GRB 250721A

A. Holzmann Airasca (UniTrento and INFN Bari) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 09:37:46.53 UT on 21 July 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250721A (trigger 774783471/250721401).
which was also detected by MAXI (M. Nakajima et al. 2025, GCN 41129
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 ).
The Fermi GBM on-ground location is consistent with the MAXI position.

The angle from the Fermi LAT boresight is 70 degrees.

The GBM light curve consists of a single emission episode with a duration (T90)
of about 54 s (50-300 keV). The time-averaged spectrum
from T0-3.1 to T0+1.5 s is best fit by
a simple power law function with index -1.46 +/- 0.07.

The event fluence (10-1000 keV) in this time interval is
(9.8 +/- 1.6)E-07 erg/cm^2. The 1-sec peak photon flux measured
starting from T0-2.5 s in the 10-1000 keV band is 1.7 +/- 0.2 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html

For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), T. Sbarrato
(INAF-OAB), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), D.N.
Burrows (PSU), M.A. Williams (PSU), S. Dichiara (PSU) 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 250721A in a series of observations tiled on the sky.  The
total exposure time is	1.4 ks, distributed over 5 tiles; the maximum
exposure at a single sky location in the tiling was 569 s. The data
were collected between T0+16.2 ks and T0+22.9 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
6.5e-13 to 2.2e-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_GRB00137.

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

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 250721A. Automated analysis of the XRT data will
be presented online at https://www.swift.ac.uk/xrt_products/TILED_GRB00137

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.

C. Kang (Ehime U.), M. Serino, Y. Kawakubo (AGU), H. Negoro (Nihon U.),
M. Nakajima, K. Takagi, H. Takahashi, K. Tatano, H. Nishio (Nihon U.),
T. Mihara, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN),
T. Sakamoto, S. Sugita, H. Hiramatsu, Y. Kondo, A. Yoshida (AGU),
Y. Tsuboi, H. Sugai, N. Nagashima (Chuo U.),
M. Shidatsu, Y. Niida, T. Nakamoto (Ehime U.),
I. Takahashi, Y. Yatsu (Science Tokyo),
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.),
W. Iwakiri (Chiba U.),
T. Kawamuro (Osaka U.),
S. Yamada (Tohoku U)

The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient 
source at 09:38:05 UT on July 21, 2025. Assuming that the source flux was constant
over the transit, we obtain the source position at
(R.A., Dec) = ( 82.805, -25.592) = (05 31 13, -25 35 31) (J2000)
with a statistical 90% C.L. elliptical error region with long and short radii of 0.37 deg
and 0.32 deg, respectively.
The roll angle of long axis from the north direction is 24.0 deg counterclockwise.
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).
The X-ray flux averaged over the scan was 119 +- 23 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:
(81.815, -25.748) deg = (05 27 15, -25 44 52) (J2000)
(82.171, -26.333) deg = (05 28 41, -26 19 58) (J2000)
(84.986, -24.908) deg = (05 39 56, -24 54 28) (J2000)
(84.621, -24.329) deg = (05 38 29, -24 19 44) (J2000)

There was no significant excess flux in the next transit at 11:10 UT with an upper limit of 20 mCrab.

We also note that a transient was detected by Fermi (GCN Circular #41128
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 ), 
which is probably associated with the event.

The Fermi GBM team reports the detection of a likely LONG GRB

"At 09:37:46.53 UT on 21 July 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250721A (trigger 774783471/250721401).

The on-ground calculated location, using the Fermi GBM trigger data,
is RA = 76.94, Dec = -12.37 (J2000 degrees, equivalent to J2000 5h 7m, -12d 22'),
with a statistical uncertainty of 10.20 degrees.

The angle from the Fermi LAT boresight is 82 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250721401/quicklook/glg_skymap_all_bn250721401.png

The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250721401/quicklook/glg_healpix_all_bn250721401.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250721401/quicklook/glg_lc_medres34_bn250721401.gif"