Gamma-ray Burst 251011A

(All information courtesy of the instrument teams.)

Previous IAU Circulars

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Results of Observations

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Sat 11 Oct 25 03:42:09 UT
NOTICE_TYPE:     Fermi-GBM Flight Position
RECORD_NUM:      48
TRIGGER_NUM:     781846905
GRB_RA:            6.167d {+00h 24m 40s} (J2000),
                   6.514d {+00h 26m 03s} (current),
                   5.496d {+00h 21m 59s} (1950)
GRB_DEC:         +47.150d {+47d 08' 60"} (J2000),
                 +47.293d {+47d 17' 33"} (current),
                 +46.873d {+46d 52' 23"} (1950)
GRB_ERROR:       6.65 [deg radius, statistical plus systematic]
GRB_INTEN:       525 [cnts/sec]
DATA_SIGNIF:     24.70 [sigma]
INTEG_TIME:      1.024 [sec]
GRB_DATE:        20959 TJD;   284 DOY;   25/10/11
GRB_TIME:        13300.09 SOD {03:41:40.09} UT
GRB_PHI:         180.00 [deg]
GRB_THETA:        60.00 [deg]
DATA_TIME_SCALE: 1.0240 [sec]
HARD_RATIO:      0.92
LOC_ALGORITHM:   3 (version number of)
MOST_LIKELY:      97%  GRB
2nd_MOST_LIKELY:   1%  Generic Transient
DETECTORS:       0,0,0, 0,0,0, 1,1,0, 1,0,0, 0,0,
SUN_POSTN:       196.68d {+13h 06m 43s}   -7.09d {-07d 05' 27"}
SUN_DIST:        138.87 [deg]   Sun_angle= -11.3 [hr] (East of Sun)
MOON_POSTN:       71.51d {+04h 46m 01s}  +27.55d {+27d 33' 12"}
MOON_DIST:        53.55 [deg]
MOON_ILLUM:      78 [%]
GAL_COORDS:      118.21,-15.48 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       27.94, 40.06 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251011154/quicklook/glg_lc_medres34_bn251011154.gif
COMMENTS:        Fermi-GBM Flight-calculated Coordinates.  
COMMENTS:        This trigger occurred at longitude,latitude = 148.88,25.62 [deg].  
COMMENTS:        The LC_URL file will not be created until ~15 min after the trigger.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Sat 11 Oct 25 03:42:28 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      59
TRIGGER_NUM:     781846905
GRB_RA:            0.950d {+00h 03m 48s} (J2000),
                   1.283d {+00h 05m 08s} (current),
                   0.307d {+00h 01m 14s} (1950)
GRB_DEC:         +43.950d {+43d 57' 00"} (J2000),
                 +44.093d {+44d 05' 37"} (current),
                 +43.672d {+43d 40' 18"} (1950)
GRB_ERROR:       3.08 [deg radius, statistical only]
DATA_SIGNIF:     31.00 [sigma]
DATA_INTERVAL:   4.096 [sec]
GRB_DATE:        20959 TJD;   284 DOY;   25/10/11
GRB_TIME:        13300.09 SOD {03:41:40.09} UT
GRB_PHI:         175.00 [deg]
GRB_THETA:        62.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   4173 (Gnd S/W Version number)
SUN_POSTN:       196.68d {+13h 06m 43s}   -7.09d {-07d 05' 27"}
SUN_DIST:        140.63 [deg]   Sun_angle= -11.0 [hr] (East of Sun)
MOON_POSTN:       71.51d {+04h 46m 02s}  +27.55d {+27d 33' 14"}
MOON_DIST:        57.50 [deg]
MOON_ILLUM:      78 [%]
GAL_COORDS:      113.94,-18.09 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       21.74, 39.20 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251011154/quicklook/glg_lc_medres34_bn251011154.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_781846905.fits
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file will not be created/available until ~15 min after the trigger.  
COMMENTS:        The POS_MAP_URL file will not be created/available until ~1.5 min after the notice.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Sat 11 Oct 25 03:42:58 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      0
TRIGGER_NUM:     781846905
GRB_RA:            0.580d {+00h 02m 19s} (J2000),
                   0.912d {+00h 03m 39s} (current),
                 359.938d {+23h 59m 45s} (1950)
GRB_DEC:         +44.920d {+44d 55' 12"} (J2000),
                 +45.063d {+45d 03' 49"} (current),
                 +44.642d {+44d 38' 30"} (1950)
GRB_ERROR:       1.76 [deg radius, statistical only]
DATA_SIGNIF:     34.50 [sigma]
DATA_INTERVAL:   3.072 [sec]
GRB_DATE:        20959 TJD;   284 DOY;   25/10/11
GRB_TIME:        13300.09 SOD {03:41:40.09} UT
GRB_PHI:         175.00 [deg]
GRB_THETA:        61.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41731 (Gnd S/W Version number)
SUN_POSTN:       196.68d {+13h 06m 43s}   -7.09d {-07d 05' 27"}
SUN_DIST:        139.64 [deg]   Sun_angle= -10.9 [hr] (East of Sun)
MOON_POSTN:       71.52d {+04h 46m 04s}  +27.55d {+27d 33' 16"}
MOON_DIST:        57.63 [deg]
MOON_ILLUM:      78 [%]
GAL_COORDS:      113.87,-17.09 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       22.10, 40.17 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251011154/quicklook/glg_lc_medres34_bn251011154.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_781846905.fits
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        This is likely a Long GRB.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file will not be created/available until ~15 min after the trigger.  
COMMENTS:        The POS_MAP_URL file will not be created/available until ~1.5 min after the notice.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Sat 11 Oct 25 03:50:58 UT
NOTICE_TYPE:     Fermi-GBM Final Position
RECORD_NUM:      0
TRIGGER_NUM:     781846905
GRB_RA:            2.590d {+00h 10m 22s} (J2000),
                   2.927d {+00h 11m 42s} (current),
                   1.939d {+00h 07m 45s} (1950)
GRB_DEC:         +43.270d {+43d 16' 12"} (J2000),
                 +43.413d {+43d 24' 48"} (current),
                 +42.992d {+42d 59' 31"} (1950)
GRB_ERROR:       2.44 [deg radius, statistical only]
GRB_DATE:        20959 TJD;   284 DOY;   25/10/11
GRB_TIME:        13300.09 SOD {03:41:40.09} UT
GRB_PHI:         176.00 [deg]
GRB_THETA:        63.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41731 (Gnd S/W Version number)
SUN_POSTN:       196.68d {+13h 06m 44s}   -7.09d {-07d 05' 35"}
SUN_DIST:        141.72 [deg]   Sun_angle= -11.1 [hr] (East of Sun)
MOON_POSTN:       71.61d {+04h 46m 25s}  +27.57d {+27d 33' 56"}
MOON_DIST:        56.48 [deg]
MOON_ILLUM:      78 [%]
GAL_COORDS:      115.04,-18.97 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       22.60, 38.01 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251011154/quicklook/glg_lc_medres34_bn251011154.gif
LOC_URL:         http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251011154/quicklook/glg_locplot_all_bn251011154.png
COMMENTS:        Fermi-GBM Final Position.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file should be available by the time this FINAL notice is produced.  
COMMENTS:        This notice was generated completely by automated pipeline processing.  
COMMENTS:        This is likely a Long GRB.

GCN Circular #42195
The Fermi GBM team reports the detection of a likely LONG GRB
At 03:41:40 UT on 11 Oct 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251011A (trigger 781846905.087994 / 251011154).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 2.6, Dec = 43.3 (J2000 degrees, equivalent to J2000 00h 10m, 43d 17'), with a statistical uncertainty of 2.4 degrees.
The angle from the Fermi LAT boresight is 63.0 degrees.
The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251011154/quicklook/glg_skymap_all_bn251011154.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/bn251011154/quicklook/glg_healpix_all_bn251011154.fit
The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251011154/quicklook/glg_lc_medres34_bn251011154.gif
GCN Circular #42199
M. Tembhurnikar (IUCAA), S. Salunke (IUCAA), Harsha K. H. (IUCAA), A. Arya (IITB), A. Goyal (IITB), U. Pathak (IITB), G. Waratkar (Caltech/IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:
Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a long-duration GRB 251011A which was also detected by Fermi GBM (GCN Circ. 42195).
The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2025-10-11 03:41:37.50 UTC. The measured peak count rate associated with the burst is 200 (+32, -29) counts/s above the background in the combined data three quadrants out of four, with a total of 743 (+117, -127) counts. The local mean background count rate was 208 (+3, -3) counts/s. Using cumulative rates, we measure a T90 of 6.5 (+2.1, -1.9) s.
The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve showed a peak of emission at 2025-10-11 03:41:37 UTC. The measured peak count rate associated with the burst is 356 (+68, -62) counts/s above the background in the combined data of all quadrants, with a total of 952 (+242, -256) counts. The local mean background count rate was 1335 (+7, -8) counts/s. We measure a T90 of 5.9 (+2.1, -2.9) s from the cumulative Veto light curve.
CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.
CZTI GRB detections are reported regularly on the payload site at: http://astrosat.iucaa.in/czti/?q=grb
GCN Circular #42208
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 251011A ( Fermi GBM team, GCN 42195) errorbox 1 days 37513 sec after notice time and 1 days 37548 sec after trigger time at 2025-10-12 14:07:28 UT, with upper limit up to 19.0 mag. The observations began at zenith distance = 21 deg. The sun altitude is -34.7 deg.
The galactic latitude b = -19 deg., longitude l = 116 deg.
Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3012037
We obtain a following upper limits.
```
Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

  123979 | 2025-10-12 14:07:28 |        MASTER-Tunka | (00h 19m 58.96s , +42d 27m 50.2s) |   C |    60 | 19.0 |        
```
Filter C is a clear (unfiltred) band.
The observation and reduction will continue. The message may be cited.
GCN Circular #42210
Chen-Wei Wang, Xing-Hao Luo, Cheng-Kui Li, Shao-Lin Xiong, and Chao Zheng report on behalf of the Insight-HXMT team:
At 2025-10-11T03:41:40.000 (T0), Insight-HXMT/HE detected the burst GRB 251011A, which is also detected by Fermi/GBM (Fermi GBM team, GCN#42195) and AstroSat (M. Tembhurnikar et al., GCN#42199).
The Insight-HXMT/HE light curve mainly consists of a single pulse with a T90 of 3.6 +1.6/-1.4 s. The 1s peak rate, measured from T0, is 788 cnts/sec. The total counts from this burst is 1444 counts.
The HXMT/HE light curve can be found here: https://www.bursthub.cn//admin/static/hxmtgrb251011A.png
All measurements above are made with the CsI detectors operating in the regular mode with the energy range of about 60-900 keV (deposited energy). Only gamma-rays with energy greater than about 200 keV can penetrate the spacecraft and leave signals in the CsI detectors installed inside of the telescope.
Insight-HXMT is the first Chinese space X-ray telescope, which was funded jointly by the China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS). More information about it could be found at: http://www.hxmt.org.
GCN Circular #42245
James DeLaunay (PSU), Aaron Tohuvavohu (Caltech), Jamie A. Kennea (PSU), Samuele Ronchini (GSSI), Gayathri Raman (PSU), Tyler Parsotan (GSFC), Maia Williams (Northwestern) report:
Swift/BAT did not localize GRB 251011A onboard (T0: 2024-10-11T03:41:40.09 UTC, Fermi GCN 42195, AstroSat CZTI GCN 42199, Insight-HXMT GCN 42210).
The Fermi/GBM 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 burst occurred during a Swift slew. The arcmin position of the burst is found with the newly developed pipeline BAT-GLIMPSE: Gamma-ray Localization using Imaging and Mosaic techniques for Pointing and Slew Epochs (Ronchini et. al, in prep). The pipeline makes use of the tools from BatAnalysis (Parsotan et al. 2025). A confident location is found for the burst with SNR of 23.5. The burst is detected in BAT with a duration of ~ 5 seconds.
The BAT position is RA, Dec = 4.7878, +45.1894 deg which is
```
   RA(J2000)  = 00h 19m 09.07s
   Dec(J2000) =  +45d 11’ 21.84″
```
with an estimated uncertainty of 5 arcmin.
This position is consistent with the Ferm/GBM localization (GCN 42195).
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/
GCN Circular #42340
R. Hamburg (USRA) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:
"At 03:41:40.09 UT on 11 October 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251011A (trigger 781846905/251011154), which was also detected by AstroSat (Tembhurnikar et al. 2025, GCN 42199), Insight-HMXT (Wang et al. 2025, GCN 42210), and Swift-BAT/GUANO (DeLaunay et al. 2025, GCN 42245). The Fermi GBM on-ground location is consistent with the Swift position.
The angle from the Fermi LAT boresight is 62 degrees.
The GBM light curve consists of a multi-peaked structure with a duration (T90) of about 4.9 s (50-300 keV). The time-averaged spectrum from T0-0.7 to T0+4.9 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.76 +/- 0.01 and the cutoff energy, parameterized as Epeak, is 136 +/- 1 keV.
The event fluence (10-1000 keV) in this time interval is (3.26 +/- 0.07)E-06 ergs/cm^2 erg/cm^2. The 1-sec peak photon flux measured starting from T0+1.8 s in the 10-1000 keV band is 11.5 +/- 0.3 ph/s/cm^2.
A Band function fits the spectrum equally well with Epeak = 125 +/- 1 keV, alpha = -0.69 +/- 0.01 and beta = -2.70 +/- 0.04.
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/"

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