MAXI/GSC observations of the X-ray transient Swift J1858.6-0814

ATel #12163; H. Negoro (Nihon U.), M. Sugizaki (Tokyo Tech), M. Nakajima, A. Sakamaki, W. Maruyama, M. Aoki, K. Kobayashi (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, Y. Tachibana, K. Morita (Tokyo Tech), S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai, T. Midooka (JAXA), Y. Ueda, A. Tanimoto, T. Morita, S. Yamada, S. Ogawa (Kyoto U.), Y. Tsuboi, W. Iwakiri, R. Sasaki, H. Kawai, T. Sato (Chuo U.), H. Tsunemi, T. Yoneyama, K. Asakura, S. Ide (Osaka U.), M. Yamauchi, K. Hidaka, S. Iwahori (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), M. Shidatsu (Ehime U.) report on behalf of the MAXI team:
on 3 Nov 2018; 05:36 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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Following the discovery of the X-ray transient Swift J1858.6-0814 (Krimm et al. ATel #12151, also Ludlum et al. #12158 and Kennea et al. #12160), we investigated the source activity after October 9 (MJD 58400) using MAXI/GSC data. We find that the 2-10 keV X-ray flux tends to increase since around October 24 (MJD 58415), and decrease after October 30 (MJD 58421). The 1-day averaged 2-10 keV X-ray fluxes on October 24 and 29 (around the peak) were 0.009 (+0.009, - 0.008) c/s/cm2 (~ 4 mCab) and 0.017 (+0.010,-0.009) c/s/cm2 (8 mCrab), respectively. This soft X-ray fluxes lower than the hard ones Swift/BAT obtained (ATel #12151) are consistent with a very hard spectrum in the faint state when NICER (ATel #12158) and Swift/XRT (#12160) observed.

MAXI/GSC also marginally detected two bright flares from the region consistent with the source position at 09:32 UT on October 27 and 02:25 on October 30. The flares have different spectral properties though the difference is not significant. The former flare has a soft spectrum, similar to flares NICER detected (ATel #12158), and average count-rates are 0.099 (+0.035,-0.029) and 0.048 (+0.029,-0.022) c/s/cm2 at 2-4 keV and 4-10 keV, respectively. While the latter has a hard spectrum having count-rates of 0.018 (+0.021,-0.014) and 0.105 (+0.041,-0.034) c/s/cm2 at 2-4 keV and 4-10 keV, respectively.