MAXI/GSC detection of a hard-to-soft state transition of MAXI J1727-203, suggesting a black-hole binary

ATel #11696; H. Negoro (Nihon U.), M. Shidatsu (Ehime U.), T. Mihara (RIKEN), M. Serino (AGU), M. Nakajima, A. Sakamaki, W. Maruyama (Nihon U.), S. Nakahira, F. Yatabe, Y. Takao, M. Matsuoka (RIKEN), N. Kawai, M. Sugizaki, Y. Tachibana, K. Morita (Tokyo Tech), T. Sakamoto, S. Sugita, Y. Kawakubo, T. Hashimoto, A. Yoshida (AGU), S. Ueno, H. Tomida, M. Ishikawa, N. Isobe, Y. Sugawara, 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), and K. Yamaoka (Nagoya U.)
on 7 Jun 2018; 10:08 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

Tweet

MAXI/GSC observations of the newly discovered X-ray nova MAXI J1727-203 (Yoneyama et al. ATel. #11683, also see ATel #11689, #11690, #11691, #11692) showed energy spectral softening starting from the middle of June 5 (MJD 58274). The 2-20 keV X-ray counts has been almost linearly increasing, but the hardness ratio (4-10 keV/2-4 keV) has changed from 0.80+/-0.20 at scans from 11:14 to 17:25 June 5 to 0.33 +/- 0.05 at scans from 07:18 to 19:40 on June 6. Thus, NICER (Ludlam et al. ATel. #11689) is likely to have observed the source at the beginning of the transition.
The current GSC spectrum obviously consists of soft and hard components. If we fit the 2-20 keV spectrum obtained at 7:18-19:40 on June 6 with an absorbed power-law plus disk-blackbody (diskbb) model with n_H fixed at 4.6e21 cm^-2 (ATel #11689), we obtained the inner disk temperature of 0.39+/-0.11 keV (the power-law index, Gamma, is fixed at 2.0), or 0.32+/-0.14 keV (Gamma is fixed at 2.5). Here, due to the poor statistics, we fixed the power-law index. The diskbb component is dominant below 3 keV in the former case (Gamma=2.0), and in both cases the diskbb normalization parameters indicate relatively large disk inner radii, about 200 and 300 [(d/10 kpc)/sqrt(cos(theta))] km, where d is the distance to the source and theta is the disk inclination, for Gamma = 2.0 and 2.5, respectively. We also obtained the 90% lower limits of the radius, 32 and 29 [(d/10 kpc)/sqrt(cosA)] km for Gamma = 2.0 and 2.5, respectively. An absorbed blackbody plus power-law model also yields a similar large spherical radius. An unabsorbed 2-10 keV flux was 2.2e-9 erg/s/cm2 for Gamma=2.0, corresponding to the 2-10 keV luminosity of 1.9e37 erg/s for the 8 kpc distance source.

The ultrasoft component with the rather low disk temperature and the large inner disk radius (for d > 3 kpc and the face-on disk (theta = 0 deg)) in the soft-state spectrum (e.g., Tanaka & Shibazaki 1996, ARAA, 34, 607) strongly suggests that MAXI J1727-203 is a binary hosting a black hole. Further multi-wavelength observations are encouraged.

MAXI data of MAXI J1727-203