the end of the last November, around MJD 58100 (ATel #11020). On MJD 58120-58130, the X-ray flux increased by a factor of about two with the spectrum unchanged, but afterward the flux continued to decrease almost exponentially (Nakahira et al. PASJ, submitted, arXiv:1804.00800). Recent MAXI/GSC data show that the flux decreased rapidly after MJD 58190-58200, and that the 2-10 keV flux has been below the detection limit of GSC for 4days (about 8 mCrab) since April 16, MJD 58224, when the flux was lower than the extrapolation of the exponential decay by one order of magnitude. The spectrum tended to soften with the decrease, and no soft-to-hard transition was recognized.

From previous outburst study of X-ray novae, the soft-to-hard transition is known to occur at 1-4% of the Eddington luminosity (Maccarone 2003, A&A, 409, 697), and such a transition was expected to occur until the end of March even if the observed peak flux corresponded to the Eddington luminosity. Rapid decrease at the end of an outburst after the soft-to-hard transition was often observed. This time, however, the source faded away remaining in the soft state, suggesting that for instance the disk evaporation triggering the hard state (e.g., Meyer et al. 2000, A&A, 361, 175) did not work even in low mass accretion rates.

We encourage multi-wavelength observations to understand what happens and know the current status of the accretion disk of the rare bright black hole X-ray nova.

MAXI data for MAXI J1535-571