MAXI/GSC detection of (quasi-)periodic outbursts in 2014-2015 and the current status of Swift J1626.6-5156

ATel #14498; H. Negoro, K. Seino. M. Nakajima (Nihon U.), N. Kawai (Tokyo Tech), M. Aoki, K. Kobayashi, R. Takagi, K. Asakura, K. Seino (Nihon U.), T. Mihara, T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Nishida, K. Komachi, A. Yoshida (AGU), Y. Tsuboi, W. Iwakiri, R. Sasaki, H. Kawai, Y. Okamoto, S. Kitakoga (Chuo U.), M. Shidatsu (Ehime U.), R. Adachi, M. Niwano, R. Hosokawa (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, Y. Goto, R. Uematsu (Kyoto U.), H. Tsunemi (Osaka U.), M. Yamauchi, K. Kurogi, K. Miike (Miyazaki U.), T. Kawamuro (UDP/NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), and M. Sugizaki (NAOC)
on 31 Mar 2021; 14:46 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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We reinvestigate long-term variations of the Be X-ray pulsar Swift J1626.6-5156 currently in outburst (ATel #14454, #14457, #14462, #14489). From MAXI/GSC 11-years data, we find (quasi-)periodic 4 or 5 outbursts starting from September 23, 2014 (MJD 56923). The highest peak flux of the outbursts is about 30 mCrab in the 4-10 keV band at ~ MJD 56940 during the first outburst. The other peak fluxes are about 20 mCrab or less. Fitting a 2-10 keV X-ray light curve from MJD 56910 to 57250 with a sine plus constant model gives a periodicity of 71.1 +/- 0.8 days. This periodicity is similar to that of "mini"-outbursts of ~72.5 days, observed at a late stage of the outburst in 2005-2009 (DeCesar et al. ATel #2036; also see DeCesar et al. 2013, ApJ, 762, 61). From this behavior, we conclude that these (quasi-)periodic outbursts originate from Swift J1626.6-5156 though we regarded the current outburst as new activity after 15 years in the previous telegram (ATel #14454).

Currently, Swift J1626.6-5156 is in the decline phase of the outburst. The peak flux of this outburst was about 25 mCrab in the 4-10 keV band. This peak flux level similar to those in the outbursts in 2006-2008 and 2014-2015 when multiple mini-outbursts were observed suggests further periodic mini-outburst events.
Further multi-wavelength monitoring observations are encouraged to understand physics of mini-outbursts (e.g., Osaki et al. 2001, A&A, 370, 488).

MAXI data for Swift J1626.6-5156