GRB 250828B

M. J. Moss (GSFC), S. D. Barthelmy (GSFC), R. Gupta (GSFC), P. A. Evans (U
Leicester), H. A. Krimm (NSF), S. Laha (GSFC/UMBC), A. Y. Lien (U Tampa),
C. B. Markwardt (GSFC), D. M. Palmer (LANL), T. Parsotan (GSFC), D. Sadaula
(GSFC/UMBC), T. Sakamoto (AGU) (i.e. the Swift-BAT team):

Using the data set from T-239 to T+963 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 250828B (trigger #1345836)
(Evans et al., GCN Circ. 41581
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 ).  The BAT ground-calculated position is
RA, Dec = 124.867, -17.352 deg which is
   RA(J2000)  =  08h 19m 28.2s
   Dec(J2000) = -17d 21' 07.8"
with an uncertainty of 2.4 arcmin, (radius, sys+stat, 90% containment). The
partial coding was 64%.

The mask-weighted light curve shows a complex structure. The main pulse
starts and peaks at ~T0, and ends at ~T+5 s. In addition, there are some
weak emission that starts prior to the main pulse at ~T-60 s and persists
after the main pulse until ~T+60 s. T90 (15-350 keV) is 94.09 +- 32.03 sec
(estimated error including systematics).

The time-averaged spectrum from T-56.23 to T+76.66 sec is best fit by a
simple power-law model.  The power law index of the time-averaged spectrum
is 2.13 +- 0.19.  The fluence in the 15-150 keV band is 1.5 +- 0.2 x 10^-06
erg/cm2. The 1-sec peak photon flux measured from T+0.12 sec in the 15-150
keV band is 1.7 +- 0.2 ph/cm2/sec.  All the quoted errors are at the 90%
confidence level.

The results of the batgrbproduct analysis are available at
https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1345836

V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, 
G.Antipov,  A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
C.Francile,  F. Podesta, R.Podesta, E. Gonzalez  (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A.Sosnovskij (CrAO),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) 

MASTER-OAFA robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Argentina (OAFA observatory of San Juan National University) was pointed to the Swift GRB 250828B ( P. A. Evans et al., GCN 41581
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 ) errorbox  36970 sec after notice time and 36991 sec after trigger time at 2025-08-29 08:50:09 UT, with upper limit up to  18.0 mag. The observations began at zenith distance = 76 deg. The sun  altitude  is -27.3 deg. 

The galactic latitude b = 11 deg., longitude l = 239 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2978941

We obtain a following upper limits.  

Tmid-T0  |          Site       |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________

   37021 |         MASTER-OAFA |   C |    60 | 17.9 |        
   37101 |         MASTER-OAFA |   C |    60 | 18.0 |        
Filter C is a clear (unfiltred) band. 


The observation and reduction will continue. 
The message may be cited.

P. A. Evans (U Leicester), R. Gupta (NASA GSFC), J. A. Kennea (PSU),
K. L. Page (U Leicester), D. M. Palmer (LANL) and M. H. Siegel (PSU)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 22:33:37 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250828B (trigger=1345836).  Swift could not slew to the
burst due to an observing constraint.
The BAT on-board calculated location is 
RA, Dec 124.852, -17.388 which is 
   RA(J2000) = 08h 19m 24s
   Dec(J2000) = -17d 23' 17"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a duration of about 8 sec.  The peak count rate
was ~1700 counts/sec (15-350 keV), at ~0 sec after the trigger.

Due to a Sun observing constraint, Swift cannot slew to the BAT
position until 17:55 UT on 2025 September 07. There will thus be no XRT
or UVOT data for this trigger before this time.

We note that this GRB is coincident within one minute with a 
subthreshold Gravitational Wave trigger S250828ef, at 
22:32:44 UT. However, this GW trigger is of low confidence, rated
as 99% likely to be Terrestrial background noise.

Burst Advocate for this burst is P. A. Evans (pae9 AT leicester.ac.uk).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)