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[vsnet-grb-info 40290] LIGO/Virgo/KAGRA S251013x: Updated Sky localization and Source Classification
by GCN Circulars 13 Oct '25

13 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42220 SUBJECT: LIGO/Virgo/KAGRA S251013x: Updated Sky localization and Source Classification DATE: 25/10/13 16:23:04 GMT FROM: Nihar.Gupte(a)aei.mpg.de The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S251013x (GCN Circular 42212). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page: https://gracedb.ligo.org/superevents/S251013x For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 1361 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 3775 +/- 1200 Mpc (a posteriori mean +/- standard deviation). For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/. [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040 View this GCN Circular online at https://gcn.nasa.gov/circulars/42220. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40289] GRB 251013B: SVOM/GRM observation
by GCN Circulars 13 Oct '25

13 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42219 SUBJECT: GRB 251013B: SVOM/GRM observation DATE: 25/10/13 15:20:56 GMT FROM: Yue Wang <m18509381757(a)163.com> SVOM/GRM team: Yue Wang, Chen-Wei Wang, Wen-Jun Tan, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP) SVOM/ECLAIRs team: Stéphane Schanne (CEA), Hui Yang (IRAP) Report on behalf of the SVOM team: SVOM/GRM was triggered on-ground by GRB 251013B at 2025-10-13T08:24:36.250 (T0), which is also detected by Fermi/GBM (Fermi GBM Team, GCN #42215). With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of mutli-peaks with a T90 of 55 +16/-7 s in the 15-5000 keV band. The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb251013B.png In addition, the position of this burst, as determined by Fermi/GBM (RA=356.5, DEC=7.5, GCN #42215), is located at about 68.2 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view. With this localization, the time-averaged spectrum from T0-30 to T0+50 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.63 +0.10/-0.13 and the cutoff energy, parameterized as Epeak, is 324 +567/-129 keV. The event fluence (10-1000 keV) in this time interval is (1.08 +0.09/-0.11)E-05 erg/cm^2. The localization of GRB 251013B in the 'Amati' relation diagram is shown at: https://www.bursthub.cn//admin/static/svgrb251013B_amati.png 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. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS. The SVOM/GRM point of contact for this burst is: Yue Wang (IHEP) (yuewang(a)ihep.ac.cn) View this GCN Circular online at https://gcn.nasa.gov/circulars/42219. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40288] GRB 251013A: SVOM/GRM observation
by GCN Circulars 13 Oct '25

13 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42218 SUBJECT: GRB 251013A: SVOM/GRM observation DATE: 25/10/13 15:18:58 GMT FROM: Yue Wang <m18509381757(a)163.com> SVOM/GRM team: Yue Wang, Chen-Wei Wang, Wen-Jun Tan, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP) SVOM/ECLAIRs team: Stéphane Schanne (CEA), Hui Yang (IRAP) Report on behalf of the SVOM team: SVOM/GRM was triggered both in-flight by GRB 251013A (SVOM trigger reference: sb25101308) at 2025-10-13T08:04:05.800 (T0), which is also detected by Fermi/GBM (Fermi GBM Team, GCN #42213). With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of a single pulse with a T90 of 3.5 +0.9/-0.6 s in the 15-5000 keV band. The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb251013A.png In addition, the position of this burst, as determined by Fermi/GBM (RA=19.9, DEC=-1.6, GCN #42213), is located at about 57 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view. Marginal signal seen by ECLAIRs through the shields With this localization, the time-averaged spectrum from T0-2 to T0+4 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -0.68 +0.13/-0.13 and the cutoff energy, parameterized as Epeak, is 142 +15/-12 keV. The event fluence (10-1000 keV) in this time interval is (2.96 +0.19/-0.17)E-06 erg/cm^2. The localization of GRB 251013A in the 'Amati' relation diagram is shown at: https://www.bursthub.cn//admin/static/svgrb251013A_amati.png 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. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS. The SVOM/GRM point of contact for this burst is: Yue Wang (IHEP) (yuewang(a)ihep.ac.cn) View this GCN Circular online at https://gcn.nasa.gov/circulars/42218. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40287] GRB 251003A: Mondy optical upper limit
by GCN Circulars 13 Oct '25

13 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42217 SUBJECT: GRB 251003A: Mondy optical upper limit DATE: 25/10/13 14:51:03 GMT FROM: Alina Volnova at IKI RAS <alinusss(a)gmail.com> A. Volnova (IKI), N. Pankov (HSE, IKI), E. Klunko (ISTP), A. Pozanenko (IKI) report on behalf of IKI-GRB-FuN: We performed optical observations of the field of GRB 251003A, detected by Fermi (Fermi team, GCNC 42069) and Swift (Beardmore et al., GCNC 42070), in the R-filter with the AZT-33IK 1.5m telescope of the Sayan Solar Observatory (Mondy). The observations began on 2025-10-06 13:10 UT, i.e. ~3.5 days since the trigger and consisted of 90 exposures by 80 s each. The optical source reported previously (Aceituno et al., GCN 42072; Méndez et al., GCN 42073; Strausbaugh et al., GCN 42074; Freeberg et al., GCN 42085; Sanchez-Ramirez et al., GCN 42086; Worssam et al., GCN 42094; Schneider et al., GCN 42095; Mo et al., GCN 42106; Konno et al., GCN 42126) is not detected in the stacked frame. The preliminary photometry is based on nearby USNO-B stars (R2 magnitudes) and is the following: Date UT start t-T0 Exp. Filter OT Err. UL (mid, days) (n*s) (3sigma) 2025-10-06 13:10:16 3.50116 90*80 R n/d n/d 22.0 View this GCN Circular online at https://gcn.nasa.gov/circulars/42217. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40286] MASTER OT J011025.98-583210.6 : SVOM/VT optical observation
by GCN Circulars 13 Oct '25

13 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42216 SUBJECT: MASTER OT J011025.98-583210.6 : SVOM/VT optical observation DATE: 25/10/13 14:25:57 GMT FROM: Yinuo Ma <mayn(a)bao.ac.cn> Y. N. Ma, L. P. Xin, H. L. Li, Z. H. Yao, Y. L. Qiu, C. Wu, X. H. Han, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team. SVOM/VT performed a Target of Opportunity observation of MASTER OT J011025.98-583210.6 (Kechin et al., GCN 42211). SVOM/VT began observing the field at 2025-10-13T04:48:11 UTC in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. With X-band data, at the position of the optical transient (Kechin et al., GCN 42211) we don't detect any uncatalogued source, compared to the Legacy survey. But we noticed that there is a catalogued red source with an angular distance of 1.0 seconds from the source reported (Kechin et al., GCN 42211). The position is R.A.=01:10:26.11, DEC. = -58:32:10.49, J2000. The brightness was 20.31+/-0.07 in VT_R band, but VT_B>23 mag. The analysis showed that this source was stable during our 3.7 hours observation. Our photometry was not corrected for Galactic extinction. Given the source with red color at the position which is very near the MASTER OT J011025.98-583210.6, it is possible associated. A stellar flare from this late type star is possible. 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/42216. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40285] GRB 251013B: Fermi GBM Final Real-time Localization
by GCN Circulars 13 Oct '25

13 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42215 SUBJECT: GRB 251013B: Fermi GBM Final Real-time Localization DATE: 25/10/13 08:33:39 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 08:24:27 UT on 13 Oct 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251013B (trigger 782036672.403254 / 251013350). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 356.5, Dec = 7.5 (J2000 degrees, equivalent to J2000 23h 45m, 7d 30'), with a statistical uncertainty of 5.7 degrees. The angle from the Fermi LAT boresight is 87.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251013350/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251013350/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251013350/… View this GCN Circular online at https://gcn.nasa.gov/circulars/42215. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40284] Fermi GRB 251013A: Global MASTER-Net observations report
by GCN Circulars 13 Oct '25

13 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42214 SUBJECT: Fermi GRB 251013A: Global MASTER-Net observations report DATE: 25/10/13 08:30:31 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 (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) started inspect of the Fermi GRB 251013A ( Fermi GBM team, GCN 42213) errorbox 544 sec after notice time and 582 sec after trigger time at 2025-10-13 08:13:47 UT, with upper limit up to 19.4 mag. The observations began at zenith distance = 61 deg. The sun altitude is -22.2 deg. The galactic latitude b = -63 deg., longitude l = 140 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3013630 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 642 | 2025-10-13 08:13:47 | MASTER-OAFA | (01h 20m 06.37s , -01d 28m 39.3s) | C | 120 | 19.1 | 778 | 2025-10-13 08:15:53 | MASTER-OAFA | (01h 20m 06.37s , -01d 28m 38.7s) | C | 140 | 19.3 | 940 | 2025-10-13 08:18:19 | MASTER-OAFA | (01h 20m 06.39s , -01d 28m 38.6s) | C | 170 | 19.4 | 1061 | 2025-10-13 08:21:15 | MASTER-OAFA | (01h 20m 06.42s , -01d 28m 38.7s) | C | 60 | 19.0 | 1126 | 2025-10-13 08:22:21 | MASTER-OAFA | (01h 20m 06.43s , -01d 28m 38.9s) | C | 60 | 18.9 | 1191 | 2025-10-13 08:23:26 | MASTER-OAFA | (01h 20m 06.45s , -01d 28m 38.8s) | C | 60 | 18.9 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/42214. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40283] GRB 251013A: Fermi GBM Final Real-time Localization
by GCN Circulars 13 Oct '25

13 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42213 SUBJECT: GRB 251013A: Fermi GBM Final Real-time Localization DATE: 25/10/13 08:14:36 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 08:04:05 UT on 13 Oct 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251013A (trigger 782035450.15137 / 251013336). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 19.9, Dec = -1.6 (J2000 degrees, equivalent to J2000 01h 19m, -1d 36'), with a statistical uncertainty of 2.6 degrees. The angle from the Fermi LAT boresight is 95.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251013336/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251013336/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251013336/… View this GCN Circular online at https://gcn.nasa.gov/circulars/42213. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40282] LIGO/Virgo/KAGRA S251013x: Identification of a GW compact binary merger candidate
by GCN Circulars 12 Oct '25

12 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42212 SUBJECT: LIGO/Virgo/KAGRA S251013x: Identification of a GW compact binary merger candidate DATE: 25/10/13 03:31:33 GMT FROM: Kento IMAI <kent0311p(a)g.ecc.u-tokyo.ac.jp> The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We identified the compact binary merger candidate S251013x during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-10-13 02:44:14.281 UTC (GPS time: 1444358672.281). The candidate was found by the Aframe [1], GstLAL [2], MBTA [3], and PyCBC Live [4] analysis pipelines. S251013x is an event of interest because its false alarm rate, as estimated by the online analysis, is 1.5e-08 Hz, or about one in 2 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S251013x The classification of the GW signal, in order of descending probability, is BBH (99%), Terrestrial (1%), NSBH (<1%), or BNS (<1%). Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [5] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [5] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%. The source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin. Two sky maps are available at this time and can be retrieved from the GraceDB event page: * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [6], distributed via GCN and SCiMMA notices about 30 seconds after the candidate event time. * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [6], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time. The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 1589 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1946 +/- 505 Mpc (a posteriori mean +/- standard deviation). For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/. [1] Marx et al. PRD 111, 042010 (2025) doi:10.1103/PhysRevD.111.042010 [2] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008 [3] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234 [4] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a [5] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe [6] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013 View this GCN Circular online at https://gcn.nasa.gov/circulars/42212. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40281] GRB 251011B inspection: MASTER orphan GRB detection
by GCN Circulars 12 Oct '25

12 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42211 SUBJECT: GRB 251011B inspection: MASTER orphan GRB detection DATE: 25/10/13 00:15:06 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> Ya.Kechin, V.Lipunov, P.Balanutsa, A.Kuznetsov, G.Antipov, I.Panchenko, K.Zhirkov, E.Gorbovskoy, N.Tiurina, A.Sankovich, Ya.Kechin, M.Shilova, A.Chasovnikov, V.Topolev, V.Senik, D.Vlasenko, Yu.Tselik (Lomonosov MSU), R.Podesta, C.Francile, F. Podesta, E. Gonzalez (OAFA, San Juan Uni.,Argentina); D.Buckley, (SAAO, South Africa) O.Gress, N.Budnev (ISU, Irkutsk), A. Sosnovskiy (CrAO RAS), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), V.M.Pillet, R.Rebolo Lopez (The Instituto de Astrofisica de Canarias, Spain), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez,J.Martinez,A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysic Observatory, Mexico) The MASTER-OAFA robotic telescope of the MASTER Global Robotic Net [1,2,3,4] started Swift GRB 251011B (Ttrigger=2025-10-11 07:08:22UT; XRT RA,Dec(2000)=00h 52m 56.33s,-62d 02'00.2" GCN 42197, GCN 42201) alert observations at 2025-10-11 07:09:27 and observed it up to sunrise (Balanutsa et al. GCN 42203). Next night MASTER automatic planner continued Fermi error-box (GCN 32196) inspection as independent alert MASTER OT J011025.98-583210.6 discovery MASTER-OAFA auto-detection system of MASTER Global Robotic Net [1,2,3,4] during Fermi GRB 251011B (GCN42204, also detected by Swift (GCN42197) and observed by MASTER in alert mode (GCN 42203)) inspection discovered OT source at (RA, Dec) = 01h 10m 25.98s -58d 32m 10.6s on 2025-10-12.16208 UT with light curve, that is typical for gamma-ray bursts optical counterpart. The OT unfiltered magnitud m=17.8 (first image) at 2025-10-12 03:53:24UT (mlim=19.3 at single image). The OT is seen in 8 images with typical GRB counterpart decay. There is no minor planet at this place. We have reference image on 2025-09-16.19346 UT with unfiltered mlim=20.3m. So as the Ttrigger for GRB 251011B was 19h earlier our discovery time (2025-10-11 07:08:22.33UT) and Swift error-box center was at 00h 52m 56.33s -62d 02' 00.2" (Swift GCN 42197, GCN 42201) MASTER OT looks not connected with just GRB 251011B, but can be the orphan one [5]. Spectral observations and deep photometry are required. [1] Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L [2] Lipunov et al. 2022, Universe, Vol. 8(5), id.271 [3] Lipunov et a. 2019, ARep, vol.63, 293 [4] Lipunov V., Kornilov V., Gorbovskoy E., Tiurina N., Kuznetsov A. 2023, Astronomical Robotic Networks and Operative Multichanel Astrophysics,Lomonosov MSU PRESS, 591pp. http://www.pereplet.ru/lipunov/625.html#625 [5] Lipunov et al. 2022, MNRAS, 516, 4980 https://ui.adsabs.harvard.edu/abs/2022MNRAS.516.4980L/abstract View this GCN Circular online at https://gcn.nasa.gov/circulars/42211. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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