MAXI J1535-571 currently shows softest spectrum

ATel #11020; M. Shidatsu, S. Nakahira (RIKEN), H. Negoro (Nihon U.), N. Kawai (Tokyo Tech), S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai (JAXA), T. Mihara, M. Sugizaki, W. Iwakiri, F. Yatabe, Y. Takao, M. Matsuoka (RIKEN), S. Sugita, T. Yoshii, Y. Tachibana, S. Harita, Y. Muraki, K. Morita (Tokyo Tech), A. Yoshida, T. Sakamoto, M. Serino, Y. Kawakubo, Y. Kitaoka, T. Hashimoto (AGU), H. Tsunemi, T. Yoneyama (Osaka U.), M. Nakajima, T. Kawase, A. Sakamaki (Nihon U.), Y. Ueda, T. Hori, A. Tanimoto, S. Oda, T. Morita, S. Yamada (Kyoto U.), Y. Tsuboi, Y. Nakamura, R. Sasaki, H. Kawai (Chuo U.), M. Yamauchi, C. Hanyu, K, Hidaka (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.)
on 28 Nov 2017; 03:10 UT
Credential Certification: Megumi Shidatsu (shidatsu@kusastro.kyoto-u.ac.jp)

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We report the recent MAXI/GSC monitoring of the black hole candidate MAXI J1535-571 discovered on 2017 September 2 (GCN #21788, ATels #10699, #10700, #10702, #10704, #10708, #10711, #10714, #10716, #10734, #10745, #10761, #10768, #10816, #10899). The source showed rapid X-ray spectral softening in September 18-22 when its flux was at around the maximum, and exhibited hardening from around October 13 as it decreased its flux. However, MAXI J1535-571 started to exhibit steady spectral softening since October 25. The hardness ratio of the MAXI/GSC fluxes (6-20 keV/2-6 keV) in the last 10 days until November 27 reached ~0.03, the lowest value in the current outburst, and significantly lower than the value of ~0.1 at the previous minimum near the flux peak before the temporary hardening occurred. The time-averaged GSC spectrum obtained in November 23-25 was well reproduced by a multi-color disk model with an inner temperature of ~0.8 keV, convolved with a power-law component with a fixed photon index of 2.4, both absorbed by interstellar matter with a fixed hydrogen column density of 2.6e22 /cm2. The inner disk radius was estimated as ~110 km (i=30 deg and D=10 kpc were assumed), consistent with the values obtained in the first softening. However, the fraction of the power-law component (<~0.02), was much smaller compared with that in the previous softening (~0.1). These results indicate that the source is now in the high/soft state, and this would be a good timing of soft X-ray observations to determine the disk parameters. Further multi-wavelength observations are encouraged, as the source would make the soft-to-hard transition soon. The MAXI/GSC data of MAXI J1535-571 are available at http://maxi.riken.jp/nakahira/1535monitor/ .