MAXI/GSC detection of an intense X-ray flare from the RS CVn binary CF Tuc

ATel #12009; W. Iwakiri, Y. Tsuboi, R. Sasaki, H. Kawai, T. Sato (Chuo U.), M. Serino(AGU), H. Negoro, M. Nakajima, A. Sakamaki, W. Maruyama (Nihon U.), T. Mihara, S. Nakahira, F. Yatabe, Y. Takao, M. Matsuoka (RIKEN), T. Sakamoto, S. Sugita, Y. Kawakubo, T. Hashimoto, A. Yoshida (AGU), N. Kawai, M. Sugizaki, Y. Tachibana, K. Morita (Tokyo Tech), S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai (JAXA), Y. Ueda, A. Tanimoto, T. Morita, S. Yamada (Kyoto U.), H. Tsunemi, T. Yoneyama, K. Asakura, S. Ide (Osaka U.), M. Yamauchi, K. Hidaka, S. Iwahori (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), M. Shidatsu (Ehime U.) report on behalf of the MAXI team:
on 5 Sep 2018; 06:07 UT
Credential Certification: Wataru Iwakiri (wataru.iwakiri@riken.jp)

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The MAXI/GSC nova alert system triggered on the flare-like event from the position consistent with the RS CVn binary CF Tuc during a scan transit at 07:24 UT on Sept 3 2018. Without assumptions on the source constancy, we obtain a rectangular error box for the transient source with the following corners:
(R.A., Dec) = (12.498, -75.690) deg = (00 49 59, -75 41 23) (J2000)
(R.A., Dec) = (15.398, -75.527) deg = (01 01 35, -75 31 37) (J2000)
(R.A., Dec) = (13.924, -74.117) deg = (00 55 41, -74 07 01) (J2000)
(R.A., Dec) = (11.278, -74.281) deg = (00 45 06, -74 16 51) (J2000).
Since CF Tuc is the only known X-ray source in this error region, we concluded that this is an X-ray flare from CF Tuc.
The observed peak X-ray flux is 147+/-29 mCrab (1 sigma error) in the 2 - 20 keV band. The peak flux is about 2 times higher than the previous flare detected by the MAXI/GSC at Jan 30 2012 (ATel #3897). Assuming a distance of 89 pc and Crab-like spectrum, the peak luminosity is 4.6 +/- 0.3 x 10³² erg/s. The e-folding decay time of this flare is about 9000 sec.