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

[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 13 Oct '25

13 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 13 Oct '25

13 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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[vsnet-grb-info 40280] GRB 251011A: Insight-HXMT detection
by GCN Circulars 12 Oct '25

12 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42210 SUBJECT: GRB 251011A: Insight-HXMT detection DATE: 25/10/12 16:37:23 GMT FROM: Chenwei Wang at IHEP <cwwang(a)ihep.ac.cn> Chen-Wei Wang, Xing-Hao Luo, Cheng-Kui Li, Shao-Lin Xiong, and Chao Zheng report on behalf of the Insight-HXMT team: At 2025-10-11T03:41:40.000 (T0), Insight-HXMT/HE detected the burst GRB 251011A, which is also detected by Fermi/GBM (Fermi GBM team, GCN#42195) and AstroSat (M. Tembhurnikar et al., GCN#42199). The Insight-HXMT/HE light curve mainly consists of a single pulse with a T90 of 3.6 +1.6/-1.4 s. The 1s peak rate, measured from T0, is 788 cnts/sec. The total counts from this burst is 1444 counts. The HXMT/HE light curve can be found here: https://www.bursthub.cn//admin/static/hxmtgrb251011A.png All measurements above are made with the CsI detectors 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/42210. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40279] GRB 251011B: Swift/UVOT Upper Limits
by GCN Circulars 12 Oct '25

12 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42209 SUBJECT: GRB 251011B: Swift/UVOT Upper Limits DATE: 25/10/12 15:56:51 GMT FROM: Mike Siegel at PSU/Swift MOC <mhs18(a)psu.edu> M. H. Siegel (PSU) and A. D'Ai (INAF-IASFPA) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 251011B 107 s after the BAT trigger (D'Ai et al., GCN Circ. 42197). No optical afterglow consistent with the XRT position (Evans et al., GCN Circ. 42201) is detected in the initial UVOT exposures. Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white_FC 107 257 147 >20.9 white 107 257 147 >20.9 u 320 463 141 >20.4 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.017 in the direction of the burst (Schlegel et al. 1998). View this GCN Circular online at https://gcn.nasa.gov/circulars/42209. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40278] Fermi GRB 251011A: Global MASTER-Net observations report
by GCN Circulars 12 Oct '25

12 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42208 SUBJECT: Fermi GRB 251011A: Global MASTER-Net observations report DATE: 25/10/12 15:31:51 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-Tunka robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Applied Physics Institute, Irkutsk State University) started inspect of the Fermi GRB 251011A ( Fermi GBM team, GCN 42195) errorbox 1 days 37513 sec after notice time and 1 days 37548 sec after trigger time at 2025-10-12 14:07:28 UT, with upper limit up to 19.0 mag. The observations began at zenith distance = 21 deg. The sun altitude is -34.7 deg. The galactic latitude b = -19 deg., longitude l = 116 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3012037 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 123979 | 2025-10-12 14:07:28 | MASTER-Tunka | (00h 19m 58.96s , +42d 27m 50.2s) | C | 60 | 19.0 | 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/42208. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40277] GRB 251012A: Fermi GBM Final Real-time Localization
by GCN Circulars 12 Oct '25

12 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42207 SUBJECT: GRB 251012A: Fermi GBM Final Real-time Localization DATE: 25/10/12 14:56:50 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 SHORT GRB At 14:46:27 UT on 12 Oct 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251012A (trigger 781973192.204998 / 251012616). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 291.6, Dec = 7.5 (J2000 degrees, equivalent to J2000 19h 26m, 7d 30'), with a statistical uncertainty of 9.9 degrees. The angle from the Fermi LAT boresight is 59.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251012616/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251012616/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251012616/… View this GCN Circular online at https://gcn.nasa.gov/circulars/42207. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40276] GRB 251011B: SVOM/GRM observation
by GCN Circulars 12 Oct '25

12 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42206 SUBJECT: GRB 251011B: SVOM/GRM observation DATE: 25/10/12 08:26:34 GMT FROM: xueyuan zao <zaoxueyuan(a)gmail.com> SVOM/GRM team: Xue-Yuan Zao, Zheng-Hang Yu, Chen-Wei Wang, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP) SVOM/ECLAIRs team: Sebastien Guillot (IRAP), Stéphane Schanne (CEA), Nicolas Dagoneau Report on behalf of the SVOM team: SVOM/GRM detected the burst GRB 251011B at 2025-10-11T07:08:22.000 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM Team, GCN #42196) and Swift/BAT (A. D'Ai et al., GCN #42197). With the event-by-event data downloaded through the X-band ground station, we conducted the standard analysis pipeline of GRB 251011B. The GRM light curve shows that this burst consists of a single pulse with a T90 of 48 +9/-10 s in the 15-5000 keV band. The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb251011B.png In addition, the position of this burst, as determined by Fermi/GBM (RA= 5.3, DEC= -56.1, GCN#42196), is located at about 85.17 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view. With this localization, the time-averaged spectrum from T0-15 to T0+30 s is best fitted by a power law function. The power law index is 2.22 +0.23/-0.19. The event fluence (10-1000 keV) in this time interval is (3.56 +0.56/-0.82)E-06 erg/cm^2. 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: Xue-Yuan Zao (IHEP) (zaoxy(a)ihep.ac.cn) View this GCN Circular online at https://gcn.nasa.gov/circulars/42206. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40275] GRB 251011B: Swift-XRT refined Analysis
by GCN Circulars 12 Oct '25

12 Oct '25

TITLE: GCN CIRCULAR NUMBER: 42205 SUBJECT: GRB 251011B: Swift-XRT refined Analysis DATE: 25/10/12 07:29:18 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> B. Sbarufatti (INAF-OAB), C. Salvaggio (INAF-OAB), S. Lanava (PSU), S. Dichiara (PSU), J.A. Kennea (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), M. Ferro (INAF-OAB) and P.A. Evans report on behalf of the Swift-XRT team: We have analysed 5.1 ks of XRT data for GRB 251011B, from 85 s to 61.7 ks after the trigger. The data comprise 112 s in Windowed Timing (WT) mode (the first 9 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The light curve can be modelled with an initial power-law decay with an index of alpha=3.12 (+/-0.08), followed by a break at T+579 s to an alpha of 0.57 (+0.07, -0.20). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.15 (+/-0.12). The best-fitting absorption column is 1.8 (+0.4, -0.3) x 10^21 cm^-2, in excess of the Galactic value of 1.7 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 3.1 x 10^-11 (4.5 x 10^-11) erg cm^-2 count^-1. A summary of the WT-mode spectrum is thus: Total column: 1.8 (+0.4, -0.3) x 10^21 cm^-2 Galactic foreground: 1.7 x 10^20 cm^-2 Excess significance: 8.4 sigma Photon index: 2.15 (+/-0.12) If the light curve continues to decay with a power-law decay index of 0.57, the count rate at T+24 hours will be 0.013 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.0 x 10^-13 (5.7 x 10^-13) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at https://www.swift.ac.uk/xrt_products/01403191. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/42205. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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