The brightest X-ray hard state of AT 2019wey observed with the MAXI/GSC

ATel #15776; H. Negoro, M. Nakajima, K. Kobayashi, M. Tanaka, Y. Soejima (Nihon U.), T. Mihara, T. Kawamuro, S. Yamada, T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Hiramatsu, H. Nishikawa, A. Yoshida (AGU), Y. Tsuboi, W. Iwakiri, J. Kohara, S. Urabe, S. Nawa, N. Nemoto (Chuo U.), M. Shidatsu, M. Iwasaki (Ehime U.), N. Kawai, M. Niwano, R. Hosokawa, Y. Imai, N. Ito, Y. Takamatsu (Tokyo Tech), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, T. Kurihara (JAXA), Y. Ueda, S. Ogawa, K. Setoguchi, T. Yoshitake, K. Inaba, Y. Nakatani (Kyoto U.), M. Yamauchi, T. Sato, R. Hatsuda, R. Fukuoka, Y. Hagiwara, Y. Umeki (Miyazaki U.), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), and M. Sugizaki (NAOC)
on 25 Nov 2022; 13:52 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

Tweet

The optical transient AT 2019wey (Tonry et al. 2019, TNSTR, 2019-2553) is a peculiar X-ray source showing a gradual rise and complex time variations (Yao et al. 2021, ApJ, 920, 121). The source is likely to be a low-mass X-ray binary hosting a neutron star or a black hole. Recently, the source often triggered the MAXI/GSC Nova-Alert System, and we found that after a rapid flux decrease around MJD 59600 (here we define this as the end of the primary outburst) the X-ray flux has been increasing again. On November 23, the flux reaches about 50 mCrab at 2-20 keV, being in the brightest state ever observed.

Average X-ray fluxes from MJD 59900 (November 17, 2022) to 59907 (November 24) are (7.5 +/- 0.3)e-2 ph/cm2/s (~36 mCrab) at 2-4 keV, (5.7 +/- 0.3)e-2 ph/cm2/s (~46 mCrab) at 4-10 keV, and (3.2 +/- 0.4)e-2 ph/cm2/s (~79 mCrab) at 10-20 keV. Those in a relatively bright plateau phase in the primary outburst from MJD 59100 to 59110 were (4.7 +/- 0.2)e-2 ph/cm2/s, (2.8 +/- 0.2)e-2 ph/cm2/s, and (0.4 +/- 0.3)e-2 ph/cm2/s, respectively.

Yao et al. reported that the source spectrum became softer when the source was brighter. The above data apparently show an opposite behavior. An average energy spectrum obtained in MJD 59800-59907 is well represented by an absorbed power-law model with a photon index of 1.73 +/- 0.08 and a column density of (0.15 +/- 0.34)e22 cm^-2, which is a typical spectrum of black hole binaries in the hard state. We also note that the spectrum is also similar to that in the rising phase in the primary outburst observed with NICER (see the left panel of Figure 12 by Yao et al.) though the flux at that time was about half of the current one. This may indicate that the current bright hard state results from the hysteretic behavior in the rising phase of the bright outburst (Miyamoto et al. 1995, ApJ, 442, L13).

Followup observations are encouraged to reveal the nature of the source.

MAXI data for AT 2019wey