MAXI/GSC observations indicate further softening of MAXI J1535-571 toward the high/soft state

ATel #10761; M. Shidatsu, S. Nakahira (RIKEN), H. Negoro (Nihon U.), S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai (JAXA), T. Mihara, M. Sugizaki, W. Iwakiri, M. Matsuoka (RIKEN), N. Kawai, 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 (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 20 Sep 2017; 06:43 UT
Credential Certification: Satoshi NAKAHIRA (nakahira@crab.riken.jp)

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As reported by Nakahira et al. (ATel #10729) and Kennea et al. (ATel #10731), the new X-ray transient MAXI J1535-571 (GCN #21788, ATels #10699, #10700, #10702, #10704, #10708, #10711, #10714, #10716, #10734, #10745) exhibited an X-ray spectral softening on September 10. Since then, the MAXI/GSC has observed a continuous decrease of the source flux in the 10-20 keV band and a further spectral softening of MAXI J1535-571.
The MAXI/GSC time-averaged spectrum produced from the scans in 2017 September 19 UT 22:00 and 23:32 has a steep power-law-like shape. Fitting it with an absorbed power-law model, we obtained a photon index of 2.90 ± 0.08, where the absorption column density was fixed at 2.4×10²² cm^-2 (Kennea et al., ATel #10731). The spectrum was better reproduced by a multi-color disk blackbody plus power-law model, which yielded an inner disk temperature of 1.5^+0.6_-0.4, an inner radius of 18⁺¹⁸_-9 km (where an inclination of 0 deg and a distance of 8 kpc are assumed) and a photon index of 2.9^+0.4_-0.3. The fit was improved significantly by adding the disk blackbody component, with an F-test probability of 7×10^-4 (where the chi-squared value and the degree of freedom were reduced by 15 and 2 from the single power-law model, respectively).
These results suggest that the source is now in the intermediate state and likely to enter into the high/soft state soon. As of September 19 UT 21:45, the source intensity in 2-20 keV is ~5 Crab, which corresponds to an absorbed flux of ~1.2×10^-7 erg cm^-2 sec, by adopting the best-fit diskbb+power-law model above.

The MAXI/GSC light curves of MAXI J1535-571 are available at:
http://maxi.riken.jp/star_data/J1535-572/J1535-572.html .
Also the spectral information from the MAXI/GSC data can be obtained at:
http://maxi.riken.jp/nakahira/1535monitor .