MAXI/GSC detection of a possible X-ray flare from the RS CVn binary system SS Boo or a GRB

ATel #13730; S. Kitakoga, R. Sasaki, W. Iwakiri (Chuo U.), S. Yamada (Kyoto U.), H. Negoro, M. Nakajima, M. Aoki, K. Kobayashi, R. Takagi, K, Asakura, K, Seino, S. Mokumoto (Nihon U.), T. Mihara, C. Guo, Y. Zhou, T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Nishida, A. Yoshida (AGU), Y. Tsuboi, H. Kawai, Y. Okamoto (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. Ogawa, K. Setoguchi, T. Yoshitake (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 13 May 2020; 13:35 UT
Credential Certification: Wataru Iwakiri (wataru.iwakiri@riken.jp)

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The MAXI/GSC nova alert system triggered an X-ray flare-like event at 14:06 UT on May 1th 2020. The estimated X-ray flux in the 4 - 10 keV band was about 180 mCrab. No significant X-ray variation was observed during this scan transit (about 30 sec). Assuming that the source flux was constant over the transit, we obtain the source position at
(R.A., Dec) = (228.623 deg, 38.261 deg) = (15 14 29, +38 15 39) (J2000)
with a statistical 90% C.L. elliptical error region with long and short radii of 0.29 deg and 0.28 deg, respectively. The roll angle of the long axis from the north direction is 46.0 deg counterclockwise. Considering an additional systematic uncertainty of 0.1 deg (90% containment radius), this position is consistent with that of the eclipsing binary of the RS-CVn type, SS Boo which is listed in the ROSAT Bright Source Catalog. During the next transit at 15:38 UT, any source was not detected significantly (< 39 mCrab at 4 - 10 keV) with MAXI/GSC.

The spectrum was well reproduced with a thin thermal plasma model (Smith et al. 2001, ApJ, 556, 91S) for an assumed X-ray flare from SS Boo. The 2 - 10 keV flux over the one scan transit was about 4.7 (+0.2, -2.1 ) x 10^-9 ergs cm^-2 s^-1, corresponding to a luminosity of 3.1 (+0.1, -1.4) x 10³⁴ ergs s^-1 for an assumed distance of 237 pc (Gaia Collaboration et al. 2018, A&A, 616, 1G). The temperature (kT) and the emission measure (EM) were 5.2 +/- 2.3 keV and 2.9 (+/- 1.0) x 10⁵⁷ cm^-3, respectively. The reduced chi-square and the degree of freedom were 1.6 and 5, respectively. The kT and EM were typical values compared with previous results (Shibata and Yokoyama 1999, ApJ, 526, 49S; Tsuboi et al. 2016, PASJ, 68, 90S). However, the duration of this flare was too short compared with the typical one; the typical duration, which was expected from the relation of the duration and X-ray bolometric luminosity (Tsuboi et al. 2016, PASJ, 68, 90S), was 25,000 sec. On the other hand, the upper limit of the e-folding decay time is less than about 3600 sec because the X-ray flux decreased to less than 39 mCrab at the next scan transit of MAXI (92 minutes after). Therefore, if this flare occurs on SS Boo, it is an unusual short stellar flare .

To evaluate the possibility that the event originates with a GRB, we also fit the spectrum using a power-law model. The spectrum was also well reproduced with a power-law model with a photon index of 2.3 +/- 0.3 . The 2 - 10 keV flux is 5.3 (+0.7, -1.6) x 10^-9 ergs cm^-2 s^-1. The reduced chi-square and the degree of freedom were 1.4 and 5, respectively. Since the slope is steeper than the typical GRB, it might be an X-ray rich GRB or an X-ray Flash.

Summarizing the above discussion, we conclude that the short flare event was associated with a short unusual stellar flare from SS Boo or a GRB. If the transient event is an X-ray flare from SS Boo, this is the first detection of this source with MAXI/GSC.