GRB 250728A

M.A. Williams (PSU), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), A.P. Beardmore (U. Leicester), A. Melandri (INAF-OAR), T.
Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), S. Dichiara (PSU), J.A.
Kennea (PSU) and P.A. Evans (U. Leicester) reports on behalf of the
Swift-XRT team:

Swift-XRT has performed follow-up observations of the
Fermi/GBM-detected burst GRB 250728A, collecting  3.1 ks of Photon
Counting (PC) mode data between T0+14.6 ks and T0+28.2 ks. 

One uncatalogued X-ray source has been detected within the estimated
3-sigma Swift/BAT-GUANO error region (394 arcsec), it is below the RASS limit
and shows no definitive signs of fading. Therefore, at the present time
we cannot confirm this as the afterglow. Details of this source are
given below:

Source 3:
  RA (J2000.0):  260.9895  =  17:23:57.48
  Dec (J2000.0): +16.2004  =  +16:12:01.6
  Error: 6.7 arcsec (radius, 90% conf.)
  Count-rate: (1.98 [+1.26, -0.93])e-3 ct s^-1	 
  Distance: 154 arcsec from Fermi/GBM position.
  Flux: (1.34 [+0.85, -0.63])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Two uncatalogued sources were also detected too far from the BAT
position to be likely afterglow candidates.

The results of the XRT-team automatic analysis of the XRT observations,
including a position-specific upper limit calculator, are available at
https://www.swift.ac.uk/ToO_GRBs/00021852.

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

Camila Angulo (UNAM), Francesco Magnani (CPPM), Rosa L. Becerra (U Roma), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Fredd Alvarez (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (AUS), Sarah Antier (OCA), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM):

We imaged the field of the Fermi/Swift GRB 250728A (Pathak et al., GCN Circ. 41190
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 ; Ronchini et al., GCN Circ. 41185
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 ) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-07-29 03:29 to 05:34 UTC (from 18.07 to 20.15 hours after the trigger) and obtained 96 minutes of exposure in the i filter.

The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.

In the stacked image, we do not detect any new source in the BAT uncertainty region (Ronchini et al., GCN Circ. 41185
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 ) to a 5-sigma limit of:

i > 24.20

Furthermore, at the position of Swift/XRT source #3 (Evans et al., GCN Circ. 41186
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 ) at 17:23:57.48 +16:12:01.6 we do not detect a new source to a 3-sigma limit of:

i > 24.75

Further observations are ongoing.

We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.

COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.

U. Pathak (IIT Bombay), V. Sharma (NASA GSFC/UMBC) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 09:25:45.57 UT on 28 July 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250728A (trigger 775387550/250728393),
which was also detected by Swift/BAT-GUANO (Samuele Ronchini et al. 2025, GCN 41185
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 ).
The Fermi GBM on-ground location is consistent with the Swift/BAT-GUANO position.

The angle from the Fermi LAT boresight is 80 degrees.

The GBM light curve of a short complex pulse with a duration (T90)
of about 0.6 s (50-300 keV). 

The time-averaged spectrum from T0-0.58 to T0+0.32 s is best fit by
a power law function. The power law index is -1.6 +/- 0.1.

The event fluence (10-1000 keV) in this time interval is
(3.5 +/- 0.5)E-07 erg/cm^2. The 64-ms peak photon flux measured
starting from T0-0.064 s in the 10-1000 keV band is 7 +/- 1.2 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/"

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

Swift has initiated a ToO observation of the Fermi/GBM-detected event
GRB 250728A. Automated analysis of the XRT data will be presented online at
https://www.swift.ac.uk/ToO_GRBs/00021852
Any uncatalogued X-ray sources detected in this analysis will be
reported on this website and via GCN COUNTERPART notices. These are
not necessarily related to the Fermi/GBM event. 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.

Samuele Ronchini (PSU), James DeLaunay (PSU), Aaron Tohuvavohu (Caltech), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC), Maia Williams (PSU) report:

Swift/BAT did not localize GRB 250728A onboard (T0: 2025-07-28T09:25:45.57 UTC).
The Fermi notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; Tohuvavohu et al. 2020, ApJ, 900, 1).
Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.

The arcmin position of the burst is found with the newly developed pipeline BAT-GLIMPSE: Gamma-ray Localization using Imaging and Mosaic techniques for Pointing and Slew Epochs (Ronchini et. al, in prep). The pipeline makes use of the tools from BatAnalysis ([Parsotan et al. 2025](https://iopscience.iop.org/article/10.3847/1538-4357/ade240)). The source is found with an SNR = 8.1

Independently, the BAT likelihood search, NITRATES (DeLaunay + Tohuvavohu 2022, ApJ, 941, 169), confirms the detection of the burst in a 0.512 s analysis time bin starting at T0 - 0.512 s with a sqrt(TS) of 16.4. An arcminute localization is found with DeltaLLHOut of 42.69 and a DeltaLLHPeak of 35.31.
See Section 9.1 and Figures 10 and 17 in the NITRATES paper for brief descriptions and interpretations of sqrt(TS), DeltaLLHPeak, and DeltaLLHOut.

The BAT position is:

RA, Dec = 260.945, 16.2049 deg 

which is

RA(J2000) = 17h 23m 46.80s 

Dec(J2000) = 16d 12’ 17.6″

with an estimated uncertainty of 3 arcmin radius.

More details about this burst can be found on the trigger report page here: https://guano.swift.psu.edu/trigger_report?id=775387581

XRT and UVOT follow-up has been requested. Results of follow-up observations will be reported in future circulars.

GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches. A live reporting of Swift/BAT event data recovered by GUANO can be found at: https://www.swift.psu.edu/guano/