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

[vsnet-grb-info 42702] GRB 260511B: LCO optical observations
by GCN Circulars 12 May '26

12 May '26

TITLE: GCN CIRCULAR NUMBER: 44574 SUBJECT: GRB 260511B: LCO optical observations DATE: 26/05/12 09:48:43 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> A. Saccardi (CEA/Irfu), C. Wu (NAOC), J. X. Cao (GXU), L. Zhang (IHEP), D. Kong (GXU), report on behalf of the SVOM mission team: We observed the field of the SVOM/ECLAIRs GRB 260511B (Godet et al., GCN 44543), also detected by Fermi/GBM (Fermi GBM team, GCN 44542; Roberts et al., GCN 44554), NuSTAR (Waratkar et al., GCN 44556), and AstroSat (Arya et al., GCN 44560), with the LCO 1m telescope at Cerro Tololo Observatory equipped with the Sinistro instrument. Our observation started on 2026-05-12 at 00:14:08 UT (about 13.23 hr after the trigger) and we obtained 3x200 s exposures in the SDSS r and 3x200 s exposures in the Pan-STARRS z filters. The optical counterpart (Wu et al., GCN 44547; Sasada et al., GCN 44549; Jiang et al., GCN 44551; Wu et al., GCN 44552; Li et al., GCN 44557; Vijaykumar et al., GCN 44561; de Ugarte Postigo et al., GCN 44565; Lee et al. GCN 44567; Li et al., GCN 44568; Wang et al., GCN 44571; Belkin et al., GCN 44572; Adami et al., GCN 44573) is clearly detected in our images. We measure the following magnitudes calibrated against the Pan-STARRS DR1 catalog, and not corrected for Galactic extinction: r = 19.68 +/- 0.02 AB (mid-time 13.29 hr after the trigger); z = 19.71 +/- 0.09 AB (mid-time 18.14 hr after the trigger). This project is funded by the SVOM collaboration. View this GCN Circular online at https://gcn.nasa.gov/circulars/44574. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42701] GRB 260511B: OHP/T193 spectroscopic observations
by GCN Circulars 12 May '26

12 May '26

TITLE: GCN CIRCULAR NUMBER: 44573 SUBJECT: GRB 260511B: OHP/T193 spectroscopic observations DATE: 26/05/12 09:05:55 GMT FROM: Christophe Adami at LAM <christophe.adami(a)lam.fr> C. Adami (LAM/Pytheas/AMU), D. Russeil (LAM/Pytheas/AMU), E. Le Floc'h (CEA-Saclay) report on behalf of a larger collaboration and on behalf of the transient team of the OHP 2026 special call: We observed the optical counterpart (Wu et al., GCN 44547 ; Sasada et al., GCN 44549 ; Jiang et al., GCN 44551 ; Wu et al., GCN 44552 ; Li et al., GCN 44557 ; Vijaykumar et al., GCN 44561 ; de Ugarte Postigo et al., GCN 44565 ; Lee et al., GCN 44567; Li et al., GCN 44568; Wang et al., GCN 44571; Belkin et al., GCN 44572) of GRB 260511B (FERMI team, GCN 44542 ; Godet et al., GCN 44543; Roberts et al., GCN 44554; Waratkar et al., GCN 44556; Arya et al., GCN 44560) in spectroscopic mode using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. We obtained a total of 145 min of exposures (10min + 15min + 4 x 30min) with the MISTRAL spectroscopic blue setting from 4200 to 8000 AA, starting at 2026-05-11 22:04:34 UT under modest weather conditions. The host environment at z=2.006 detected by the GTC and reported by de Ugarte Postigo et al. (GCN 44565) and Wang et al. (GCN 44571) is visible in our data, in particular with the 2344 AA and 2600 AA (rest-frame) FeII lines observed at a consistent redshift of z=2.007 +/- 0.001. 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/44573. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42700] GRB 260511B: GOTO optical counterpart detection
by GCN Circulars 12 May '26

12 May '26

TITLE: GCN CIRCULAR NUMBER: 44572 SUBJECT: GRB 260511B: GOTO optical counterpart detection DATE: 26/05/12 08:15:30 GMT FROM: Sergey Belkin at Monash University <sergey.belkin(a)monash.edu> S. Belkin, G. Ramsay, D. O'Neill, B. P. Gompertz, M. Wortley, R. Starling, K. Ackley, M. Dyer, J. Lyman, K. Ulaczyk, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, B. Godson, T. Killestein, A. Kumar, M. Pursiainen, on behalf of GOTO collaboration We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to the Fermi/GBM (Fermi GBM team, GCN 44542) and SVOM/ECLAIRs (Godet et al. GCN 44543) triggers for GRB 260511B. Observations started at 2026-05-11 21:09:05 UT, (+10.13h post trigger) in the GOTO L-band (400-700 nm). Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations (Lyman et al. 2026). We detect the previously reported optical counterpart of GRB 260511B at a position consistent with the source reported by Wu et al. (GCN 44547), Sasada et al. (GCN 44549), Jiang et al. (GCN 44551), Wu et al. (GCN 44552), Vijaykumar et al. (GCN 44561), de Ugarte Postigo et al. (GCN 44565), Lee et al. (GCN 44567), and Li et al. (GCN 44568). The source is detected in the GOTO L-band (400-700 nm) with L=19.45 ± 0.09 AB mag at a mid-time of 2026-05-11 21:44:06 UT (+10.73 h). GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester, the University of Birmingham and the Instituto de Astrofisica de Canarias (IAC). View this GCN Circular online at https://gcn.nasa.gov/circulars/44572. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42699] GRB 260511B: Xinglong 2.16m/BEFOSC redshift confirmation
by GCN Circulars 12 May '26

12 May '26

TITLE: GCN CIRCULAR NUMBER: 44571 SUBJECT: GRB 260511B: Xinglong 2.16m/BEFOSC redshift confirmation DATE: 26/05/12 07:55:07 GMT FROM: Liping Xin at NAOC, SVOM <xlp(a)nao.cas.cn> J. Wang, L. P. Xin, J. Zheng, H. L. Li, C. Wu, Y. L. Qiu, X. H. Han, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, J. Y. Wei (NAOC) reports on behalf of SVOM follow-up team The follow-up for the optical counterpart (Wu et al. GCN 44547; Sasada et al. GCN 44549; Jiang et al. GCN 44551; Wu et al. GCN 44552; Waratkar et al. GCN 44556; Li et al. GCN 44557; Vijaykumar et al. GCN 44561) of GRB 260511B (Fermi GBM team GCN 44542; Godet et al. GCN 44543; Roberts et al. GCN 44544; Arya et al. GCN 44560; de Ugarte Postigo et al., GCN 44565; Lee et al, GCN 44567; Li et al., GCN 44568) within the errorbox of X-ray counterpart (Godet et al. GCN 44543; Li et al., GCN 44566;) was performed at 2026-05-11T12:46:21 UTC, about 1.76 hours after the trigger, using the NAOC 2.16m telescope at Xinglong observatory, China. A total of 2*1800s exposure was performed by BEFOSC spectrograph in our spectroscopic observation. The wavelength ranges from 4000-8800A in the obsever frame. After combining the two frames, absorption features of multiple ions: CIV, FeII and MgII can be identified at a common redshift of ~2.006. In addition, there is a feature possibly due to MgII double let resulted from an intervening system at redshift of 1.435. Our results are consistent with the report of redshift (de Ugarte Postigo et al., GCN 44565). The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495) View this GCN Circular online at https://gcn.nasa.gov/circulars/44571. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42698] GRB 260510C: SVOM/COLIBRÍ (FM-GFT) further optical observations
by GCN Circulars 12 May '26

12 May '26

TITLE: GCN CIRCULAR NUMBER: 44570 SUBJECT: GRB 260510C: SVOM/COLIBRÍ (FM-GFT) further optical observations DATE: 26/05/12 05:34:29 GMT FROM: William H. Lee at UNAM <wlee(a)astro.unam.mx> William H. Lee (UNAM), Dalya Akl (NYUAD), Rosa L. Becerra (UNAM), Stéphane Basa (UAR Pytheas), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), 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), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM), and Andrea Saccardi (CEA/Irfu) report: We reimaged the field of the SVOM/ECLAIRs and Fermi/GBM GRB 260510C (Fermi GBM Team, GCN Circ. 44520; Gotz et al., GCN Circ. 44521) and also detected by AstroSat (Angulo et al. GCN Circ. 44544) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-05-12 03:45 to 04:44 UTC (from 34.52 to 35.53 hours after the trigger) and obtained 47 minutes of simultaneous exposure in r and z filters. The data were reduced, coadded, calibrated, and analysed with the COLIBRÍ ASU pipeline. 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 detect the source reported by Angulo et al. (GCN Circ. 44530) with preliminary unsubtracted magnitudes of: r = 22.36 + /- 0.07, z = 21.33 + /- 0.08. Compared with our first epoch (Angulo et al. GCN Circ. 44530), we estimate an optical decay temporal index of alpha ≈ -0.9 for the source. We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM. 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/44570. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42697] GRB 260510A: AstroSat CZTI detection
by GCN Circulars 12 May '26

12 May '26

TITLE: GCN CIRCULAR NUMBER: 44569 SUBJECT: GRB 260510A: AstroSat CZTI detection DATE: 26/05/12 05:10:46 GMT FROM: Anuraag Arya at IIT Bombay <aryaanuraag910(a)gmail.com> M. Tembhurnikar (IUCAA), Harsha K. H. (IUCAA), S. Salunke (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 GRB 260510A which is also detected by Fermi/GBM (Fermi GBM team, GCN Circ. 44508), and GECAM-B (Luo et al., GCN Circ. 44519). The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2026-05-10 02:51:20.28 UTC. The measured peak count rate associated with the burst is 98 (+27, -16) counts/s above the background in the combined data of three quadrants (out of four), with a total of 1136 (+230, -210) counts. The local mean background count rate was 180 (+2, -2) counts/s. We measure a T90 of 30 (+5, -8) s from the cumulative CZT light curve. The source was also faintly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. 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 data products like interactive and downloadable light curves for this GRB can be found at: https://astrosat.iucaa.in/cift/cift_products/516077480.79/S516077480.79_det… 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/44569. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42696] GRB 260511B: SVOM/VT optical observations
by GCN Circulars 12 May '26

12 May '26

TITLE: GCN CIRCULAR NUMBER: 44568 SUBJECT: GRB 260511B: SVOM/VT optical observations DATE: 26/05/12 04:28:34 GMT FROM: Huali Li at at NAOC, SVOM <lhl(a)nao.cas.cn> H. L. Li, C. Wu, Y. L. Qiu, L. P. Xin, Y. N. Ma, Z. H. Yao, J. R. Xu, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA/Irfu), L. Zhang (IHEP) and D. Kong (GXU) report on behalf of the SVOM/VT team. SVOM/VT performed observations of GRB260511B triggered by SVOM/ECLAIRs (sb26051102, Godet et al., GCN 44543). The burst was also detected by Fermi-GBM (Fermi GBM team, GCN 44542; Roberts et al., GCN 44554)，AstroSat (Arya et al., GCN 44560) and NuSTAR (Waratkar et al. GCN 44556). Our available data via automatic slew started at 2026-05-11T14:26:43 UTC, approximately 3.45 hours post trigger, in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously. The ToO observations started at 2026-05-11T20:37:09 UTC, about 9.62 hours post trigger. The afterglow (Wu et al. GCN 44547; Sasada et al. GCN 44549; Jiang et al. GCN 44551; Wu et al. GCN 44552; Li et al. GCN 44557; Vijaykumar et al. GCN 44561) with the redshift of z=2.006 (Postigo et al., GCN 44565) was clearly detected in both channels. The following measurements are in the AB magnitude and are not corrected for Galactic extinction: Mid time | Band | Exposure Time | Brightness 3.450 h VT_B 100 s 18.00 +/- 0.03 mag 3.450 h VT_R 100 s 17.61 +/- 0.03 mag 10.159 h VT_B 100 s 19.52 +/- 0.05 mag 10.159 h VT_R 100 s 19.05 +/- 0.05 mag The source showed a fading slope of about -1.3 during the observations above. The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS. View this GCN Circular online at https://gcn.nasa.gov/circulars/44568. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42695] GRB 260511B: SVOM/COLIBRÍ (FM-GFT) optical observations
by GCN Circulars 12 May '26

12 May '26

TITLE: GCN CIRCULAR NUMBER: 44567 SUBJECT: GRB 260511B: SVOM/COLIBRÍ (FM-GFT) optical observations DATE: 26/05/12 04:22:36 GMT FROM: William H. Lee at UNAM <wlee(a)astro.unam.mx> William H. Lee (UNAM), Dalya Akl (NYUAD), Rosa L. Becerra (UNAM), Stéphane Basa (UAR Pytheas), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), 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), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM), L. Zhang (IHEP), and D. Kong (GXU) report: We imaged the field of the SVOM/ECLAIRs and Fermi/GBM GRB 260511B (Fermi GBM Team, GCN Circ. 44529; Godet et al., GCN Circ. 44532) and also detected by AstroSat (Arya et al. GCN Circ. 44560) and NuSTAR (Waratkar et al. GCN Circ. 44556) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-05-12 03:24 to 03:43 UTC (from 16.35 to 16.67 hours after the trigger) and obtained 16 minutes of simultaneous exposure in r and z filters. The data were reduced, coadded, calibrated, and analysed with the COLIBRÍ ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. At the position of the optical counterpart reported by SVOM/C-GFT (Wu et al. GCN Circ. 44547), MITSuME (Sasada et al. GCN Circ. 44549), TRT (Jiang et al. GCN Circ. 44551), SVOM/VT (Wu et al. GCN Circ. 44552), we detect a source with preliminary magnitudes of: r = 19.99 +/- 0.01, z = 19.67 +/- 0.02. Compared with the optical observations reported by de Ugarte Postigo (GCN Circ. 44565), we estimate an optical decay temporal index of alpha ≈ -1.3 for the source. We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM. 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/44567. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42694] GRB 260510B: EP-FXT follow-up observation
by GCN Circulars 12 May '26

12 May '26

TITLE: GCN CIRCULAR NUMBER: 44566 SUBJECT: GRB 260510B: EP-FXT follow-up observation DATE: 26/05/12 02:31:52 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> A. Li (BNU), S.-Y. Fu (HUST), C.-Y. Wang (THU), R. Shi (PMO), Y. Liu (NAOC, CAS) on behalf of the Einstein Probe (EP) team: EP-FXT performed a follow-up observation of the SVOM/ECLAIRs-detected burst GRB 260510B (SVOM/sb26051001, Saccardi et al., GCN 44515). The follow-up obervation started at 2026-05-10T13:15:11 UTC, approximately 55 minutes after the SVOM trigger, with a total exposure time of 6.1 ks. On-ground analysis of the FXT data found an uncatalogued fading source at R.A., Dec. = 173.2323, -34.5399 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic) within the ECLAIRs error circle. The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power-law model, with the hydrogen column density fixed at the Galactic value and a photon index of 2.24 (-0.56/+0.60). The derived average unabsorbed flux in the 0.5-10 keV band is 1.17 (-0.3/+1.7)×10^-12 erg s^-1 cm^-2. 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/44566. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42693] GRB 260511B: OSIRIS+/GTC redshift z=2.006
by GCN Circulars 11 May '26

11 May '26

TITLE: GCN CIRCULAR NUMBER: 44565 SUBJECT: GRB 260511B: OSIRIS+/GTC redshift z=2.006 DATE: 26/05/11 23:06:28 GMT FROM: Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo(a)gmail.com> A. de Ugarte Postigo (LAM), C. C. Thoene (AbAO), A. Martin-Carrillo (UCD), S. Geier (GTC), M. A. Aloy (UV), J. P. U. Fynbo (DAWN/NBI), L. Galbany (IEEC-CSIC), L. Izzo (INAF/OACN and DARK/NBI), G. Lombardi (GTC), D. B. Malesani (DAWN/NBI and Radboud), N. A. Rakotondrainibe (LAM), B. Schneider (LAM), N. R. Tanvir (Univ. Leicester), A. M. García Rodríguez (GTC) and D. González González (GTC) report: We observed the optical counterpart (Wu et al. GCN 44547; Sasada et al. GCN 44549; Jiang et al. GCN 4455; Wu et al. GCN 44552; Waratkar et al. GCN 44556; Li et al. GCN 44557; Vijaykumar et al. GCN 44561) of GRB 260511B (Fermi GBM team GCN 44542; Godet et al. GCN 44543; Roberts et al. GCN 44554; Arya et al. GCN 44560) using the Gran Telescopio Canarias (GTC) equipped with the OSIRIS+ instrument. In the 30-s acquisition image (beginning on 2026-05-11 at 21:05:13.010 UT, that is 10.08 hr after trigger), the optical afterglow is well detected with a preliminary magnitude of r = 19.30 ± 0.05 (AB), calibrated against nearby SDSS objects, and not corrected for Galactic extinction. A total of 3 spectra of 900 s were secured, starting on 2026-05-11 at 21:11:43.011 UT (10.19 hr after trigger), using grism R1000B. Continuum is visible over the complete wavelength range 3620-7800 AA. A trough is detected at the blue edge of the spectrum at ~3650 AA, which we identify as a DLA. A number of strong metal absorption features are detected, which we interpret as due to SII, SiII, SiII*, OI, CII, SiIV, CIV, FeII, FeII* AlII, NiII, NiII*, AlIII, CrII, ZnII, and MnII all at a common redshift of 2.006 +/- 0.001, which we suggest to be the redshift of GRB 260511B. Multiple intervening systems are also detected in the spectrum, including a strong system at z = 1.437 with lines due to SiII, CIV, FeII, AlII, AlIII, MgII. This work has used the GRBspec database at http://grbspec.eu (de Ugarte Postigo et al. 2014, doi:10.1117/12.2055774). View this GCN Circular online at https://gcn.nasa.gov/circulars/44565. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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