NuSTAR detection and localization of MAXI J1810-222

ATel #12283; H. Negoro (Nihon U.), T. Mihara (RIKEN), F. Harrison, K. Forster, B. Grefenstette, K. Madsen, H. Miyasaka (Caltech), M. Nakajima, A. Sakamaki, W. Maruyama, M. Aoki, K. Kobayashi (Nihon U.), 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, T. Oeda, K. Shiraishi (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, Y. Kurihara (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), M. Shidatsu (Ehime U.)
on 11 Dec 2018; 00:23 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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Following the discovery of the soft X-ray transient MAXI J1810-222 (Negoro et al. ATel #12254; Maruyama et al. #12264), we requested NuSTAR ToO observations. From 08:57 to 20:20 on 2018 December 9, NuSTAR performed mosaic 5x5 observations, approximately 600 sec each. It covered a 52'x52' region centering at (R.A., Dec) =(273.006, -22.418), which was obtained from the latest MAXI data at that time.

NuSTAR FPMA and FPMB clearly detected a source at the position (R.A., Dec) = (273.1226, -22.3274) with an uncertainty of 2 arcmin. In the observations, the optical star camera used for aspect reconstruction was unavailable because of the proximity to the Sun, resulting in the relatively large errors. This position is within the MAXI error region above, but outside of the refined position (ATel #12264). We note that there is no known X-ray source within the error region.

The preliminary analysis shows a steep energy spectrum with no apparent hard tail. The 3-10 keV spectrum is represented by a power-law with a photon index of 5.5 +/- 0.3, or a blackbody or disk blackbody model with 0.6-0.7 keV. The 4-10 keV flux was 1.3e-11 erg/s. These are consistent with a recent MAXI/GSC spectrum. The nature of the source is still unknown.

A further deep NuSTAR observation will be carried out, followup observations, e.g., in the radio band, are highly encouraged to reveal this peculiar soft X-ray transient.