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[vsnet-grb-info 42692] GRB 260510C: Fermi GBM Observation
by GCN Circulars 11 May '26

11 May '26

TITLE: GCN CIRCULAR NUMBER: 44564 SUBJECT: GRB 260510C: Fermi GBM Observation DATE: 26/05/11 20:51:58 GMT FROM: Eva MP at INAOE <eva.palafox(a)gmail.com> E. Palafox (INAOE, Mexico) reports on behalf of the Fermi Gamma-ray Burst Monitor Team: "At 17:13:43.99 UT on 10 May 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260510C (trigger 800126028/260510718), which was also detected by SVOM/ECLAIRs (Gotz et al. 2026, GCN 44521), MAXI (Iwakiri et al. 2026, GCN 44526), COLIBRI (Angulo et al. 2026, GCN 44530), SVOM/VT (Li et al. 2026, GCN 44527 and GCN 44540), EP-FXT (Fu et al. 2026, GCN 44541) and AstroSat CZTI (Arya et al. 2026,GCN 44544). The Fermi GBM on-ground location is consistent with the SVOM/ECLAIRs position. The angle from the Fermi LAT boresight is 68 degrees. The GBM light curve consists of two bright emission episodes with a duration (T90) of about 162 s (50-300 keV). The time-averaged spectrum from T0-8.2 to T0+143.4 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.3 +/- 0.2 and the cutoff energy, parameterized as Epeak, is 176 +/- 55 keV. The event fluence (10-1000 keV) in this time interval is (4.9 +/- 0.7)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+120 s in the 10-1000 keV band is 2.6 +/- 0.3 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog: https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html For Fermi GBM data and info, please visit the official Fermi GBM Support Page: https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/" View this GCN Circular online at https://gcn.nasa.gov/circulars/44564. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42691] GRB 260509A: SAO RAS optical observations
by GCN Circulars 11 May '26

11 May '26

TITLE: GCN CIRCULAR NUMBER: 44563 SUBJECT: GRB 260509A: SAO RAS optical observations DATE: 26/05/11 18:46:55 GMT FROM: Alexander Moskvitin at SAO RAS <mosk(a)sao.ru> A. Moskvitin and O. Spiridonova (SAO RAS), report on behalf of the GRB follow-up team. We observed the field of the GRB 260509A (Götz et al., GCN 44504) with the Zeiss-1000, 1m telescope of the SAO RAS equipped with the CCD photometer. We obtained 12 * 300 sec. images in Rc band on May 10, 18:18:31--19:49:52 UT (21.22 hr after the trigger). The OT (O'Neill et al., GCN 44505; Saccardi et al., GCN 44506; Wu et al., GCN 44507; Sánchez Álvarez et al., GCN 44509; Zheng et al., GCN 44510; Kechin et al., GCN 44512; Belkin, GCN 44513; Corcoran et al., GCN 44514; Bochenek and Perley, GCN 44516) is marginally detected with the brightness of R = 24.0 +/- 0.3. This preliminary photometry is based on nearby stars from the USNO-B1 catalogue (R2 magnitudes) and is not corrected for the Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/44563. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42690] GRB 260511A: REM near-infrared detection
by GCN Circulars 11 May '26

11 May '26

TITLE: GCN CIRCULAR NUMBER: 44562 SUBJECT: GRB 260511A: REM near-infrared detection DATE: 26/05/11 18:00:39 GMT FROM: Riccardo Brivio at INAF-OAB <riccardo.brivio(a)inaf.it> R. Brivio, M. Ferro, P. D'Avanzo, S. Covino, D. Fugazza (INAF-OAB), A. Melandri (INAF-OAR) report on behalf of the REM team: We observed the field of GRB 260511A, detected by SVOM/ECLAIRs (Godet et al., GCN 44532) and Fermi/GBM (Fermi GBM Team, GCN 44529) with the REM 60 cm robotic telescope located at the ESO observatory of La Silla (Chile). The observations were carried out in the H band, started on 2026-05-11 at 06:15:22 UT (i.e. 110 s after the burst) and lasted for about 10 minutes. From preliminary photometry, we detect the optical/NIR counterpart (Jiang et al., GCN 44533; Akl et al. GCN 44534; Saccardi et al. GCN 44535; Zheng et al. GCN 44536; Wu et al., GCN 44538, Harper et al., GCN 44545) with the following magnitude: H = 12.9 +/- 0.2 (Vega; calibrated against the 2MASS catalogue), at a mid-time of 115 s after the trigger. View this GCN Circular online at https://gcn.nasa.gov/circulars/44562. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42689] GRB 260511B: GROWTH-India Telescope optical observations
by GCN Circulars 11 May '26

11 May '26

TITLE: GCN CIRCULAR NUMBER: 44561 SUBJECT: GRB 260511B: GROWTH-India Telescope optical observations DATE: 26/05/11 17:39:26 GMT FROM: V. Swain at IIT Bombay <vishwajeet.s(a)iitb.ac.in> V. Vijaykumar (IITB), A.P. Saikia (IITB), V. Swain (IITB), T. Mohan (IITB), S. Patil (IITB), V. Bhalerao (IITB), G.C. Anupama (IIA), S. Barway (IIA) and R. Norbu (IAO) report on behalf of the GIT team: We observed the field of GRB 260511B detected by Fermi (Fermi GBM Team, GCN 44542), with 0.7m GROWTH-India Telescope (GIT). We started the observation at 2026-05-11 16:19:32 (UTC), i.e., 5.32 hours after the trigger, and obtained multiple exposures in the SDSS r', g', and i' filters. We detect the optical counterpart in our images at the position reported by Wu et al., GCN 44547. The photometric upper limit is as follows: | MJD (mid) | Filter | tmid-t0 (hours) | Exposure Time (sec) | Magnitude (AB) | | -------------- | ------ | --------------- | ------------------- | ---------------- | | 61171.68255 | r' | 5.37 | 1x400 | 18.27 +/- 0.05 | | 61171.68758 | g' | 5.49 | 1x400 | 18.54 +/- 0.06 | | 61171.69242 | i' | 5.61 | 1x360 | 18.10 +/- 0.10 | The measurement is calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction. Our results are consistent with other optical observations (Wu et al., GCN 44547 and GCN 44552; Sasada et al. GCN 44549; Jiang et al., GCN 44551.) 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/44561. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42688] GRB 260511B: AstroSat CZTI detection
by GCN Circulars 11 May '26

11 May '26

TITLE: GCN CIRCULAR NUMBER: 44560 SUBJECT: GRB 260511B: AstroSat CZTI detection DATE: 26/05/11 17:32:28 GMT FROM: Anuraag Arya at IIT Bombay <aryaanuraag910(a)gmail.com> A. Arya (IITB), A. Goyal (IITB), M. Tembhurnikar (IUCAA), Harsha K. H. (IUCAA), S. Salunke (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 long GRB 260511B which is also detected by Fermi/GBM (Fermi GBM Team, GCN 44542), SVOM/ECLAIRs (Godet et al., GCN 44543), and NuSTAR (Waratkar et. al. , GCN Circ. 44556). 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 2026-05-11 11:01:02.99 UTC. The measured peak count rate associated with the burst is 398 (+41, -36) counts/s above the background in the combined data of all quadrants, with a total of 3131 (+342, -481) counts.The local mean background count rate was 282 (+2, -4) counts/s. We measure a T90 of 39 (+3, -2) s from the cumulative CZT light curve. The source was 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 2026-05-11 11:01:02.74. The measured peak count rate associated with the burst is 1362 (+85, -90) counts/s above the background in the combined data of all quadrants, with a total of 7777 (+766, -805) counts.The local mean background count rate was 1287 (+4, -5) counts/s. We measure a T90 of 37 (+16, -3) 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 data products like interactive and downloadable light curves for this GRB can be found at: https://astrosat.iucaa.in/cift/cift_products/516193231.75/S516193231.75_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/44560. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42687] GRB 260506B: Glowbug gamma-ray detection
by GCN Circulars 11 May '26

11 May '26

TITLE: GCN CIRCULAR NUMBER: 44559 SUBJECT: GRB 260506B: Glowbug gamma-ray detection DATE: 26/05/11 17:12:04 GMT FROM: C.C. Cheung at Naval Research Lab <Teddy.Cheung(a)nrl.navy.mil> C.C. Cheung, R. Woolf, M. Kerr, J.E. Grove (NRL), A. Goldstein (USRA), C.A. Wilson-Hodge, D. Kocevski (MSFC), and M.S. Briggs (UAH) report: The Glowbug gamma-ray telescope [1,2,3], operating on the International Space Station, reports the detection of GRB 260506B, which was also detected by Fermi/GBM (GCN 44484, 44485) and AstroSat/CZTI (GCN 44503). Using an adaptive window with a resolution of 32-ms, the burst onset is determined to be 2026-05-06 13:07:30.824 with a duration of 20.5 s and a total significance of about 34 sigma. The lightcurve comprises a faint initial peak at ~T0+1s, a complex structure from ~T0+7s to +15s, and a peak at ~T0+21s — the peaks correspond to the brightest features seen in the GBM lightcurve. Note that data from ~T0-1s to +1s suffered from deadtime in various detectors. The analysis results presented here are preliminary and use a response function that lacks a detailed characterization of the surrounding passive structure of the ISS. Glowbug is a NASA-funded technology demonstrator for sensitive, low-cost gamma-ray transient telescopes developed, built, and operated by the U.S. Naval Research Laboratory (NRL) with support from the University of Alabama in Huntsville, USRA, and NASA MSFC. It was launched on 2023 March 15 aboard the Department of Defense Space Test Program’s STP-H9 to the ISS and operated until 2024 April when it was put in safe storage on orbit. Glowbug was removed from storage and resumed operation on 2025 September 12. [1] Grove, J.E. et al. 2020, Proc. Yamada Conf. LXXI, arXiv:2009.11959 [2] Woolf, R.S. et al. 2022, Proc. SPIE, 12181, id. 121811O [3] Woolf, R.S. et al. 2024, Proc. SPIE, 13151, id. 1315108 Distribution Statement A: Approved for public release. Distribution is unlimited. View this GCN Circular online at https://gcn.nasa.gov/circulars/44559. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42686] GRB 260506A: Glowbug gamma-ray detection of a short burst
by GCN Circulars 11 May '26

11 May '26

TITLE: GCN CIRCULAR NUMBER: 44558 SUBJECT: GRB 260506A: Glowbug gamma-ray detection of a short burst DATE: 26/05/11 16:37:58 GMT FROM: C.C. Cheung at Naval Research Lab <Teddy.Cheung(a)nrl.navy.mil> C.C. Cheung, R. Woolf, M. Kerr, J.E. Grove (NRL), A. Goldstein (USRA), C.A. Wilson-Hodge, D. Kocevski (MSFC), and M.S. Briggs (UAH) report: The Glowbug gamma-ray telescope [1,2,3], operating on the International Space Station, reports the detection of the short burst GRB 260506A, which was detected by Fermi/GBM (GCN 44481). Using an adaptive window with a resolution of 32-ms, the burst onset is determined to be 2026-05-06 06:28:47.264 with a duration of 0.32 s and a total significance of about 19.0 sigma. The lightcurve comprises a single peak. The analysis results presented here are preliminary and use a response function that lacks a detailed characterization of the surrounding passive structure of the ISS. Glowbug is a NASA-funded technology demonstrator for sensitive, low-cost gamma-ray transient telescopes developed, built, and operated by the U.S. Naval Research Laboratory (NRL) with support from the University of Alabama in Huntsville, USRA, and NASA MSFC. It was launched on 2023 March 15 aboard the Department of Defense Space Test Program’s STP-H9 to the ISS and operated until 2024 April when it was put in safe storage on orbit. Glowbug was removed from storage and resumed operation on 2025 September 12. [1] Grove, J.E. et al. 2020, Proc. Yamada Conf. LXXI, arXiv:2009.11959 [2] Woolf, R.S. et al. 2022, Proc. SPIE, 12181, id. 121811O [3] Woolf, R.S. et al. 2024, Proc. SPIE, 13151, id. 1315108 Distribution Statement A: Approved for public release. Distribution is unlimited. View this GCN Circular online at https://gcn.nasa.gov/circulars/44558. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42685] GRB 260511B: J-band observations by the SYSU 80 cm infrared telescope
by GCN Circulars 11 May '26

11 May '26

TITLE: GCN CIRCULAR NUMBER: 44557 SUBJECT: GRB 260511B: J-band observations by the SYSU 80 cm infrared telescope DATE: 26/05/11 16:10:48 GMT FROM: lijj328(a)mail2.sysu.edu.cn Jin-Ji Li, Chun Chen, Duo-Le Cao, Zhong-Nan Dong, Wei-Sen Huang, Jia-Qi Lin, Pu Lin, Yun Shi, Hao-Nan Yang, Yan Yu, P H Thomas Tam, Rong-Feng Shen, Bin Ma (Sun Yat-sen University) report on behalf of the SYSU 80cm infrared telescope team: We observed the field of GRB 260511B (SVOM burst-id sb26051102) detected by SVOM/ECLAIRs (Godet et al., GCN 44543) and Fermi (Fermi GBM team, GCN 44542), using the Sun Yat-sen University (SYSU) 80 cm infrared telescope. Our observations started on 2026 May 11 at 13:51 UT, 2.9 hours after the GRB trigger, with a total exposure time of 1800 s in the J-band images. We clearly detect the source at the position of the optical counterpart (Wu et al., GCN 44547; Sasada et al., GCN 44549; Jiang et al., GCN 44551; Wu et al., GCN 44552), and we measure J = 16.02 +/- 0.07 Vega mag. The photometry was calibrated against nearby stars from the 2MASS catalog and is reported in the Vega system, without correction for Galactic extinction. Further observations are planned. The SYSU 80 cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University. View this GCN Circular online at https://gcn.nasa.gov/circulars/44557. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42684] GRB 260511B: NuSTAR detection of prompt emission
by GCN Circulars 11 May '26

11 May '26

TITLE: GCN CIRCULAR NUMBER: 44556 SUBJECT: GRB 260511B: NuSTAR detection of prompt emission DATE: 26/05/11 16:08:21 GMT FROM: Gaurav Waratkar at Caltech <gauravwaratkar(a)iitb.ac.in> G. Waratkar (Caltech) and B. Grefenstette (Caltech) report on behalf of the NuSTAR Search for INteresting Gamma-ray Signals (SINGS) working group: The NuSTAR SINGS working group reports the detection of prompt emission from the long-duration GRB 260511B in both the NuSTAR CsI anti-coincidence shields. Details of the search algorithm will be described in a future paper. The NuSTAR SINGS algorithm, triggered at 2026-05-11T11:00:31.7 UTC, shows a detection of GRB 260511B, also detected by Fermi/GBM (Fermi GBM Team, GCN 44542) and SVOM (Godet et al., GCN 44532). The NuSTAR CsI shield data are recorded at 1 Hz. We detect an initial faint burst followed by a 20-s long brighter burst consistent with the Fermi/GBM lightcurve. The peak count rate is ~2500-cps with a baseline rate of ~1000-cps during this time period. We also see marginal evidence in the signal above 100 keV in the CZT detectors. The optical counterpart candidate (Wu et al., GCN 44547) at RA = 192.57345, Dec = 4.05512 implies an offset from the NuSTAR boresight of 93-deg (i.e. from the side of the instrument) and an offset from the geocenter of 164-deg. Lightcurves and analysis for this GRB can be found here: https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2026/260511B Information on NuSTAR SINGS can be found here: https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/ NuSTAR is a NASA Small Explorer mission led by Caltech and managed by JPL for NASA's Science Mission Directorate in Washington. View this GCN Circular online at https://gcn.nasa.gov/circulars/44556. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42683] GRB 260511A: NuSTAR detection of prompt emission
by GCN Circulars 11 May '26

11 May '26

TITLE: GCN CIRCULAR NUMBER: 44555 SUBJECT: GRB 260511A: NuSTAR detection of prompt emission DATE: 26/05/11 16:06:48 GMT FROM: Gaurav Waratkar at Caltech <gauravwaratkar(a)iitb.ac.in> G. Waratkar (Caltech) and B. Grefenstette (Caltech) report on behalf of the NuSTAR Search for INteresting Gamma-ray Signals (SINGS) working group: The NuSTAR SINGS working group reports the detection of prompt emission from the long-duration GRB 260511A in both the NuSTAR CsI anti-coincidence shields. Details of the search algorithm will be described in a future paper. The NuSTAR SINGS algorithm, triggered at 2026-05-11T06:13:30.0 UTC, shows a detection of GRB 260511A, also detected by Fermi/GBM (Fermi GBM Team, GCN 44529) and SVOM (Godet et al., GCN 44532). The NuSTAR CsI shield data are recorded at 1 Hz. We detect a ~10-s burst consistent with Fermi/GBM. The peak count rate is ~3500-cps with a baseline rate of ~1000-cps during this time period. We do not see evidence in the signal above 100 keV in the CZT detectors. The optical counterpart candidate (Akl et al., GCN 44534) at RA = 197.8996, Dec = -33.80446 implies an offset from the NuSTAR boresight of 57-deg and an offset from the geocenter of 144-deg. Lightcurves and analysis for this GRB can be found here: https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2026/260511A Information on NuSTAR SINGS can be found here: https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/ NuSTAR is a NASA Small Explorer mission led by Caltech and managed by JPL for NASA's Science Mission Directorate in Washington. View this GCN Circular online at https://gcn.nasa.gov/circulars/44555. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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