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[vsnet-grb-info 40085] LIGO/Virgo/KAGRA S250927ck: Identification of a GW compact binary merger candidate
by GCN Circulars 27 Sep '25

27 Sep '25

TITLE: GCN CIRCULAR NUMBER: 42015 SUBJECT: LIGO/Virgo/KAGRA S250927ck: Identification of a GW compact binary merger candidate DATE: 25/09/27 16:38:19 GMT FROM: avanivikrambhai.patel(a)ligo.org The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We identified the compact binary merger candidate S250927ck during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-09-27 15:24:48.939 UTC (GPS time: 1443021906.939). The candidate was found by the Aframe [1], cWB BBH [2], and GstLAL [3] analysis pipelines. S250927ck is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.1e-08 Hz, or about one in 1 year, 3 days. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S250927ck The classification of the GW signal, in order of descending probability, is BBH (98%), Terrestrial (2%), NSBH (<1%), or BNS (<1%). Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [4] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [4] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%. The source chirp mass falls with highest probability in the bin (44.0, 88.0) solar masses, assuming the candidate is astrophysical in origin. Two sky maps are available at this time and can be retrieved from the GraceDB event page: * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [5], distributed via GCN and SCiMMA notices about 26 seconds after the candidate event time. * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [5], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time. The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is 177 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 4333 +/- 1544 Mpc (a posteriori mean +/- standard deviation). For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/. [1] Marx et al. PRD 111, 042010 (2025) doi:10.1103/PhysRevD.111.042010 [2] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018 [3] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008 [4] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe [5] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013 View this GCN Circular online at https://gcn.nasa.gov/circulars/42015. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40084] IceCube-250926A: Upper limits from a search for additional neutrino events in IceCube
by GCN Circulars 27 Sep '25

27 Sep '25

TITLE: GCN CIRCULAR NUMBER: 42014 SUBJECT: IceCube-250926A: Upper limits from a search for additional neutrino events in IceCube DATE: 25/09/27 16:01:16 GMT FROM: Yuhua Yao at IceCube/UW-Madison <yyao255(a)icecube.wisc.edu> The IceCube Collaboration (http://icecube.wisc.edu/) reports: IceCube has performed a search [1] for additional track-like muon neutrino events arriving from the direction of IceCube-250926A (https://gcn.nasa.gov/circulars/42003) in a time range of 1000 seconds centered on the alert event time (2025-09-26 12:20:35.170 UTC to 2025-09-26 12:37:15.170 UTC) during which IceCube was collecting good quality data. Excluding the event that prompted the alert, zero track-like events are found within the 90% containment region of IceCube-250926A. We report a p-value of 1.00 in this time window. IceCube’s sensitivity to neutrino point sources with an E^-2.5 spectrum, expressed as E^2 dN/dE evaluated at 1 TeV, is 1.3e-01 GeV cm^-2 within the 90% spatial containment region of IceCube-250926A in a 1000 second time window. 90% of events IceCube would detect from a source at this declination with an E^-2.5 spectrum have energies in the approximate energy range between 2e+02 GeV and 1e+05 GeV. A subsequent search was performed including 2 days of data centered on the alert event time (2025-09-25 12:28:55.170 UTC to 2025-09-27 12:28:55.170 UTC). In this case, we report a p-value of 1.00, consistent with no significant excess of track events. IceCube’s sensitivity to neutrino point sources with an E^-2.5 spectrum, expressed as E^2 dN/dE evaluated at 1 TeV, is 1.5e-01 GeV cm^-2 within the 90% spatial containment region of IceCube-250926A in a 2 day time window. 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] IceCube Collaboration, R. Abbasi et al., ApJ 910 4 (2021) View this GCN Circular online at https://gcn.nasa.gov/circulars/42014. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40083] GRB 250926A: Insight-HXMT detection
by GCN Circulars 27 Sep '25

27 Sep '25

TITLE: GCN CIRCULAR NUMBER: 42013 SUBJECT: GRB 250926A: Insight-HXMT detection DATE: 25/09/27 15:01:50 GMT FROM: Chenwei Wang at IHEP <cwwang(a)ihep.ac.cn> Chen-Wei Wang, Cheng-Kui Li, Shao-Lin Xiong, and Chao Zheng report on behalf of the Insight-HXMT team: At 2025-09-26T00:21:24.600 (T0), Insight-HXMT/HE was trigger by the burst GRB 250926A, which is also detected by Fermi/GBM (Fermi GBM Team, GCN#41993) and AstroSat (A. Goyal et al., GCN#42008). The Insight-HXMT/HE light curve mainly consists of a precursor followed by a main pulse with a T90 of 2.1 +2.3/-0.7 s. The 1s peak rate, measured from T0-0.050 s, is 3838 cnts/sec. The total counts from this burst is 4746 counts. The HXMT/HE light curve can be found here: https://www.bursthub.cn//admin/static/hxmtgrb250926A.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. View this GCN Circular online at https://gcn.nasa.gov/circulars/42013. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40082] GRB 250926B: Fermi GBM Final Localization
by GCN Circulars 26 Sep '25

26 Sep '25

TITLE: GCN CIRCULAR NUMBER: 42012 SUBJECT: GRB 250926B: Fermi GBM Final Localization DATE: 25/09/26 21:05:58 GMT FROM: Matt Godwin <msg0028(a)uah.edu> The Fermi GBM team reports the detection of a likely LONG GRB "At 19:50:31.07 UT on 26 September 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250926B (trigger 780609036/250926827). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 291.66, Dec = -80.93 (J2000 degrees, equivalent to J2000 19h 26m, -80d 55'), with a statistical uncertainty of 7.60 degrees. The angle from the Fermi LAT boresight is 23 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250926827/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250926827/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250926827/…" View this GCN Circular online at https://gcn.nasa.gov/circulars/42012. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40081] GRB 250926B: Fermi GBM Final Localization
by GCN Circulars 26 Sep '25

26 Sep '25

TITLE: GCN CIRCULAR NUMBER: 42012 SUBJECT: GRB 250926B: Fermi GBM Final Localization DATE: 25/09/26 21:05:58 GMT FROM: Matt Godwin <msg0028(a)uah.edu> The Fermi GBM team reports the detection of a likely LONG GRB "At 19:50:31.07 UT on 26 September 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250926B (trigger 780609036/250926827). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 291.66, Dec = -80.93 (J2000 degrees, equivalent to J2000 19h 26m, -80d 55'), with a statistical uncertainty of 7.60 degrees. The angle from the Fermi LAT boresight is 23 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250926827/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250926827/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250926827/…" View this GCN Circular online at https://gcn.nasa.gov/circulars/42012. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40080] Fermi trigger No 780609036: Global MASTER-Net observations report
by GCN Circulars 26 Sep '25

26 Sep '25

TITLE: GCN CIRCULAR NUMBER: 42011 SUBJECT: Fermi trigger No 780609036: Global MASTER-Net observations report DATE: 25/09/26 20:30:38 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> 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-SAAO robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in South Africa (South African Astronomical Observatory) started inspect of the Fermi GRB250926.83 (trigger No 780609036,19h 26m 38.40s , -80d 55m 48.0s, R=7.6) errorbox 112 sec after notice time and 144 sec after trigger time at 2025-09-26 19:52:55 UT, with upper limit up to 18.2 mag. The observations began at zenith distance = 54 deg. The sun altitude is -40.0 deg. The galactic latitude b = -29 deg., longitude l = 313 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3000539 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 160 | 2025-09-26 19:52:55 | MASTER-SAAO | (00h 31m 12.67s , -84d 57m 34.6s) | C | 30 | 16.7 | 205 | 2025-09-26 19:52:55 | MASTER-SAAO | (00h 31m 12.74s , -84d 57m 34.5s) | C | 120 | 17.8 | Coadd 160 | 2025-09-26 19:52:55 | MASTER-SAAO | (00h 52m 18.08s , -85d 11m 39.8s) | C | 30 | 17.1 | 205 | 2025-09-26 19:52:55 | MASTER-SAAO | (00h 52m 18.11s , -85d 11m 39.9s) | C | 120 | 18.2 | Coadd 214 | 2025-09-26 19:53:44 | MASTER-SAAO | (00h 31m 11.30s , -84d 57m 37.0s) | C | 40 | 17.0 | 214 | 2025-09-26 19:53:44 | MASTER-SAAO | (00h 52m 17.26s , -85d 11m 41.8s) | C | 40 | 17.4 | 278 | 2025-09-26 19:54:43 | MASTER-SAAO | (00h 31m 10.47s , -84d 57m 38.0s) | C | 50 | 17.4 | 278 | 2025-09-26 19:54:43 | MASTER-SAAO | (00h 52m 16.41s , -85d 11m 42.8s) | C | 50 | 17.8 | 352 | 2025-09-26 19:55:52 | MASTER-SAAO | (00h 31m 09.08s , -84d 57m 36.9s) | C | 60 | 17.1 | 352 | 2025-09-26 19:55:53 | MASTER-SAAO | (00h 52m 14.97s , -85d 11m 43.7s) | C | 60 | 17.6 | 1541 | 2025-09-26 20:15:41 | MASTER-SAAO | (19h 47m 45.38s , -84d 30m 55.3s) | C | 60 | 14.1 | 1541 | 2025-09-26 20:15:41 | MASTER-SAAO | (19h 29m 45.92s , -84d 14m 40.6s) | C | 60 | 15.1 | 1796 | 2025-09-26 20:19:57 | MASTER-SAAO | (16h 56m 37.37s , -84d 03m 27.9s) | C | 60 | 17.4 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/42011. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40079] GRB 250920C: PRIME near-infrared detection
by GCN Circulars 26 Sep '25

26 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41943 SUBJECT: GRB 250920C: PRIME near-infrared detection DATE: 25/09/22 20:28:38 GMT FROM: O. Guiffreda at UMD <oriogui(a)umd.edu> O. Guiffreda (UMD), J. Durbak (UMD), A. S. Kutyrev (NASA/GSFC), N. Passaleva (U Rome), M. El Kabir (U Rome), E. Troja (U Rome), S. B. Cenko (NASA/GSFC) The following correction is primarily the result of an error in Vega to AB magnitude conversion in J and H filters. Following the Fermi GBM (GCN 41903) and Swift BAT (GCN 41904), we observed the transient field in Z, Y, J and H filters with PRIME ~7 hours after the initial Fermi and Swift detection. At the position of the optical counterpart reported by GOTO (GCN 41907), we detect an uncatalogued source in all aforementioned bands. Using nearby 2MASS and PanSTARRS stars for preliminary calibration we derive the following magnitudes and limits, not corrected for Galactic extinction: |Filter | Mag(AB) | |-------|---------------| |Z | 19.8 +/- 0.10 | |Y | 19.7 +/- 0.07 | |J | 19.8 +/- 0.10 | |H | 19.4 +/- 0.07 | PRIME is a 1.8m telescope with 1.56 square degree FOV (0.5 arcsec/pixel) located in Sutherland, South Africa at the South African Astronomical Observatory (SAAO) (Kutyrev et al. 2023, Yama et al. 2023, Durbak et al. 2024). We thank the Osaka University observers at PRIME and the staff at SAAO for their support with these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41943. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40078] GRB 250925A: Mondy and AbAO optical observations
by GCN Circulars 26 Sep '25

26 Sep '25

TITLE: GCN CIRCULAR NUMBER: 42005 SUBJECT: GRB 250925A: Mondy and AbAO optical observations DATE: 25/09/26 15:27:34 GMT FROM: Nicolai Pankov at HSE, IKI RAS <colinsergesen(a)gmail.com> N. Pankov (HSE, IKI), A. Pozanenko (IKI), E. Klunko (ISTP), R. Ya. Inasaridze (AbAO) report on behalf of GRB-IKI-FuN: We observed the field of GRB 250925A detected by Swift (Dichiara et. al, GCN 41985) at z=3.899 (Castro-Tirado et. al, GCN 41997) with the AZT-33IK 1.5m telescope of the Sayan Solar Observatory (Mondy) and the AS-32 0.7m telescope of the Abastumani Observatory (AbAO). The observations started at Mondy on 2025-09-25 at 20:00 UT, i.e. 0.118 days since trigger. In total we obtained 30x120 s exposures with AZT-33IK, and 85x60 s exposures with AS-32. The optical afterglow initially detected by NOT/ORM (Corcoran et. al, GCN 41990) and also observed by (Saikia et. al, GCN 41986; Pavoni & Moretti et. al, GCN 41989; Bochenek & Perley, GCN 41991; Masi et. al, GCN 41992; Mendez et. al, GCN 41995; Castro-Tirado et. al, GCN 41997; Liu et. al, GCN 41998; Ochi et. al, GCN 41999; Xin et. al., GCN 42002; Rakotondrainibe et. al, GCN 42004) is clearly visible in the stacked images from Mondy. Preliminary photometry is as follows: Date UTstart Exptime t-T0 Filter OT Err. UL Site/Telescope (nxs) (mid, days) (3sigma) 2025-09-25 20:00:15 6x120 0.12626 R 22.3 0.2 23.1 Mondy/AZT-33IK 2025-09-25 20:12:15 6x120 0.13460 R 22.1 0.2 23.0 Mondy/AZT-33IK 2025-09-25 20:24:16 6x120 0.14294 R 22.1 0.2 23.1 Mondy/AZT-33IK 2025-09-25 20:36:16 6x120 0.15128 R 22.1 0.1 23.1 Mondy/AZT-33IK 2025-09-25 20:48:16 6x120 0.15962 R 22.6 0.2 23.1 Mondy/AZT-33IK 2025-09-25 19:24:44 79x60 0.12486 R n/d n/d 21.7 AbAO/AS-32 The photometry was calibrated against nearby USNO-B1.0 stars (R2-mags) and not corrected for the Galactic extinction. Using all available r-data we estimate a temporal slope of alpha = -1.17 +/- 0.05. View this GCN Circular online at https://gcn.nasa.gov/circulars/42005. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40077] IceCube Alert 250926.52: Global MASTER-Net observations report
by GCN Circulars 26 Sep '25

26 Sep '25

TITLE: GCN CIRCULAR NUMBER: 42010 SUBJECT: IceCube Alert 250926.52: Global MASTER-Net observations report DATE: 25/09/26 20:00:38 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> 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) was pointed to the IceCube Alert 250926.52 (trigger No 64842455,08h 44m 07.68s , +10d 06m 07.2s, R=0.99) errorbox 26203 sec after notice time and 26266 sec after trigger time at 2025-09-26 19:46:41 UT, with upper limit up to 19.0 mag. The observations began at zenith distance = 79 deg. The sun altitude is -29.0 deg. The galactic latitude b = 30 deg., longitude l = 217 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3000303 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 26297 | 2025-09-26 19:46:41 | MASTER-Tunka | (08h 41m 52.46s , +09d 47m 01.3s) | C | 60 | 19.0 | 26297 | 2025-09-26 19:46:41 | MASTER- | (08h 42m 05.85s , +09d 40m 54.7s) | C | 60 | 18.5 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/42010. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40076] GRB 250920C: ULL-ASTRO-MASTER early detection of the optical afterglow with LCO 40-cm telescope at Siding Spring Observatory
by GCN Circulars 26 Sep '25

26 Sep '25

TITLE: GCN CIRCULAR NUMBER: 42009 SUBJECT: GRB 250920C: ULL-ASTRO-MASTER early detection of the optical afterglow with LCO 40-cm telescope at Siding Spring Observatory DATE: 25/09/26 19:40:45 GMT FROM: Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf(a)iac.es> V. Ghiraldo, J. Basurto Merino, P.G. Berdayes, A. Caballero-Almagro, A. Cerón, M. Contreras, F. Díaz-Segado, T. Ferrer-Laviña, B. Gandolfi, J. Hernández Fung, L. Juliá-Maroto, E. Lekaroz-Urriza, M. Manzano García, E. Mejía-Martínez, J. Prieto Polo, M. Pulido-Torres, M. Quintana-Ansaldo, A. Schenone-Zanuzzi, A. Selezneva, T. Tundidor Rodríguez, E. Urquijo-Rodríguez (all ULL), M. Abdul-Masih (IAC and ULL), and I. Pérez-Fournon (IAC and ULL). We report on a Las Cumbres Observatory (LCO) 40-cm telescope early observation of the long-duration Gamma-Ray Burst GRB 250920C, detected by Fermi (Fermi GBM team, GCN circ. 41903; and Mailyan et al., GCN circ. 41917), Swift (Gupta et al., GCN circ. 41904; Kuin et al., GCN circ. 41920; Goad et al., GCN circ. 41922; Gupta et al., GCN circ. 41924; and D'Ai et al. GCN circ. 41935), SVOM (Wang et al., GCN circ. 41929), and Konus-Wind (Panteleeva et al., GCN circ. 41957). We observed the field of GRB 250920C with one of the two LCO 40-cm telescopes (Planewave Delta Rho 350 telescopes equipped with CMOS QHY600 cameras) located at the LCO node at Siding Spring Observatory near Coonabarabran, New South Wales, Australia. The observation, a single exposure of 300 sec in the SDSS r filter, started on 2025-09-20 at 17:21:51 UT, about 1.94 hours after the Fermi and Swift trigger. The optical and near-infrared counterpart reported first by Wortley et al. using GOTO (GCN circ. 41907) and with observations by several other groups (GCN circ. 41911, GCN circ. 41913, GCN circ. 41916, GCN circ. 41920, GCN circ. 41925, GCN circ. 41928, GCN circ. 41943, GCN circ. 41947, GCN circ. 41951, GCN circ. 41952, GCN circ. 41955, GCN circ. 41956, GCN circ. 41965, and GCN circ. 41967) is clearly detected in our image, with a magnitude of r = 18.77 +/- 0.10, calibrated against PanSTARRS-1 DR2 and not corrected for galactic extinction, that is consistent with another reported r-band detection at a similar epoch. A redshift of z = 1.399 - 1.40 has been measured by Sánchez-Ramírez et al. (GCN circ. 41928), de Wet et al. (GCN circ. 41952), and Izzo et al. (GCN circ. 41955). Based on observations made with the Las Cumbres Observatory’s education network telescopes that were upgraded through generous support from the Gordon and Betty Moore Foundation (program IAC2025B-010). These observations are part of a course in Astrophysical Techniques of the Master in Astrophysics of the Astrophysics Department of the University of La Laguna in collaboration with the Instituto de Astrofísica de Canarias (Tenerife, Canary Islands, Spain). View this GCN Circular online at https://gcn.nasa.gov/circulars/42009. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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