MAXI observations of two bright X-ray flares from MAXI J0709-159

ATel #15188; K. Kobayashi, H. Negoro (Nihon U.), S. Sugita, M. Serino (AGU), T. Mihara (RIKEN), W. Iwakiri (Chuo U.), M. Nakajima, K. Asakura, K. Seino (Nihon U.), T. Tamagawa, J. Li, M. Matsuoka (RIKEN), T. Sakamoto, K. Komachi, H. Hiramatsu, A. Yoshida (AGU), Y. Tsuboi, H. Kawai, Y. Okamoto, S. Kitakoga, J. Kohara (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, M. Tominaga, T. Nagatsuka, T. Kurihara (JAXA), Y. Ueda, S. Yamada, S. Ogawa, K. Setoguchi, T. Yoshitake, Y. Goto, R. Uematsu, K. Inaba (Kyoto U.), H. Tsunemi (Osaka U.), M. Yamauchi, Y. Nonaka, T. Sato, R. Hatsuda, R. Fukuoka (Miyazaki U.), T. Kawamuro (UDP/NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC) report on behalf of the MAXI team:
on 27 Jan 2022; 14:27 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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We further report on MAXI observations of MAXI J0709-159 discovered by MAXI/GSC on 2022 January 25 (Serino et al. ATel #15178). Using the source position determined by NICER (Iwakiri et al. ATel #15181), we reconstructed light curves of MAXI/GSC, and found that the X-ray enhancement at 10:42 UT was not a short burst-like event, but a long-lasting flaring event continuing longer than the MAXI scan for about 50 s. Furthermore, we found another flaring event from the source at 13:48 UT on January 25, about 6 min before the NICER detection during the raster scan observations. Rapid time variations on timescales of seconds are observed in both flares, though it is not statistically confirmed, yet.

The GSC energy spectrum of the first flaring event is well represented by an absorbed power-law model with a photon index of 1.9 (+0.6/-0.5) and an equivalent hydrogen column density nH of 3.0 (+5.2/-3.0) e22 cm-2, or a single blackbody model with a temperature of 1.6 +/- 0.2 keV. 2-10 keV fluxes using these models are 5.2 (+0.8/-0.7) e-9 erg/cm2/s and 5.4 +/-0.8 e-9 erg/cm2/s, respectively. An unabsorbed 2-10 keV flux of the power-law model is 6.2 (+2.7/-1.6) e-9 erg/cm2/s.

On the other hand, the energy spectrum of the second flaring event is heavily absorbed. An absorbed power-law model fit gives a photon index of 2.5 (+1.1/-1.0) and nH = 5.4 (+3.9/-2.8) e23 cm-2, and an absorbed blackbody model fit a temperature of 2.3 (+0.8/-0.5) keV and nH = 2.3 (+2.8/-1.9) e23 cm-2. Absorbed 2-10 keV fluxes are 3.0 (+0.7/-0.6) e-9 erg/cm2/s and 3.2 (+0.7/-0.6) e-9 erg/cm2/s, respectively, and unabsorbed 2-10 keV fluxes are 16 (+41/-10) e-9 erg/cm2/s and 5.3 (+4.1/-2.0) e-9 erg/cm2/s, respectively. We note that the shape and fluxes of the two energy spectra above 5 keV are very similar.

The observed energy spectra are similar to those of low-mass X-ray binaries hosting a neutron star in the hard state rather than those of X-ray binary pulsars. We note that the Be star HD 54786 is located in the error region obtained by NICER (ATel #15178). If HD 54786 is a companion star of MAXI J0709-159, the observed rapid variability suggests that the source is a kind of Super-giant Fast X-ray Transients (SFXTs), or a Cir X-1-like object.

Further followup observations with multi-wavelengths are encouraged.

MAXI data for MAXI J0709-159