GCN Circular 35721

H. Negoro, M. Nakajima (Nihon U.), W. Iwakiri (Chiba U.), T. Mihara (RIKEN),
K. Kobayashi, M. Tanaka, Y. Soejima, Y. Kudo (Nihon U.),
T. Kawamuro, S. Yamada, S. Wang, 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, E.Goto (Chuo U.), M. Shidatsu, 
Y. Niida (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, Y. Okada (Kyoto U.),
M. Yamauchi, Y. Hagiwara, Y. Umeki, Y. Otsuki (Miyazaki U.),
K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), and M. Sugizaki (NAOC)

The MAXI/GSC nova alert system triggered on an uncatalogued soft X-ray transient source at
01:43:39 UT on February 14, 2024. Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (73.197 deg, -55.923 deg) = (04 52 47, -55 55 22) (J2000)
with a 90% C.L. statistical error of 0.42 deg and an additional systematic uncertainty of
0.1 deg (90% containment radius).
The X-ray flux averaged over the scan was 109 +- 24 mCrab (4.0-10.0keV, 1 sigma error).
Without assumptions on the source constancy, we obtain a rectangular error box 
for the transient source with the following corners:
(73.301, -55.429) deg = (04 53 12, -55 25 44) (J2000)
(72.444, -56.103) deg = (04 49 46, -56 06 10) (J2000)
(74.543, -56.916) deg = (04 58 10, -56 54 57) (J2000)
(75.381, -56.228) deg = (05 01 31, -56 13 40) (J2000)

The burst showed a spectral softening during the burst duration of about 30 sec (in the MAXI 
scan window), and the average spectrum is characterized by a power-law with a photon index 
of 2.3 (+0.8, -0.6)  at a 90% confidence level.

An X-ray flux of about 20 mCrab at 2-10 keV was also recognized in the next transit at 03:16 UT.
There was no significant excess flux in the previous transit at 00:10 UT, and in the transit 
at 04:49 UT with an upper limit of 20 mCrab for each.

We note that 1H 0451-560 is located near the center of the error region. 
1H 0451-560 is a short-lived transient X-ray source with a Crab-like spectrum (Kaluzienski 1978, 
IAUC 3174).  Thus, 1H 0451-560 is most likely. We also tentatively name the source MAXI J0452-559.

Swift/XRT followup observation is requested.

We crosspost this content to the Astronomer's Telegram because of the possible nature of the source.