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MAXI/GSC detection of the magnetar AX J1846.4-0258

ATel #13922; T. Mihara (RIKEN), H. Negoro (Nihon U.), M. Tominaga, T. Nagatsuka (JAXA), M. Nakajima, M. Aoki, K. Kobayashi, R. Takagi, K, Asakura, K, Seino (Nihon U.), 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.), N. Kawai, R. Adachi, M. Niwano (Tokyo Tech), S. Nakahira, Y. Sugawara, S. Ueno, H. Tomida, M. Ishikawa (JAXA), M. Shidatsu (Ehime U.), Y. Ueda, S. Yamada, S. Ogawa, K. Setoguchi, T. Yoshitake (Kyoto U.), H. Tsunemi (Osaka U.), M. Yamauchi, K. Kurogi, K. Miike (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC) report on behalf of the MAXI team
on 7 Aug 2020; 13:04 UT
Credential Certification: Tatehiro Mihara (mihara@crab.riken.jp)

Subjects: X-ray, Neutron Star, Transient, Pulsar, Magnetar

Referred to by ATel #: 13950, 13988

The MAXI/GSC nova alert system triggered a weak X-ray transient source at 07:49 UT on 2020-07-30 (59060 MJD) in the one-day accumulation. Assuming that the source flux was constant over the transits, the source position was (R.A., Dec) = (281.47 deg, -2.63 deg) = (18 45 53, -02 37 33) (J2000) with a statistical 90% C.L. elliptical error region with long and short radii of 0.26 deg and 0.24 deg, respectively. The roll angle of long axis from the north direction is 11.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The averaged flux over the three scans (2020-07-30 01:38, 03:11, and 04:44 UT) with a significant X-ray flux was 50 +- 15 mCrab (4.0-10.0keV, 1 sigma error). There was no significant excess flux in the previous transit at 00:01 UT and in the next transit at 06:21 UT with an upper limit of 20 mCrab for each. However, there were less significant (~2 sigma) fluxes (~30 mCrab) in the previous scans at 07-28 00:03 and 01:36, 07-29 11:41 and 14:47, and in the next scan at 07-30 21:46.
There were three known sources in the error region. On 2020-08-01 20:11 UT Swift/BAT detected a flare from AX J1846.4-0258, which is one of the three sources (GCN #28187 and ATel #13913). It is the second youngest pulsar in our galaxy (and also a magnetar) and shows a magnetar-like activity (Gotthelf et al. 2000). Intermittent detections with MAXI from 2020-07-28 to 07-30 are consistent with the flaring behavior of the source. The X-ray fluxes with MAXI would also come from AX J1846.4-0258.