September 2025 - vsnet-grb-info - ooruri.kusastro.kyoto-u.ac.jp

[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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[vsnet-grb-info 39927] EP250916a: EP-FXT follow-up observation
by GCN Circulars 18 Sep '25

18 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41861 SUBJECT: EP250916a: EP-FXT follow-up observation DATE: 25/09/18 07:46:30 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> C. Y. Dai (NJU), Q. Y. Wu, D. Y. Li, H. W. Pan (NAO, CAS) on behalf of the Einstein Probe (EP) team: EP-FXT performed a follow-up observation of EP250916a (GCN 41841) at 2025-09-17 11:34:24 (UTC), about 32 hours after the WXT detection, with an exposure time of 3.1 ks. FXT detected a bright source within the WXT error circle at R.A. = 268.2379 and Dec. = –35.3222 (J2000), with a positional uncertainty of 10 arcseconds (radius, 90% confidence level, including both statistical and systematic errors). The fit with a power-law spectrum yields Nh = 3.1 × 10^21 cm^-2 and a photon index of 1.66 ± 0.02, with a 0.5–10 keV flux of 5.4 × 10^-10 erg/s/cm^2. Additionally, we note that this source keeps brightening in the WXT monitoring, with the flux rising from approximately 3 × 10^-11 erg/s/cm^2 at 2025-09-16 03:33:13 to about 2 × 10^-10 erg/s/cm^2 at 2025-09-17 17:54:10 (UTC). Further follow-up observations are encouraged to identify the nature of this source. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41861. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39926] GRB 250916A: MAO/AZT-22 optical observations
by GCN Circulars 18 Sep '25

18 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41860 SUBJECT: GRB 250916A: MAO/AZT-22 optical observations DATE: 25/09/18 05:09:34 GMT FROM: Yodgor Rajabov at UBAI <rajabov(a)astrin.uz> O. Burkhonov, Y. Rajabov, B.Abidkhanov, S. Eghamberdiev, Y. Tillayev (UBAI), A. Shaymanov (Maidanak Observatory/UBAI) report on behalf of UBAI team. 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 1.5-m at Maidanak Observatory telescope AZT-22 equipped with the 4kx4k CCD SNUCAM camera(Im et al., 2010). We obtained series of 300 sec. images in Bessel R filter on September 17, 18:35:18-19:02:20 UTC. The OT (Belkin et al., GCN 41847; Mohan et al., GCN 41858, Moskvitin et al. GCN 41859) is clearly detected in the individual and stacked frames with the brightness of R = 19.76 +/- 0.03 (t_mid - T0 = 1.2229 days = 29.4917 hours). All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained in Johnson Cousin filters were calibrated using the Gaia DR3 Synphot catalog. For image template subrtaction we used PanSTARRS DR2 catalog. The data has not corrected for the Galactic extinction. Maidanak astronomical observatory (MAO) is an observational facility of the Ulugh Beg Astronomical Institute (UBAI), Uzbekistan Academy of Sciences (http://maidanak.uz/) View this GCN Circular online at https://gcn.nasa.gov/circulars/41860. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39925] GRB 250916A: SAO RAS optical observations
by GCN Circulars 17 Sep '25

17 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41859 SUBJECT: GRB 250916A: SAO RAS optical observations DATE: 25/09/17 23:24:23 GMT FROM: Alexander Moskvitin at SAO RAS <mosk(a)sao.ru> A. Moskvitin, O. Spiridonova (SAO RAS), A. Pozanenko (IKI), A. Ghosh, S. Razzaque (CAPP, University of Johannesburg), Tao An and Yuanqi Liu (Shanghai Astronomical Observatory) 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 1-m SAO RAS telescope Zeiss-1000 equipped with CCD-photometer. We obtained series of 300 sec. images in BVRcIc bands on September 17, 20:21:44--21:42:59 UT. The OT (Belkin et al., GCN 41847; Mohan et al., GCN 41858) is clearly detected in the individual and stacked frames with the brightness of R = 19.60 +/- 0.02 (t_mid - T0 = 1.3059 days = 31.3419 hours). Preliminary photometry is based on the nearby objects from PanSTARRS (magnitudes converted with Lupton 2005 equations) and has not corrected for the Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41859. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39924] GRB 250916A : GIT optical observations
by GCN Circulars 17 Sep '25

17 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41858 SUBJECT: GRB 250916A : GIT optical observations DATE: 25/09/17 21:43:01 GMT FROM: V. Swain at IIT Bombay <vishwajeet.s(a)iitb.ac.in> T. Mohan, V. Swain, A.P. Saikia, V. Bhalerao (IITB), G.C. Anupama, S. Barway (IIA) and R. Norbu (IAO) report on behalf of the GIT team: We observed the field of Fermi GRB 250916A (Fermi GBM team, GCN 41839) with 0.7m GROWTH-India Telescope (GIT) in r' filter. We started the observation at 2025-09-17 17:34:08 UT, i.e., 28.08 hours after the Fermi GBM trigger. The source is detected at the position reported by GOTO (Belkin et al., GCN 41847) and photometry result follows as: | MJD (mid) | tmid-t0 (hours) | Filter | Exposure (s) | Mag (AB) | | ------------- | ----------- | ------ | ------------ | -------------- | | 60935.73411 | 28.13 | r' | 360 | 19.73 +- 0.06 | The source is fading and photometric result is consistent with Belkin et al., GCN 41847. More broadband observations are under way. The measurement is calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction. The GROWTH India Telescope (GIT; Kumar et al. 2022) is a 70-cm telescope with a 0.7-degree field of view, set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay (IITB) with funding from DST-SERB and IUSSTF. It is located at the Indian Astronomical Observatory (Hanle), operated by IIA. We acknowledge funding by the IITB alumni batch of 1994, which partially supports the operations of the telescope. Telescope technical details are available at https://sites.google.com/view/growthindia/. View this GCN Circular online at https://gcn.nasa.gov/circulars/41858. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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