MAXI/GSC detection of a new X-ray transient MAXI J1803-298

ATel #14587; M. Serino (AGU), H. Negoro, M. Nakajima, K. Kobayashi, K. Asakura, K. Seino (Nihon U.), T. Mihara, T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, S. Sugita, K. Komachi, A. Yoshida (AGU), Y. Tsuboi, W. Iwakiri, H. Kawai, Y. Okamoto, S. Kitakoga (Chuo U.), M. Shidatsu (Ehime U.), N. Kawai, R. Adachi, M. Niwano, R. Hosokawa (Tokyo Tech), S. Nakahira, Y. Sugawara, S. Ueno, H. Tomida, M. Ishikawa, M. Tominaga, T. Nagatsuka (JAXA), Y. Ueda, S. Yamada, S. Ogawa, K. Setoguchi, T. Yoshitake, Y. Goto, R. Uematsu (Kyoto U.), H. Tsunemi (Osaka U.), M. Yamauchi, Y. Nonaka, T. Sato, R. Hatsuda, R. Fukuoka (Miyazaki U.), T. Kawamuro (UDP/NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC) report on behalf of the MAXI team
on 2 May 2021; 00:51 UT
Credential Certification: Motoko Suzuki (motoko@crab.riken.jp)

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The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient source at 19:50 UT on 2021 May 1. Assuming that the source flux was constant over each transit, we obtain the source position at

(R.A., Dec) = (270.923 deg, -29.804 deg) = (18 03 41, -29 48 14) (J2000)

with a statistical 90% C.L. elliptical error region with long and short radii of 0.31 deg and 0.26 deg, respectively. The roll angle of long axis from the north direction is 71.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). We name the source MAXI J1803-298.
The source was not significant before 12 UT on May 1, and the source flux has increased rapidly since then. The flux at 22:55 was 46+-18 mCrab in 4-10 keV. We encourage followup observations.