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[vsnet-grb-info 41424] EP260105a: refined analysis of the EP-WXT and EP-FXT observations
by GCN Circulars 05 Jan '26

05 Jan '26

TITLE: GCN CIRCULAR NUMBER: 43333 SUBJECT: EP260105a: refined analysis of the EP-WXT and EP-FXT observations DATE: 26/01/05 14:30:43 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> H. Zhou (PMO, CAS), Z. C. Zou (NJU), D. Y. Li, Z. X. Ling (NAO, CAS) on behalf of the Einstein Probe (EP) team: The fast X-ray transient EP260105a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Zhou et al., GCN 43327). The refined analysis of the WXT data shows that the event started at T0=2026-01-05T05:47:50 (UTC) and lasted for 100 seconds. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 2.4 x 10^20 cm^-2 and a photon index of 0.89(-0.65/+0.67). The derived average unabsorbed 0.5-4 keV flux is (1.1+/-0.5) x 10^(-9) erg/s/cm^2. The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously at 2026-01-05T05:52:07 (UTC), about 4.3 minutes after T0. The exposure time of this observation is 2698s. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 181.0148, DEC = 22.7092 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with the WXT position. A significant decay was detected in the light curve during the initial phase of FXT observation. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 2.4 x 10^20 cm^-2 and a photon index of 2.07 (-0.56/+0.58). The derived average unabsorbed 0.5-10 keV flux is 4.2 (-1.7/+2.1) x 10^(-13) erg/s/cm^2. No optical counterpart of EP260105a has been detected so far (Lipunov et al., GCN 43328; Corcoran et al., GCN 43332) , and further mutil-band follow-up observations are encouraged to explore the nature of EP260105a. Another ToO observation with EP-FXT is being performed start from 2026-01-05T12:31:46 (UTC). Further information will be updated when the telemetry data is received. 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/43333. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41423] EP260105a: LCO Optical Upper limit
by GCN Circulars 05 Jan '26

05 Jan '26

TITLE: GCN CIRCULAR NUMBER: 43332 SUBJECT: EP260105a: LCO Optical Upper limit DATE: 26/01/05 11:05:43 GMT FROM: Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo(a)ucd.ie> G. Corcoran (UCD), J. Quirola-Vasquez (Radboud), F. E. Bauer (SSI and UTA), A. Levan (Radboud), A. Martin-Carrillo (UCD), P.G. Jonker (Radboud) report on behalf of a larger collaboration: We observed the field of the fast X-ray transient EP260105a (Zhou et al. GCN 43327) using the Las Cumbres Observatory (LCO) network at the Cerro Tololo Inter-American Observatory node using the Sinistro instrument on the 1m telescope. A series of 6x300s exposures were taken in the SDSS-r filter starting at 2026-01-05T07:10:04.556 UTC (1.326 hours after the WXT trigger). We do not detect any sources within the EP/FXT uncertainty region. From the stacked image we obtain a 3 sigma upper limit of: r > 21.5. This upper limit is in AB magnitudes, was calibrated using nearby stars from the Pan-STARRS catalog and is not corrected for Galactic extinction. Our observations are consistent with the upper limits reported by Lipunov et al. GCN 43328. View this GCN Circular online at https://gcn.nasa.gov/circulars/43332. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41422] EP260104a: Global MASTER-Net observations report
by GCN Circulars 05 Jan '26

05 Jan '26

TITLE: GCN CIRCULAR NUMBER: 43331 SUBJECT: EP260104a: Global MASTER-Net observations report DATE: 26/01/05 09:18:34 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-Kislovodsk robotic telescope [1] located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the EP260104a ( EP Team et al., GCN 43330) errorbox 56335 sec after trigger time at 2026-01-05 02:04:39 UT, with upper limit up to 15.3 mag. The observations began at zenith distance = 28 deg. The sun altitude is -27.9 deg. The galactic latitude b = 73 deg., longitude l = 251 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3096369 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 56366 | 2026-01-05 02:04:39 | MASTER-Kislovodsk | (11h 56m 25.18s , +14d 49m 37.7s) | C | 60 | 15.3 | 80735 | 2026-01-05 08:49:48 | MASTER- | (11h 58m 24.52s , +15d 34m 21.5s) | C | 180 | 17.0 | 81514 | 2026-01-05 09:02:47 | MASTER- | (11h 58m 27.43s , +15d 32m 44.3s) | C | 180 | 17.9 | 81514 | 2026-01-05 09:02:47 | MASTER- | (11h 59m 39.45s , +14d 59m 25.9s) | C | 180 | 17.9 | 81896 | 2026-01-05 09:09:09 | MASTER- | (11h 59m 35.99s , +14d 58m 57.1s) | C | 180 | 17.3 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. [1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023, Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp. http : // www.pereplet.ru/lipunov/625.html View this GCN Circular online at https://gcn.nasa.gov/circulars/43331. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41421] EP260104a: Einstein Probe detection of a fast X-ray transient
by GCN Circulars 05 Jan '26

05 Jan '26

TITLE: GCN CIRCULAR NUMBER: 43330 SUBJECT: EP260104a: Einstein Probe detection of a fast X-ray transient DATE: 26/01/05 08:41:44 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> H. Zhou (PMO, CAS), Z. C. Zou (NJU), J. P. Chen (SYSU), Y. J. Yi (BNU), D. Y. Li, C. L. Guo, Z. X. Ling, Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team: We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP260104a. The transient triggered EP-WXT (ID: 01709250979) at 2026-01-04T10:25:44 (UTC). The WXT position of the source is R.A. = 178.535 deg, DEC = 16.491 deg (J2000) with an uncertainty of 2.8 arcmin in radius (90% C.L. statistical and systematic). Since EP260104a was triggered in SAA region, no autonomous follow-up observation was performed. We analyzed the telemetry data, and the 0.5-4 keV rate light curve shows a single pulse lasting for about 150 seconds. The absorbed powerlaw model with a fixed Galactic column density of Hydrogen of 4.57 x 10^20 cm^-2 was adopted to fit the average WXT spectrum. The best-fitted photon index is 2.44(+1.26/-0.96), the average flux (0.5-4 keV) is (2.4+/-1.1) x 10^-10 erg s^-1 cm^-2, and no additional absorption is required. All uncertainties are reported in 90% C.L. We have arranged X-ray follow-up observations and will update the information upon receiving the data. 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/43330. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41420] GRB 260101A: COLIBRÍ optical upper limits
by GCN Circulars 05 Jan '26

05 Jan '26

TITLE: GCN CIRCULAR NUMBER: 43329 SUBJECT: GRB 260101A: COLIBRÍ optical upper limits DATE: 26/01/05 06:51:12 GMT FROM: Jean-Luc Atteia at IRAP <jean-luc.atteia(a)irap.omp.eu> Jean-Luc Atteia (IRAP), Noémie Globus (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Dalya Akl (NYUAD), Camila Angulo (UNAM), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Nikos Mandarakas (LAM), Kin Ocelotl López (UNAM), 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), report: We imaged the field of GRB 260101A (Fermi GBM team, GCN Circ. 43284; Cenko et al., GCN Circ. 43285; Pérez-Fournon et al., GCN Circ. 43286; de Ugarte Postigo et al., GCN Circ. 43287; Beardmore et al., GCN Circ. 43289; Lipunov et al., GCN Circ. 43290; D'Avanzo et al., GCN Circ. 43291; SVOM/GRM team et al., GCN Circ. 43292; Saccardi et al., GCN Circ. 43293; Wang et al., GCN Circ. 43294; Hristov et al., GCN Circ. 43297; Globus et al., GCN Circ. 43312; Klinger et al., GCN Circ. 43318) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2026-01-05 03:38 to 05:46 UTC (98.7 hours to 100.8 hours after the trigger) and obtained 94 minutes of simultaneous exposure in the r and z filters. The data were reduced and coadded 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. In our stacked images, the previously detected afterglow (Cenko et al., GCN Circ. 43285; Pérez-Fournon et al., GCN Circ. 43286; Globus et al., GCN Circ. 43312; Klinger et al., GCN Circ. 43318) has faded beyond our 5-sigma detection limit: r > 22.96, z > 22.42. 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/43329. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41419] EP260105a: Global MASTER-Net observations report
by GCN Circulars 05 Jan '26

05 Jan '26

TITLE: GCN CIRCULAR NUMBER: 43328 SUBJECT: EP260105a: Global MASTER-Net observations report DATE: 26/01/05 06:36:38 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-OAFA robotic telescope [1] located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP260105a ( EP Team et al., GCN 43327) errorbox 302 sec after trigger time at 2026-01-05 05:55:32 UT, with upper limit up to 17.7 mag. The observations began at zenith distance = 76 deg. The sun altitude is -33.4 deg. The galactic latitude b = 79 deg., longitude l = 235 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3096319 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 333 | 2026-01-05 05:55:32 | MASTER-OAFA | (12h 04m 39.81s , +22d 45m 13.6s) | C | 60 | 17.0 | 404 | 2026-01-05 05:56:38 | MASTER-OAFA | (12h 04m 39.97s , +22d 45m 19.5s) | C | 70 | 17.2 | 490 | 2026-01-05 05:57:55 | MASTER-OAFA | (12h 04m 40.18s , +22d 45m 26.3s) | C | 90 | 17.3 | 597 | 2026-01-05 05:59:31 | MASTER-OAFA | (12h 04m 40.39s , +22d 45m 34.0s) | C | 110 | 17.3 | 723 | 2026-01-05 06:01:28 | MASTER-OAFA | (12h 04m 40.60s , +22d 45m 42.6s) | C | 130 | 17.4 | 874 | 2026-01-05 06:03:44 | MASTER-OAFA | (12h 04m 40.84s , +22d 45m 52.0s) | C | 160 | 17.5 | 991 | 2026-01-05 06:06:30 | MASTER-OAFA | (12h 04m 41.03s , +22d 45m 59.4s) | C | 60 | 17.5 | 1056 | 2026-01-05 06:07:36 | MASTER-OAFA | (12h 04m 41.10s , +22d 46m 03.1s) | C | 60 | 17.5 | 1121 | 2026-01-05 06:08:41 | MASTER-OAFA | (12h 04m 41.18s , +22d 46m 07.0s) | C | 60 | 17.5 | 1188 | 2026-01-05 06:09:47 | MASTER-OAFA | (12h 04m 41.28s , +22d 46m 10.9s) | C | 60 | 17.5 | 1253 | 2026-01-05 06:10:53 | MASTER-OAFA | (12h 04m 41.36s , +22d 46m 14.7s) | C | 60 | 17.6 | 1319 | 2026-01-05 06:11:58 | MASTER-OAFA | (12h 04m 41.43s , +22d 46m 18.6s) | C | 60 | 17.7 | 1385 | 2026-01-05 06:13:05 | MASTER-OAFA | (12h 04m 41.51s , +22d 46m 22.1s) | C | 60 | 17.7 | 1451 | 2026-01-05 06:14:10 | MASTER-OAFA | (12h 04m 41.60s , +22d 46m 25.6s) | C | 60 | 17.6 | 1516 | 2026-01-05 06:15:16 | MASTER-OAFA | (12h 04m 41.63s , +22d 46m 29.0s) | C | 60 | 17.7 | 1582 | 2026-01-05 06:16:22 | MASTER-OAFA | (12h 04m 41.67s , +22d 46m 32.3s) | C | 60 | 17.7 | 1648 | 2026-01-05 06:17:27 | MASTER-OAFA | (12h 04m 41.73s , +22d 46m 35.5s) | C | 60 | 17.7 | 1714 | 2026-01-05 06:18:33 | MASTER-OAFA | (12h 04m 41.76s , +22d 46m 38.8s) | C | 60 | 17.7 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. [1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023, Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp. http : // www.pereplet.ru/lipunov/625.html View this GCN Circular online at https://gcn.nasa.gov/circulars/43328. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41418] EP260105a: Einstein Probe detection of an X-ray transient
by GCN Circulars 05 Jan '26

05 Jan '26

TITLE: GCN CIRCULAR NUMBER: 43327 SUBJECT: EP260105a: Einstein Probe detection of an X-ray transient DATE: 26/01/05 06:17:04 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> H. Zhou (PMO, CAS), Z. C. Zou (NJU), D. Y. Li, Z. X. Ling (NAO, CAS) on behalf of the Einstein Probe (EP) team: We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP260105a. The transient triggered EP-WXT (ID: 01709250995) at 2026-01-05T05:50:30 (UTC). The WXT position of the source is R.A. = 181.036 deg, DEC = 22.723 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 181.0158 deg, DEC = 22.7089 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). Further information will be updated when the telemetry data is received. 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/43327. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41417] GRB 260103A: Insight-HXMT/HE detection
by GCN Circulars 05 Jan '26

05 Jan '26

TITLE: GCN CIRCULAR NUMBER: 43326 SUBJECT: GRB 260103A: Insight-HXMT/HE detection DATE: 26/01/05 05:48:29 GMT FROM: Xinghao Luo at IHEP <2952704891(a)qq.com> Xing-hao Luo, Chen-Wei Wang, Chao Zheng, and Shao-Lin Xiong (IHEP) report on behalf of the Insight-HXMT team: At 2026-01-03T01:46:53.800 (T0), Insight-HXMT/HE detected the long burst GRB 260103A, which is also detected by SVOM/GRM (Chen-Wei Wang et.al., GCN #43314) , GECAM-B (Xing-Hao Luo et.al., GCN #43317) The Insight-HXMT/HE light curve mainly consists of multiple pulses with a T90 of 4.6 +1.8/-1.0 s. The 1s peak rate, measured from T0-0.25 s, is 1848 cnts/sec. Insight-HXMT/HE detected a total of 4704 counts form this burst. The HXMT/HE light curve can be found here: https://www.bursthub.cn//admin/static/hxmtgrb260103A.png All measurements above are made with the CsI detectors of Insight-HXMT/HE 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/43326. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41416] GRB 260104A: COLIBRÍ optical upper limits
by GCN Circulars 05 Jan '26

05 Jan '26

TITLE: GCN CIRCULAR NUMBER: 43325 SUBJECT: GRB 260104A: COLIBRÍ optical upper limits DATE: 26/01/05 04:32:13 GMT FROM: globus(a)astro.unam.mx Noémie Globus (UNAM), Jean-Luc Atteia (IRAP), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Dalya Akl (NYUAD), Camila Angulo (UNAM), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Nikos Mandarakas (LAM), Kin Ocelotl López (UNAM), 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), report: We imaged the field of GRB 260104A (Caputo et al., GCN Circ. 43315; Evans et al., GCN Circ. 43319; Kennea et al., GCN Circ. 43320), using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-01-05 at 01:46 to 02:44 UTC (from 20.16 to 21.12 hr after the trigger) and obtained 41 minutes of simultaneous exposure in the r and z filters. The data were reduced and coadded with the COLIBRÍ ASU pipeline and analyzed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and was not corrected for Galactic extinction. In the stacked image, we do not detect any new source at the XRT uncertainty region (Caputo et al., GCN Circ. 43315) down to the following 5-sigma limit: r > 21.90, z > 20.96. 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/43325. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41415] GRB 260104C: Fermi GBM Final Real-time Localization
by GCN Circulars 05 Jan '26

05 Jan '26

TITLE: GCN CIRCULAR NUMBER: 43324 SUBJECT: GRB 260104C: Fermi GBM Final Real-time Localization DATE: 26/01/05 00:08:31 GMT FROM: Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply(a)GIOC.nsstc.nasa.gov> The Fermi GBM team reports the detection of a likely LONG GRB At 23:57:57 UT on 4 Jan 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260104C (trigger 789263882.41794 / 260104999). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 312.7, Dec = -26.1 (J2000 degrees, equivalent to J2000 20h 50m, -26d 06'), with a statistical uncertainty of 9.3 degrees. The angle from the Fermi LAT boresight is 101.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260104999/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260104999/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260104999/… View this GCN Circular online at https://gcn.nasa.gov/circulars/43324. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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