MAXI/GSC detection of a short soft X-ray flash XRF 230419A/MAXI J2108-395

ATel #16027; H. Negoro, K. Kobayashi, M. Nakajima, M. Tanaka, Y. Soejima, Y. Kudo (Nihon U.), T. Mihara, T. Kawamuro, S. Yamada, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, 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), and W. Iwakiri (Chiba U.)
on 5 May 2023; 01:06 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

Tweet

The MAXI/GSC nova alert system triggered on an uncatalogued soft X-ray transient source at 12:22:07 UT on 2023 April 19. Assuming that the source flux was constant over the transit, we obtain the source position at
(R.A., Dec) = (317.247 deg, -39.549 deg) = (21 08 59, -39 32 56) (J2000)
with a statistical 90% C.L. elliptical error region with long and short radii of 0.37 deg and 0.28 deg, respectively. The roll angle of the long axis from the north direction is 83.0 deg counterclockwise.
Without assumptions on the source constancy, we obtain a rectangular error box for the transient source with the following corners:
(R.A., Dec) = (316.736, -40.795) deg = (21 06 56, -40 47 42) (J2000)
(R.A., Dec) = (317.697, -40.778) deg = (21 10 47, -40 46 40) (J2000)
(R.A., Dec) = (317.602, -38.179) deg = (21 10 24, -38 10 44) (J2000)
(R.A., Dec) = (316.676, -38.195) deg = (21 06 42, -38 11 42) (J2000)
In both cases, there is an additional systematic uncertainty of 0.1 deg (90% containment radius).

This burst-like event shows a symmetric sharp profile and a soft energy spectrum. A Gaussian fit to the profile in the 2-4 keV band gives a width of 6.4 +/- 1.8 sec (FWHM) with a peak at 12:22:04.5 +/- 1.2 (UT). The numbers of photons are 9 and 5 in the 2-4 keV and 4-6 (4.65) keV band, respectively. Probabilities for the background to produce such events with more than 8 counts and 4 counts in 14 seconds are 2.6e-11 and 5.2e-5, respectively. Fitting the energy spectrum with a blackbody model gives a temperature of 0.7 +/- 0.2 keV. For a power-law model, we obtained a photon index of 3.2 +/- 1.2. Taking account of the short profile and the collimator response, we roughly estimate the 2-4 keV peak flux at more than 1 Crab.

A possible candidate of a nonastrophysical event is reflection of solar X-rays by the solar paddle of the ISS, a part of which was close to the event. The GOSE data (https://www.swpc.noaa.gov/products/goes-x-ray-flux), however, did not show any significant solar activity. We also searched a transient event using the TNS (https://www.wis-tns.org/), but no counterpart was found. Such a soft and short event is quite uncommon in more than 13 years observation of MAXI. MAXI J1501-026 (ATel #7954) showed a similar soft X-ray spectrum (kT ~ 0.53 keV), but a longer duration of about 40 sec or more. The nature of both is unknown.
We name this soft X-ray flash event XRF 230419/MAXI J2108-395.

Trigger Informaiton about XRF 230419/MAXI J2108-395