MAXI/GSC detection of a new outburst from SAX J1810.8-2609
H. Negoro (Nihon U.), T. Mihara, S. Nakahira, F. Yatabe, Y. Takao, M. Matsuoka (RIKEN),  N. Kawai, M. Sugizaki, Y. Tachibana, K. Morita (Tokyo Tech),  T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, T. Hashimoto, A. Yoshida (AGU),  M. Nakajima, A. Sakamaki, W. Maruyama (Nihon U.),  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.)
We report a new X-ray outburst from the low-mass X-ray binary SAX J1810.8-2609 (aka V4722 Sgr; Ubertini et al. 1998, IAUC 6838 ) observed with MAXI/GSC. The enhancement was recognized from 2018 April 23 (MJD 58231), and X-ray count rates in the 2-4 keV and 4-10 keV bands peaked on April 26 at 0.085 +/- 0.008 c/s/cm2 (~ 80 mCrab) and 0.096+/-0.008 c/s/cm2 (~ 82 mCrab), respectively. The hardness of the energy spectrum did not vary significantly during the outburst. The time-averaged energy spectrum on April 26-29 was well represented by an absorbed power-law with a photon index of 2.04 +/- 0.10 and a hydrogen column density nH of 0.3e22 fixed, and the 2-10 keV luminosity was 4.7e36 (d/4.9 kpc)^2 erg/s where d is the distance to the source (see Allen et al. 2018, ApJ, 854, 58, and references therein). No X-ray burst was observed. This is the second detection of the outbursts from this source with MAXI/GSC, and the first detection was in May, 2012 (Degenaar & Wijnalds 2013, IAU Symp. 291). Reanalysis of GSC data show that the peak count rates were observed on May 12 (MJD 56059), and 0.298 (+0.033, -0.031) c/s/cm2 and 0.330 (+0.035, -0.034) c/s/cm2 in the 2-4 keV and 4-10 keV bands, respectively. Finally, we note that weak (~0.01 c/s/cm2 at 4-10 keV) enhancement from the region was also recognized on 2015 January 20-24 (MJD 57042-57046).
x-ray,binary,neutron star,star,transient