GRB 251018A

Anurag Talele (University of Potsdam),

Martin Jelinek, Alzbeta Malenakova, Jan Strobl 
(ASU CAS Ondrejov, CZ), 

Filip Novotny (ASU Ondrejov + Masaryk University),

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

report:

The 30 cm robotic telescope at the FRAM-Auger in
Malargue (Argentina) reacted robotically to the
Swift/BAT alert of GRB251018A (trigger 1405083, Gupta
et. al. GCN42343
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 ) issued at 03:12:02 UT.

FRAM obtained its first unfiltered exposure 42 seconds
after the trigger, at 03:12:44 UT, followed by a
sequence of 20s and 60s exposures.

An optical transient is detected at the UVOT position.
It appears at R(Vega) roughly 15.5 mag at T0 + 78s and
fades to roughly 18.0 mag by the end of our 28 minutes
observing run consistent with detections by Lipunov et.
al. GCN42346
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 , Hu et. al. GCN42363
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  and Kuin et. al.
GCN42353
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 .

B. Hristov (UAH) and C. Meegan (UAH) report on behalf of the Fermi GBM Team:

"At 03:12:00.63 UT on 18 October 2025 , the Fermi Gamma-Ray Burst Monitor (GBM)
triggered and located GRB 251018A (trigger 782449925 / 251018133),
which was also detected by the Swift/XRT (R. Gupta et al. 2025, GCN 42343
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 )
The GBM on-ground location is consistent with the Swift position.

The angle from the Fermi LAT boresight at the GBM trigger time is 42 degrees.

The GBM light curve consists of multiple peaks,
with a duration (T90) of about 27.1 s  (50-300 keV).
The time-averaged spectrum from T0-6.1 s to T0+21.5 s
is well fit by a power law function with an exponential high-energy cutoff.
The power law index is -0.91 +/- 0.09 and the cutoff energy, parameterized as Epeak, is 715 +/- 162 keV.

The event fluence (10-1000 keV) in this time interval is
(6.8 +/- 0.3)E-6 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+3 s in the 10-1000 keV band 0.95 +/- 0.04 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/"

Y.-D. Hu (GXU), I. Perez-Garcia, A. J. Castro-Tirado, E. J. Fernandez-Garcia, M. D. Caballero-Garcia, S.-Y. Wu, R. Sanchez-Ramirez, and S. Guziy (IAA-CSIC), I. M. Carrasco (SMA), M. Gritsevich (Univ. of Helsinki), C. Perez del Pulgar (Univ. of Malaga), S. Jeong (ADD, Daejeon), G. Garcia-Segura and D. Hiriart (IA-UNAM, Ensenada), W. H. Lee (UNAM, Mexico DF), D.-R. Xiong (YNAO), B.-B. Zhang (NJU), and A. Maury (Space Obs., San Pedro de Atacama) on behalf of a larger collaboration, report:

Following the detection of GRB251018A by Swift (Gupta et al., GCN 42343
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 ) and Fermi (Fermi GBM team, GCN 42342
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 ), the 0.6m BOOTES-5/JGU robotic telescope at San Pedro Martir Observatory (Mexico) automatically responded to this high-energy event starting on Oct 18, 03:12:45 UT (i.e., 43 sec after detection). Series of images in clear filter were gathered and we detect an optical source. Using USNO B-1.0 as a reference, we measure an initial magnitude of 16.78 +/- 0.16 in the first 1 x 5 s stacked image. 

Meanwhile, the 0.6m BOOTES-7 robotic telescope at Space Observatory (San Pedro de Atacama, Chile) also automatically observed the GRB location starting on Oct. 18, 03:13:22 UT (i.e. 80 s after trigger). The afterglow is also detected with 16.04 +/- 0.11. These detections are consistent with the one reported by MASTER (Lipunov et al., GCN 42346
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 ), TRT (Fu et al., GCN 42345
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 ), and UVOT (Kuin et al., GCN 42353
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 ). Further analysis of the additional images is ongoing.

We would like to thank the staff at San Pedro Martir Observatory and San Pedro de Atacama Space Observatory for their excellent support.

E. Ambrosi  (INAF-IASFPA) , J.A. Kennea (PSU), D.N. Burrows (PSU), S.
Lanava (PSU), A.P. Beardmore (U. Leicester), J.P. Osborne (U.
Leicester), M. Capalbi (INAF-OAR), M. Perri (SSDC & INAF-OAR) and P.A.
Evans report on behalf of the Swift-XRT team:

We have analysed 7.5 ks of XRT data for GRB 251018A, from 63 s to 50.2
ks after the   trigger. The data comprise 259 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 a power-law decay with a decay
index of alpha=1.122 (+0.031, -0.029).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.68 (+0.15, -0.14). The
best-fitting absorption column is  1.4 (+0.6, -0.5) x 10^21 cm^-2,
consistent with the Galactic value of 9.4 x 10^20 cm^-2 (Willingale et
al. 2013). The PC mode spectrum has a photon index of 1.81 (+0.25,
-0.23) and a best-fitting absorption column of 2.1 (+1.1, -0.9) x 10^21
cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 4.0 x 10^-11 (5.2 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.1 (+1.1, -0.9) x 10^21 cm^-2
Galactic foreground: 9.4 x 10^20 cm^-2
Excess significance: 2.0 sigma
Photon index:	     1.81 (+0.25, -0.23)

If the light curve continues to decay with a power-law decay index of
1.122, the count rate at T+24 hours will be 6.4 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.6 x
10^-13 (3.3 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/01405083.

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

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

Using 2660 s of XRT Photon Counting mode data and 4 UVOT
images for GRB 251018A, 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 = 282.04220, -37.02921 which is equivalent
to:

RA (J2000): 18h 48m 10.13s
Dec (J2000): -37d 01' 45.2"

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.

N.P.M. Kuin (MSSL-UCL) and R. Gupta (NASA GSFC) report on behalf of the
Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 251018A 84 s
after the BAT trigger (Gupta et al., GCN Circ. 42343
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 ).  A source consistent
with the XRT position is detected in the initial UVOT exposures.

Preliminary detections 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               84          128           44         17.98 +/- 0.08

The magnitudes in the table are not corrected for the Galactic extinction due
to the reddening of E(B-V) = 0.101 in the direction of the burst (Schlegel
et al. 1998).

 V. Lipunov, I.Panchenko, A.Kuznetsov, G.Antipov, P.Balanutsa, E.Gorbovskoy,
 K.Zhirkov,  N.Tiurina, A.Sankovich, Ya.Kechin,  Yu. Tselik, M.Shilova, A.Chasovnikov, V.Topolev,
 V.Senik, D.Vlasenko, V.Shumkov, K.Vetrov, P.Rudominskaya (Lomonosov MSU, SAI, Moscow),
 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.Sosnovskij (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  MASTER Global Robotic Net [1-4]
 started Fermi (GCN 42342
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 ) and Swift  GRB 251018A (Gupta et al. GCN 42343
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 , Trigger time 2025-10-18 03:12:02UT)
105s after notice time (141s after trigger time) at 2025-10-18 03:14:24 UT (Lipunov et al. GCN 42344
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 , cover map and OT https://master.sai.msu.ru/site/master2/observ.php?id=3017251

There is MASTER OT J184810.01-370141.6,detected since first image (03:14:24) with m_OT=16.5 at 2025-10-18 03:16:46 (unfiltered, automatically calculated)
This OT was discovered by Swift-UVOT (Gupta et al. GCN 42343
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 ) and observed by TRT (S.Y.Fu et al. GCN 42345
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 ) at the same time.

Observation and reduction will be continued.


[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

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

We observed the field of GRB 251018A (Gupta et al., GCN 423243
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 ) using the 0.7-m telescope of the Thai Robotic Telescope (TRT) network, located at Cerro Tololo Inter-American Observatory, Chile. Observations started at 2025-10-17 03:14:29.2 UT, i. e., 147 s after the trigger, and a series frames in the Sloan r filter were obtained.

An uncatalogued and varying optical transient (OT) is clearly detected at the Swift-UVOT position (Gupta et al., GCN 42343
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 ), with r = 17.96 +/-0.03 (AB) at ~ 865 s post-burst, calibrated with nearby SKYMAPPER-DR2 catalog and without the Galactic extinction correction. We thus confirm that the OT is indeed the optical afterglow of the burst.

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-OAFA robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Argentina (OAFA observatory of San Juan National University) was pointed to the Swift GRB 251018A ( R. Gupta et al., GCN 42343
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 ) errorbox  105 sec after notice time and 141 sec after trigger time at 2025-10-18 03:14:24 UT, with upper limit up to  18.4 mag. The observations began at zenith distance = 68 deg. The sun  altitude  is -46.5 deg. 

The galactic latitude b = -16 deg., longitude l = 359 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=3017251

We obtain a following upper limits.  

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

     147 |         MASTER-OAFA |   C |    10 | 17.8 |        
     168 |         MASTER-OAFA |   C |    20 | 18.3 |        
     194 |         MASTER-OAFA |   C |    20 | 18.2 |        
     225 |         MASTER-OAFA |   C |    30 | 18.4 |        
     263 |         MASTER-OAFA |   C |    30 | 18.4 |        
Filter C is a clear (unfiltred) band. 


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

R. Gupta (NASA GSFC), R. Brivio (INAF-OAB), S. Dichiara (PSU),
M. Ferro (INAF-OAB), D. M. Palmer (LANL), T. Sbarrato (INAF-OAB) and
M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 03:12:02 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 251018A (trigger=1405083).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 282.071, -37.028 which is 
   RA(J2000) = 18h 48m 17s
   Dec(J2000) = -37d 01' 41"
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 60 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~1 sec after the trigger.

The XRT began observing the field at 03:13:16.7 UT, 74.4 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 282.0433, -37.0265 which is equivalent to:
   RA(J2000)  = 18h 48m 10.39s
   Dec(J2000) = -37d 01' 35.4"
with an uncertainty of 6.3 arcseconds (radius, 90% containment). This
location is 79 arcseconds from the BAT onboard position, within the BAT
error circle. 
The initial flux in the 2.5 s image was 5.19e-10 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 83 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	18:48:10.04 = 282.04183
  DEC(J2000) = -37:01:42.2  = -37.02840
with a 90%-confidence error radius of about 0.62 arc sec. This position is 8.0
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.96 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.100.

Burst Advocate for this burst is R. Gupta (rahulbhu.c157 AT gmail.com).
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 03:12:00 UT on 18 Oct 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251018A (trigger 782449925.633958 / 251018133).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 294.7, Dec = -40.3 (J2000 degrees, equivalent to J2000 19h 38m, -40d 17'), with a statistical uncertainty of 6.8 degrees.

The angle from the Fermi LAT boresight is 36.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251018133/quicklook/glg_skymap_all_bn251018133.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/bn251018133/quicklook/glg_healpix_all_bn251018133.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251018133/quicklook/glg_lc_medres34_bn251018133.gif