GRB 250904A

D. M. Palmer (LANL), S. D. Barthelmy (GSFC), 
R. Gupta (GSFC), H. A. Krimm (NSF),
S. Laha (GSFC/UMBC), A. Y. Lien (U Tampa),
C. B. Markwardt (GSFC), M. J. Moss (GSFC),
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 250904 A (trigger #1347179)
(Salvaggio, et al., GCN Circ. 41704
Loading...
 
 ).  The BAT ground-calculated position is
RA, Dec = 84.377, 3.575 deg which is 
   RA(J2000)  =  05h 37m 30.5s 
   Dec(J2000) = +03d 34' 29.9" 
with an uncertainty of 1.0 arcmin (radius, sys+stat, 90% containment).
The partial coding was 100%.
 
The BAT light curve showed a complex structure with a duration of about 50 sec.
T90 (15-350 keV) is 37.11 +- 13.89 sec (estimated error including systematics).
 
The time-averaged spectrum from T-19.68 to T+54.44 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.40 +- 0.11.  The fluence in the 15-150 keV band is 1.7 +- 0.1 x 10^-06 erg/cm2.
The 1-sec peak photon flux measured from T+5.11 sec in the 15-150 keV band
is 1.1 +- 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/1347179

A. Bochenek and D. A. Perley (LJMU) report:
 
We observed the field of GRB250904A (Salvaggio et al., GCN 41704
Loading...
 
 ) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 4x120s exposures in the SDSS i’ filter starting at 2025-09-05 05:47:54 UT, approximately 12.2 hours after the trigger.

We do not detect the reported optical counterpart (Fu et al., GCN 41709
Loading...
 
 ; Oates et al., GCN 41721
Loading...
 
 ) or any new source near the XRT position (Evans et al., GCN 41706
Loading...
 
 ; Goad et al., GCN 41711
Loading...
 
 ). The 3-sigma limiting magnitude on the stacked images is i > 22.4 mag.

S. R. Oates (Lancaster Uni.) and C. Salvaggio (INAF-OAB)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250904A
86 s after the BAT trigger (Salvaggio et al., GCN Circ. 41704
Loading...
 
 ).
A source consistent with the XRT position (Goad et al., GCN Circ. 41711
Loading...
 
 )
is detected in white and marginally in u.

Preliminary detections and 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: 

Filter         T_start(s)   T_stop(s)      Exp(s)           Mag/3sigUL
white (FC)          86           235            147             20.23+\-0.36
u   (FC)            298           441             141            19.14+\-0.41 (2.7 sigma)
white              4990      10603           1097             20.60+\-0.16
v                   5401         5601             197             >19.5
b                   4786         6421             393             >20.4
u                   6016         6215             197             >19.6
uvw1             5811         6011             197             >19.5
uvm2             5606        5806             197             >19.4
uvw2             5196       10942             524             >20.2

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.284 in the direction of the burst
(Schlegel et al. 1998).

J.P. Osborne (U. Leicester), P. D'Avanzo (INAF-OAB), M.G. Bernardini
(INAF-OAB), E. Ambrosi	(INAF-IASFPA) , S. Lanava (PSU), S. Dichiara
(PSU), J.A. Kennea (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U.
Leicester) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 7.0 ks of XRT data for GRB 250904A, from 66 s to 45.6
ks after the   trigger. The data comprise 9 s in Windowed Timing (WT)
mode (taken while Swift was slewing), with the remainder in Photon
Counting (PC) mode. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.85 (+0.05, -0.04).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.05 (+0.27, -0.25). The
best-fitting absorption column is  4.4 (+1.4, -1.0) x 10^21 cm^-2,
consistent with the Galactic value of 3.3 x 10^21 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 4.0 x 10^-11 (6.5 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.4 (+1.4, -1.0) x 10^21 cm^-2
Galactic foreground: 3.3 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.05 (+0.27, -0.25)

If the light curve continues to decay with a power-law decay index of
0.85, the count rate at T+24 hours will be 7.0 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.8 x
10^-13 (4.5 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01347179.

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

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-Tavrida robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Russia (Lomonosov MSU, SAI Crimea astronomical station) was pointed to the Swift GRB 250904A ( C. Salvaggio et al., GCN 41704
Loading...
 
 ) errorbox  19454 sec after notice time and 19494 sec after trigger time at 2025-09-04 23:01:57 UT, with upper limit up to  15.5 mag. The observations began at zenith distance = 81 deg. The sun  altitude  is -35.9 deg. 

The galactic latitude b = -14 deg., longitude l = 201 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2984839

We obtain a following upper limits.  

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

   19585 |      MASTER-Tavrida |   C |   180 | 15.5 |        
Filter C is a clear (unfiltred) band. 


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

M.R. Goad, J.P. Osborne, A.P. Beardmore and P.A. Evans (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 343 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 250904A, we find an astrometrically corrected X-ray
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 84.38044, +3.58445 which is equivalent
to:

RA (J2000): 05h 37m 31.31s
Dec (J2000): +03d 35' 04.0"

with an uncertainty of 2.5 arcsec (radius, 90% confidence).

This position may be improved as more data are received. The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).

This circular was automatically generated, and is an official product of the
Swift-XRT team.

S.Y. Fu (HUST), K. Noysena, K. Chanchaiworawit, S. Tinyanont (NARIT), L.B. He, X. Liu, Z.P. Zhu, S.Q. Jiang, J. An, D. Xu (NAOC) report on behalf of a large collaboration:

We observed the field of GRB 250904A (Salvaggio et al., GCN 41704
Loading...
 
 ), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at New South Wales, Australia (SBO). Observations started at 17:49:41.450 UTC on 2025-09-04, i.e., ~ 12.6 min after the Swift/BAT trigger and a series of frames in the R band were obtained. 

An uncatalogued source is detected in our stacked image within the enhanced Swift/XRT error circle (Evans et al., GCN 41706
Loading...
 
 ) with magnitude of R = 20.1 +/- 0.2 (AB) at 18.97 min post-trigger, calibrated with Pan-STARRS1 DR2 catalog converted using Lupton (2005) equations and not corrected for Galactic extinction.

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

Using 343 s of promptly downlinked XRT event data for GRB 250904A, we
find an enhanced XRT position of the afterglow: RA, Dec: 84.38029,
3.58466 which is equivalent to:
   RA (J2000)  = 05 37 31.27
   Dec (J2000) = +03 35 04.8
with an uncertainty of 2.2 arcseconds (radius, 90% confidence).
Analysis of the promptly available data is online at
http://www.swift.ac.uk/sper/1347179.

Position enhancement is described by Goad et al. (2007, A&A, 476, 1401)
and Evans et al. (2009, MNRAS, 397, 1177).

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

C. Salvaggio (INAF-OAB), R. Brivio (INAF-OAB), S. Dichiara (PSU),
R. Gupta (NASA GSFC), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL),
K. L. Page (U Leicester), D. M. Palmer (LANL) and
T. Sbarrato (INAF-OAB) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 17:37:03 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250904A (trigger=1347179).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 84.358, +3.584 which is 
   RA(J2000) = 05h 37m 26s
   Dec(J2000) = +03d 35' 04"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single complex peak
structure with a duration of about 50 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~8 sec after the trigger.

The XRT began observing the field at 17:38:25.4 UT, 82.4 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 84.38001, 3.58520 which is
equivalent to:
   RA(J2000)  = 05h 37m 31.20s
   Dec(J2000) = +03d 35' 06.7"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 79 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at https://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 3.34
x 10^21 cm^-2 (Willingale et al. 2013).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 85 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag.
The 8'x8' region for the list of sources generated on-board covers 100% of the
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.292.

Burst Advocate for this burst is C. Salvaggio (chiara.salvaggio AT inaf.it).
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/)