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[vsnet-grb-info 39937] GRB 250916A: NuSTAR detection of the prompt emission
by GCN Circulars 18 Sep '25

18 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41871 SUBJECT: GRB 250916A: NuSTAR detection of the prompt emission DATE: 25/09/18 19:45:02 GMT FROM: Gaurav Waratkar at Caltech <gauravwaratkar(a)iitb.ac.in> 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 250916A 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 2025-09-16 13:33:27.000 (with a resolution ~5-seconds). This is consistent with the detections by Fermi-GBM (Fermi GBM Team, GCN Circ., 41839), Astrosat-CZTI (Arya et al., GCN Circ. 41843), Glowbug (Cheung et al., GCN Circ. 41855), and CALET-GBM (Trig ID. 1442064382). The NuSTAR CsI shield data are recorded at 1 Hz. The burst appears to be composed of two significantly-detected peaks. We do not detect the initial faint burst detected by Fermi-GBM which occurred ~250-s before our detection. The total duration for the event is at roughly 50-s. The peak count rate is ~500-cps over a baseline rate of ~1,000-cps during this time period. We do not see clear evidence in the signal above 100 keV in the CdZnTe detectors. The localization from the optical counterpart candidate (Belkin et al., GCN Circ. 41847) at RA = 26.57, Dec = 36.16 implies an offset from the NuSTAR boresight of 69-deg (e.g., through the side of the instrument) and the offset from the geocenter of 142-deg. Lightcurves and analysis for this GRB can be found here: https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2025/250916A/ 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/41871. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39936] GRB 250916A: further SAO RAS optical observations
by GCN Circulars 18 Sep '25

18 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41870 SUBJECT: GRB 250916A: further SAO RAS optical observations DATE: 25/09/18 15:39:58 GMT FROM: Alexander Moskvitin at SAO RAS <mosk(a)sao.ru> A. Moskvitin (SAO RAS), A. Pozanenko (IKI), N. Pankov (HSE, IKI) report on behalf of GRB follow-up collaboration and IKI-GRB-FuN. We observed the field of the GRB 250916A (Fermi GBM team, GCN 41839; Arya et al., GCN 41843; Cheung et al., GCN 41855) with the BTA, 6-m telescope of SAO RAS equipped with SCORPIO-1 focal reducer. We obtained 3 x 60 sec. images in Rc band under mediocre weather conditions on September 18, 00:07:09--00:15:29 UT (t_mid - T0 = 1.4458 days = 34.6994 hours). The OT (Belkin et al., GCN 41847; Mohan et al., GCN 41858; Burkhonov et al., GCN 41860) at the redshift z = 2.015 (de Ugarte Postigo et al., GCN 41863) is clearly detected in the stacked frame with a brightness of R = 19.72 +/- 0.05. The preliminary photometry is based on PanSTARRS data on nearby objects (magnitudes calculated using the Lupton 2005 transformations) and does not take into account Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41870. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39935] GRB 250912A: SVOM/ECLAIRs refined analysis
by GCN Circulars 18 Sep '25

18 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41869 SUBJECT: GRB 250912A: SVOM/ECLAIRs refined analysis DATE: 25/09/18 15:09:27 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> Hui Yang, Olivier Godet, Marius Brunet, Jean-Luc Atteia (IRAP), Floriane Cangemi (APC), Jean-Grégoire Ducoin, Damien Dornic (CPPM) Using the event-by-event data downloaded through the X-band ground station, we report further analysis of ECLAIRs observations of GRB 250912A (SVOM burst-id sb25091206, GCN 41820) detected at T0 = 2025-09-12T21:06:24.69, which was also detected by SVOM/GRM (GCN 41826) and Fermi/GBM (GCN 41819). The burst that triggered ECLAIRs consists of a short main peak starting from T0-1.19 s and lasting about 0.6 s in the 4-120 keV energy band, followed by extended emission continuing until T0+18.3 s. We note a weak precursor at T0-11.1 s, detected mainly in the 8-20 keV band by ECLAIRs and also by GRM, whose position is consistent with that of the main event. The spectrum of the main peak (from T0-1.19 s to T0-0.55 s) in the 4-120 keV energy range is well fitted by a power-law model with a photon index of 1.2 +/-0.2. With this model, the 4-120 keV flux is (1.7+/-0.4)e-07 erg/cm^2/s. The spectrum of the extended emission following the main peak (from T0-0.55 s to T0+18.31 s) in the 4-120 keV band is well fitted by a power-law model with a photon index of 1.4+/-0.1, yielding a 4-120 keV flux of (3.3+/-0.5)e-08 erg/cm^2/s. The spectral index is consistent with that measured by SVOM/GRM and Fermi/GBM (GCN 41828). The combined fluence of both components in the 4-120 keV is (7.3+/-1.0)e-07 erg/cm^2. The low count rates prevent a spectral analysis of the precursor and the time-resolved spectral analysis of the burst. All the quoted errors are at the 68% confidence level. The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. The SVOM/ECLAIRs point of contact for this burst is: Hui Yang (IRAP) (hui.yang(a)irap.omp.eu) View this GCN Circular online at https://gcn.nasa.gov/circulars/41869. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39934] GRB 250917B: Swift-BAT refined analysis
by GCN Circulars 18 Sep '25

18 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41868 SUBJECT: GRB 250917B: Swift-BAT refined analysis DATE: 25/09/18 13:56:31 GMT FROM: Amy <yarleen(a)gmail.com> T. Parsotan (GSFC), S. D. Barthelmy (GSFC), R. Caputo (GSFC), R. Gupta (GSFC), H. A. Krimm (NSF), S. Laha (GSFC/UMBC), A. Y. Lien (U Tampa), C. B. Markwardt (GSFC), M. J. Moss (GSFC), D. M. Palmer (LANL), D. Sadaula (GSFC/UMBC), T. Sakamoto (AGU) (i.e. the Swift-BAT team): Using the data set from T-240 to T+962 sec from the recent telemetry downlink, we report further analysis of BAT GRB 250917B (trigger #1350381) (Caputo et al., GCN Circ. 41848). The BAT ground-calculated position is RA, Dec = 291.382, 11.117 deg which is RA(J2000) = 19h 25m 31.8s Dec(J2000) = +11d 06' 59.9" with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 48%. The mask-weighted light curve shows a single FRED-like structure that starts at ~T-0.1, peaks at ~T0, and ends at ~T+0.4. T90 (15-350 keV) is 0.29 +- 0.08 sec (estimated error including systematics). The time-averaged spectrum from T+0.00 to T+0.37 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.66 +- 0.17. The fluence in the 15-150 keV band is 1.7 +- 0.2 x 10^-07 erg/cm2. The 1-sec peak photon flux measured from T-0.31 sec in the 15-150 keV band is 2.4 +- 0.3 ph/cm2/sec. All the quoted errors are at the 90% confidence level. The results of the batgrbproduct analysis are available at https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1350381 View this GCN Circular online at https://gcn.nasa.gov/circulars/41868. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39933] LIGO/Virgo/KAGRA S250918aw: 1 counterpart neutrino candidate event from an IceCube neutrino search
by GCN Circulars 18 Sep '25

18 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41867 SUBJECT: LIGO/Virgo/KAGRA S250918aw: 1 counterpart neutrino candidate event from an IceCube neutrino search DATE: 25/09/18 13:48:08 GMT FROM: Zsuzsa Marka at IceCube/Columbia University <zsuzsa(a)astro.columbia.edu> IceCube Collaboration (http://icecube.wisc.edu/) reports: We have performed a search for track-like muon neutrino candidate events detected by IceCube consistent with the sky localization of the low-significance gravitational-wave candidate event S250918aw in a time range of 1000 seconds centered on the alert event time (2025-09-18 07:55:35 UTC to 2025-09-18 08:12:15 UTC) [1,2]. During this time period, IceCube was collecting good quality data. A single hypothesis test was conducted using a Bayesian approach to quantify the joint GW + neutrino event significance, which assumes a binary merger scenario and accounts for known astrophysical priors, such as GW source distance, in the statistical significance estimation [3]. One track-like event was found in spatial and temporal coincidence with the gravitational-wave candidate S250918aw calculated from the map circulated in the S250918aw-2-Preliminary notice. This represents an overall p-value of 0.00023 for the Bayesian search. The p-value measures the consistency of the observed track-like events with the known atmospheric backgrounds for this single map (not accounting for statistical trials from multiple GW events). Further details are available at https://gcn.nasa.gov/missions/icecube and at https://roc.icecube.wisc.edu/public/LvkNuTrackSearch. Properties of the coincident event are shown below: dt(s) RA(deg) Dec(deg) Angular uncertainty(deg) p-value(Bayesian) ------------------------------------------------------------------------------------- -74.35 159.8 44.3 0.66 0.00023 where: dt = Time of track event minus time of GW trigger (sec) Angular uncertainty = Angular uncertainty of track event: the radius of a circle representing 90% CL containment by area. p-value = the individual p-value for the specific track event from this search. The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector operating at the geographic South Pole, Antarctica. The IceCube realtime alert point of contact can be reached at roc(a)icecube.wisc.edu [1] M. G. Aartsen et al 2020 ApJL 898 L10 [2] Abbasi et al. Astrophys.J. 944 (2023) 1, 80 [3] I. Bartos et al. 2019 Phys. Rev. D 100, 083017 View this GCN Circular online at https://gcn.nasa.gov/circulars/41867. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39932] GRB 250918A: BALROG localization (Fermi Trigger 779891543 / GRB 250918522)
by GCN Circulars 18 Sep '25

18 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41866 SUBJECT: GRB 250918A: BALROG localization (Fermi Trigger 779891543 / GRB 250918522) DATE: 25/09/18 13:33:35 GMT FROM: Jochen Greiner at MPE <jcgrog(a)mpe.mpg.de> T. Preis (University of Innsbruck) & J. Greiner (MPE Garching) report: The public trigdat data of the Fermi Gamma-ray Burst Monitor (GBM) trigger 779891543 at 12:32:18 on 18 Sept. 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) = 225.6 deg Decl.(2000.0) = -16.2 deg The 1 sigma statistical error radius is 4.1 deg. We estimate an additional systematic error of 2 deg. Further details are available at: https://grb.mpe.mpg.de/grb/GRB250918522/ The Healpix map can be downloaded from: https://grb.mpe.mpg.de/grb/GRB250918522/healpix The location parameters are available as JSON at: https://grb.mpe.mpg.de/grb/GRB250918522/json View this GCN Circular online at https://gcn.nasa.gov/circulars/41866. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39931] GRB 250918A: Fermi GBM Final Real-time Localization
by GCN Circulars 18 Sep '25

18 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41865 SUBJECT: GRB 250918A: Fermi GBM Final Real-time Localization DATE: 25/09/18 12:42:57 GMT FROM: Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply(a)GIOC.nsstc.nasa.gov> The Fermi GBM team reports the detection of a likely LONG GRB At 12:32:18 UT on 18 Sep 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250918A (trigger 779891543.0931 / 250918522). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 221.8, Dec = -15.9 (J2000 degrees, equivalent to J2000 14h 47m, -15d 54'), with a statistical uncertainty of 3.4 degrees. The angle from the Fermi LAT boresight is 93.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250918522/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250918522/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250918522/… View this GCN Circular online at https://gcn.nasa.gov/circulars/41865. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39930] GRB 250917B: Swift/UVOT Upper Limits
by GCN Circulars 18 Sep '25

18 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41864 SUBJECT: GRB 250917B: Swift/UVOT Upper Limits DATE: 25/09/18 11:35:38 GMT FROM: Alice Breeveld at MSSL-UCL <a.breeveld(a)ucl.ac.uk> A.A. Breeveld (UCL-MSSL) and R. Caputo (GSFC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 250917B 139 s after the Swift/BAT trigger (Caputo et al., GCN Circ. 41848) and the Fermi GBM trigger (Fermi GBM Team, GCN 41846). No optical afterglow consistent sources 1, 5 or 6 identified by the Swift-XRT (Kennea et al., GCN Circ. 41851) is detected in the initial UVOT exposures. Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white_FC 139 289 147 >20.5 u_FC 351 601 246 >19.9 white 139 5391 492 >21.2 v 681 1059 39 >18.1 b 606 5225 236 >20.1 u 351 5020 462 >20.1 w1 730 4815 216 >18.9 m2 705 1084 39 >18.8 w2 831 1035 39 >19.0 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 1.983 in the direction of the burst (Schlegel et al. 1998). View this GCN Circular online at https://gcn.nasa.gov/circulars/41864. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39929] GRB 250916A: OSIRIS+/GTC redshift z = 2.015
by GCN Circulars 18 Sep '25

18 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41863 SUBJECT: GRB 250916A: OSIRIS+/GTC redshift z = 2.015 DATE: 25/09/18 10:56:55 GMT FROM: Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo(a)gmail.com> A. de Ugarte Postigo (LAM), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), J. F. Agui Fernandez (CAHA), C. C. Thoene (AbAO), M. A. Aloy (UV), J. P. U. Fynbo (DAWN/NBI), L. Galbany (IEEC-CSIC), S. Geier (GTC), L. Izzo (INAF/OACN and DARK/NBI), G. Lombardi (GTC), N. A. Rakotondrainibe (LAM), B. Schneider (LAM), N. R. Tanvir (Univ. Leicester), F. Pérez Toledo (GTC) and D. González Gonzalez (GTC) report: We observed the optical afterglow (Belkin et al., GCN 41847) of GRB 250916A (Fermi GBM team, GCN 41839) with OSIRIS+, mounted on the 10.4 m GTC telescope, at Roque de los Muchachos Observatory, on the island of La Palma (Spain). The observation consisted of an r-band acquisition image followed by 3x900 s of spectroscopy using grism R1000B, with a spectral coverage between 3600 and 7800 AA and a resolving power of ~600. The observation started at 2025-09-18 04:15:12.011 UT (1.61 days after the burst onset). In the acquisition image we detect the afterglow at r = 20.0 +/- 0.1 mag (AB), as compared to Pan-STARRS field stars and not corrected by Galactic extinction. The spectrum shows a clear continuum over the entire spectral range. The spectrum continuum shows a moderate curvature, which could be indicative of host galaxy extinction. We detect multiple spectral features that correspond to Lyman alpha, S II, Si II, Si II*, O I, C II, Ni II, Ni II*, Si IV, C IV, Fe II, Fe II*, Al II, Al III, Cr II, Zn II, and Mn II, all at a common redshift of 2.015, which we identify as the redshift of the GRB. We also note the presence of at least one intervening system with C IV features at z = 1.853. View this GCN Circular online at https://gcn.nasa.gov/circulars/41863. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39928] GRB 250916A: Swift-XRT observations
by GCN Circulars 18 Sep '25

18 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41862 SUBJECT: GRB 250916A: Swift-XRT observations DATE: 25/09/18 07:58:29 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), C. Salvaggio (INAF-OAB), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB), 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/GBM-detected burst GRB 250916A, collecting 1.6 ks of Photon Counting (PC) mode data between T0+101.9 ks and T0+107.2 ks. One uncatalogued X-ray source has been detected within the estimated 3-sigma GOTO error region (0.8 arcsec) given in GCN Circ. 41847, it is below the RASS limit and shows no definitive signs of fading. However, given its coincident location with the fading optical source, it is indeed likely to be the afterglow. Details of this source are given below: Source 1: RA (J2000.0): 26.5708 = 01h 46m 16.99s Dec (J2000.0): +36.1652 = +36d 09' 54.6" Error: 3.7 arcsec (radius, 90% conf. [Enhanced position]) Count-rate: 0.0274 +/- 0.0049 ct s^-1 Distance: 2 arcsec from GOTO position. Flux: (9.2 +/- 1.6)e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV) An uncatalogued source was also detected, however this was too far from the GRB position to be the afterglow. 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/00021859. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41862. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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