GRB 260925A

D. Sadaula (GSFC/UMBC), E. Ambrosi (INAF-IASFPA), 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), D. M. Palmer (LANL), T.
Parsotan (GSFC), T. Sakamoto (AGU) (i.e. the Swift-BAT team):

Using the data set from T-119 to T+723 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 260925A (trigger #1513316)
(Ambrosi et al., GCN Circ. 45739
Loading...
 
 ).  The BAT ground-calculated position is
RA, Dec = 132.475, 24.972 deg which is
   RA(J2000)  =  08h 49m 54.0s
   Dec(J2000) = +24d 58' 18.5"
with an uncertainty of 2.5 arcmin, (radius, sys+stat, 90% containment). The
partial coding was 100%.

The mask-weighted light curve shows a weak pulse that starts at ~T0, peaks
at ~T+1 s, and end at ~T+3 s. The pulse is potentially followed by some
weak tail emission that lasts till ~T+40 s.  T90 (15-350 keV) is 35.42 +-
10.55 sec (estimated error including systematics).

The time-averaged spectrum from T+0.48 to T+40.41 sec is best fit by a
simple power-law model.  The power law index of the time-averaged spectrum
is 1.60 +- 0.44.  The fluence in the 15-150 keV band is 2.4 +- 0.7 x 10^-07
erg/cm2. The 1-sec peak photon flux measured from T+1.10 sec in the 15-150
keV band is 0.6 +- 0.1 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/1513316

Rui-Chen Gao, Jin-Ji Li, Zhong-Nan Dong, Jia-Qi Lin, Pu Lin, Yun Shi, Hao-Nan Yang, Hao-Ran Zhang, P H Thomas Tam, Rong-Feng Shen, Bin Ma (Sun Yat-sen University) report on behalf of the SYSU 80cm infrared telescope team:

We observed the field of the transient GRB 260925A (Ambrosi et al., GCN 45739
Loading...
 
 ) using the Sun Yat-sen University (SYSU) 80cm infrared telescope. Our observations were carried out on 2026 Sep 25 20:44 UT, 13.5 hours after the trigger, using stacked images in the J band.

We do not detect any counterpart at the position of GRB260925A with enhanced XRT position (Goad et al., GCN 45746
Loading...
 
 ), down to a 5-sigma depth of J ~ 18.49 mag (Vega). 

The SYSU 80 cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University.

J.A. Kennea (PSU), A.P. Beardmore (U. Leicester), J.P. Osborne (U.
Leicester), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB), A. D'Ai
(INAF-IASFPA), D.N. Burrows (PSU), S. Lanava (PSU) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 4.2 ks of XRT data for GRB 260925A, from 119 s to 28.6
ks after the   trigger. The data are entirely in Photon Counting (PC)
mode. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=2.95 (+0.30, -0.25).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.6 (+0.5, -0.4). The
best-fitting absorption column is  9 (+24, -6) x 10^20 cm^-2,
consistent with the Galactic value of 3.6 x 10^20 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 4.3 x 10^-11 (4.8 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     9 (+24, -6) x 10^20 cm^-2
Galactic foreground: 3.6 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.6 (+0.5, -0.4)

If the light curve continues to decay with a power-law decay index of
2.95, the count rate at T+24 hours will be 8.9 x 10^-9 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.8 x
10^-19 (4.3 x 10^-19) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/01513316.

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

N. Klingler (NASA-GSFC / UMBC / CRESST II) and E. Ambrosi (INAF-IASFPA) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 260925A 115 s after the BAT trigger (Ambrosi et al., GCN Circ. 45739
Loading...
 
 ).  No optical afterglow consistent with the enhanced XRT position (Goad et al., GCN Circ. 45746
Loading...
 
 ) is detected in the initial UVOT exposures.  Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white_FC           115          265          147         >20.0
u_FC               273          523          246         >19.1
white              115         1372          358         >20.9
v                  603         1249           78         >17.9
b                  529         1348           62         >18.2
u                  273         1323          304         >19.3
w1                 652         1299           55         >18.3
m2                1079         1099           19         >17.6
w2                 578          771           39         >18.3

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.039 in the direction of the burst (Schlegel et al. 1998).

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

Using 1269 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 260925A, 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 = 132.45679, +24.91132 which is equivalent
to:

RA (J2000): 08h 49m 49.63s
Dec (J2000): +24d 54' 40.7"

with an uncertainty of 2.6 arcsec (radius, 90% confidence).

This position may be improved as more data are received. The latest
position can be viewed at https://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.

Stéphane Basa (UAR Pytheas), Enrique Moreno Méndez (UNAM), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM),  Camila Angulo (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP),  Leonardo García-García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), and Alan M. Watson (UNAM) report:

We imaged the field of the Swift GRB260925A (Ambrosi at al., GCN Circ. 45739
Loading...
 
 ) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-09-25 10:30:27 to 12:20:36 UTC (from 3.29 to 5.14 hours after the trigger) and obtained 64 minutes of simultaneous exposure in the r and z filters.

The data were reduced and coadded with the COLIBRÍ ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.

We do not detect any source within the enhanced XRT error circle (Ambrosi at al., GCN Circ. 45739
Loading...
 
 ), while taking account of the warning regarding the reliability of this position, down to the following 5-sigma limits:

r > 23.17
z > 22.46

We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM.

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.

SUBJECT: GRB 260925A: FRAM-Auger optical limit

Filip Novotny, Martin Jelinek and Jan Strobl
(ASU CAS Ondrejov, CZ),

Martin Masek, Sergey Karpov, Petr Janecek, Jakub Jurysek,
Jan Ebr, Ronan Cunniffe, Petr Travnicek and Michael Prouza
(Institute of Physics, Prague, CZ)

report:

The 30cm robotic telescope FRAM-Auger at the Pierre Auger
Observatory (Cerro Los Leones, Malargue, Argentina) reacted
robotically to the Swift/BAT alert of GRB 260925A (trigger
1513316, Ambrosi et al., GCN 45739
Loading...
 
 ). The field was below the
horizon at the time of the trigger; we obtained a series of
60s R-band images starting at 08:41:36 UT, i.e. ~89 min post
trigger, as the field was rising in morning twilight. The first
images, taken at elevations of 2-4 deg, are of too poor quality
to be used. The usable part of the series was obtained between
08:54:41 and 09:14:59 UT at elevations of 4.4-7.7 deg
(airmass ~12-7).

We do not detect any source at the Swift/XRT enhanced position
(Ambrosi et al., GCN 45739
Loading...
 
 ), neither in single images nor in the
combined frame, with the following limit:

T-T0[mid]  Images  Span[s]  Limit (Vega)
~112 min   14      1218     R_C > 15.3

Single 60s images reach R_C > ~14.

The photometry is calibrated against the Atlas-REFCAT2
catalogue (Tonry et al. 2018, ApJ, 867, 105) and is not
corrected for Galactic extinction.

E. Ambrosi (INAF-IASFPA), S. Dichiara (PSU), A. Melandri (INAF-OAR),
K. L. Page (U Leicester), B. Sbarufatti (INAF-OAB) and
M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 07:13:02 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 260925A (trigger=1513316).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 132.462, +24.971,  which is 
   RA(J2000) = 08h 49m 51s
   Dec(J2000) = +24d 58' 16"
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 15 sec.  The peak count rate
was ~1200 counts/sec (15-350 keV), at ~1 sec after the trigger.

The XRT began observing the field at 07:14:52.2 UT, 110.7 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 132.45674,
24.91182 which is equivalent to:
   RA(J2000)  = 08h 49m 49.62s
   Dec(J2000) = +24d 54' 42.6"
with an uncertainty of 2.3 arcseconds (radius, 90% containment). This
location is 213 arcseconds from the BAT onboard position. This position
may be improved as more data are received; the latest position is
available at https://www.swift.ac.uk/sper. This position is more than
2-sigma away from the BAT position, and so possibly unrelated to the
trigger. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 3.64
x 10^20 cm^-2 (Willingale et al. 2013).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 119 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 none of
the XRT error circle. 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 0.038.

Burst Advocate for this burst is E. Ambrosi (elena.ambrosi 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/)