MAXI/GSC detection of AT 2019wey in an extremely soft state

ATel #16197; H. Negoro, M. Nakajima, K. Kobayashi, M. Tanaka, Y. Soejima, Y. Kudo (Nihon U.), T. Mihara, T. Kawamuro, S. Yamada, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Hiramatsu, H. Nishikawa, A. Yoshida (AGU), Y. Tsuboi, S. Urabe, S. Nawa, N. Nemoto (Chuo U.), M. Shidatsu (Ehime U.), I. Takahashi, M. Niwano, S. Sato, N. Higuchi, Y. Yatsu (Tokyo Tech), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, T. Kurihara (JAXA), Y. Ueda, K. Setoguchi, T. Yoshitake, Y. Nakatani (Kyoto U.), M. Yamauchi, Y. Hagiwara, Y. Umeki, Y. Otsuki (Miyazaki U.), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC), and W. Iwakiri (Chiba U.)
on 20 Aug 2023; 14:04 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

Tweet

The optical and X-ray transient AT 2019wey (Tonny et al. 2019, TNSTR, 2553; Mereminskiy et al. ATel. #13571) is still bright in X-rays (e.g., ATel #15776, also in other wavelengths #15816, #15824, #15971). The MAXI/GSC detected AT 2019wey in an extremely soft state in 2023 August 7-15 (no data from July 8 until August 7).

No significant X-rays are recognized above 5 keV in a GSC spectrum obtained in August 8-14. If we fit the spectrum with an absorbed disk-blackbody (diskbb) plus power-law model with a fixed column density at 4.17e21 (Yao et al. 2021, ApJ, 922, a121), we find that the power-law component with a photon index of 4.7 (+1.4, -1.3) dominates over the disk component with a temperature of 0.12 (+0.09, -0.07) keV at energies above 2.3 keV. Because the disk temperature is much below the GSC energy band (2-20 keV), we only obtain a lower limit of the disk normalization of 1.4e6, corresponding to the inner disk radius of about 1200 km by assuming a 10 kpc distance and a face-on disk, which is about nine times larger than that in Chandra and NICER observations on 2020 September 20 (Yao et al.) An absorbed and unabsorbed 2-10 keV fluxes are 2.6e-10 erg/cm2/s and 2.9e-10 erg/cm2/s, respectively.

Previous observational properties are more consistent with those of black hole X-ray binaries than neutron star ones (Yao et al. 2021; Yao et al. 2021, ApJ, 920, a120; Mereminskiy et al. 2022, A&A, 661, A32), but the nature is still unknown because of its peculiar properties. The spectrum becomes harder, but still softer than before. Followup observations are encouraged.

MAXI/GSC data for AT 2019wey