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[vsnet-grb-info 40025] GRB 250920B: AbAO optical upper limits
by GCN Circulars 23 Sep '25

23 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41958 SUBJECT: GRB 250920B: AbAO optical upper limits DATE: 25/09/23 21:32:13 GMT FROM: Nicolai Pankov at HSE, IKI RAS <colinsergesen(a)gmail.com> N. Pankov (HSE, IKI), A. Pozanenko (IKI), A. Moskvitin (SAO RAS), R. Ya. Inasaridze (AbAO) report on behalf of GRB-IKI-FuN: We observed the field of a long GRB 250920B detected by Fermi/GBM (The Fermi GBM team GCN 41897), Swift (Gupta et. al, GCN 41898), with the AS-32 0.7m telescope of AbAO. We obtained two series of images in the R band on epochs 2025-09-22 and 2025-09-23 (~2.3 and ~3.3 days since trigger, respectively). The optical afterglow initially detected by Swift/UVOT (Gupta et. al, GCN 41898) and further observed by (WU et. al, GCN 41908; Hall et. al, GCN 41914; Pankov et. al, GCN 41931; Reguitti et. al, GCN 41932; Moskvitin et. al, GCN 41948; Pankov et. al, GCN 41953) is not detected in the stacked images of both epochs. Preliminary upper limits are as follows: Date UTstart Exptime t-T0 Filter OT Err. UL (nxs) (mid, days) (3sigma) 2025-09-22 16:09:15 20x2+97x30 2.32458 R n/d n/d 20.3 2025-09-23 16:04:34 153*30 3.33076 R n/d n/d 20.3 The photometry was calibrated against nearby PS1 DR2 stars (R-mags obtained via Lupton 2005 transformations) and not corrected for the Galactic extinction. Ref. stars PS1 DR2 (Lupton 2005) RA Dec R R_err 187.24325 47.82521 15.134 0.007 187.41433 47.78151 14.974 0.008 View this GCN Circular online at https://gcn.nasa.gov/circulars/41958. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40024] Konus-Wind detection of GRB 250920C
by GCN Circulars 23 Sep '25

23 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41957 SUBJECT: Konus-Wind detection of GRB 250920C DATE: 25/09/23 19:12:38 GMT FROM: Valentina Panteleeva at Ioffe Institute <panteleevav228(a)gmail.com> V. Panteleeva, D. Frederiks, A. Lysenko, A. Ridnaia, D. Svinkin, A. Tsvetkova, M. Ulanov, and T. Cline, on behalf of the Konus-Wind team, report: The long-duration GRB 250920C (Fermi-GBM detection: The Fermi GBM team, GCN 41903; Mailyan & Meegan, GCN 41917; Swift-BAT detection: Gupta et al., GCN 41904; Gupta et al., GCN 41924; SVOM-GRM observaion: Wang et al., GCN 41929) triggered Konus-Wind at T0=55519.312 s UT (15:25:19.312). The burst light curve shows a multipeaked structure which starts at ~T0-0.4 s and has a total duration of ~41 s. The emission is seen up to ~2 MeV. The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB250920_T55519/ As observed by Konus-Wind, the burst had the total fluence of 2.91(-0.21,+0.29)x10^-5 erg/cm2, and the 64-ms peak flux, measured from T0+25.904 s, of 8.27(-0.78,+0.78)x10^-6 erg/cm2/s (both in the 20 keV - 10 MeV energy range). The time-averaged spectrum of the burst (measured from T0 to T0+41.216 s) is best fit in the 20 keV - 10 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -1.22(-0.07,+0.09), the high energy photon index beta = -3.19(-6.81,+0.51), the peak energy Ep = 160(-12,+9) keV (chi2 = 104/97 dof). The spectrum near the maximum count rate (measured from T0+24.832 to T0+33.024 s) is best fit in the 20 keV - 10 MeV range by a power law with exponential cutoff model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha = -1.14(-0.07,+0.07) and Ep = 220(-14,+16) keV (chi2 = 96/81 dof). Fitting by a GRB (Band) model yields the same alpha and Ep, and an upper limit on the high energy photon index: beta < -3.15 (chi2 = 97/80 dof). Assuming the redshift z=1.399 (Sanchez-Ramirez et al., GCN 41928; Izzo et al., GCN 41955) and a standard cosmology with H_0 = 67.3 km/s/Mpc, Omega_M = 0.315, and Omega_Lambda = 0.685 (Planck Collaboration, 2014), we estimate the following rest-frame parameters: the isotropic energy release E_iso is 1.54(-0.11,+0.16)x10^53 erg, the peak luminosity L_iso is 1.05(-0.10,+0.10)x10^53 erg/s, the rest-frame peak energy of the time-averaged spectrum Ep,i,z is 383(-30,+22) keV and the spectrum near the maximum count rate Ep,p,z is 528(-33,+38) keV. With the obtained estimates, GRB 250920C is inside 68% prediction bands for both the 'Amati' and the 'Yonetoku' relations derived for the sample of >300 long KW GRBs with known redshifts (Tsvetkova et al., 2017; Tsvetkova et al., 2021), see http://www.ioffe.ru/LEA/GRBs/GRB250920_T55519/GRB250920C_rest_frame.pdf All the quoted errors are at the 68% confidence level. All the quoted values are preliminary. View this GCN Circular online at https://gcn.nasa.gov/circulars/41957. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40023] GRB 250920C: KAIT optical observations
by GCN Circulars 23 Sep '25

23 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41956 SUBJECT: GRB 250920C: KAIT optical observations DATE: 25/09/23 18:23:18 GMT FROM: Weikang Zheng at UC Berkeley <weikang(a)berkeley.edu> WeiKang Zheng (UCB) and Alexei V. Filippenko (UCB) report on behalf of the KAIT GRB team: The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at Lick Observatory, observed the field of GRB 250920C (Fermi GBM team, GCN 41903; Gupta et al., GCN 41904; Mailyan et al., GCN 41917; Gupta et al., GCN 41924; SVOM Team, GCN 41929) around mid time of ~0.778d after the burst. A set of 90x60s clear (roughly R) filter images were obtained. We marginally detected the optical afterglow (Wortley et al., GCN 41907; Strausbaugh et al., GCN 41911; Ghosh et al., GCN 41913; Lipunov et al., GCN 41916; Kuin et al., GCN 41920; Mohan et al., GCN 41925; Sanchez-Ramirez et al., GCN 41928; Guiffreda et al., GCN 41943; de Ugarte Postigo et al., GCN 41951). We estimate the afterglow to be 21.0 +/- 0.3 mag (Vega). View this GCN Circular online at https://gcn.nasa.gov/circulars/41956. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40022] GRB 250920C: VLT redshift confirmation z = 1.399
by GCN Circulars 23 Sep '25

23 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41955 SUBJECT: GRB 250920C: VLT redshift confirmation z = 1.399 DATE: 25/09/23 16:00:11 GMT FROM: Valentina Abril-Melgarejo at Obs. Paris/LUX <valentina.abril-melgarejo(a)obspm.fr> L. Izzo (INAF/OACn and DARK/NBI), D. Xu (NAOC), V. Abril-Melgarejo (LUX-Paris Obs.), G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), N. R. Tanvir (Leicester), V. D’Elia (ASI/SSDC), report on behalf of the Stargate collaboration: We observed the optical afterglow (Wortley et al., GCN 41907) of GRB 250920C (Gupta et al., GCN 41904), using the ESO VLT UT4 (Yepun) equipped with the MUSE spectrograph. The mid time of our observation was Jan 21.209 UT (10.41 hr after the GRB trigger), and consisted of 4 exposures of 19 min each. We extracted a 1D spectrum using a circular aperture of 0.4 arcsec radius, centered on the afterglow emission. In the reduced spectrum, which covers the wavelength range 4750 - 9330 AA, we detect clear continuum and a number of absorption features, which we identify, among others, as Mg II, Mg I, Fe II, Fe II*, Ni II*, Cr II, Zn II, Mn II, all at a common redshift z = 1.399. The detection of Fe II fine-structure lines cements the association of the GRB with this absorption system. Furthermore, an emission line consistent with [O II] at the same redshift is also detected, likely from the underlying host galaxy (Wortley et al., GCN 41907). Our result is thus fully consistent, and strengthens, the previous reports by GTC (Sanchez-Ramirez et al., GCN 41928) and SALT (De Wet et al.. GCN 41952). We acknowledge expert support from the observing staff in Paranal, in particular Boris Haeussler, Enrico Congiu, Ana Jimenez Gallardo, and Marcelo Lopez. View this GCN Circular online at https://gcn.nasa.gov/circulars/41955. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40021] GRB 250920C: VLT redshift confirmation z = 1.399
by GCN Circulars 23 Sep '25

23 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41955 SUBJECT: GRB 250920C: VLT redshift confirmation z = 1.399 DATE: 25/09/23 16:00:11 GMT FROM: Valentina Abril-Melgarejo at Obs. Paris/LUX <valentina.abril-melgarejo(a)obspm.fr> L. Izzo (INAF/OACn and DARK/NBI), D. Xu (NAOC), V. Abril-Melgarejo (LUX-Paris Obs.), G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), N. R. Tanvir (Leicester), V. D’Elia (ASI/SSDC), report on behalf of the Stargate collaboration: We observed the optical afterglow (Wortley et al., GCN 41907) of GRB 250920C (Gupta et al., GCN 41904), using the ESO VLT UT4 (Yepun) equipped with the MUSE spectrograph. The mid time of our observation was Jan 21.209 UT (10.41 hr after the GRB trigger), and consisted of 4 exposures of 19 min each. We extracted a 1D spectrum using a circular aperture of 0.4 arcsec radius, centered on the afterglow emission. In the reduced spectrum, which covers the wavelength range 4750 - 9330 AA, we detect clear continuum and a number of absorption features, which we identify, among others, as Mg II, Mg I, Fe II, Fe II*, Ni II*, Cr II, Zn II, Mn II, all at a common redshift z = 1.399. The detection of Fe II fine-structure lines cements the association of the GRB with this absorption system. Furthermore, an emission line consistent with [O II] at the same redshift is also detected, likely from the underlying host galaxy (Wortley et al., GCN 41907). Our result is thus fully consistent, and strengthens, the previous reports by GTC (Sanchez-Ramirez et al., GCN 41928) and SALT (De Wet et al.. GCN 41952). We acknowledge expert support from the observing staff in Paranal, in particular Boris Haeussler, Enrico Congiu, Ana Jimenez Gallardo, and Marcelo Lopez. View this GCN Circular online at https://gcn.nasa.gov/circulars/41955. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40020] EP250911a: e-MERLIN radio observations
by GCN Circulars 23 Sep '25

23 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41954 SUBJECT: EP250911a: e-MERLIN radio observations DATE: 25/09/23 12:59:20 GMT FROM: Giulia Gianfagna at INAF-IAPS <giulia.gianfagna(a)inaf.it> G. Gianfagna, G. Bruni, L. Piro, A. L. Thakur (INAF-IAPS) report: We observed the Fast X-ray Transient EP250911a (Liang et al., GCN 41788) with the e-MERLIN radio telescope under the project CY20206 (PI: Gianfagna) at 5 GHz starting on Sep. 17, 16:00 UT (~6 days after the EP trigger) for a total of 11 hours. 1331+3030 was used for flux scale calibration, and 2344+2952 for complex gain. The beam size was 63x37 milli-arcsec. Data was reduced with the e-MERLIN pipeline and imaged with CASA. The image RMS was 18 uJy/beam. We did not find statistically significant emission at a level above 5-sigma at the position of the optical counterpart reported by Quirola-Vasquez et al. (GCN 41790), Angulo et al (GCN 41793), Lai et al. (GCN 41801), Moskvitin et al. (GCN 41805), Ma et al. (GCN 41807), Malesani et al. (GCN 41811). Thus, we estimate a 5-sigma upper limit of ~ 90 uJy at 5 GHz. Further e-Merlin observations are planned. We thank the e-Merlin staff for the prompt scheduling of these observations. e-MERLIN is a National Facility operated by the University of Manchester at Jodrell Bank Observatory on behalf of STFC, part of UK Research and Innovation. View this GCN Circular online at https://gcn.nasa.gov/circulars/41954. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40019] GRB 250920B: CrAO ZTSH optical observations
by GCN Circulars 23 Sep '25

23 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41953 SUBJECT: GRB 250920B: CrAO ZTSH optical observations DATE: 25/09/23 12:38:45 GMT FROM: Nicolai Pankov at HSE, IKI RAS <colinsergesen(a)gmail.com> N. Pankov (HSE, IKI), A. Pozanenko (IKI), A. Moskvitin (SAO RAS), V. Rumyantsev (CrAO) report on behalf of GRB-IKI-FuN: We observed the field of a long GRB 250920B detected by Fermi/GBM (The Fermi GBM team GCN 41897), Swift (Gupta et. al, GCN 41898), with the ZTSH telescope of CrAO. We obtained series of images in R,V bands starting on 2025-09-21 at 16:28:47.74 UT, i.e. 1.321 days since Fermi/GBM trigger. The optical afterglow initially detected by Swift/UVOT (Gupta et. al, GCN 41898) and further observed by (WU et. al, GCN 41908; Hall et. al, GCN 41914; Pankov et. al, GCN 41931; Reguitti et. al, GCN 41932; Moskvitin et. al, GCN 41948) is clearly visible in individual 120 s exposures taken in the R-band. Preliminary photometry of the OT in the R-band is as follows: Date UTstart Exptime t-T0 Filter OT Err. UL (s) (mid, days) (3sigma) 2025-09-21 17:21:58 120 1.35870 R 20.7 0.1 22 The photometry was calibrated against nearby USNO-B1.0 stars (R2 mags) and not corrected for the Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41953. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40018] GRB 250920C: SALT redshift confirmation
by GCN Circulars 23 Sep '25

23 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41952 SUBJECT: GRB 250920C: SALT redshift confirmation DATE: 25/09/23 12:27:05 GMT FROM: Simon de Wet at DTU Space <simdewet(a)gmail.com> S. de Wet (DTU Space), D. A. H. Buckley (SAAO), D. B. Malesani (DAWN/NBI and Radboud), P. J. Groot (Radboud, UCT, and SAAO) report: We observed the optical afterglow (Wortley et al., GCN 41907) of GRB 250920C (Gupta et al., GCN 41904), using the Southern African Large Telescope (SALT) equipped with the Robert Stobie Spectrograph (RSS). Observations started at 23:11:13 UT on 2025 Sep 20 (7.8 hr after the GRB), and consisted of 2x1200 s of spectroscopy using the PG700 grating (which covers the wavelength range 3600-7500 AA). In the coadded spectrum, continuum is detected across the whole wavelength range. A few narrow absorption features are visible which we interpret as due to C IV 1548,1550 (very strong), Fe II 2344 and 2374, Mg II 2796,2803 and Mg I 2852, all at a redshift of 1.399. Our result is thus consistent with, and confirms, the earlier measurement reported by Sanchez-Ramirez et al. (GCN 41928) using the GTC. View this GCN Circular online at https://gcn.nasa.gov/circulars/41952. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40017] GRB 250920C: COLIBRÍ optical observations
by GCN Circulars 23 Sep '25

23 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41951 SUBJECT: GRB 250920C: COLIBRÍ optical observations DATE: 25/09/23 11:33:46 GMT FROM: Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo(a)gmail.com> Antonio de Ugarte Postigo (LAM), Diego López-Cámara (UNAM), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Jean-Luc Atteia (IRAP), Fredd Alvarez (UNAM), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (OCA), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), and Benjamin Schneider (LAM): We imaged the field of the Fermi/GBM GRB 250920C (Fermi GBM team, GCN Circ. 41903; Mailyan et al. GCN 41917), also detected by Swift/BAT (Gupta et al. GCN 41904; Gupta et al. GCN 41924) and SVOM/GRM (Wang et al. GCN 41929) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-09-23 08:01:47 to 10:19:49 UTC (from 2.69 to 2.75 days after the trigger) and obtained 64 minutes of simultaneous exposures in the r and z filters. 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. We detected the optical counterpart reported by Wortley et al. (GCN Circ. 41907), at preliminary magnitudes of: r = 22.70 +/- 0.14 z = 22.54 +/- 0.25 We note that the source is still significantly brighter than the candidate host galaxy identified by Wortley et al. (GCN Circ. 41907) which appears in the Legacy Survey with r ~ 25.1 mag, so our observations are still afterglow dominated. Further observations and analysis are ongoing. 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/41951. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40016] GRB 250922C: AstroSat CZTI detection of a short burst
by GCN Circulars 23 Sep '25

23 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41950 SUBJECT: GRB 250922C: AstroSat CZTI detection of a short burst DATE: 25/09/23 05:51:16 GMT FROM: Anuraag Arya at IIT Bombay <aryaanuraag910(a)gmail.com> A. Goyal (IITB), A. Arya (IITB), S. Salunke (IUCAA), M. Tembhurnikar (IUCAA), Harsha K. H. (IUCAA), 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 short-duration GRB 250922C. The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2025-09-22 19:53:50.45 UTC. The measured peak count rate associated with the burst is 407 (+136, -92) counts/s above the background in the combined data of three quadrants (out of four), with a total of 137 (+46, -45) counts. The local mean background count rate was 253 (+8, -11) counts/s. Using cumulative rates, we measure a T90 of 0.43 (+0.45, -0.15) s. The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2025-09-22 19:53:50.73 UTC. The measured peak count rate associated with the burst is 291 (+66, -72) counts/s above the background in the combined data of all quadrants, with a total of 477 (+186, -198) counts. The local mean background count rate was 1436 (+10, -12) counts/s. Due to the intrinsic 1 s binning of veto data, we cannot reliably estimate a T90 from it. 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/41950. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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