GRB 251011B

B. Sbarufatti (INAF-OAB), C. Salvaggio (INAF-OAB), S. Lanava (PSU), S.
Dichiara (PSU), J.A. Kennea (PSU), J.P. Osborne (U. Leicester), K.L.
Page (U. Leicester), A.P. Beardmore (U. Leicester), M. Ferro (INAF-OAB)
and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 5.1 ks of XRT data for GRB 251011B, from 85 s to 61.7
ks after the   trigger. The data comprise 112 s in Windowed Timing (WT)
mode (the first 9 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The light curve can be modelled with an initial power-law decay with an
index of alpha=3.12 (+/-0.08), followed by a break at T+579 s to an
alpha of 0.57 (+0.07, -0.20).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.15 (+/-0.12). The
best-fitting absorption column is  1.8 (+0.4, -0.3) x 10^21 cm^-2, in
excess of the Galactic value of 1.7 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 3.1 x 10^-11 (4.5 x 10^-11) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     1.8 (+0.4, -0.3) x 10^21 cm^-2
Galactic foreground: 1.7 x 10^20 cm^-2
Excess significance: 8.4 sigma
Photon index:	     2.15 (+/-0.12)

If the light curve continues to decay with a power-law decay index of
0.57, the count rate at T+24 hours will be 0.013 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.0 x
10^-13 (5.7 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/01403191.

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

O. Mukherjee (USRA), C. Meegan (UAH), U. Pathak (IIT Bombay) and L. Scotton (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 07:08:21.91 UT on 11 October 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 251011B (trigger 781859306/251011297).
which was also detected by Swift BAT (A. D'Ai et al. 2025, GCN 42197
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 ), 
and Swift XRT (P.A. Evans et al. 2025, GCN 42201
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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 81.8 degrees.

The GBM light curve consists of a single pulse with a duration (T90)
of about 35 s (50-300 keV). The time-averaged spectrum
from T0-8.0 to T0+26.5 s is best fit by
a Band function with Epeak = 36 +/- 5 keV,
alpha = 0.4 +/- 0.7, and beta = -2.2 +/- 0.1.

The event fluence (10-1000 keV) in this time interval is
(3.3 +/- 0.3)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+0.58 s in the 10-1000 keV band is 3 +/- 0.2 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/"

P.Balanutsa, V.Lipunov, A.Kuznetsov, G.Antipov, I.Panchenko, K.Zhirkov, N.Tiurina, A.Sankovich, Ya.Kechin, M.Shilova, A.Chasovnikov, V.Topolev,
V.Senik, D.Vlasenko, Yu.Tselik, E.Gorbovskoy(Lomonosov MSU),
R.Podesta, C.Francile,  F. Podesta, E. Gonzalez (OAFA, San Juan Uni.,Argentina);
D.Buckley, (SAAO, South Africa)
O.Gress, N.Budnev (ISU, Irkutsk),
A. Sosnovskiy (CrAO RAS),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
V.M.Pillet, R.Rebolo Lopez (The Instituto de Astrofisica de Canarias, Spain),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez,J.Martinez,A.R.Corella,
L.H.Rodriguez (INAOE, Guillermo Haro  Astrophysic Observatory, Mexico)

The MASTER-OAFA robotic telescope of the MASTER Global Robotic Net [1,2,3,4]
started GRB 251011B (Fermi GCN 42196
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 , Swift: D'Ai et al. GCN 42197
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 , XRT Evans et al. GCN 42201
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 ,
Ttrigger=2025-10-11 07:08:22UT    XRT RA,Dec(2000)=00h 52m 56.33s,-62d 02' 00.2")
at 2025-10-11 07:09:27 with 10s expositions at 48 deg.altitude
There is no optical counterpart on a single images
Observations and reduction will be continued.

Tstart(+exp), UT        RADec(2000) of MASTERsquarecent. Exp,s mlim
2025-10-11 07:09:07	00h 53m 53.80s -61d 58m 00.8s 	10	18.0
2025-10-11 07:09:23	00h 53m 53.92s -61d 58m 00.0s 	10	18.0
2025-10-11 07:09:39	00h 53m 53.86s -61d 58m 00.6s 	20	18.6
2025-10-11 07:10:05	00h 53m 53.91s -61d 58m 00.5s 	20	18.9
2025-10-11 07:10:31	00h 53m 54.05s -61d 57m 59.3s 	30	18.0
2025-10-11 07:11:06	00h 53m 54.18s -61d 57m 59.2s 	30	18.0
2025-10-11 07:11:43	00h 53m 54.30s -61d 57m 59.1s 	40	18.3
2025-10-11 07:12:29	00h 53m 54.50s -61d 57m 58.5s 	50	18.8
2025-10-11 07:13:25	00h 53m 54.74s -61d 57m 58.3s 	60	19.2
2025-10-11 07:14:32	00h 53m 55.02s -61d 57m 57.6s 	70	19.6
2025-10-11 07:15:47	00h 53m 55.39s -61d 57m 57.1s 	90	19.6
2025-10-11 07:17:23	00h 53m 55.79s -61d 57m 56.7s 	110	19.6
2025-10-11 07:19:19	00h 53m 56.33s -61d 57m 55.8s 	130	19.6
2025-10-11 07:21:35	00h 53m 57.00s -61d 57m 55.1s 	160	19.6
up to 2025-10-11 09:17:08.



[1] Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L
[2] Lipunov et al. 2022, Universe, Vol. 8(5), id.271
[3] Lipunov et a. 2019, ARep, vol.63, 293
[4] Lipunov V., Kornilov V., Gorbovskoy E., Tiurina N., Kuznetsov A.
 2023,  Astronomical Robotic Networks and Operative Multichanel Astrophysics,
Lomonosov MSU PRESS, 591pp.
http://www.pereplet.ru/lipunov/625.html#625

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

Using 243 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 251011B, 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 = 13.23469, -62.03340 which is equivalent
to:

RA (J2000): 00h 52m 56.33s
Dec (J2000): -62d 02' 00.2"

with an uncertainty of 2.4 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.

A. D'Ai (INAF-IASFPA), E. Ambrosi (INAF-IASFPA),
V. D'Elia (SSDC & INAF-OAR), R. Gupta (NASA GSFC),
N. P. M. Kuin (UCL-MSSL), A. Melandri (INAF-OAR),
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 07:08:22.33 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 251011B (trigger=1403191).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 13.287, -62.032 which is 
   RA(J2000) = 00h 53m 09s
   Dec(J2000) = -62d 01' 55"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty). The BAT light curve showed a complex structure
with a duration of about 150 seconds.  The peak count rate was ~3000
counts/sec (15-350 keV) at ~0 sec after the trigger.

The XRT began observing the field at 07:09:58.8 UT, 96.5 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 13.23523,
-62.03269 which is equivalent to:
   RA(J2000)  = 00h 52m 56.46s
   Dec(J2000) = -62d 01' 57.7"
with an uncertainty of 3.8 arcseconds (radius, 90% containment). This
location is 87 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 in excess of the Galactic value (1.68 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.5
(+3.47/-2.92) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 1.95e-09 erg cm^-2 s^-1 (0.2-10
keV).

UVOT data are unavailable at this time.

Burst Advocate for this burst is A. D'Ai (antonino.dai 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/)

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

At 07:08:21 UT on 11 Oct 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251011B (trigger 781859306.91454 / 251011297).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 5.3, Dec = -56.1 (J2000 degrees, equivalent to J2000 00h 21m, -56d 06'), with a statistical uncertainty of 5.3 degrees.

The angle from the Fermi LAT boresight is 78.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251011297/quicklook/glg_skymap_all_bn251011297.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/bn251011297/quicklook/glg_healpix_all_bn251011297.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251011297/quicklook/glg_lc_medres34_bn251011297.gif