MAXI/GSC detection of an X-ray flare from possible new X-ray transient MAXI J2320-268

ATel #16144; W. Iwakiri (Chiba U.) , M. Nakajima (Nihon U.), M. Serino (AGU), H. Negoro, K. Kobayashi, M. Tanaka, Y. Soejima, Y. Kudo (Nihon U.), T. Mihara, T. Kawamuro, S. Yamada, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, S. Sugita, H. Hiramatsu, H. Nishikawa, A. Yoshida (AGU), Y. Tsuboi, S. Urabe, S. Nawa, N. Nemoto (Chuo U.), M. Shidatsu (Ehime U.), I. Takahashi, M. Niwano, S. Sato, N. Higuchi, Y. Yatsu (Tokyo Tech), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, T. Kurihara (JAXA), Y. Ueda, K. Setoguchi, T. Yoshitake, Y. Nakatani (Kyoto U.), M. Yamauchi, Y. Hagiwara, Y. Umeki, Y. Otsuki (Miyazaki U.), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC) report on behalf of the MAXI team:
on 21 Jul 2023; 14:23 UT
Credential Certification: Wataru Iwakiri (wataru.iwakiri@riken.jp)

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The MAXI/GSC nova alert system triggered an uncatalogued X-ray transient source at 16:16:15 UT on 2023 July 20. The X-ray peak time was 13:10:37 UT before the trigger time, and the X-ray emission continued for three consecutive scans. We estimated the location of this X-ray transient using the three MAXI scan data from 13:10 to 16:16; we obtained the source position at
(R.A., Dec) = (350.215 deg, -26.805 deg) = (23 20 51, -26 48 17) (J2000)
with a statistical 90% C.L. elliptical error region
with long and short radii of 0.43 deg and 0.42 deg, respectively.
The roll angle of the long axis from the north direction is 167.0 deg counterclockwise.
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).
The peak X-ray flux is about 50 mCrab in the 2 - 10 keV band. Assuming the flux was exponentially decaying, the estimated e-folding time is roughly 2.5 hours. The averaged X-ray spectrum is well represented by a powerlaw model with a photon index of 2.4 +0.7, -0.6, or a blackbody model with a temperature of 1.1 +/- 0.5 keV (1 sigma error).

We tentatively name the source MAXI J2320-268 because no known X-ray transient has been reported within this error region. Since the nature of this source is completely unknown, we strongly encourage follow-up observations.