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

[vsnet-grb-info 40504] GRB 251024A: Swift detection of a burst
by GCN Circulars 24 Oct '25

24 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42426 SUBJECT: GRB 251024A: Swift detection of a burst DATE: 25/10/24 18:09:58 GMT FROM: K.L. Page at U Leicester <klp5(a)leicester.ac.uk> M. J. Moss (GSFC), S. Dichiara (PSU), R. A. J. Eyles-Ferris (U Leicester), R. Gupta (NASA GSFC), J. A. Kennea (PSU), A. Y. Lien (U Tampa), K. L. Page (U Leicester) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 17:47:02.25 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 251024A (trigger= 1406589). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 306.622, -68.574 which is RA(J2000) = 20h 26m 29s Dec(J2000) = -68d 34' 25" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with a duration of about 30 sec. The peak count rate was ~1000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 17:49:11.3 UT, 129.1 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 306.59743, -68.57225 which is equivalent to: RA(J2000) = 20h 26m 23.38s Dec(J2000) = -68d 34' 20.1" with an uncertainty of 1.9 arcseconds (radius, 90% containment). This location is 32 arcseconds from the BAT onboard position, within the BAT error circle. This position may be improved as more data are received; the latest position is available at https://www.swift.ac.uk/sper. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (3.73 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 3 (+2.09/-1.77) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 132 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.044. Burst Advocate for this burst is M. J. Moss (mikejmoss3 AT gmail.com). Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/) View this GCN Circular online at https://gcn.nasa.gov/circulars/42426. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40503] Swift GRB251024.74: Global MASTER-Net observations report
by GCN Circulars 24 Oct '25

24 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42425 SUBJECT: Swift GRB251024.74: Global MASTER-Net observations report DATE: 25/10/24 17:54:39 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) was pointed to the Swift GRB251024.74 (trigger No 1406589,20h 26m 29.26s , -68d 34m 26.4s, R=0.05) errorbox 10 sec after notice time and 73 sec after trigger time at 2025-10-24 17:48:15 UT, with upper limit up to 16.5 mag. Observations started at twilight. The observations began at zenith distance = 38 deg. The sun altitude is -11.1 deg. The galactic latitude b = -34 deg., longitude l = 327 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3022698 We obtain a following upper limits. Tmid-T0 | Site |Filt.| Expt. | Limit| Comment _________|_____________________|_____|_______|______|________ 78 | MASTER-SAAO | P\ | 10 | 16.2 | 78 | MASTER-SAAO | C | 10 | 16.1 | 112 | MASTER-SAAO | P\ | 20 | 16.5 | 112 | MASTER-SAAO | C | 20 | 16.3 | 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/42425. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40502] GRB 251023B / EP251023b: COLIBRÍ Confirmation of the Fading Nature of the Optical Candidate
by GCN Circulars 24 Oct '25

24 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42424 SUBJECT: GRB 251023B / EP251023b: COLIBRÍ Confirmation of the Fading Nature of the Optical Candidate DATE: 25/10/24 16:50:02 GMT FROM: Rosa Leticia Becerra Godínez at Instituto de Astronomía, UNAM <rbecerra(a)astro.unam.mx> Enrique Moreno Méndez (UNAM), Nikos Mandarakas (LAM), Alan M. Watson (UNAM), Rosa L. Becerra (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM): We reimaged the field of the GRB 251023B / EP251022b (Kang et al., GCN Circ. 42392; Nakahira et al., GCN Circ. 42402; Wu et al., GCN Circ. 42415; Cheung et al., GCN Circ. 42407), using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-10-24 07:06 to 08:12 UTC (from 22.3 to 23.4 hours after the trigger) and obtained 16 minutes of exposure in each of the r and i filters and 32 minutes of exposure in the z filter. The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. In our second epoch, we do not detect the optical candidate reported by Mandarakas et al. (GCN Circ. 42404) down to the following 3-sigma limits: r > 22.8 i > 22.4 z > 22.0 These values are consistent with the non-detection reported by Adami et al. (GCN Circ. 42421) and confirm the fading nature of the source, supporting its association with GRB 251023B / EP251022b. We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams. COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico. View this GCN Circular online at https://gcn.nasa.gov/circulars/42424. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40501] GRB 251022A: COLIBRÍ Further Multi-frequency Optical Observations
by GCN Circulars 24 Oct '25

24 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42423 SUBJECT: GRB 251022A: COLIBRÍ Further Multi-frequency Optical Observations DATE: 25/10/24 15:05:07 GMT FROM: Rosa Leticia Becerra Godínez at Instituto de Astronomía, UNAM <rbecerra(a)astro.unam.mx> Enrique Moreno Méndez (UNAM), Nikos Mandarakas (LAM), Alan M. Watson (UNAM), Rosa L. Becerra (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM): We reobserved the field of the Fermi GRB 251022A (Fermi GBM Team, GCN Circ. 42380; Di Lalla et al., GCN Circ. 42384) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-10-24 07:25 to 08:30 UTC (from 32.8 to 33.9 hours after the trigger) and obtained 10 minutes of exposure in each of the g, r, i, and z filters and 20 minutes of exposure in the y filter. The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. In our second epoch, we still detect the optical counterpart firstly reported by GOTO (O’Neill et al. GCN Circ. 42386) and subsequently followed-up by DDOTI (Becerra et al. GCN Circ. 42394), MASTER (Lipunov et al. GCN Circ. 42395), COLIBRÍ (Mandarakas et al. GCN Circ. 42397), and GROWTH (Mohan et al. GCN Circ. 42413) at preliminary magnitudes and upper limits of: g = 22.52 +/- 0.23 r = 22.05 +/- 0.20 i = 21.99 +/- 0.31 z = 21.95 +/- 0.65 y > 20.7 (3-sigma) Compared with the values reported in our first epoch (Mandarakas et al. GCN Circ. 42397), we estimate a temporal decay index of ~-2, consistent with a jet break. Our detection in g suggests that this is not a high-redshift GRB, but instead has z < 3. We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams. COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico. View this GCN Circular online at https://gcn.nasa.gov/circulars/42423. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40500] EP251023a: MAO/AZT-22 optical observations
by GCN Circulars 24 Oct '25

24 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42422 SUBJECT: EP251023a: MAO/AZT-22 optical observations DATE: 25/10/24 15:02:32 GMT FROM: Yodgor Rajabov at UBAI <rajabov(a)astrin.uz> Y. Rajabov, O. Burkhonov, B.Abidkhanov, S. Ehgamberdiev, Y. Tillayev (UBAI), T. Boyqobilov, A. Shaymanov (Maidanak Observatory/UBAI) report on behalf of UBAI team. We observed the field of EP transient EP251023a (Wu et al., GCN 42388), with the AZT-22 1.5m telescope of the Maidanak Observatory (MAO) starting on 2025-10-23 at 22:52:18 UT, i.e. ~10.37 hours since trigger. In total we obtained 6x300 s exposures in the R-band using 4kx4k CCD SNUCAM camera (Im et al., 2010). The optical afterglow initially detected by GOTO (Wortley et al., GCN 42387) and also observed (Li et al., GCN 42389; Liu et al., GCN 42391; Mandarakas et al., GCN 42400; Lipunov et al., GCN 42405; Selezneva et al., GCN 42406; Mohan et al., GCN 42411; Rakotondrainibe et al., GCN 42420) is detected in our individual and stacked frames. Preliminary photometry is as follows: Date UTstart Exptime t-T0 Filter OT Err. UL Site/Telescope (nxs) (mid, days) 2025-10-23 23:08:31 6x300 0.85928 R 20.56 0.03 22.99 MAO/AZT-22 All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained in Johnson Cousins filters were calibrated using the Gaia DR3 Synphot 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/42422. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40499] GRB 251023B: OHP/T193 optical observations
by GCN Circulars 24 Oct '25

24 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42421 SUBJECT: GRB 251023B: OHP/T193 optical observations DATE: 25/10/24 13:26:31 GMT FROM: Ny Avo Rakotondrainibe at LAM <nyavo.rakotobe(a)gmail.com> C. Adami (LAM/Pytheas/AMU), N.A. Rakotondrainibe (LAM), E. Le Floc'h (CEA/Irfu), report on behalf of the MISTRAL GRB collaboration: We carried out observations of GRB 251023B (Cheung et al., GCN 42407; Mandarakas et al., GCN 42404; Nakahira et al., GCN 42402; Evans et al., GCN 42401; Kang et al., GCN 42392) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. We obtained 6 exposures of 15 min in the r-band at a midtime of 2025 10 24 01:51:23 UT, corresponding to T-T0 = 17.04 hours. We do not significantly detect the optical source identified by Mandarakas et al. (GCN 42404) at the 3-sigma upper limit of: r’ < 21.78 mag (AB) The photometric calibration was performed using nearby stars from the PanSTARRS catalog and the magnitudes are not corrected for Galactic extinction. We used the STDWeb/STDPipe tools (Karpov 2025). We acknowledge the excellent support from the Observatoire de Haute-Provence and in particular Jean Balcaen. View this GCN Circular online at https://gcn.nasa.gov/circulars/42421. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40498] EP251023a: OHP/T193 optical observations
by GCN Circulars 24 Oct '25

24 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42420 SUBJECT: EP251023a: OHP/T193 optical observations DATE: 25/10/24 13:25:17 GMT FROM: Ny Avo Rakotondrainibe at LAM <nyavo.rakotobe(a)gmail.com> N.A. Rakotondrainibe (LAM), C. Adami (LAM/Pytheas/AMU), E. Le Floc'h (CEA/Irfu), report on behalf of the MISTRAL GRB collaboration: We carried out observations of EP251023a (Mohan et al., GCN 42411; Selezneva et al., GCN 42406; Lipunov et al., GCN 42405; Mandarakas et al., GCN 42400; Liu et al., GCN 42391; Li et al., GCN 42389; Wu et al., GCN 42388; Wortley et al., GCN 42387) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. We obtained 4 exposures of 15 min in the r-band at a midtime of 2025 10 24 03:17:52UT corresponding to T-T0 = 24.78 hours. The afterglow is well detected in r’ and we measured the following preliminary magnitude: r’ = 21.55+/-0.10 mag (AB) The photometric calibration was performed using nearby stars from the PanSTARRS catalog and the magnitudes are not corrected for Galactic extinction. We used the STDWeb/STDPipe tools (Karpov 2025). We acknowledge the excellent support from the Observatoire de Haute-Provence and in particular Jean Balcaen. View this GCN Circular online at https://gcn.nasa.gov/circulars/42420. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40497] GRB 251023C: AstroSat CZTI detection
by GCN Circulars 24 Oct '25

24 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42419 SUBJECT: GRB 251023C: AstroSat CZTI detection DATE: 25/10/24 12:15:05 GMT FROM: Anuraag Arya at IIT Bombay <aryaanuraag910(a)gmail.com> S. Salunke (IUCAA), M. Tembhurnikar (IUCAA), Harsha K. H. (IUCAA), A. Arya (IITB), A. Goyal (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,bright GRB 251023C which was also detected by SWIFT BAT (DeLaunay et. al., GCN Circ. 42412), and CALET (Asaoka et. al., GCN Circ. 42414). 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 2025-10-23 16:24:32.50 UTC. The measured peak count rate associated with the burst is 682 (+55, -51) counts/s above the background in the combined data of all quadrants, with a total of 4344 (+401, -229) counts. The local mean background count rate was 294 (+1, -7) counts/s. Using cumulative rates, we measure a T90 of 18.2 (+2.8, -1.6) s. The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2025-10-23 16:24:28.05 UTC. The measured peak count rate associated with the burst is 942 (+83, -89) counts/s above the background in the combined data of all quadrants, with a total of 5554 (+521, -528) counts. The local mean background count rate was 1676 (+7, -8) counts/s. We measure a T90 of 15.8 (+1.9, -6.2) 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 View this GCN Circular online at https://gcn.nasa.gov/circulars/42419. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40496] GRB 251023C: AstroSat CZTI detection
by GCN Circulars 24 Oct '25

24 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42419 SUBJECT: GRB 251023C: AstroSat CZTI detection DATE: 25/10/24 12:15:05 GMT FROM: Anuraag Arya at IIT Bombay <aryaanuraag910(a)gmail.com> S. Salunke (IUCAA), M. Tembhurnikar (IUCAA), Harsha K. H. (IUCAA), A. Arya (IITB), A. Goyal (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,bright GRB 251023C which was also detected by SWIFT BAT (DeLaunay et. al., GCN Circ. 42412), and CALET (Asaoka et. al., GCN Circ. 42414). 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 2025-10-23 16:24:32.50 UTC. The measured peak count rate associated with the burst is 682 (+55, -51) counts/s above the background in the combined data of all quadrants, with a total of 4344 (+401, -229) counts. The local mean background count rate was 294 (+1, -7) counts/s. Using cumulative rates, we measure a T90 of 18.2 (+2.8, -1.6) s. The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2025-10-23 16:24:28.05 UTC. The measured peak count rate associated with the burst is 942 (+83, -89) counts/s above the background in the combined data of all quadrants, with a total of 5554 (+521, -528) counts. The local mean background count rate was 1676 (+7, -8) counts/s. We measure a T90 of 15.8 (+1.9, -6.2) 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 View this GCN Circular online at https://gcn.nasa.gov/circulars/42419. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40495] EP251023b/GRB 251023B: JinShan optical observation
by GCN Circulars 24 Oct '25

24 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42418 SUBJECT: EP251023b/GRB 251023B: JinShan optical observation DATE: 25/10/24 08:38:09 GMT FROM: L. B. He at NAOC <helb(a)bao.ac.cn> L.B. He, J. An, S.Q. Jiang, X. Liu, Z.P. Zhu, D. Xu (NAOC), S.Y. Fu, A.D. Zhu, L. Lei, H.Z. Wu (HUST), J.Z. Liu (XAO) report on behalf of a large collaboration: We observed the field of EP251023b/GRB 251023B (Kang et al., GCN 42392; Nakahira et al., GCN 42402; Wu et al., GCN 42415) using the 100C telescope of the JinShan project, located at Altay, Xinjiang, China. Observations started at 2025-10-23 16:01:41 UT, i. e. 7.3 hours after the trigger. The previously reported optical counterpart (Mandarakas et al., GCN 42404) was detected in our stacked z-band frame with a magnitude z~21.4 at a mid-time 9.75 hrs post-burst. The magnitude was callibrated with nearby PanSTARRS1 dr2 catalog without the Galactic extinction correction. We acknowledge the excellent support from T.Q. Chen and J.F. Zhang for enabling these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/42418. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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