GRB 250527B

T. Kawamuro (Osaka U.), M. Nakajima (Nihon U.), M. Serino (AGU), W. Iwakiri (Chiba U.),
H. Negoro, K. Takagi, H. Takahashi, K. Tatano, H. Nishio (Nihon U.),
T. Mihara, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN),
T. Sakamoto, S. Sugita, Y. Kawakubo, H. Hiramatsu, Y. Kondo, A. Yoshida (AGU),
Y. Tsuboi, H. Sugai, N. Nagashima (Chuo U.),
M. Shidatsu, Y. Niida, C. Kang, T. Nakamoto (Ehime U.),
I. Takahashi, M. Niwano, N. Higuchi, Y. Yatsu (Tokyo Tech),
S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, M. Kurihara (JAXA),
Y. Ueda, K. Fujiwara (Kyoto U.),
M. Yamauchi, M. Nishio, C. Hiraizumi (Miyazaki U.),
K. Yamaoka (Nagoya U.),
M. Sugizaki (Kanazawa U.),
S. Yamada (Tohoku U.)

The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient source at 2025-05-27 17:38:05 UT.
Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (222.622 deg, -52.704 deg) = (14 50 29, -52 42 14) (J2000)
with a statistical 90% C.L. elliptical error region
with long and short radii of 0.12 deg and 0.11 deg, respectively.
The roll angle of long axis from the north direction is 90.0 deg counterclockwise.
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).
The X-ray flux averaged over the scan was 350 +- 36 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:
(222.154, -51.832) deg = (14 48 36, -51 49 55) (J2000)
(221.831, -51.930) deg = (14 47 19, -51 55 48) (J2000)
(222.883, -53.207) deg = (14 51 31, -53 12 25) (J2000)
(223.212, -53.106) deg = (14 52 50, -53 06 21) (J2000)

There was no significant excess flux in the previous transit at 17:17 UT
and in the next transit at 18:50 UT with an upper limit of 20 mCrab for each.

P. A. Evans (U. Leicester) reports on behalf of the Swift team:

Swift has initiated a series of observations, tiled on the sky, of the
MAXI GRB 250527B. Automated analysis of the XRT data will
be presented online at https://www.swift.ac.uk/xrt_products/TILED_GRB00136

Any uncatalogued X-ray sources detected in this analysis will be
reported on this website and via GCN COUNTERPART notices. The probability of finding
serendipitous sources, unrelated to the MAXI event is high: any X-ray source
considered to be a probable afterglow candidate will be reported via a GCN Circular
after manual consideration.

Details of the XRT automated analysis methods are detailed in Evans et
al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).

This circular is an official product of the Swift-XRT team.

J.A. Kennea (PSU), D.N. Burrows (PSU), M.A. Williams (PSU), A.P.
Beardmore (U. Leicester), J.P. Osborne (U. Leicester), B. Sbarufatti
(INAF-OAB), C. Salvaggio (INAF-OAB), M. Ferro (INAF-OAB) and P.A. Evans
(U. Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the MAXI-detected
burst GRB 250527B in a series of observations tiled on the sky.  The
total exposure time is	3.5 ks, distributed over 12 tiles; the maximum
exposure at a single sky location in the tiling was 620 s. The data
were collected between T0+57.5 ks and T0+65.2 ks, and are entirely in
Photon Counting (PC) mode. 

No X-ray sources have been detected. The 3-sigma upper limit in the
field (not including the regions where the tiles overlap) ranges from
~0.02 to ~0.05 ct s^-1, corresponding to a 0.3-10 keV observed flux of
9.2e-13 to 2.0e-12 erg cm^-2 s^-1 (assuming a typical GRB spectrum).

The results of the XRT-team automatic analysis of the tiled XRT
observations, including a position-specific upper limit calculator, are
available at https://www.swift.ac.uk/xrt_products/TILED_GRB00136.

This circular is an official product of the Swift-XRT team.