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

[vsnet-grb-info 39713] IceCube-Cascade 250828A: Upper limits from a search for additional neutrino events in IceCube
by GCN Circulars 02 Sep '25

02 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41647 SUBJECT: IceCube-Cascade 250828A: Upper limits from a search for additional neutrino events in IceCube DATE: 25/09/02 00:57:15 GMT FROM: Alicia Mand at IceCube/UW-Madison <aemand(a)wisc.edu> The IceCube Collaboration (http://icecube.wisc.edu/) reports: IceCube has performed a search [1] for additional track-like muon neutrino events arriving from the direction of IceCube-Cascade 250828A (https://gcn.gsfc.nasa.gov/notices_amon_icecube_cascade/141290_67759305.amon) in a time range of 1000 seconds centered on the alert event time (2025-08-28 19:22:57.520 UTC to 2025-08-28 19:39:37.520 UTC) during which IceCube was collecting good quality data. Excluding the event that prompted the alert, one track-like event is found within the 90% containment region of IceCube-Cascade 250828A. We find that this additional event is well described by atmospheric background expectations, with a p-value of 0.28. The IceCube sensitivity to neutrino point sources with an E^-2.5 spectrum (E^2 dN/dE at 1 TeV) within the locations spanned by the 90% spatial containment region of IceCube-Cascade 250828A ranges from 1.3e-01 to 6.1e+00 GeV cm^-2 in a 1000 second time window. 90% of events IceCube would detect from a source at this declination with an E^-2.5 spectrum have energies in the approximate energy range between 2e+02 GeV and 3e+06 GeV. A subsequent search was performed including 2 days of data centered on the alert event time (2025-08-27 19:31:17.520 UTC to 2025-08-29 19:31:17.520 UTC). In this case, we report a p-value of 0.63, consistent with no significant excess of track events. The IceCube sensitivity to neutrino point sources with an E^-2.5 spectrum (E^2 dN/dE at 1 TeV) within the locations spanned by the 90% spatial containment region of IceCube-Cascade 250828A ranges from 1.5e-01 to 6.2e+00 GeV cm^-2 in a 2 day time window. The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector operating at the geographic South Pole, Antarctica. The IceCube realtime alert point of contact can be reached at roc(a)icecube.wisc.edu. [1] IceCube Collaboration, R. Abbasi et al., ApJ 910 4 (2021) View this GCN Circular online at https://gcn.nasa.gov/circulars/41647. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39712] GRB 250901A: SVOM/VT optical afterglow with VHF data
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41646 SUBJECT: GRB 250901A: SVOM/VT optical afterglow with VHF data DATE: 25/09/01 23:27:01 GMT FROM: Liping Xin at NAOC, SVOM <xlp(a)nao.cas.cn> L.P. Xin, H.L. Li, C. Wu. Y. L. Qiu, Z.H. Yao, Y.N. Ma, X.H. Han, J. Wang, W.J. Xie, Y. Xu, H.B. Cai, J. Y. Wei (NAOC), J. Palmerio (CEA) , F. Piron (LUPM), C. Guépin (LUPM) report on behalf of the SVOM team. SVOM/VT performed an automatic slew on the burst triggered by SVOM/ECLAIRs (Piron et al., GCN 41634). SVOM/VT began observing the field automatically at 2025-09-01T18:20:17.000 UTC, 355 seconds after the T0, with the slew of the platform triggered on-board, in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. With downlinked VHF data, an uncatalogued source is found in stacked images within the errorbox of SVOM/MXT (Piron et al., GCN 41634) and Swift/XRT (Osborne et al., GCN 41645) compared to Legacy survey. It is located at R.A. and DEC. 356.18453, -15.74171 deg or R.A., (J2000) = 23:44:44.28 DEC. (J2000) = -15:44:30.16 The error is about 0.5 arcseconds. The AB magnitudes were derived as follows: --------------------------------------------------------- (T-T0)_mid exptime band mag 505 sec 300 sec VT_R 19.66+/-0.03 505 sec 300 sec VT_B 21.75+/-0.05 Given the color of VT_B-VT_R~2.0 mag, it might be a medium redshift GRB (Wang et al., 2020, RAA). More follow-ups are encouraged. 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/41646. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39711] GRB 250901A: Swift-XRT afterglow detection
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41645 SUBJECT: GRB 250901A: Swift-XRT afterglow detection DATE: 25/09/01 22:33:36 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> J.P. Osborne (U. Leicester), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), D.N. Burrows (PSU), S. Lanava (PSU), S. Dichiara (PSU), K.L. Page (U. Leicester) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of GRB 250901A. We searched for X-ray sources in 754 s of Photon Counting (PC) mode data. The total exposure at the position of the afterglow (see below) is 754 s, obtained between T0+2.6 ks and T0+3.3 ks. An uncatalogued X-ray source is detected and is above the LSXPS 3-sigma upper limit at this position, and is therefore likely the GRB afterglow. The position of this source is RA, Dec=356.1837, -15.7410 which is equivalent to: RA (J2000): 23h 44m 44.08s Dec(J2000): -15d 44' 27.7" with an uncertainty of 3.9 arcsec (radius, 90% confidence). The light curve can be modelled with a power-law decay with a decay index of alpha=3.2 (+1.8, -3.0). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.9 (+0.5, -0.4). The best-fitting absorption column is 6 (+12, -4) x 10^20 cm^-2, consistent with the Galactic value of 1.9 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 3.5 x 10^-11 (4.0 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 6 (+12, -4) x 10^20 cm^-2 Galactic foreground: 1.9 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.9 (+0.5, -0.4) If the light curve continues to decay with a power-law decay index of 3.2, the count rate at T+24 hours will be 2.9 x 10^-6 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.0 x 10^-16 (1.1 x 10^-16) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/03000066. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/SVOM_FIELD00033. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41645. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39710] EP250901a: Swift-XRT observations
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41644 SUBJECT: EP250901a: Swift-XRT observations DATE: 25/09/01 22:18:32 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> P.A. Evans (U. Leicester), J.A. Kennea (PSU), E. Ambrosi (INAF-IASFPA) , A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), R. Brivio (INAF-OAB), D.N. Burrows (PSU), S. Campana (INAF-OAB), M. Capalbi (INAF-OAR), A. D'Ai (INAF-IASFPA), P. D'Avanzo (INAF-OAB), V. D'Elia (SSDC & INAF-OAR), S. Dichiara (PSU), M. Ferro (INAF-OAB), A. Melandri (INAF-OAR), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M. Perri (SSDC & INAF-OAR), C. Salvaggio (INAF-OAB), T. Sbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), M.A. Williams (PSU) report on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of the Einstein Probe/WXT-detected source EP250901a, collecting 2.0 ks of Photon Counting (PC) mode data between T0+6.2 ks and T0+12 ks after the trigger. We have detected 1 source. This has been automatically classified as an known X-ray source. Known X-ray sources ------------------- Source 1 (SWIFT J181210.9+184216): ================================== RA (J2000.0): 273.0457 = 18 12 10.97 Dec (J2000.0): +18.7047 = +18 42 16.9 Error: 5.5 (arcsec, radius, 90% confidence). Detect flag: GOOD Distance: 2.1 arcmin from the Einstein Probe/WXT position. Mean rate: 0.0570 +/- 0.0097 ct s^-1 Mean flux: (2.06 +/- 0.35)e-12 erg cm^-2 s^-1 Peak rate: 0.061 +/- 0.015 ct s^-1 Peak flux: (2.21 +/- 0.54)e-12 erg cm^-2 s^-1 ECF: 3.62e-11 erg cm^-2 ct^-1 assuming NH=4.95e+21 cm^-2, gamma=2.27 determined from a spectral fit. This matches a catalogued X-ray source 1RXS J181210.7+184228 in the ROSAT/RASSFSC catalogue. Details: Separation: 12.4" from the XRT source Cat Rate: 2.0e-02 +/- 8.7e-03 ct s^-1 Cat Flux: 6.7e-13 +/- 2.8e-13 erg cm^-2 s^-1 (0.3-10 keV) so the source is 2.5-sigma above the catalogued flux. The source may be fading, at the 0.9-sigma level. A SIMBAD object `1RXS J181210.7+184228' is 12.4" away. There are 3 2MASS objects within the source's 3-sigma error radius. All fluxes are 0.3-10 keV, observed. For all flux conversions and comparisons with catalogues and upper limits from other missions, we assumed a power-law spectrum with NH=3x10^20 cm^-2 and photon index (Gamma)=1.7 unless otherwise stated. The results of the XRT-team automatic analysis of the XRT observations, including a position-specific upper limit calculator, are available at https://www.swift.ac.uk/EP/EP_FIELD00067. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41644. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39709] LIGO/Virgo/KAGRA S250830bp: DECam GW-MMADS candidates
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41643 SUBJECT: LIGO/Virgo/KAGRA S250830bp: DECam GW-MMADS candidates DATE: 25/09/01 22:04:52 GMT FROM: xjh(a)andrew.cmu.edu Xander J. Hall (CMU), Lei Hu (CMU), Tomás Cabrera (CMU), Brendan O’Connor (CMU), Antonella Palmese (CMU), Igor Andreoni (UNC), James Freeburn (UNC), Keerthi Kunnumkai (CMU), on behalf of the Gravitational Wave MultiMessenger Astronomy DECam Survey (GW-MMADS) team report: DECam observed the high probability area of the LVK gravitational wave candidate S250830bp (GCN 41606) using the wide-field Dark Energy Camera (DECam) on the 4m Blanco telescope. Observations started at 2025-09-01T03:39 UTC (Prop ID: 2025B-485252; PI Soares-Santos) and covered the highest 90% probability region of the event (GCN 41607). We run the SFFT difference imaging (Hu et al. 2022) on the available images, filter out likely stars and moving objects, and then visually inspect the remaining transients. We report on TNS new transients within the LVK 99% CI area, and we report here select transients of high quality and interest: First, one matched to a NED galaxy within the 90% volume (GCN 41608): | id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | NED separation | | ------------------------ | ---------- | ---------- | ---------- | ----------------------- | ------ | ----- | ----- | ----- | | T202509012137497m775946 | AT 2025wpv | 324.457179 | -77.996131 | 2025-09-01 03:57:43.776 | 22.6 | 0.3 | 1.16 | 1.6” Next, a transient of a known Gaia quasar (Gavras et al. 2023): | id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | Gaia separation | | ------------------------ | ---------- | ---------- | ---------- | ----------------------- | ------ | ----- | ----- | ----- | | A202509012159035m780432 | AT 2025wpq | 329.764660 | -78.075678 | 2025-09-01 03:41:20.544 | 20.3 | 0.3 | 0.6 | 0.165” Finally, we confirm the transient reported by McMahon et al. (GCN 41640): | id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | | ------------------------ | ---------- | ---------- | ---------- | ----------------------- | ------ | ----- | ----- | | T202509012157542m772445 | AT 2025wpk | 329.475736 | -77.412522 | 2025-09-01 03:49:33.024 | 22.2 | 0.2 | 1.2 Further analysis is underway. We thank the CTIO and NOIRLab staff for supporting these observations and the data calibration. View this GCN Circular online at https://gcn.nasa.gov/circulars/41643. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39708] GRB 250901A: LCO optical upper limit
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41642 SUBJECT: GRB 250901A: LCO optical upper limit DATE: 25/09/01 22:02:00 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> A. Saccardi (CEA/Irfu), B. Schneider (LAM), D. Turpin (CEA/Irfu), B. Cordier (CEA/Irfu), report on behalf of the SVOM mission team: We observed the field of the GRB 250901A detected by SVOM/ECLAIRs (Piron et al., GCN 41634) with the LCO 1m telescope at South African Astronomical Observatory equipped with the Sinistro instrument. Our observation started on 2025-09-01 at 19:22:42 UT (68 minutes after the trigger) and we obtained 3x200 s exposures using the SDSS r filter. In the stacked image, we did not detect any new optical source at the position of the Swift/XRT source #1 (https://www.swift.ac.uk/SVOM/SVOM_FIELD00033/) We derived the following upper limit r > 21.3 AB mag (3-sigma) calibrated using nearby stars from the PanSTARRS DR1 catalog and not corrected for Galactic extinction. This project is funded by the SVOM collaboration. View this GCN Circular online at https://gcn.nasa.gov/circulars/41642. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39707] EP250827b: Swift observations of AT 2025wkm
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41641 SUBJECT: EP250827b: Swift observations of AT 2025wkm DATE: 25/09/01 20:57:31 GMT FROM: xjh(a)andrew.cmu.edu Xander J. Hall (Carnegie Mellon U.), Tomas Ahumada (Caltech), and Robert Stein (UMD) on behalf of the ZTF collaboration: We requested Swift ToO observations of the X-ray flare EP250827b and optical transient AT 2025wkm reported by Schroeder et al. (GCN 41635). The observation was taken with a midpoint time of 2025-08-31 at 21:13:29 UT, approximately 4.62 days after the flare, with a duration of 4ks. The source was not detected with XRT with an upper limit of 2.5 * 10^-3 cts/s at 0.3 - 10 keV. Given the redshift reported by Sevilla & Ho (GCN 41639) of z = 0.1200 this gives a rough luminosity upper limit of 4.5 * 10^42 erg/s. We clearly detect a source at the position of AT 2025wkm in UVOT observations. We use a synthetic galaxy model to produce subtracted magnitudes and correct for galactic extinction. We report in the UVW1 filter, a magnitude of m= 21.7 +/- 0.4 AB and in the UVM2 filter, a magnitude of m = 22.2 +/- 0.3. We thank the Swift team, Observatory Duty Scientists and the PI for promptly scheduling and making these observations possible. View this GCN Circular online at https://gcn.nasa.gov/circulars/41641. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39706] LIGO/Virgo/KAGRA S250830bp: DECam DESGW Epoch 1 Candidates
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41640 SUBJECT: LIGO/Virgo/KAGRA S250830bp: DECam DESGW Epoch 1 Candidates DATE: 25/09/01 20:32:52 GMT FROM: Isaac McMahon at University of Zürich <isaac.mcmahon(a)ligo.org> Isaac McMahon, Sean MacBride, Marcelle Soares-Santos (UZH), Lillian Joseph (Benedictine U.), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) Team: At 2025-09-01 03:39:18 UTC, the Dark Energy Camera (DECam) began the first epoch of observations in response to the LVK alert issued for the candidate gravitational-wave event S250830bp (GCN 41606). We observed four fields centered on the following ICRS coordinates: | RA | DEC | |----------|----------| | 324.165777 | -77.51123 | | 327.955221 | -77.54517 | | 323.965777 | -77.61123 | | 327.755221 | -77.64517 | These pointings cover the 90% localization region of candidate gravitational-wave event S250830bp. All fields were observed in DECam g, r, i, z filters with 90-second exposures. The limiting magnitudes achieved for epoch 1 were 21.8 in g, 21.8 in r, and 21.5 in i, and 20.7 in z. We process the images with our difference imaging pipeline (Herner et al. 2020) using DES and public DECam images as templates. We employ the autoscan machine learning code (Goldstein et al 2015) to reject subtraction artifacts. Candidates were initially selected by requiring at least two high signal to noise detections, which were separated in time in order to reject moving objects. We also require an autoscan score of at least 0.7 on at least one of those detections. We also match our candidates against the ALLWISE, Milliquas, and Quaia AGN catalogs (Secrest et al 2015, Flesch 2023, Storey-Fisher et al 2024) within the LVK localization volume to determine if any correspond to known active galactic nuclei. Poor observing conditions during observing complicated positive identification of transient candidates. However, we report one candidate below with good detections in multiple bands. The candidate is separated by 5.58 arcseconds from the center of the apparent host galaxy LEDA 236249, which does not have a reported redshift. | ID | ATNAME | RA | DEC | MAG_G | MAG_G_ERR | MAG_R | MAG_R_ERR | MAG_I | MAG_I_ERR | MAG_Z | MAG_Z_ERR | |--------- |-------- |------------ |----------- |------- |----------- |------- |----------- |------- |----------- |------- |----------- | | DESGW_3336579 | AT2025wpk | 329.476163 | -77.41256 | 22.41 | 0.24 | 21.39 | 0.09 | 21.1 | 0.06 | 20.93 | 0.14 | Further observations are ongoing and we encourage followup of the event region and the above candidate. The DECam Search & Discovery Program for Optical Signatures of Gravitational Wave Events (DESGW) is carried out by the Dark Energy Survey (DES) collaboration in partnership with wide-ranging groups in the community. DESGW uses data obtained with the Dark Energy Camera (DECam), which was constructed by the DES collaboration with support from the Department of Energy and member institutions, and utilizes data as distributed by the Science Data Archive at NOIRLAB. NOIRLAB is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. We thank the Cerro Tololo observatory staff for their support in acquiring these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41640. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39705] EP250827b: Gemini Spectroscopy of AT 2025wkm
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41639 SUBJECT: EP250827b: Gemini Spectroscopy of AT 2025wkm DATE: 25/09/01 20:27:47 GMT FROM: Anna Y Q Ho at Cornell University <ayh24(a)cornell.edu> Cassie Sevilla and Anna Ho (Cornell) report on behalf of the ZTF collaboration: We acquired long-slit spectroscopy of AT 2025wkm (possibly associated with Einstein Probe X-ray flare EP250827b; Schroeder et al. GCN #41635) using GMOS-N at Gemini Observatory on 2025-08-31 12:15:31 UTC, under the target-of-opportunity program GN-2025B-Q-130 (PI Ho). The observation was carried out with a 1" slit, the R400 grating, and the GG455 filter. We took four 450s exposures, two at a central wavelength of 710 nm and two at 720 nm. The data was reduced using DRAGONS. The spectrum shows a blue continuum with narrow emission lines of H alpha, H beta, [O III], and [S II] at a consistent redshift of z=0.120. The implied peak measured absolute magnitude of AT 2025wkm is M_g = -19.5 mag, corrected for Milky Way extinction. The rapid rise (~3d) to such a high luminosity is unusual for extragalactic transients. We thank the Gemini Observatory staff for assistance in rapidly obtaining these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41639. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39704] LIGO/Virgo/KAGRA S250901cb: Identification of a GW compact binary merger candidate
by GCN Circulars 01 Sep '25

01 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41638 SUBJECT: LIGO/Virgo/KAGRA S250901cb: Identification of a GW compact binary merger candidate DATE: 25/09/01 19:47:05 GMT FROM: shoki.iwaguchi(a)ligo.org The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We identified the compact binary merger candidate S250901cb during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-09-01 18:59:41.965 UTC (GPS time: 1440788399.965). The candidate was found by the Aframe [1], cWB BBH [2], GstLAL [3], MBTA [4], PyCBC Live [5], and SPIIR [6] analysis pipelines. S250901cb is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.2e-14 Hz, or about one in 1e6 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S250901cb 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%. [7] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [7] 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 (11.0, 22.0) 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: * amplfi.multiorder.fits,0, an initial localization generated by AMPLFI [8], distributed via GCN and SCiMMA notices about 24 seconds after the candidate event time. * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [9], distributed via GCN and SCiMMA notices about 4 minutes after the candidate event time. The preferred sky map at this time is bayestar.multiorder.fits,0. For the bayestar.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 935 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis): icrs; ellipse(13h05m, -07d35m, 30.96d, 9.71d, 118.56d) Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1960 +/- 524 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] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018 [3] 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 [4] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234 [5] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a [6] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023 [7] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe [8] Chatterjee et al. MLST 5, 045030 (2024) doi:10.1088/2632-2153/ad8982 [9] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013 View this GCN Circular online at https://gcn.nasa.gov/circulars/41638. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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