GRB 251013D

M. H. Siegel (PSU) reports on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 251013D 71 ks after the Fermi/GBM trigger (GCN Circ. 42224
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 ; DeLaunay et al., GCN Circ. 42244
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 ). We detect an uncatalogued optical source near the position of XRT source #2 (Burrows et al., GCN Circ. 42291
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 ). There is no evidence of fading in the UVOT data thus far.

The preliminary UVOT position is:
    RA  (J2000) =  03:07:52.73 =  46.96971 (deg.)
    Dec (J2000) = +08:48:57.4  =   8.81595 (deg.)
with an estimated uncertainty of 0.42 arc sec. (radius, 90% confidence).

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

white            71081        72734         1622      20.98+/-0.16
white            76655        78444         1758      20.78+/-0.13
white            82245        83695         1423      20.68+/-0.13

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.312 in the direction of the burst
(Schlegel et al. 1998).

D.N. Burrows (PSU), S. Lanava (PSU), J.P. Osborne (U. Leicester), K.L.
Page (U. Leicester), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), S.
Campana (INAF-OAB), S. Dichiara (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 and
Swift/BAT-GUANO detected burst GRB 251013D, collecting	4.9 ks of
Photon Counting (PC) mode data between T0+71.0 ks and T0+83.7 ks. 

Two uncatalogued X-ray sources have been detected within the estimated
3-sigma BAT-GUANO error region (394 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):  46.8919  =  03h 07m 34.05s
  Dec (J2000.0): +8.7859  =  +08d 47' 09.2"
  Error: 5.6 arcsec (radius, 90% conf.)
  Count-rate: (3.42 [+1.17, -0.97])e-3 ct s^-1	 
  Distance: 37 arcsec from Fermi/GBM position.
  Flux: (1.83 [+1.3, -1.1])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 2:
  RA (J2000.0):  46.9703  =  03h 07m 52.86s
  Dec (J2000.0): +8.8156  =  +08d 48' 56.0"
  Error: 6.0 arcsec (radius, 90% conf.)
  Count-rate: (2.00 [+0.95, -0.74])e-3 ct s^-1	 
  Distance: 331 arcsec from Fermi/GBM position.
  Flux: (3.4 [+6.2, -1.8])e-14 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Two 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/ToO_GRBs/00021861.

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

E. Palafox (INAOE) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 18:17:00.39 UT on 13 October 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 251013D (trigger 782072225/251013762).
which was also detected by Swift BAT (J. DeLaunay et al. 2025, GCN 42244
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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 70 degrees.

The GBM light curve consists of a single emission episode with a duration (T90)
of about 42 s (50-300 keV). 
The time-averaged spectrum from T0-2.0 to T0+53.2 s is best fit by a Comptonized function with Epeak = 129 +/- 12 keV, alpha = -0.9 +/- 0.1.
A Band function fits equally well with Epeak = 103 +/- 16 and alpha = -0.8 +/- 0.2, and 
beta -2.3 +/- 0.2.

The event fluence (10-1000 keV) in this time interval is
(4.4 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+4.9 s in the 10-1000 keV band is 0.83 +/- 0.03 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/"

R. Konno (WIS), S. Garrappa (WIS), E. A. Zimmerman (WIS), A. Horowicz (WIS), E. O. Ofek (WIS), S. Ben-Ami (WIS), D. Polishook (WIS), O. Yaron (WIS), S. Fainer (WIS), A. Krassilchtchikov (WIS), Y. M. Shani (WIS), E. Segre (WIS), A. Gal-Yam (WIS), and S. Spitzer (WIS) report on behalf of the LAST Collaboration.

We report observations of GRB 251013D, detected by Fermi (Fermi GBM Team, GCN 42224
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 ). Observations were conducted with the Large Array Survey Telescope (LAST; Ofek et al. 2023, PASP 135, 5001; Ben-Ami et al. 2023, PASP 135, 5002).

We observed the field of GRB 251013D using 20 divergent telescopes (each with a 7.4 deg^2 FoV) in clear band (similar to the Gaia Bp band) over several epochs. Each coadd consists of 20x20 s exposures, yielding a limiting magnitude of about 20.5 AB mag.

Our observations cover about 80% of the localization region within the HEALPix FITS file glg_healpix_all_bn251013762.fit (Fermi GBM Team, GCN 42224
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 ) and include the Swift/BAT-GUANO localization (DeLaunay et al., GCN 42244
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 ). We coadded the Swift/BAT-GUANO coverage obtained between 2025-10-13 22:22:33 UTC (T − T0 = 4.09 h) and 2025-10-14 01:02:54 UTC (T − T0 = 6.77 h) for a total integration of 120 x 20 s and performed image subtraction using reference images of this field. No new source was detected within the Swift/BAT-GUANO region down to a limiting magnitude of 21.14 (AB). No source brighter than 19.44 mag (AB) with significant variability (> 0.5 mag) was observed between the start and end of the observations of the same field.

LAST is a survey telescope array of the Weizmann Astrophysical Observatory (https://www.weizmann.ac.il/wao/).

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 251013D. Automated analysis of the XRT data will be presented online at
https://www.swift.ac.uk/ToO_GRBs/00021861
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.

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

Swift/BAT did not localize GRB 251013D onboard (T0: 2025-10-13T18:17:00.39 UTC, Fermi Trig 782072225).

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 BAT likelihood search, NITRATES (DeLaunay + Tohuvavohu 2022, ApJ, 941, 169), detects the burst in a 16.384 s analysis time bin starting at T0 - 0.0 s with a sqrt(TS) of 27.6.
An arcminute localization is found with DeltaLLHOut of 159 and a DeltaLLHPeak of 130.

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 = 46.901, 8.800 deg which is
   RA(J2000)  = 03h 07m 36.24s
   Dec(J2000) = 8d 48’ 00.0″
with an estimated uncertainty of 5 arcmin radius.

More details about this burst can be found on the trigger report page here:

https://guano.swift.psu.edu/trigger_report?id=782072256

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/

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-SAAO robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in South Africa (South African Astronomical Observatory) started inspect of the Fermi GRB 251013D ( Fermi GBM team, GCN 42224
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 ) errorbox  12470 sec after notice time and 12506 sec after trigger time at 2025-10-13 21:45:26 UT, with upper limit up to  19.2 mag. The observations began at zenith distance = 54 deg. The sun  altitude  is -48.9 deg. 

The galactic latitude b = -41 deg., longitude l = 167 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=3013985

We obtain a following upper limits.  

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

   12536 | 2025-10-13 21:45:26 |         MASTER-SAAO | (02h 43m 45.87s , +12d 29m 46.5s) |   C |    60 | 18.9 |        
   12536 | 2025-10-13 21:45:26 |         MASTER-SAAO | (02h 45m 35.22s , +12d 14m 30.3s) |   C |    60 | 19.0 |        
   12729 | 2025-10-13 21:48:39 |         MASTER-SAAO | (02h 42m 32.93s , +10d 34m 14.1s) |   C |    60 | 19.1 |        
   12729 | 2025-10-13 21:48:39 |         MASTER-SAAO | (02h 44m 21.71s , +10d 18m 58.2s) |   C |    60 | 19.2 |        
Filter C is a clear (unfiltred) band. 


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

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

At 18:17:00 UT on 13 Oct 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251013D (trigger 782072225.388414 / 251013762).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 44.4, Dec = 10.2 (J2000 degrees, equivalent to J2000 02h 57m, 10d 11'), with a statistical uncertainty of 5.4 degrees.

The angle from the Fermi LAT boresight is 73.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251013762/quicklook/glg_skymap_all_bn251013762.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/bn251013762/quicklook/glg_healpix_all_bn251013762.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251013762/quicklook/glg_lc_medres34_bn251013762.gif