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

[vsnet-grb-info 37668] IPN triangulation of GRB 250309B
by GCN Circulars 10 Mar '25

10 Mar '25

TITLE: GCN CIRCULAR NUMBER: 39652 SUBJECT: IPN triangulation of GRB 250309B DATE: 25/03/10 13:17:34 GMT FROM: Dmitry Svinkin at Ioffe Institute <svinkin(a)mail.ioffe.ru> A.S. Kozyrev, D.V. Golovin, M.L. Litvak, I.G. Mitrofanov, and A.B. Sanin on behalf of the HEND/Mars Odyssey team, D. Svinkin, D. Frederiks, A. Lysenko, A. Ridnaia, and T. Cline on behalf of the Konus-Wind team, A. Goldstein, M. S. Briggs, C. Wilson-Hodge, and E. Burns on behalf of the Fermi GBM team, and W. Boynton, C. Fellows, K. Harshman, H. Enos, R. Starr, and A.S. Gardner on on behalf of the GRS-Odyssey GRB team, report: The long-duration GRB 250309B (Fermi-GBM BALROG localization: Preis and Greiner, GCN 39629; Fermi-GBM detection: The Fermi GBM team, GCN 39635; McDermott et al., GCN 39642; GECAM-B detection: Wang et al., GCN 39648) was detected by Fermi (GBM trigger 763198715), Konus-Wind, Mars-Odyssey (HEND), and GECAM-B at about 27510 s UT (07:38:30). We have triangulated it to a preliminary, 3 sigma error box whose coordinates are: ------------------------------- RA(2000), deg Dec(2000), deg ------------------------------ Center: 208.8101 -12.9141 Corners: 204.1541 -23.3442 204.3621 -23.0319 213.4312 -2.9483 213.5345 -2.5970 ------------------------------- The error box area is 1.3 sq. deg, and its maximum dimension is 23 deg (the minimum one is 3.5 arcmin). The Sun distance was 135 deg. This localization may be improved. The IPN localization is consistent with, but reduces the area of, the final Fermi-GBM (GCN 39635) and BALROG (GCN 39629) localizations. A triangulation map and HEALPix FITS file are posted at http://www.ioffe.ru/LEA/GRBs/GRB250309_T27513/IPN/ The HEALPix triangulation map is the multi-order HEALPix in units of probability density. The localization of IceCube-250309A (IceCube Collaboration, GCN 39631) is inconsistent with the IPN localization. The optical transient ZTF25aaitvjt/AT2025dws (Stein et al., GCN 39639) is inside the IPN box, which further strengthen the interpretation of transient as the burst afterglow. The Konus-Wind time history and spectrum will be given in a forthcoming GCN Circular. View this GCN Circular online at https://gcn.nasa.gov/circulars/39652. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 37667] IceCube-241224A: MASTER detection of PKS 1217+02 blazar flare with Zhirkov effect
by GCN Circulars 10 Mar '25

10 Mar '25

TITLE: GCN CIRCULAR NUMBER: 39651 SUBJECT: IceCube-241224A: MASTER detection of PKS 1217+02 blazar flare with Zhirkov effect DATE: 25/03/10 12:01:17 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> K. Labzina, K.Zhirkov, V.Lipunov, E.Gorbovskoy, A.Kuznetsov, G.Antipov, D.Vlasenko, A.Kuznetsov, N.Tyurina, P.Balanutsa, A.Chasovnikov, V.Topolev, Ya.Kechin, V.Senik, Yu.Tselik (Lomonosov MSU), O.Gress, N.Budnev, O.Ershova (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), D.Buckley (SAAO), A.Sosnovskij (Crao RAS), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), 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 (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) started IceCube-241224A (IC GCN #38664, trigger No 303893, trigger time 07:10:04UT, 12h 20m 13.44s , +02d 46m 01.2s, R=0.74) errorbox 16 sec after notice time (178 sec after trigger time) at 2024-12-24 07:13:02 UT, with upper limit up to 19.6 mag (Lipunov et al.GCN#38663, MASTER cover map: https://master.sai.msu.ru/site/master2/event.php?id=2720792 ) We observed it in MASTER-SAAO, MASTER-OAFA and analyzed MASTER archive images since 2016 of the sources, that can be related with this event. There is quasar PKS 1217+02 inside 3-sigma IC error-box https://vizier.cds.unistra.fr/viz-bin/VizieR?-source=&-out.add=_r&-out.add=…) We detected its brightening up to 15.7m near alert time. MASTER archive light curve from 2016 year is available at http://observ.pereplet.ru/IC/MASTER_IC241224A.jpg We report the observation of an Zhirkov effect, discovered earlier (The Astrophysic Jornal Letters, 896, L19, 2020 https://ui.adsabs.harvard.edu/abs/2020ApJ...896L..19L/abstract ). View this GCN Circular online at https://gcn.nasa.gov/circulars/39651. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 37666] GRB 250309B/AT2025dws : COLIBRÍ Detection of the Optical Counterpart
by GCN Circulars 10 Mar '25

10 Mar '25

TITLE: GCN CIRCULAR NUMBER: 39650 SUBJECT: GRB 250309B/AT2025dws : COLIBRÍ Detection of the Optical Counterpart DATE: 25/03/10 11:48:47 GMT FROM: F. Fortin at IRAP <ffortin.sci.edu(a)gmail.com> Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Kin Ocelotl López (UNAM), Francesco Magnani (CPPM), Stéphane Basa (UAR Pytheas), Benjamin Schneider (LAM), Alan M. Watson (UNAM), William H. Lee (UNAM), Camila Angulo (UNAM), Dalya Akl (AUS), Sarah Antier (OCA), Jean-Luc Atteia (IRAP), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Noémie Globus (UNAM), Diego López-Cámara (UNAM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), and Antonio de Ugarte Postigo (LAM) We observed the field of GRB 250309B (Preis et al., GCN 39629) using the OGSE engineering camera on the COLIBRÍ telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico. We observed from 2025-03-10T07:34:55 to 2025-03-10T08:46:13 UTC (23h 56min after the trigger) and obtained 3000 seconds of exposure in the r filter. The data were analyzed in STDWeb/STDPipe (Karpov 2021), with photometric calibration against Pan-STARRS DR1 and image subtraction against Pan-STARRS DR2. Our photometry is in the AB system and is not corrected for Galactic extinction. In our stacked image, after the subtraction, we clearly detect a source at the position of the optical candidate AT2025dws (reported by Stein et al., GCN 39639, Pérez-Fournon et al. GCN 39643, Moskvitin et al. GCN 39645, Perley et al. GCN 39646) with magnitude: r = 20.76 +/- 0.10 Further observations are planned to monitor the evolution of the transient. We warmly thank the COLIBRÍ and DDRAGO engineering teams and the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir. View this GCN Circular online at https://gcn.nasa.gov/circulars/39650. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 37665] GRB 250309B: Swift-XRT refined Analysis
by GCN Circulars 10 Mar '25

10 Mar '25

TITLE: GCN CIRCULAR NUMBER: 39649 SUBJECT: GRB 250309B: Swift-XRT refined Analysis DATE: 25/03/10 07:19:06 GMT FROM: K.L. Page at U Leicester <klp5(a)leicester.ac.uk> K.L. Page and P.A. Evans report on behalf of the Swift-XRT team: We have analysed 3.9 ks of XRT data for the Fermi/GBM-detected GRB 250309B, from 46.1 to 57.7 ks after the Fermi trigger (GCN Circs. 39635, 39642). The data are entirely in Photon Counting (PC) mode. We detect a previously uncatalogued X-ray source at the following coordinates: RA/Dec(J2000) = 210.80129, -8.50302, which is equivalent to RA (J2000): 14 03 12.31 Dec(J2000): -08 30 10.9 with an uncertainty of 3.0 arcsec (radius, 90% confidence). This source is 1.8 arcsec from the optical counterpart AT2025dws discovered by the Zwicky Transient Facility (GCN Circ. 39639), and also detected by LCO (GCN Circ. 39643), SAO RAS (GCN Circ. 39645), the Liverpool telescope (GCN Circ. 39646) and the Swift UVOT (GCN Circ. 39644). A redshift of 1.898 has been reported from OSIRIS+/GTC spectroscopy in GCN Circ. 39647. The light curve can be modelled with a power-law decay with a decay index of alpha=2.7 (+1.1, -2.6). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.74 (+0.24, -0.23). The best-fitting absorption column is consistent with the Galactic value of 3.0 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.8 x 10^-11 (4.0 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Galactic foreground column: 3.0 x 10^20 cm^-2 Photon index: 1.74 (+0.24, -0.23) If the light curve continues to decay with a power-law decay index of 2.7, the count rate at T+24 hours will be 0.015 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.7 x 10^-13 (6.0 x 10^-13) erg cm^-2 s^-1. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/39649. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 37664] GRB 250309B / IceCube-250309A: GECAM-B observations
by GCN Circulars 10 Mar '25

10 Mar '25

TITLE: GCN CIRCULAR NUMBER: 39648 SUBJECT: GRB 250309B / IceCube-250309A: GECAM-B observations DATE: 25/03/10 07:04:11 GMT FROM: Yue Wang <m18509381757(a)163.com> Yue Wang, Jia-Cong Liu, Jin-Peng Zhang, Chen-Wei Wang, Wen-Jun Tan, Wang-Chen Xue, Shao-Lin Xiong (IHEP), report on behalf of the GECAM team: GECAM-B was triggered on-ground by a long burst, GRB 250309B, at 2025-03-09T07:38:31.050 UTC (T0), which was also observed by Fermi/GBM (McDermott et al., GCN 39642). According to GECAM-B light curve, this burst mainly consists of multiple pulses with a duration (T90) of about 6.74 (+/-0.15) sec (50-300 keV). The GECAM-B light curve of GRB 250309B could be found at: https://www.bursthub.cn//admin/static/gecamgrb250309B.png The GECAM-B localization of GRB 250309B is consistent with the Fermi/GBM localization of this burst (McDermott et al., GCN 39642) and the position of optical counterpart AT 2025dws (Stein et al., GCN 39639) as well as the IceCube-250309A (The IceCube Collaboration, GCN 39631). Thus, we confirm that GRB 250309B is spatially and temporally coincident with IceCube-250309A. The time-integrated spectrum of GRB 250309B from T0-1.0 s to T0+6.0 s is best fitted by a cutoff-powerlaw function with Epeak = 109.3 +/- 6.8 keV and alpha = -1.00 +/- 0.27. The event fluence (10-1000 keV) in this time interval is (7.65 +/- 0.56)E-06 erg/cm^2. With the measured redshift z=1.898 (D. B. Malesani et al., GCN 39647), we calculate the isotropic energy Eiso is (7.4 ± 0.8)x10^52 erg. Then we note that GRB 250309B is well consistent with Type II GRBs in the 'Amati' relation diagram, as shown at: https://www.bursthub.cn//admin/static/grb250309B_amati.png Further, we note that there is no GECAM-B trigger just around the event time of IceCube-250309A (2025-03-09 at 07:36:04.75 UT, The IceCube Collaboration, GCN 39631). We implemented a targeted search [1] for burst activities from T0-330 s to T0-30 s embracing this IceCube event, but identified no candidate above 3 sigma. Thus, we calculated the upper limit of a potential precursor of GRB 250309B which is simultaneously associated with IceCube-250309A. Considering three typical GRB spectral models (i.e. soft, normal and hard Band functions), three timescales and the center region of GRB localization (RA= 211.07, Dec = -10.73), the 3 sigma upper limits of the potential GRB precursor energy flux (15 keV-5000 keV, in units of 10^-7 erg/s/cm^2) are reported below: |Timescale (s)|Soft|Normal|Hard| | ------------ | ------------ | ------------ | ------------ | |0.1|3.86|3.92|6.33| |1 |1.22|1.24|2.00| |10 |0.38|0.39|0.63| We note that these results are preliminary. Refined analysis will be reported. Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two microsatellites (GECAM-A and GECAM-B) launched in Dec. 2020. As the third member of GECAM constellation, GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS). View this GCN Circular online at https://gcn.nasa.gov/circulars/39648. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 37663] GRB 250309B / AT2025dws: OSIRIS+/GTC spectroscopy z=1.898
by GCN Circulars 10 Mar '25

10 Mar '25

TITLE: GCN CIRCULAR NUMBER: 39647 SUBJECT: GRB 250309B / AT2025dws: OSIRIS+/GTC spectroscopy z=1.898 DATE: 25/03/10 05:47:31 GMT FROM: Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo(a)gmail.com> D. B. Malesani (DAWN/NBI and Radboud Univ.), A. de Ugarte Postigo (LAM), A. Martin-Carrillo (UCD), C. C. Thoene (AbAO), J. F. Agui Fernandez (CAHA), N. R. Tanvir (Univ. Leicester), L. Izzo (INAF/OACn and DARK/NBI), J. P. U. Fynbo (DAWN/NBI), S. Geier (GTC), G. Lombardi (GTC), N. A. Rakotondrainibe (LAM), F. Perez Toledo (GTC), and A. Perez (GTC) report: We observed the afterglow of GRB 250309B (Preis et al. GCN 39629; Fermi GBM team GCN 39635; Stein et al. GCN 39639; McDermott et al. GCN 39642; Perez-Fournon et al. GCN 39643; Stain et al. GCN 39644; Moskvitin et al. GCN 39645; Perley et al. GCN 39646) with OSIRIS+, mounted on te 10.4.m GTC telescope at the Roque de los Muchachos Observatory in the island of La Palma (Spain). The observation started at 2025-03-10T03:28:33.011 UT (19.83 h after the burst) and consisted of 2x900 s spectroscopy with grism R1000B, covering the range between 3600 and 7880 AA. A preliminary reduction shows a continuum across the complete spectral range with multiple spectral features which we interpret as due to SiII, SiIV, OI, CII, CIV, FeII, AlII, AlIII, ZnII, and MnII at a common redshift of 1.898, which we identify as the redshift of the GRB. Additionally, we detect a strong intervening system with features of CIV, AlII, FeII, MgII and MgI at a a redshift of 1.634. View this GCN Circular online at https://gcn.nasa.gov/circulars/39647. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 37662] Liverpool Telecope observations of AT 2025dws / GRB 250309B
by GCN Circulars 10 Mar '25

10 Mar '25

TITLE: GCN CIRCULAR NUMBER: 39646 SUBJECT: Liverpool Telecope observations of AT 2025dws / GRB 250309B DATE: 25/03/10 03:52:06 GMT FROM: Daniel Perley at Liverpool JMU <d.a.perley(a)ljmu.ac.uk> D. A. Perley (LJMU), A. Bochenek (LJMU), and A. Y. Q. Ho (Cornell) report: We obtained imaging observations of AT2025dws (Stein et al., GCN 39639), a possible optical counterpart of GRB 250309B (Preis & Greiner, GCN 39629; McDermott et al., GCN 39635) with IO:O on the Liverpool Telescope on UT 2025 March 10 between 01:46:53 and 02:01:08 UT. Two 100s exposures were acquired in each of the g, r, i, and z bands. The source was weakly detected in all four filters. We report the following photometry, calibrated relative to SDSS standards (dt is reported relative to the GRB 250309B GBM trigger time; McDermott et al., GCN 39642): MJD dt filter mag err 60744.07423 0.7558 g 20.47 0.19 60744.07708 0.7587 r 20.30 0.14 60744.07991 0.7615 i 20.07 0.19 60744.08274 0.7643 z 19.44 0.17 The red colours and significant fading are consistent with the interpretation of this source as the afterglow (see also Pérez-Fournon et al., GCN 39643; Stein et al., GCN 39644; Alexander et al., 39645). No host galaxy subtraction or extinction correction has been performed. View this GCN Circular online at https://gcn.nasa.gov/circulars/39646. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 37661] GRB 250309B / AT2025dws: SAO RAS optical observations
by GCN Circulars 10 Mar '25

10 Mar '25

TITLE: GCN CIRCULAR NUMBER: 39645 SUBJECT: GRB 250309B / AT2025dws: SAO RAS optical observations DATE: 25/03/10 02:45:01 GMT FROM: Moskvitin Alexander at SAO RAS <mosk(a)sao.ru> A. S. Moskvitin, O. I. Spiridonova (SAO RAS), A. S. Pozanenko (IKI), A. Ghosh, S. Razzaque (CAPP, University of Johannesburg), Yu. V. Sotnikova (SAO RAS) report on behalf of a larger collaboration. We partially covered the error circle of the GRB 250309B (Preis and Greiner, GCN 39629; Fermi GBM team, GCN 39635; McDermott et al., GCN 39642) with 1-m telescope of SAO RAS, Zeiss-1000, equipped with the CCD-photometer. The fields of ZTF25aaitvjt / AT2025dws (Stein et al., GCN 39639), XRT sources #1 and #3 (Evans et al., GCN 39633) were observed in Rc band on March 9/10 night. The AT2025dws source does not fit into the error circle of ground-based GBM localization (Fermi GBM group, GCN 39635), but falls into the BALROG localization field (Preis et al., GCN 39629). We obtained two epochs of observations for ZTF25aaitvjt / AT2025dws field. The 1st: on March 9, 21:58:17 -- March 9, 22:29:28 UT, t_mid - T0 = 14.5893 h = 0.60789 days (5 x 300 sec. exposure); the 2nd: on March 9, 23:52:03 -- March 10, 00:47:22 UT, t_mid - T0 = 16.6865 h = 0.69527 days (10 x 300 sec. exposure). We clearly detected ZTF25aaitvjt / AT2025dws (Stein et al., GCN 39639; Pérez-Fournon et al., GCN 39643; Stein and Ahumada, GCN 39644) in the stacked images. Preliminary photometry is the following. Date UT start t-T0 Exptime Filter OT Err UL (mid, days) (s) (3sigma) 2025-03-09 21:58:17 0.60789 5 x 300 Rc 19.52 +/- 0.04 22.0 2025-03-09 23:52:03 0.69527 10 x 300 Rc 19.66 +/- 0.02 22.6 The above photometry includes the OT and a possible host galaxy. The photometry is based on nearby stars from the USNO-B1.0 catalog (R2 magnitudes) and has not been corrected for the Galactic extinction. USNO-B1.0 reference stars RA Dec R2 14:03:06.24 -08:27:38.9 15.67 14:03:08.07 -08:26:47.1 17.30 View this GCN Circular online at https://gcn.nasa.gov/circulars/39645. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 37660] GRB 250309B / AT2025dws: Swift-UVOT detection
by GCN Circulars 10 Mar '25

10 Mar '25

TITLE: GCN CIRCULAR NUMBER: 39644 SUBJECT: GRB 250309B / AT2025dws: Swift-UVOT detection DATE: 25/03/10 02:12:00 GMT FROM: Robert David Stein at JSI <rdstein(a)umd.edu> Robert Stein (JSI), Tomas Ahumada (Caltech) report, On behalf of the Zwicky Transient Facility (ZTF) and Global Relay of Observatories Watching Transients Happen (GROWTH) collaborations: We requested observations of AT2025dws (Stein et al., GCN 39639), the candidate counterpart to GRB 250309B (Preis & Greiner, GCN 39629; McDermott et al., GCN 39635), with the Neil Gehrels Swift Observatory. Observations were conducted beginning 2025-03-09 20:26:46 UT, ~13 hours after the GRB time (2025-03-09 07:38:30.66 UT). These observations had an exposure time of 30 minutes, in the U-band filter. We reduced the Swift UVOT data (Roming et al. 2005) using HEASoft. A source is clearly visible at the position of AT2025dws, with an apparent magnitude of m = 21.5 +/- 0.2 [AB Mag]. These observations had a limiting magnitude of m = 22.4 [AB Mag]. While we have not performed host subtraction, the source is marginally detected in archival SDSS imaging with a u-band magnitude of m=24.02. This suggests our detection is dominated by transient light. AT2025dws has been already been confirmed to be fast-fading by recent optical observations with LCO (Perez-Fournon et al., GCN 39643). While there are no earlier U-band detection of this transient, our UVOT detection is fainter than the optical detections reported in GCNs 39639 and 39643. This confirms the red nature of the transient, and is consistent with the expected red/fast-fading behaviour of a GRB aferglow. View this GCN Circular online at https://gcn.nasa.gov/circulars/39644. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 37659] AT 2025dws, likely optical counterpart of GRB 250309B: LCO confirmation of fast fading
by GCN Circulars 10 Mar '25

10 Mar '25

TITLE: GCN CIRCULAR NUMBER: 39643 SUBJECT: AT 2025dws, likely optical counterpart of GRB 250309B: LCO confirmation of fast fading DATE: 25/03/10 01:28:25 GMT FROM: Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf(a)iac.es> I. Pérez-Fournon, F. Poidevin (IAC and ULL), D. Cano-Morales, I. Correa-Plasencia, and A.E. Hernández-Díaz (ULL) We report Las Cumbres Observatory Global Telescope network (LCOGT) observations of ZTF25aaitvjt / AT 2025dws, that has been proposed by Stein et al. (GCN circ. 39639) as the candidate optical counterpart of the Fermi GBM likely long Gamma-Ray Burst GRB 250309B (Preis & Greiner, GCN circ. 39629; Fermi GBM team, GCN circ. 39635; and McDermott et al., GCN circ. 39642), that may be related with the IceCube high-energy neutrino IceCube-250309A (The IceCube Collaboration, GCN circ. 39631). We observed the field of ZTF25aaitvjt / AT 2025dws with one of the LCOGT 1-m telescopes, equipped with Sinistro cameras, located at the LCOGT node at Sutherland Observatory (South Africa). We obtained a 180-sec exposure in each of the SDSS g', r', and i' filters starting at about 21.77 hours after the Fermi trigger. We detect ZTF25aaitvjt / AT 2025dws in the three filters. We measure the following magnitudes, calibrated against Pan-STARRS DR2 stars and not corrected for Galactic extinction: Date | UT start | t_mid - t0 (hours) | mag | error | filter | ---------------------------------------------------------------------- 2025-03-09 22:28:06 21.79 20.38 0.15 g' 2025-03-09 22:31:46 21.85 20.12 0.14 r' 2025-03-09 22:35:16 21.91 19.87 0.14 i' The fast fading in the g' and r' bands compared with the Zwicky Transient Facility (ZTF) detections reported by Stein et al. (GCN circ. 39639 and TNS Astronomical Transient Report No. 247224) supports that ZTF25aaitvjt / AT 2025dws is the likely optical afterglow of GRB 250309B. This work makes use of observations from the Las Cumbres Observatory global telescope network (LCOGT observing programme IAC2025A-009, SGLF). View this GCN Circular online at https://gcn.nasa.gov/circulars/39643. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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