GRB 251118A
GCN Circular 42786
Subject
GRB 251118A: Swift/UVOT Detection
Date
2025-11-20T22:23:37Z (10 days ago)
From
Mike Siegel at PSU/Swift MOC <mhs18@psu.edu>
Via
Web form
M. H. Siegel (PSU) reports on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 251118A 31.3 ks after the Fermi trigger (GCN Circ. 42743). A fading source consistent with the XRT position (Osbourne et al., GCN Circ. 42782) is detected in the UVOT exposures.
The preliminary UVOT position is:
RA (J2000) = 10:14:01.01 = 160.00422 (deg.)
Dec (J2000) = -34:31:21.1 = -34.52254 (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 31521 52651 885 18.79+/-0.05
white 108251 159027 1113 20.11+/-0.10
v 31678 42929 845 18.60+/-0.19
v 108564 147691 662 >19.73
u 31363 52627 2489 18.41+/-0.05
u 107942 158555 1113 19.61+/-0.15
The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.057 in the direction of the burst
(Schlegel et al. 1998).
GCN Circular 42784
Subject
GRB 251118A: Glowbug gamma-ray detection
Date
2025-11-20T20:44:33Z (10 days ago)
From
C.C. Cheung at Naval Research Lab <Teddy.Cheung@nrl.navy.mil>
Via
Web form
C.C. Cheung, R. Woolf, M. Kerr, J.E. Grove (NRL), A. Goldstein (USRA), C.A. Wilson-Hodge, D. Kocevski (MSFC), and M.S. Briggs (UAH) report:
The Glowbug gamma-ray telescope [1,2,3], operating on the International Space Station, reports the detection of GRB 251118A, which was also detected by Fermi/GBM (GCN 42743, 42745, 42783), Fermi/LAT (GCN 42752), NuSTAR (GCN 42774), and GECAM-B (GCN 42779).
Using an adaptive window with a resolution of 32-ms, the burst onset is determined to be 2025-11-18 20:31:43.57 with a duration of 47.1 s and a total significance of over 360 sigma. The light curve comprises two peaks at ~T0+1.5s and +37s, matching the two brightest peaks in the Fermi/GBM and GECAM-B lightcurves. The initial faint emission at the GBM trigger time (20:31:30) may be present in the Glowbug light curve at low significance. The GRB was detected despite the elevated background due to Glowbug being in and exiting the Southern polar region at the time of the burst. Note that data throughout the burst interval suffered from deadtime in various detectors.
The analysis results presented here are preliminary and use a response function that lacks a detailed characterization of the surrounding passive structure of the ISS.
Glowbug is a NASA-funded technology demonstrator for sensitive, low-cost gamma-ray transient telescopes developed, built, and operated by the U.S. Naval Research Laboratory (NRL) with support from the University of Alabama in Huntsville, USRA, and NASA MSFC. It was launched on 2023 March 15 aboard the Department of Defense Space Test Program’s STP-H9 to the ISS and operated until 2024 April when it was put in safe storage on orbit. Glowbug was removed from storage and resumed operation on 2025 September 12.
[1] Grove, J.E. et al. 2020, Proc. Yamada Conf. LXXI, arXiv:2009.11959
[2] Woolf, R.S. et al. 2022, Proc. SPIE, 12181, id. 121811O
[3] Woolf, R.S. et al. 2024, Proc. SPIE, 13151, id. 1315108
Distribution Statement A: Approved for public release. Distribution is unlimited.
GCN Circular 42783
Subject
GRB 251118A: Fermi GBM Observation
Date
2025-11-20T19:37:15Z (10 days ago)
From
Eva MP <eva.palafox@gmail.com>
Via
Web form
Eva Palafox (INAOE) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:
"At 20:31:30.09 UT on 18 November 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251118A (trigger 785190695/251118855), which was also detected by Fermi-LAT (R. Gupta et al. 2025, GCN 42752), Swift/XRT (J.P. Osborne et al. 2025, GCN 42782), GECAM-B (Yang-Zhao Ren et al., 2025, GCN 42779) and NuSTAR (G. Waratkar et al. 2025, GCN 42774).
The Fermi GBM final real-time localization (GCN 42743) is consistent with the Fermi-LAT position.
The angle from the Fermi LAT boresight is 69 degrees.
The GBM light curve consists of a faint emission episode, followed by very bright emission episodes, with a total duration (T90) of about 46.6 s (50-300 keV). The time-averaged spectrum from T0-2.8 to T0+87.3 s is best fit by a Comptonized function with Epeak = 149.3 +/- 2.0 keV, and alpha = -1.40 +/- 0.01. A Band function fits equally well with Epeak = 134.0 +/- 2.7, alpha = -1.36 +/- 0.01, and beta = -2.56 +/- 0.06.
The event fluence (10-1000 keV) in this time interval is (1.03 +/- 0.01)E-04 erg/cm^2. The 1-sec peak photon flux measured starting from T0+52 s in the 10-1000 keV band is 96 +/- 0.7 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/"
GCN Circular 42782
Subject
GRB 251118A: Swift-XRT afterglow detection
Date
2025-11-20T13:53:24Z (10 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M.G. Bernardini
(INAF-OAB), E. Ambrosi (INAF-IASFPA) , M. Capalbi (INAF-OAR), D.N.
Burrows (PSU), S. Lanava (PSU), 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/LAT-detected burst GRB 251118A. We searched for X-ray sources in
3.0 ks of Photon Counting (PC) mode data. The total exposure at the
position of the afterglow (see below) is 5.4 ks, obtained between
T0+31.3 ks and T0+108.8 ks.
Two uncatalogued X-ray sources are detected, of which one ("Source 1")
is above the RASS 3-sigma upper limit at this position, and is
therefore likely the GRB afterglow. Using 3939 s of PC mode data and 7
UVOT images, we find an enhanced XRT position (using the XRT-UVOT
alignment and matching UVOT field sources to the USNO-B1 catalogue):
RA, Dec = 160.00418, -34.52259 which is equivalent to:
RA (J2000): 10h 40m 01.00s
Dec(J2000): -34d 31' 21.3"
with an uncertainty of 2.0 arcsec (radius, 90% confidence). This
position is 5.0 arcmin from the Fermi/LAT position.
The light curve can be modelled with a power-law decay with a decay
index of alpha=1.08 (+0.18, -0.16).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.73 (+0.16, -0.15). The
best-fitting absorption column is 2.2 (+0.7, -0.6) x 10^21 cm^-2, in
excess of the Galactic value of 8.3 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 4.1 x 10^-11 (5.2 x 10^-11) erg
cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 2.2 (+0.7, -0.6) x 10^21 cm^-2
Galactic foreground: 8.3 x 10^20 cm^-2
Excess significance: 3.9 sigma
Photon index: 1.73 (+0.16, -0.15)
The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/00021890.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021890.
This circular is an official product of the Swift-XRT team.
GCN Circular 42779
Subject
GRB 251118A: GECAM-B detection
Date
2025-11-20T08:32:34Z (10 days ago)
From
renyz16607@163.com
Via
Web form
Yang-Zhao Ren, Chen-Wei Wang, Chao Zheng, Shao-Lin Xiong (IHEP) report on behalf of GECAM team:
GECAM-B was triggered in-flight by a long burst, GRB 251118A, at 2025-11-18T20:31:30.089 UTC (denoted as T0), which is also detected by Fermi/GBM (Fermi GBM team, GCN #42743), Fermi/LAT (Gupta et. al., GCN #42752), and NuSTAR (Waratkar et. al., GCN #42774).
According to the GECAM-B light curves in about 40-6000 keV, this burst mainly consists of multiple pulses with a duration (T90) of 46.2 +0.2/-0.3 s.
The GECAM-B light curve can be found here:
https://www.bursthub.cn//admin/static/gecambgrb251118A.png
The time-averaged spectrum from T0-1.9 s to T0+71.0 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.54 +0.09/-0.09 and the cutoff energy, parameterized as Epeak, is 99.8 +4.4/-4.5 keV. The event fluence (10-1000 keV) in this time interval is (1.01 +0.04/-0.04)E-04 erg/cm^2. The 'Amati' relation diagram of GRB 251118A is shown at:
https://www.bursthub.cn//admin/static/gecambgrb251118A_amati.png
Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two micro-satellites (GECAM-A and GECAM-B) launched in Dec. 2020. As the third member of GECAM constellation, GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS).
GCN Circular 42774
Subject
GRB 251118A: NuSTAR detection of bright prompt emission
Date
2025-11-20T00:26:27Z (10 days ago)
From
Gaurav Waratkar at Caltech <gauravwaratkar@iitb.ac.in>
Via
Web form
G. Waratkar (Caltech) and B. Grefenstette (Caltech) report on behalf of the NuSTAR Search for INteresting Gamma-ray Signals (SINGS) working group:
The NuSTAR SINGS working group reports the detection of prompt emission from the long-duration GRB 251118A in both the NuSTAR CsI anti-coincidence shields. This GRB was identified through a blind search using the CsI shield rates. Details of the search algorithm will be described in a future paper.
The NuSTAR SINGS algorithm triggered at 2025-11-18 20:32:21.000 (with a resolution ~5-seconds). This is consistent with the detections of GRB 251118A by Fermi/GBM (Fermi GBM Team, GCN Circ. 42743) and Fermi/LAT (Gupta et al., GCN Circ. 42752).
The NuSTAR CsI shield data are recorded at 1 Hz. We do not detect the first two bursts as seen in the Fermi/GBM lightcurve (GCN Circ. 42743) and detect the third bright burst. The burst appears to be composed of one bright peak lasting for ~9-s. The peak count rate is ~3000-cps over a baseline rate of ~1,000-cps during this time period. We do not see any evidence in the signal above 100 keV in the CdZnTe detectors.
The Fermi/LAT localization (GCN Circ. 42752) at RA = 160.08, Dec = -34.58 implies an offset from the NuSTAR boresight of 140-deg and an offset from the geocenter of 99-deg.
Lightcurves and analysis for this GRB can be found here:
https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2025/251118A
Information on NuSTAR SINGS can be found here:
https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/
NuSTAR is a NASA Small Explorer mission led by Caltech and managed by JPL for NASA's Science Mission Directorate in Washington.
GCN Circular 42752
Subject
GRB 251118A: Fermi-LAT detection
Date
2025-11-19T05:39:12Z (11 days ago)
From
Rahul Gupta at NASA GSFC <rahul.gupta@nasa.gov>
Via
Web form
R. Gupta (NASA GSFC) and T. Khalil (Univ. of Johannesburg) report on behalf of the Fermi-LAT Collaboration:
On November 18 2025, Fermi-LAT detected high-energy emission from GRB 251118A, which was also detected by Fermi-GBM (trigger 785190695 / 251118855, GCN 42743). The best LAT on-ground location is found to be:
RA, Dec = 160.08, -34.58 (J2000)
with an error radius of 0.17 deg (90 % containment, statistical error only). This was 69 deg from the LAT boresight at the time of the GBM trigger (T0 = 20:31:30.09 UT).
The data from the Fermi-LAT shows a significant increase in the event rate that is spatially and temporally correlated with the GBM emission with high significance. The photon flux above 100 MeV in the time interval 0 - 1.5 ks after the GBM trigger is (1.8 ± 0.7) E-6 ph/cm2/s. The estimated photon index above 100 MeV is -1.8 ± 0.3. The highest-energy photon is a 6.8 GeV event which is observed ~1400 seconds after the GBM trigger.
A Swift ToO has been approved for this burst.
The Fermi-LAT point of contact for this burst is Aldana Holzmann Airasca (a.holzmannairasca@unitn.it).
The Fermi-LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.
GCN Circular 42750
Subject
GRB 251118A: Swift ToO observations
Date
2025-11-19T05:10:33Z (11 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
P. A. Evans (U. Leicester) reports on behalf of the Swift team:
Swift has initiated a ToO observation of the Fermi/LAT-detected event
GRB 251118A. Automated analysis of the XRT data will be presented online at
https://www.swift.ac.uk/ToO_GRBs/00021890
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/LAT 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.
GCN Circular 42745
Subject
GRB 251118A: BALROG localization (Fermi Trigger 785190695 / GRB 251118855)
Date
2025-11-18T20:57:48Z (12 days ago)
From
Jochen Greiner at MPE <jcgrog@mpe.mpg.de>
Via
email
T. Preis (University of Innsbruck) & J. Greiner (MPE Garching) report:
The public trigdat data of the Fermi Gamma-ray Burst Monitor (GBM) trigger
785190695 at 20:31:30 on 18 Nov. 2025 were automatically fitted for spectrum
and sky location with BALROG (Burgess et al. 2018, MNRAS 476, 1427;
Berlato et al. 2019, ApJ 873, 60).
The best-fit position is:
RA(2000.0) = 160.0 deg
Decl.(2000.0) = -34.9 deg
The 1 sigma statistical error radius is 2.0 deg.
We estimate an additional systematic error of 2 deg.
Further details are available at:
https://grb.mpe.mpg.de/grb/GRB251118855/
The Healpix map can be downloaded from:
https://grb.mpe.mpg.de/grb/GRB251118855/healpix
The location parameters are available as JSON at:
https://grb.mpe.mpg.de/grb/GRB251118855/json
GCN Circular 42743
Subject
GRB 251118A: Fermi GBM Final Real-time Localization
Date
2025-11-18T20:42:00Z (12 days ago)
From
Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply@GIOC.nsstc.nasa.gov>
Via
email
The Fermi GBM team reports the detection of a likely LONG GRB
At 20:31:30 UT on 18 Nov 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251118A (trigger 785190695.089648 / 251118855).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 160.4, Dec = -36.5 (J2000 degrees, equivalent to J2000 10h 41m, -36d 30'), with a statistical uncertainty of 1.0 degrees.
The angle from the Fermi LAT boresight is 66.0 degrees.
The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251118855/quicklook/glg_skymap_all_bn251118855.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/bn251118855/quicklook/glg_healpix_all_bn251118855.fit
The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251118855/quicklook/glg_lc_medres34_bn251118855.gif