MAXI/GSC detection of a very bright X-ray flare possibly from the SFXT XTE J1739-302

ATel #13871; H. Negoro, K. Kobayashi (Nihon U.), N. Kawai (Tokyo Tech), M. Shidatsu (Ehime U.), M. Nakajima, M. Aoki, 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, A. Yoshida (AGU), Y. Tsuboi, W. Iwakiri, R. Sasaki, H. Kawai, Y. Okamoto, S. Kitakoga (Chuo U.), 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 (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) report on behalf of the MAXI team
on 17 Jul 2020; 10:16 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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At 09:34 UT on 2020 July 9, MAXI/GSC detected a bright X-ray flare-like event toward the Galactic center region (l, b) = (357.9, +0.4). The source position is consistent with that of the supergiant fast X-ray transient XTE J1739-302 (Smith et al. IAU Circ. 6748; Smith et al. 1998, ApJ, 501, L181). There was no significant excess flux in the previous transit at 08:01 UT, and in the next transit at 11:07 UT with an upper limit of 20 mCrab for each. The energy spectrum is better represented by an absorbed power-law with a photon index of 1.54 +/- 0.39 and a nH of (5.2 +/- 3.7)e22 (reduced Chi^2 = 1.22 for 4 d.o.f.) than an absorbed blackbody with a temperature of 2.45 +/- 0.35 keV and nH of less than 3.9e22 (reduced Chi^2 = 1.63 for 4 d.o.f.).

These properties are very similar to what we obtained from this region on 2015 January 11 (Negoro et al. ATel. #6900), which strongly suggests that these events originate from the same object, XTE J1739-302. Assuming that this event comes from the position of XTE J1739-302, 2-4 keV and 4-10 keV X-ray fluxes of the flare obtained with PSF image fits (Morii et al. 2016, PASJ, 86, S11) are 0.14 (+0.03, -0.03) counts/cm^2/s (~130 mCrab for the Crab count rate) and 0.37 (+ 0.05, -0.04) counts/cm^2/s (~320 mCrab), respectively. The power-law fit to the energy spectrum also gives the 2-10 keV X-ray flux of 5.7e-9 erg/cm^2/s (~240 mCrab, absorbed), or 8.0e-9 erg/cm^2/s (unabsorbed) corresponding to the luminosity of 7.0e36 erg/s for the source distance of 2.7 kpc (Rahoui et al. 2008, A&A, 484, 801).

This event is the brightest in several similar bright events which MAXI has detected for 11 years at the same position, probably from XTE J1739-302. The 4-10 keV count rate of this flare is, however, 1.5-2.0 times larger than those of the previous events. The outburst (flare) duration of XTE J1739-302 is about 1000-2000 sec (e.g., Bozzo et al. 2010, A&A, 539, A6, Sakano et al. 2002 ApJS, 138, 19). Taking this into account, the fluxes MAXI detected are lower limits for peak fluxes of the outbursts if the events came from XTE J1739-302. Nevertheless, this even is the brightest or at the high-end of the flux distribution in all the outbursts of XTE J1739-302 previously reported (e.g., Romano et al. 2014, A&A 568, A55).

Finally, we note that the longterm GSC light curve of XTE J1739-302 shows that average count rates at 2-4 keV and 4-10 keV has been increased by roughly 5e-3 counts/cm^2/s (~5 mCrab) and 6e-3 count/cm^2/s (~5 mCrab), respectively, since around 2020 April 6 (MJD 58945). This might be due to increased low-level activity of XTE J1739-302 leading to this brightest outburst, or activity of other sources unresolved with the GSC. Follow observations will clarify the cause of the low-level emission from the region.

A bright X-ray flare probably from XTE J1739-302