GRB 250917B

 N. Pankov (HSE), A. Pozanenko (IKI), A. Moskvitin (SAO RAS), E. Klunko
(ISTP), O. Spiridonova (SAO RAS) report on behalf of GRB IKI FuN:

We observed the field of Swift XRT of GRB 250917B (Fermi GBM team GCN
41846
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 ; Caputo et al. GCN 41848
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  ) with AZT-33IK telescope of Mondy
observatory in R-filter on 2025-09-17 starting (UT) 13:36:18(epoch 1) and
Zeiss-1000 of SAO RAS starting on 2025-09-17 (UT) 16:24:04 (epoch 2).

We do not detect any new sources within S1, S5, S6 XRT localisation (Kennea
et al., GCN 41851
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 ). This is consistent with optical observations (Hu et al,
GCN 41854
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 ; Breeveld et al, GCN 41864
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 ; Yadav et al., GCN 41872
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 ; Zheng et
al., GCN 41927
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 ).
However, at the boundary of the S5 XRT circle, we found an object
(coordinates are (J2000) 19:25:33.74 +11:04:24.85) that was significantly
brighter in epoch 1 compared to epoch 2. The photometry of the object is
the following

Date UT start t-T0 Filter OT Err. UL(3sigma) Telescope
(mid, days) (s)
2025-09-17 13:36:18 0.31845 R 19.97 +/- 0.08 21.4 AZT-33IK
2025-09-17 16:24:04 0.42394 Rc 20.79 +/- 0.12 21.3 Zeiss-1000

We suggest the source is an afterglow candidate of GRB 250917B.
The object is a diffuse one and most likely is the galaxy.

WeiKang Zheng (UCB) and Alexei V. Filippenko (UCB) report on

behalf of the KAIT GRB team:


The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at

Lick Observatory, automatically responded to the Swift triggered

GRB 250917B (Caputo et al., GCN 41848
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 ; see also Fermi GBM Team,

GCN 41846
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 ) starting at 06:29:01 UT, 77 seconds after the burst.

A set of 10x20s, 10x40s and 50x60s clear (roughly R) filter images

were obtained. We do not detect any new uncataloged source around

Swift-XRT sources 1, 5 or 6 (Kennea et al., GCN 41851
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 ) neither in

single image nor in coadd images. We estimate the limiting mag of

~21.5 (Vega) at a mid time of 45 minutes after the burst in our

coadd image.

Muskan Yadav  (U. Rome), Rosa L. Becerra  (U. Rome) and Eleonora Troja (U. Rome) report on behalf of the ERC BHianca team:

We observed the field of GRB 250917B (Fermi GBM Team et al., GCN 41846
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 ; Caputo et al., GCN 41848
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 ) with the HAWKI imager on the ESO VLT UT4 (Yepun). Observations started on 2025-09-17 at 23:52:22 UT, approximately 0.7 days after the trigger, and were carried out in the Ks filter at an average seeing of 0.6". 

Image subtraction against 2MASS Catalogue (Skrutskie et al. 2006) shows no new uncataloged source down to K~18.5 AB. Several fainter objects are visible within the BAT refined position (Parsotan  et al., GCN 41868
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 ), however additional observations are required to assess their variability.

At the position of the Source 1 reported by Kennea et al. GCN 41851
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 , our observation is dominated by the close bright star TYC 1059-317-1.

We thank the staff at the VLT, for the rapid execution of these observations.

T. Parsotan (GSFC), S. D. Barthelmy (GSFC), R. Caputo (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), D. Sadaula
(GSFC/UMBC), T. Sakamoto (AGU) (i.e. the Swift-BAT team):

Using the data set from T-240 to T+962 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 250917B (trigger #1350381)
(Caputo et al., GCN Circ. 41848
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 ).  The BAT ground-calculated position is
RA, Dec = 291.382, 11.117 deg which is
   RA(J2000)  =  19h 25m 31.8s
   Dec(J2000) = +11d 06' 59.9"
with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment). The
partial coding was 48%.

The mask-weighted light curve shows a single FRED-like structure that
starts at ~T-0.1, peaks at ~T0, and ends at ~T+0.4. T90 (15-350 keV) is
0.29 +- 0.08 sec (estimated error including systematics).

The time-averaged spectrum from T+0.00 to T+0.37 sec is best fit by a
simple power-law model.  The power law index of the time-averaged spectrum
is 1.66 +- 0.17.  The fluence in the 15-150 keV band is 1.7 +- 0.2 x 10^-07
erg/cm2. The 1-sec peak photon flux measured from T-0.31 sec in the 15-150
keV band is 2.4 +- 0.3 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/1350381

A.A. Breeveld (UCL-MSSL) and R. Caputo (GSFC) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250917B 139 s after the Swift/BAT trigger (Caputo et al., GCN Circ. 41848
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 ) and the Fermi GBM trigger (Fermi GBM Team, GCN 41846
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 ).
No optical afterglow consistent sources 1, 5 or 6 identified by the Swift-XRT (Kennea et al., GCN Circ. 41851
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 ) 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           139          289          147         >20.5
u_FC               351          601          246         >19.9
white              139         5391          492         >21.2
v                  681         1059           39         >18.1
b                  606         5225          236         >20.1
u                  351         5020          462         >20.1
w1                 730         4815          216         >18.9
m2                 705         1084           39         >18.8
w2                 831         1035           39         >19.0

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 1.983 in the direction of the burst (Schlegel et al. 1998).

Yan-Long Hua, Bin Li, Bin-Yang Liu, Tian-Rui Sun, Jin-Jun Geng, Xue-Feng Wu, Zhi-Ping Jin, Wen Zhao, Ji-An Jiang report on behalf of the WFST team:

Following the detection of GRB 250917B by Fermi GBM (Fermi GBM Team, GCN 41846
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 ) and Swift-BAT (R. Caputo, GCN 41848
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 ), we performed follow-up observations using the Wide Field Survey Telescope (WFST Collaboration; arXiv:2306.07590
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 ) at the Lenghu Astronomical Observation Base in Qinghai Province, China. Observations were performed in the g/r/i band, starting from 2025-09-17 13:15:02（UTC）to 2025-09-17 13:42:39（UTC）, with an exposure of 120s*3 for each band. The r-band stacked image reaches a limiting magnitude of 22.3 (AB).

We performed image subtraction using Pan-STARRS1 (PS1) reference images, but did not find any significant optical counterpart. We also examined the Swift-XRT (J.A. Kennea, GCN 41851
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 ) sources within the GRB error region: Source 1 is located near the center of a galaxy, making it difficult to justify an optical counterpart. Source 5 and Source 6 show no detectable optical counterparts in our images. These results suggest that no clear optical afterglow was detected at the positions of the X-ray sources within our observation limits.

We thank the WFST staff for supporting these observations.

R. Sonawane (IISER, TVM) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 06:27:44.33 UT on 17 September 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250917B (trigger 779783269/250917269).
which was also detected by Swift BAT (R. Caputo et al. 2025, GCN 41848
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 ).
The Fermi GBM on-ground location is consistent with the Swift BAT position.

The angle from the Fermi LAT boresight is 48 degrees.

The GBM light curve consists of a single emission episode with a duration (T90)
of about 0.2 s (50-300 keV). The time-averaged spectrum
from T0-0.2 to T0+0.3 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -1 +/- 0.2 and the cutoff energy,
parameterized as Epeak, is 120 +/- 20 keV.

The event fluence (10-1000 keV) in this time interval is
(3.2 +/- 0.3)E-07 erg/cm^2. The 64-ms peak photon flux measured
starting from T0+0.0 s in the 10-1000 keV band is 24 +/- 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.A. Kennea (PSU), A.P. Beardmore (U. Leicester), J.P. Osborne (U.
Leicester), B. Sbarufatti (INAF-OAB), C. Salvaggio (INAF-OAB), M. Ferro
(INAF-OAB), D.N. Burrows (PSU), S. Lanava (PSU) and P.A. Evans (U.
Leicester) reports on behalf of the Swift-XRT team:

We have analysed 5.5 ks of Photon Counting (PC) mode XRT data for GRB
250917B, collected between T0+142 s and T0+28.4 ks. 

Three uncatalogued X-ray sources have been detected within the
estimated 3-sigma Swift-BAT error region (296 arcsec), however none of
them is above the RASS limit or shows definitive signs of fading.
Therefore, at the present time we cannot identify which, if any, is the
afterglow. Details of these sources are given below:

Source 1:
  RA (J2000.0):  291.3458  =  19h 25m 23.00s
  Dec (J2000.0): +11.1151  =  +11d 06' 54.5"
  Error: 5.5 arcsec (radius, 90% conf. [Enhanced position])
  Count-rate: (5.5 [+1.3, -1.2])e-3 ct s^-1   
  Distance: 113 arcsec from Swift-BAT position.
  Flux: (1.26 [+0.30, -0.28])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 5:
  RA (J2000.0):  291.3929  =  19h 25m 34.30s
  Dec (J2000.0): +11.0748  =  +11d 04' 29.2"
  Error: 7.7 arcsec (radius, 90% conf.)
  Count-rate: (4.6 [+5.4, -3.9])e-4 ct s^-1   
  Distance: 147 arcsec from Swift-BAT position.

Source 6:
  RA (J2000.0):  291.4312  =  19h 25m 43.49s
  Dec (J2000.0): +11.1014  =  +11d 06' 05.0"
  Error: 8.3 arcsec (radius, 90% conf.)
  Count-rate: (7.4 [+6.4, -4.8])e-4 ct s^-1   
  Distance: 192 arcsec from Swift-BAT position.
  Flux: (1.47 [+1.27, -0.95])e-12 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Three uncatalogued sources were also detected too far from the GRB
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/01350381.

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-Tunka robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Russia (Applied Physics Institute, Irkutsk State University) started inspect of the Fermi GRB 250917B ( Fermi GBM team, GCN 41846
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 ) errorbox  21352 sec after notice time and 21386 sec after trigger time at 2025-09-17 12:24:10 UT, with upper limit up to  13.2 mag. Observations started at twilight.  The observations began at zenith distance = 22 deg. The sun  altitude  is -11.2 deg. 

The galactic latitude b = 19 deg., longitude l = 57 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2993668

We obtain a following upper limits.  

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

   21416 | 2025-09-17 12:24:10 |        MASTER-Tunka | (18h 22m 39.27s , +29d 57m 54.1s) |   C |    60 | 13.2 |        
   21498 | 2025-09-17 12:25:32 |        MASTER-Tunka | (18h 19m 56.75s , +28d 02m 47.7s) |   C |    60 | 13.0 |        
Filter C is a clear (unfiltred) band. 


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

R. Caputo (GSFC), C. Gronwall (PSU) and K. L. Page (U Leicester)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 06:27:44 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250917B (trigger=1350381).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 291.378, +11.113 which is 
   RA(J2000) = 19h 25m 31s
   Dec(J2000) = +11d 06' 48"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a short, single peaked
structure with a duration of about 0.5 sec.  The peak count rate
was ~8000 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 06:29:58.5 UT, 134.2 seconds after
the BAT trigger. No source was detected in 681 s of promptly downlinked
data. We are waiting for the full dataset to detect and localise the
XRT counterpart.

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 138 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 25% of
the BAT 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
BAT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the large, but uncertain, extinction expected.

Burst Advocate for this burst is R. Caputo (regina.caputo AT nasa.gov).
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/)

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

At 06:27:44 UT on 17 Sep 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250917B (trigger 779783269.329994 / 250917269).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 275.0, Dec = 29.0 (J2000 degrees, equivalent to J2000 18h 19m, 29d 00'), with a statistical uncertainty of 6.2 degrees.

The angle from the Fermi LAT boresight is 25.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250917269/quicklook/glg_skymap_all_bn250917269.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/bn250917269/quicklook/glg_healpix_all_bn250917269.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250917269/quicklook/glg_lc_medres34_bn250917269.gif