MAXI/GSC observation of the giant outburst from the Be/X-ray binary pulsar A 0535+262

ATel #14173; M. Nakajima, H. Negoro (Nihon U.), R. Sasaki (Chuo U.), M. Aoki, K. Kobayashi, R. Takagi, K, Asakura, K, Seino (Nihon U.), T. Mihara, C. Guo, Y. Zhou, T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Nishida, K. Komachi, A. Yoshida (AGU), Y. Tsuboi, W. Iwakiri, H. Kawai, Y. Okamoto, S. Kitakoga (Chuo U.), M. Shidatsu (Ehime U.), N. Kawai, R. Adachi, M. Niwano (Tokyo Tech), S. Nakahira, Y. Sugawara, S. Ueno, H. Tomida, M. Ishikawa, M. Tominaga, T. Nagatsuka (JAXA), Y. Ueda, S. Yamada, S. Ogawa, K. Setoguchi, T. Yoshitake, U. Goto, R. Uematsu (Kyoto U.), H. Tsunemi (Osaka U.), M. Yamauchi, K. Kurogi, K. Miike (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC)
on 14 Nov 2020; 13:58 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)

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Following the optical (ATel#14171) and X-ray (ATel#14157,#14170) observations, we report on the hardness ratio change of the ongoing giant outburst from A 0535+262 observed by MAXI/GSC. Prior to the Swift/BAT detection in the 15-50 keV energy band on 2020-10-29 (Atel#14157), the 4-10 keV source flux increased quietly to 0.034 +- 0.008 photons/s/cm² (30 +- 7 mCrab) on 2020-10-26 (MJD 59148). After the detection, the 4-10 keV / 2-4 keV hardness ratio started to increase from ∼1.7, and reached the peak value of ∼3.1 on 2020-11-07 (MJD 59160). In recent 5 days, the hardness ratio showed slight decrease, but the flux still continues to brighten (http://maxi.riken.jp/news/en/#news-1270). Since the same hardness-intensity behaviors were observed from the previous giant outburst in 2009, the current event might reach the peak within a few days. The 10-20 keV / 4-10 keV hardness ratio also remains a constant value of ∼0.7 in recent 5 days.

The source light curves can be checked at the following pages;
MAXI(http://maxi.riken.jp/pubdata/v7lrkn/J0538+263/index.html),
Swift/BAT(https://swift.gsfc.nasa.gov/results/transients/1A0535p262/),
Fermi/GBM(https://gammaray.nsstc.nasa.gov/gbm/science/pulsars/lightcurves/a0535.html), and
the BeXRB monitor page(http://integral.esac.esa.int/bexrbmonitor/webpage_oneplot.php).

MAXI/GSC lightcurve and hardness ratio