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MAXI/GSC detection of 4th renewed activity of the distant Z-source MAXI J0556-332

ATel #13628; H. Negoro (Nihon U.), T. Yoshitake (Kyoto U.), 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, W. Iwakiri, R. Sasaki, H. Kawai, Y. Okamoto, S. Kitakoga (Chuo U.), M. Shidatsu (Ehime U.), N. Kawai, M. Oeda, K. Shiraishi, R. Adachi, M. Niwano (Tokyo Tech), S. Nakahira, Y. Sugawara, S. Ueno, H. Tomida, M. Ishikawa, R. Shimomukai, M. Tominaga (JAXA), Y. Ueda, S. Yamada, S. Ogawa, K. Setoguchi (Kyoto U.), M. Yamauchi, S. Iwahori, Y. Kurihara, K. Kurogi, K. Miike (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), and M. Sugizaki (NAOC)
on 13 Apr 2020; 00:28 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

Subjects: X-ray, Neutron Star

Referred to by ATel #: 13631, 14195

At 20:55 UT on 2020 April 11 (MJD 58950), the MAXI/GSC nova alert system triggered on X-ray enhancement at the position consistent with that of the transient neutron star binary MAXI J0556-332 (Matsumura et al. ATel #3102). The 2-20 keV flux has been increasing since April 7, and the one-day average fluxes at 2-4 keV and 4-10 keV on April 11 are 0.015 +/- 0.006 photons/cm2/s (~ 9 mCrab) and 0.022 +/- 0.005 photons/cm2/s (~10 mCrab), respectively.

MAXI J0556-332 was discovered on 2011 January 11 (Matsumura et al. ATel #3102), and found to be a distant, transient Z-source (e.g., Cornelisse et al. 2012, MNRAS, 420, 3538; Sugizaki et al. 2013, PASJ, 65, 58; Homan et al. 2014, ApJ, 795, 131). After the long-term activity lasting about 500 days, small outbursts were detected roughly at intervals of four years, on 2012 October 27 (Sugizaki et al. ATel #4524), 2016 January 6 (Negoro et al. ATel. #8513), and 2020 April 11.

Peak 4-10 keV X-ray fluxes in the previous outbursts were approximately 100 mCrab, 20 mCrab, and 40 mCrab, respectively. Followup observations of this peculiar, distant transient source (e.g., Parikh et al. 2017, ApJL, 851, L28) are encouraged.

MAXI data for MAXI J0556-332