MAXI/GSC detection of a weak outburst from V4641 Sgr

ATel #11931; H. Negoro, M. Nakajima, A. Sakamaki, W. Maruyama (Nihon U.), T. Mihara, S. Nakahira, F. Yatabe, Y. Takao, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, T. Hashimoto, A. Yoshida (AGU), N. Kawai, M. Sugizaki, Y. Tachibana, K. Morita (Tokyo Tech), S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai (JAXA), Y. Ueda, A. Tanimoto, T. Morita, S. Yamada (Kyoto U.), Y. Tsuboi, W. Iwakiri, R. Sasaki, H. Kawai, T. Sato (Chuo U.), H. Tsunemi, T. Yoneyama (Osaka U.), M. Yamauchi, K. Hidaka, S. Iwahori (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), M. Shidatsu (Ehime U.)
on 10 Aug 2018; 09:22 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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We report recent activity of the micro-quasar V4641 Sgr (= SAX 1819.3–2525). Since the end of 2018 July, MAXI/GSC detected weak, soft enhancement from the position consistent with V4641 Sgr. The average 2-4 keV and 4-10 keV X-ray fluxes on August 1-8 (MJD 58331-58338) are (8.6+/-1.5)e-3 c/s/cm2 (~8 mCrab) and (5.9+/-1.7)e-3 c/s/cm2 (~ 6 mCrab), respectively.

The source also showed similar weak (6-10 mCrab) activity from 2018 April 4 to April 10 (MJD 58212-58218). We also note that the source was marginally detected between these two outbursts at a flux level of (1.6+/-0.5)e-3 c/s/cm2 at the 2-4 keV band, suggesting continuous activity of the source since the outburst in April.

The source exhibited several weak outbursts after the very bright outburst in 1999. The previous outbursts that MAXI/GSC detected at flux levels of > 3 mCrab are in 2010 September (ATel #2785, #2832, #2892), 2012 June (ATel #5803, #5836), 2014 February (ATel #7858, #7874, #7904, #7908, #7966), and 2015 July. No apparent periodicity is found. These data, however suggest that weak outbursts occur at intervals of 500-600 days though an outburst around 2017 December when the source was in an unobservable sky region was not confirmed.

Multiwavelength monitoring of the source is encouraged.

Recent MAXI observations of V4641 Sgr