November 2024 - vsnet-grb-info - ooruri.kusastro.kyoto-u.ac.jp

[vsnet-grb-info 36352] The EP-WXT trigger 01709125191 is not a real source
by GCN Circulars 28 Nov '24

28 Nov '24

TITLE: GCN CIRCULAR NUMBER: 38360 SUBJECT: The EP-WXT trigger 01709125191 is not a real source DATE: 24/11/28 04:19:36 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> B.-T. Wang (YNAO, CAS), T. C. Zheng (PMO, CAS), Y. L. Wang, S. X. Wen, W. X. Wang, W. Yuan (NAO, CAS) report on behalf of the Einstein Probe team: Further analysis of the late arrived telemetry data verifies that the EP-WXT on-board trigger 01709125191 at 2024-11-25T22:28:15.649 (UTC) is not a real source. View this GCN Circular online at https://gcn.nasa.gov/circulars/38360. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 36351] GRB 241127A: LCOGT (40-cm) optical counterpart detection
by GCN Circulars 28 Nov '24

28 Nov '24

TITLE: GCN CIRCULAR NUMBER: 38359 SUBJECT: GRB 241127A: LCOGT (40-cm) optical counterpart detection DATE: 24/11/28 04:18:13 GMT FROM: Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf(a)iac.es> I. Ortega-Casas, B. Armas-Chinea, F. Dobrindt, P. Escudero-Coca, G. Fernández-Rodríguez, Á. García Lozano, A. Huertas Ferrer, C. Méndez-Lapido, M. Torreiro Martínez, G. Villa (all ULL), S.R. Berlanas (IAC and ULL), and I. Pérez-Fournon (IAC and ULL) We report on optical follow-up observations of GRB 241127A, detected by Swift BAT, XRT, and UVOT (Salvaglio et al., GCN circular 38354) and by MASTER (Buckley et al., GCN circular 38352 and Francile et al., GCN circular 38358). We observed the field of GRB 241127A with the two Las Cumbres Observatory Global telescope network (LCOGT) Planewave Delta Rho 350 telescopes, equipped with QHY600 CMOS detectors, located at the LCOGT node at the Cerro Tololo Inter-American Observatory (Chile) in the SDSS r', i', and g' filters, with 600 sec exposures in each of the filters. We detect the optical counterpart at a position consistent with the Swift UVOT (Salvaglio et al., GCN circular 38354) and MASTER (Buckley et al., GCN circular 38352 and Francile et al., GCN circular 38358) detections. We measure the following magnitudes, calibrated using stars from the catalog of Gaia DR3 synthetic photometry generated from the Gaia BP/RP mean spectra (Gaia collaboration 2022) and without corrections for Milky Way extinction: r' = 19.42 +/- 0.08, on 2024-11-28 00:58:30 UT, 3.25 hours after the Swift trigger i' = 19.27 +/- 0.15, on 2024-11-28 01:04:49 UT, 3.36 hours after the Swift trigger g' = 19.70 +/- 0.07, on 2024-11-28 01:10:03 UT, 3.45 hours after the Swift trigger In the Legacy Surveys DR10 catalog there is a faint galaxy close to the position of the GRB optical counterpart at RA, Dec (J2000, deg) = 327.8851, -56.6653 and with magnitudes g = 25.08, r = 24.70, i = 24.38, and z = 23.57, that is the likely GRB host galaxy. These results are based on observations made with the Las Cumbres Observatory’s education network telescopes that were upgraded through generous support from the Gordon and Betty Moore Foundation and are part of a course on Astrophysical Techniques of the Master in Astrophysics of the Universidad de La Laguna, Tenerife, Spain (LCOGT observing programme IAC2024B-010, ULL-ASTRO-MASTER). View this GCN Circular online at https://gcn.nasa.gov/circulars/38359. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 36350] Swift GRB241127.91: Global MASTER-Net OT detection
by GCN Circulars 28 Nov '24

28 Nov '24

TITLE: GCN CIRCULAR NUMBER: 38358 SUBJECT: Swift GRB241127.91: Global MASTER-Net OT detection DATE: 24/11/28 03:15:55 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), V.Lipunov, E.Gorbovskoy, N.Tiurina, P.Balanutsa, D.Vlasenko, I.Panchenko, A.Kuznetsov, G.Antipov, A.Sankovich, A.Sosnovskij, Yu.Tselik, M.Gulyaev, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina (Lomonosov MSU), O.Gress, N.Budnev(ISU), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), 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) was pointed to the GRB241127.91 9640 sec after notice time and 9658 sec after trigger time at 2024-11-28 00:24:14 UT. On our 9-th (60s exposure) set , obtained 10200 sec after tigger time at 2024-11-28 00:33:17 UT, we found 1 optical transient within Swift error-box (ra=327.921 dec=-56.685 r=0.05) brighter than 19.3. T-Tmid Date Time Expt. Ra Dec Mag ---------|---------------------|-------|-----------------|-----------------|------- 10230 2024-11-28 00:33:17 60 (21h 51m 32.32s , -56d 39m 56.5s) 19.4 The 5-sigma upper limit has been about 19.3mag The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/38358. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 36349] EP241126a: WFST Observations
by GCN Circulars 28 Nov '24

28 Nov '24

TITLE: GCN CIRCULAR NUMBER: 38357 SUBJECT: EP241126a: WFST Observations DATE: 24/11/28 02:53:42 GMT FROM: Tianrui Sun at Purple Mountain Obs,CAS <trsun(a)pmo.ac.cn> Jin-Jun Geng, Ji-An Jiang, Ning Jiang, Ming-Xin Wu, Min-Xuan Cai, Jia-Zheng Zhu, Kun Yan, Qing-Yi Hai, Xue-Feng Wu on behalf of the WFST Collaboration: Following the detection of the fast X-ray transient EP241126a by WXT (Hu et al., GCN 38335) and FXT on board Einstein Probe (Zheng et al., GCN 38339), TRT (Fu et al., GCN 38337) and GSP (Li et al., GCN 38338), we carried out image observations using the newly deployed Wide Field Survey Telescope (WFST Collaboration; arXiv:2306.07590) at Lenghu Astronomical Observation Base (Qinghai province, China) to search and follow up its afterglow in the error box. The optical counterpart candidate (Fu et al., GCN 38337 and Li et al., GCN 38338) was detected in the r band on the co-added images with an exposure time of 8 x 60s. Our preliminary results are as follows: |RA |DEC | DATE-OBS |Filter | Magnitude |02:14:57.974 |+11:42:05.34 |2024-11-27T11:47 |r | 22.55+/-0.23 We used the PanStarrs DR1 (Flewelling,2020) catalog as the magnitude reference for calibration. View this GCN Circular online at https://gcn.nasa.gov/circulars/38357. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 36348] LIGO/Virgo/KAGRA S241125n: Update on Coincidence False Alarm Method
by GCN Circulars 28 Nov '24

28 Nov '24

TITLE: GCN CIRCULAR NUMBER: 38356 SUBJECT: LIGO/Virgo/KAGRA S241125n: Update on Coincidence False Alarm Method DATE: 24/11/28 02:26:46 GMT FROM: Brandon Piotrzkowski <brandon.piotrzkowski(a)ligo.org> The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration along with the Swift/BAT-GUANO Collaboration report: A search performed by the RAVEN pipeline [1] (reported in GCN Circular 38309) found a spatio-temporal coincidence between S241125n and a sub-threshold Swift/BAT trigger with ID 754189311 (DeLaunay et al., GCN Circular 38308). The estimated joint false alarm rate (FAR) for the spatial and temporal coincidence was reported to be 1.8e-11 Hz, or about one in 1e3 years (GCN 38315). See https://lscsoft.docs.ligo.org/raven/joint_far.html for a description of the Targeted Search joint FAR method. The difference between the joint FAR estimate in GCN 38315 and GCN 38308 can be accounted for by the updated gravitational-wave skymap, the use of the highest resolution information in the sky map combination, and by the fact that the GCN 38308 method included a correction for trials arising from multiple GW pipelines. The Targeted Search joint FAR presented above is dominated by the low FAR of the confident GW signal, which is much lower than the detected rate of BBH mergers. In this case of a confident GW signal, using a version of the Untargeted joint FAR method (see link above) is likely to more appropriately describe the background association rate. Using a conservative accounting of the BBH detection rate (1 per 3 days) along with the Untargeted joint FAR method increases the joint FAR to 1 per 6 years. [1] Urban, A. L. 2016, Ph.D. Thesis https://dc.uwm.edu/etd/1218 and Piotrzkowski, B. J. 2022, Ph.D. Thesis https://dc.uwm.edu/etd/3060 View this GCN Circular online at https://gcn.nasa.gov/circulars/38356. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 36347] IceCube Alert 241127.59: Global MASTER-Net observations report
by GCN Circulars 28 Nov '24

28 Nov '24

TITLE: GCN CIRCULAR NUMBER: 38355 SUBJECT: IceCube Alert 241127.59: Global MASTER-Net observations report DATE: 24/11/28 01:39:25 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, N.Tiurina, P.Balanutsa, , D.Vlasenko, I.Panchenko, A.Kuznetsov, G.Antipov, A.Sankovich, A.Sosnovskij, Yu.Tselik, M.Gulyaev, 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. 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-SAAO robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in South Africa (South African Astronomical Observatory) was pointed to the IceCube Alert 241127.59 (trigger No 41215060,10h 55m 02.88s , +05d 54m 46.8s, R=0.51) errorbox 39590 sec after notice time and 39645 sec after trigger time at 2024-11-28 01:11:59 UT, with upper limit up to 19.2 mag. The observations began at zenith distance = 65 deg. The sun altitude is -23.6 deg. The galactic latitude b = 55 deg., longitude l = 246 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2685949 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 39735 | 2024-11-28 01:11:59 | MASTER-SAAO | (10h 48m 19.09s , +05d 07m 06.3s) | C | 180 | 19.0 | 39934 | 2024-11-28 01:15:18 | MASTER-SAAO | (10h 56m 24.83s , +05d 08m 43.8s) | C | 180 | 19.2 | 40134 | 2024-11-28 01:18:38 | MASTER-SAAO | (11h 03m 32.09s , +05d 08m 51.5s) | C | 180 | 19.1 | 40915 | 2024-11-28 01:31:39 | MASTER-SAAO | (10h 48m 18.34s , +05d 06m 59.9s) | C | 180 | 17.7 | 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/38355. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 36346] GRB 241127A: Swift detection of a burst with an optical counterpart
by GCN Circulars 27 Nov '24

27 Nov '24

TITLE: GCN CIRCULAR NUMBER: 38354 SUBJECT: GRB 241127A: Swift detection of a burst with an optical counterpart DATE: 24/11/27 21:59:40 GMT FROM: K.L. Page at U Leicester <klp5(a)leicester.ac.uk> C. Salvaggio (INAF-OAB), R. Brivio (INAF-OAB), S. Dichiara (PSU), M. Ferro (INAF-OAB), R. Gupta (NASA GSFC), A. Y. Lien (U Tampa), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), A. Tohuvavohu (Caltech) and M. A. Williams (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 21:43:16 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 241127A (trigger=1270788). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 327.922, -56.685 which is RA(J2000) = 21h 51m 41s Dec(J2000) = -56d 41' 06" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single-peaked structure with a duration of about 10 sec. The peak count rate was ~2000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 21:44:22.8 UT, 66.6 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 327.88355, -56.66459 which is equivalent to: RA(J2000) = 21h 51m 32.05s Dec(J2000) = -56d 39' 52.5" with an uncertainty of 4.0 arcseconds (radius, 90% containment). This location is 105 arcseconds from the BAT onboard position, within the BAT error circle. This position may be improved as more data are received; the latest position is available at https://www.swift.ac.uk/sper. We cannot determine whether the source is fading at the present time. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (2.68 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 6.1 (+6.36/-4.83) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 2.12e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 484 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 21:51:32.32 = 327.88467 DEC(J2000) = -56:39:55.1 = -56.66531 with a 90%-confidence error radius of about 0.61 arc sec. This position is 2.1 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.14 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.034. Burst Advocate for this burst is C. Salvaggio (chiara.salvaggio AT inaf.it). Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/) View this GCN Circular online at https://gcn.nasa.gov/circulars/38354. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 36345] Swift GRB241127.91: Global MASTER-Net observations report
by GCN Circulars 27 Nov '24

27 Nov '24

TITLE: GCN CIRCULAR NUMBER: 38353 SUBJECT: Swift GRB241127.91: Global MASTER-Net observations report DATE: 24/11/27 21:55:40 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, N.Tiurina, P.Balanutsa, , D.Vlasenko, I.Panchenko, A.Kuznetsov, G.Antipov, A.Sankovich, A.Sosnovskij, Yu.Tselik, M.Gulyaev, 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. 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-SAAO robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in South Africa (South African Astronomical Observatory) was pointed to the Swift GRB241127.91 (trigger No 1270788,21h 51m 41.28s , -56d 41m 06.0s, R=0.05) errorbox 11 sec after notice time and 35 sec after trigger time at 2024-11-27 21:43:51 UT, with upper limit up to 18.3 mag. The observations began at zenith distance = 61 deg. The sun altitude is -35.8 deg. The galactic latitude b = -47 deg., longitude l = 337 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2686277 We obtain a following upper limits. Tmid-T0 | Site |Filt.| Expt. | Limit| Comment _________|_____________________|_____|_______|______|________ 41 | MASTER-SAAO | C | 10 | 13.0 | 56 | MASTER-SAAO | C | 40 | 17.9 | Coadd 70 | MASTER-SAAO | C | 10 | 17.5 | 104 | MASTER-SAAO | C | 20 | 17.9 | 143 | MASTER-SAAO | C | 20 | 16.7 | 178 | MASTER-SAAO | C | 90 | 18.3 | Coadd 187 | MASTER-SAAO | C | 30 | 17.7 | 241 | MASTER-SAAO | C | 40 | 17.9 | 305 | MASTER-SAAO | C | 50 | 15.7 | 379 | MASTER-SAAO | C | 60 | 14.9 | 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/38353. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 36344] Swift GRB241127.91: Global MASTER-Net OT detection
by GCN Circulars 27 Nov '24

27 Nov '24

TITLE: GCN CIRCULAR NUMBER: 38352 SUBJECT: Swift GRB241127.91: Global MASTER-Net OT detection DATE: 24/11/27 21:48:45 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> D. Buckley (South African Astronomical Observatory), V. Lipunov, V.Kornilov, E. Gorbovskoy, K. Zhirkov, N.Tyurina, P.Balanutsa, A.Kuznetsov, V.Senik, D. Vlasenko, G.Antipov, D.Zimnukhov, E.Minkina, A.Chasovnikov, V.Topolev, D.Kuvshinov, D. Cheryasov, Ya.Kechin, Yu.Tselik, A. Sosnovskij (Lomonosov Moscow State University, SAI, Physics Department), R. Podesta, C.Lopez, F. Podesta, C.Francile (Observatorio Astronomico Felix Aguilar OAFA), R. Rebolo, M. Serra (The Instituto de Astrofisica de Canarias), O.A. Gress, N.M. Budnev, O.Ershova (Irkutsk State University, API), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), V. Yurkov, A. Gabovich (Blagoveschensk Educational State University) MASTER-SAAO robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in South Africa (South African Astronomical Observatory) was pointed to the GRB241127.91 19 sec after notice time and 35 sec after trigger time at 2024-11-27 21:43:51 UT. On our 3-th (20s exposure) set , obtained 93 sec after tigger time at 2024-11-27 21:44:49 UT, we found 1 optical transient within Swift error-box (ra=327.921 dec=-56.685 r=0.05) brighter than 17.4. T-Tmid Date Time Expt. Ra Dec Mag ---------|---------------------|-------|-----------------|-----------------|------- 103 2024-11-27 21:44:49 20 (21h 51m 32.48s , -56d 39m 55.8s) 15.5 The 5-sigma upper limit has been about 17.4mag The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/38352. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 36343] LIGO/Virgo/KAGRA S241125n: Preliminary flux estimate and spectral analysis of the Swift/BAT-GUANO candidate counterpart
by GCN Circulars 27 Nov '24

27 Nov '24

TITLE: GCN CIRCULAR NUMBER: 38351 SUBJECT: LIGO/Virgo/KAGRA S241125n: Preliminary flux estimate and spectral analysis of the Swift/BAT-GUANO candidate counterpart DATE: 24/11/27 19:23:19 GMT FROM: Jimmy DeLaunay at Penn State <delauj2(a)gmail.com> James DeLaunay (PSU), Aaron Tohuvavohu (Caltech), Samuele Ronchini (PSU), Gayathri Raman (PSU), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC) report: We have performed a preliminary spectral study on the sub-threshold GRB candidate that was found to be spatially and temporally coincident with the GW event S241125n (GCN 38308). This candidate was identified in BAT-GUANO data by the NITRATES pipeline in an analysis time bin that starts at T_GW + 11.264 s and has a duration of 0.512 s. The NITRATES analysis uses Comptonized spectral templates that range from soft to hard GRB spectra. The maximum likelihood template found in the search had Epeak = 49 keV and spectral index = -1.2 (gamma on results webpage is -1 x spectral index). The best fit 15-350 keV flux for this spectral template at this position and analysis time bin is 1.1(-0.3, +0.2)E-7 erg/cm2/s with a 90% error bar. Note that the parameters of the template that maximized the likelihood in the low-latency search are not necessarily the true parameters of the signal. Due to the low S/N of this event, error bars on Epeak and the spectral index are large and correlated. The spectral index is unconstrained in the range of -0.4 to -2.0, but the allowed values of Epeak vary with it. With a spectral index fixed at -0.6, the 90% confidence limits of Epeak are approximately 25 to 80 keV. With a spectral index fixed at -1.6, the 90% confidence limits of Epeak are approximately 20 to 300 keV. The flux estimate varies with spectral index and Epeak. In the most recent joint localization map of the candidate GRB and S241125n, ~84% of the localization probability is within 5 arcminutes of the most likely position. The other 16% is spread out across the sky and is multimodal. Some of this remaining probability is also within BAT’s coded field of view and some is outside of it. The response changes greatly whether in or outside the coded field of view, which drives substantial range in the flux estimate depending on sky position. The Southern lobes of the joint localization are outside of the BAT’s coded field of view and thus have higher estimated flux values. This region is most likely excluded by external upper limits from Fermi-GBM (GCN 38316) and Konus-Wind (GCN 38321). The most likely position cannot be confidently excluded by these upper limits. All values in this circular are preliminary. View this GCN Circular online at https://gcn.nasa.gov/circulars/38351. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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