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

[vsnet-grb-info 39763] EP#01709201209: Swift-XRT observations
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41697 SUBJECT: EP#01709201209: Swift-XRT observations DATE: 25/09/04 13:30:47 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 EP#01709201209, collecting 1.4 ks of Photon Counting (PC) mode data between T0+4.0 ks and T0+15 ks after the trigger. We have detected 1 source inside the EP/WXT error region. This source coincides with the known source 1RXS J020013.6-084106 and the burst is likely a stellar flare from this object, as reported also in GCN 41690. Known X-ray sources ------------------- Source 1 (SWIFT J020012.7-084052): ================================== RA (J2000.0): 30.0530 = 02 00 12.72 Dec (J2000.0): -8.6811 = -08 40 52.0 Error: 3.5 (arcsec, radius, 90% confidence). Detect flag: GOOD Distance: 52 arcsec from the Einstein Probe/WXT position. Mean rate: 1.949 +/- 0.079 ct s^-1 Mean flux: (6.57 +/- 0.27)e-11 erg cm^-2 s^-1 Peak rate: 5.8 +/- 1.3 ct s^-1 Peak flux: (1.95 +/- 0.44)e-10 erg cm^-2 s^-1 ECF: 3.37e-11 erg cm^-2 ct^-1 assuming NH=1.53e+21 cm^-2, gamma=2.08 determined from a spectral fit. This matches a catalogued X-ray source CXO J020012.8-084051 in the Chandra/2CSC catalogue. Details: Separation: 1.1" from the XRT source Cat Rate: 0.0e+00 +/- 0.0e+00 ct s^-1 Cat Flux: 0.0e+00 +/- 0.0e+00 erg cm^-2 s^-1 (0.3-10 keV) so the source is not above the catalogued flux. The source IS FADING at the 4.1-sigma level. A SIMBAD object `1RXS J020013.6-084106' is 0.7" away. There is 1 2MASS object 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_FIELD00069. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41697. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39762] EP250903a: retraction of the LCO candidate, and discovery of the likely counterpart
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41696 SUBJECT: EP250903a: retraction of the LCO candidate, and discovery of the likely counterpart DATE: 25/09/04 13:25:08 GMT FROM: Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani(a)nbi.ku.dk> A. J. Levan (Radboud and Warwick), J. A. Quirola-Vasquez (Radboud), D. B. Malesani (DAWN/NBI and Radboud), G. Corcoran (UCD), A. Martin-Carrillo (UCD), R. A. J. Eyles-Ferris (Leicester), P. G. Jonker (Radbound), F. E. Bauer (SSI and UTA), and J. Chacón (PUC) report on behalf of a larger collaboration: We observed again the field of the fast X-ray transient EP250903a (Zhao et al., GCN 41674). Our first observation was carried out with LCO/Sinistro (3x300 s exposure, mean epoch 0.68 hr after the trigger) and was reported by Levan et al. (GCN 41680). We obtained a second, deeper observation with LCO/Sinistro (9x300 s exposure, mean epoch 3.79 hr after the trigger), and a follow-up observation with Gemini/GMOS-S (4x60 s exposure, mean epoch 19.38 hr after the trigger). Besides, a more accurate X-ray localization of EP250903a is now available, thanks to both Swift/XRT (https://www.swift.ac.uk/EP/) and EP/FXT (Zhang et al., GCN 41693). The candidate afterglow proposed by Levan et al. (GCN 41680) is outside both the XRT and FXT error circles. Furthermore, image subtraction with respect to the Legacy Survey leaves no detectable residual. The magnitudes of this object reported by Xin et al. (GCN 41683) using SVOM/VT are also consistent with the Legacy Survey archival values (r = 21.49, g = 21.95). Given the low S/N of the original detection, the most likely explanation is that there was no flux excess, and we hereby retract this afterglow candidate. Inspection of the second (deeper) LCO epoch and Gemini images reveals however an object not visible in the Legacy survey, at a position consistent with both the Swift/XRT and EP/FXT X-ray localization, at coordinates: RA(J2000) = 22:36:27.02 Dec(J2000) = -49:51:14.1 The estimated error is 0.5". This object is seen to fade between the two observations, and is not detected in (deeper) images from the Legacy Survey. We conclude that this is the actual optical afterglow of EP250903a. We report for the transient the following r-band magnitudes, calibrated against the SkyMapper catalog and not corrected for Galactic extinction: | Time since trigger (hr) | Telescope | Magnitude (AB) | | ----------------------- | --------- | -------------- | | 0.68 | LCO | > 21.7 | | 3.79 | LCO | 23.2 +- 0.3 | | 19.38 | Gemini | 24.3 +- 0.2 | | N/A | Legacy | > 24.8 | We apologize for any inconvenience that our previous announcement may have caused. We acknowledge excellent support from the Gemini staff, in particular Roque Ruiz. View this GCN Circular online at https://gcn.nasa.gov/circulars/41696. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39761] EP-WXT trigger 01709201209: Liverpool Telescope upper limits
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41695 SUBJECT: EP-WXT trigger 01709201209: Liverpool Telescope upper limits DATE: 25/09/04 13:04:23 GMT FROM: Rob Eyles-Ferris at U of Leicester <raje1(a)leicester.ac.uk> R. A. J. Eyles-Ferris, N. Habeeb and R. L. C. Starling (U of Leicester) report on behalf of a larger collaboration: We observed the field of the EP-WXT trigger 01709201209 with the 2m Liverpool Telescope using the IO:O instrument. We obtained 6x150s exposures in each of the SDSS r’ and SDSS g’ filters starting at 2025-08-29 04:15:08 UT, approximately 51 minutes after the X-ray detection. We compared the stacked images to reference images from Pan-STARRS. We also used the reference images to perform image subtraction but note that there is significant artefacting due to the bright stars in the centre of the field. We detect no new sources in either the subtractions or stacked images. This is consistent with 01709201209 being a stellar flare (Li et al., GCN 41690). We derive 3-sigma upper limits of r’ > 22.2 and g’ > 22.5 with photometry calibrated to Pan-STARRS and not corrected for Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41695. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39760] GRB 250830C: VZLUSAT-2 detection
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41694 SUBJECT: GRB 250830C: VZLUSAT-2 detection DATE: 25/09/04 12:09:10 GMT FROM: Jakub Ripa <ripa.jakub(a)gmail.com> J. Ripa, M. Dafcikova (Masaryk U.), A. Pal (Konkoly Observatory), N. Werner (Masaryk U.), M. Ohno (Hiroshima U.), L. Meszaros, B. Csak (Konkoly Observatory), H. Takahashi (Hiroshima U.), F. Munz , M. Topinka, F. Hroch, N. Husarikova, J.-P. Breuer (Masaryk U.), J. Hudec, J. Kapus, M. Frajt, M. Rezenov (Spacemanic s.r.o), R. Laszlo (Needronix), G. Galgoczi (Wigner Research Center/Eotvos U.), N. Uchida (ISAS/JAXA), T. Enoto (Kyoto U.), Zs. Frei (Eotvos U.), Y. Fukazawa, K. Hirose, H. Matake (Hiroshima U.), S. Hisadomi (Nagoya U.), Y. Ichinohe (Rikkyo U.), L. L. Kiss (Konkoly Observatory), T. Mizuno (Hiroshima U.), K. Nakazawa (Nagoya U.), H. Odaka (Univ of Tokyo), K. Torigoe (Hiroshima U.), P. Svoboda, V. Daniel, J. Dudas, M. Junas, J. Gromes (VZLU), I. Vertat (FEL ZCU) -- the VZLUSAT-2/GRB payload collaboration. The long-duration GRB 250830C (Konus-Wind detection trigger at 2025-08-30 14:52:17.865 UT; AstroSat detection: GCN 41628; GECAM-B detection: 41630; Insight-HXMT detection: 41632) was detected by the GRB detectors on board the VZLUSAT-2 3U CubeSat (https://www.vzlusat2.cz/en/) The data acquisition was performed by the GRB detector units no. 0 and no. 1. The detection was confirmed at the peak time 2025-08-30 14:52:19 UTC. The T90 duration is 9 s (8 s) and the significance during T90 reaches 32 sigma (18 sigma) for detector unit no. 0 (no. 1). The light curve obtained by VZLUSAT-2 is available here: https://vzlusat2.konkoly.hu/static/share/GRB250830C_GCN_VZLUSAT2.pdf All VZLUSAT-2 detections are listed at: https://monoceros.physics.muni.cz/hea/VZLUSAT-2/ The GRB detectors on VZLUSAT-2 are a demonstration payload for a future CubeSat constellation (Werner et al. Proc. SPIE 2018). Two GRB modules of VZLUSAT-2 are placed in a perpendicular manner and each consists of a 75 x 75 x 5 mm3 CsI scintillator read out by a SiPM array, covering the energy range from ~30 keV to ~1000 keV. VZLUSAT-2 was launched on 2022 January 13 from Cape Canaveral. View this GCN Circular online at https://gcn.nasa.gov/circulars/41694. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39759] EP250903a: refined analysis of the EP-WXT and EP-FXT observations
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41693 SUBJECT: EP250903a: refined analysis of the EP-WXT and EP-FXT observations DATE: 25/09/04 11:34:49 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> Y. J. Zhang (THU), T. Zhao, Y. J. Song, H. Sun(NAO, CAS), on behalf of the Einstein Probe (EP) team: The X-ray transient EP250903a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Zhao et al., GCN 41674), with several multi-wavelength follow-up observations (O'Neill et al., GCN 41672; An et al., GCN 41675; Lipunov et al., GCN 41676; Levan et al., GCN 41680; Xin et al., GCN 41683). The refined WXT data shows that the event started at T0=2025-09-03T13:59:40 (UTC) and lasted for about 200 s before the interruption of the autonomous follow-up. WXT detected the source during its rising phase with a peak flux of 6 x 10^(-10) erg/s/cm^2. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 1.0 x 10^20 cm^-2 and a photon index of 1.6 (+/-0.6). The derived average unabsorbed 0.5-4 keV flux is around 1.0 (-0.5/+1.1) x 10^(-10) erg/s/cm^2. The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 428s after T0. On-ground analysis of the FXT data found an fading source at R.A. = 339.1101, DEC = -49.8541 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), within the WXT error circle. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 1.0 x 10^20 cm^-2 and a photon index of 2.6(+/-0.1). The derived average unabsorbed 0.5-10 keV flux is around 4.3(+/-0.3) x 10^(-12) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41693. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39758] LIGO/Virgo/KAGRA S250904br: Identification of a GW compact binary merger candidate
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41692 SUBJECT: LIGO/Virgo/KAGRA S250904br: Identification of a GW compact binary merger candidate DATE: 25/09/04 11:17:57 GMT FROM: Lorenzo Aiello at University of Roma Tor Vergata <lorenzo.aiello(a)roma2.infn.it> The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We identified the compact binary merger candidate S250904br during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-09-04 10:22:08.022 UTC (GPS time: 1441016546.022). The candidate was found by the Aframe [1], cWB [2], cWB BBH [3], GstLAL [4], MBTA [5], and PyCBC Live [6] analysis pipelines. S250904br is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.2e-10 Hz, or about one in 1e2 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S250904br The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<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 (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: * amplfi.multiorder.fits,0, an initial localization generated by AMPLFI [8], distributed via GCN and SCiMMA notices about 26 seconds after the candidate event time. * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [9], 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,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is 373 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2748 +/- 681 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] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004 [3] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018 [4] 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 [5] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234 [6] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a [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/41692. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39757] GRB 250903A: VLT X-shooter spectroscopic redshift z = 1.590
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41691 SUBJECT: GRB 250903A: VLT X-shooter spectroscopic redshift z = 1.590 DATE: 25/09/04 09:50:16 GMT FROM: Andrea Saccardi at CEA/Irfu <andrea.saccardi(a)cea.fr> A. Saccardi (CEA/Irfu), J. An (NAOC), A. Martin-Carrillo (UCD), G. Corcoran (UCD), N. Habeeb (Leicester), A. J. Levan (Radboud U.), D. B. Malesani (DAWN/NBI and Radboud), G. Pugliese (API), B. Schneider (LAM), S. D. Vergani (LUX-Paris Obs.), D. Xu (NAOC) report on behalf of the Stargate collaboration: We observed the optical counterpart (An et al., GCN 41679; Xin et al., GCN 41684; Pérez-Fournon et al., GCN 41685; Saccardi et al., GCN 41686) of GRB 250903A detected by SVOM/ECLAIRs (Maiolino et al., GCN 41677), using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. From the 30 s X-Shooter acquisition image at 2025-09-04 01:36 UT (~8.18 hr post-trigger), the optical afterglow is well detected with a magnitude of r ~ 20.5 (AB) calibrated against SkyMapper nearby stars and not corrected by Galactic extinction. Our spectra cover the wavelength range 3000-21000 AA, and consist of 4 exposures of 900 s each. Observations started on 2025-09-04 at approximately 01:38 UT (~8.22 hr after the SVOM/ECLAIRs trigger). In a preliminary reduction of the spectra, we detect a continuum over the entire covered wavelength range. From detection in the blue down to ~3190 AA we set a redshift upper limit z < 1.62. Furthermore, we detect several absorption lines due to C IV, Si IV, Si II, Fe II, Al II, Mg II, Mg I, from which we infer a redshift of z = 1.590. While the lack of fine-structure lines formally prevents us from securely associating the z = 1.590 system to the GRB, the lack of any unidentified features (and the reasonable S/N of the spectra), coupled with the tight redshift upper limit, indicate that most likely GRB 250903A was indeed at z = 1.590. We acknowledge excellent support from the ESO observing staff in Paranal, in particular Cecilia Bustos, Martina Baratella, and Rob Van Holstein. The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495) View this GCN Circular online at https://gcn.nasa.gov/circulars/41691. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39756] The EP-WXT trigger 01709201209 is likely a flaring star
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41690 SUBJECT: The EP-WXT trigger 01709201209 is likely a flaring star DATE: 25/09/04 05:01:21 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> R.-Z. Li (YNAO, CAS), H. Z. Wu (HUST), M. J. Liu and H. W. Pan (NAO, CAS) on behalf of the Einstein Probe (EP) team: The EP-WXT trigger 01709201209 at the time of 2025-09-04T03:24:02, is likely a stellar flare associated with 1RXS J020013.6-084106. The estimated flux of the flare is around 4x10^-10 erg/s/cm^2 in 0.5-4.0 keV, corresponding to an X-ray luminosity of around 6.5x10^31 erg/s. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41690. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39755] LIGO/Virgo/KAGRA S250904ae: Identification of a GW compact binary merger candidate
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41689 SUBJECT: LIGO/Virgo/KAGRA S250904ae: Identification of a GW compact binary merger candidate DATE: 25/09/04 04:40:51 GMT FROM: Trang Nguyen <trang.nguyen(a)ligo.org> The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We identified the compact binary merger candidate S250904ae during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-09-04 03:33:07.667 UTC (GPS time: 1440992005.667). The candidate was found by the Aframe [1], cWB [2], cWB BBH [3], GstLAL [4], MBTA [5], and PyCBC Live [6] analysis pipelines. S250904ae is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.2e-10 Hz, or about one in 1e2 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S250904ae The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<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: * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [8], distributed via GCN and SCiMMA notices about 24 seconds after the candidate event time. * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [8], 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,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is 1021 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 3330 +/- 883 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] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004 [3] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018 [4] 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 [5] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234 [6] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a [7] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe [8] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013 View this GCN Circular online at https://gcn.nasa.gov/circulars/41689. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39754] GRB 250903A: EP-FXT counterpart detection
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41688 SUBJECT: GRB 250903A: EP-FXT counterpart detection DATE: 25/09/04 02:56:02 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> H. Z. Wu (HUST), R.-Z. Li (YNAO, CAS), M. J. Liu and H. W. Pan (NAO, CAS) on behalf of the Einstein Probe (EP) team: EP-FXT performed a follow-up observation of the SVOM/ECLAIRs-detected burst GRB 250903A (SVOM/sb25090304, Maiolino et al. GCN 41677) at 2025-09-03T18:20:54 (UTC), about 1 hour after the SVOM/ECLAIRs trigger, with an exposure time of 6300s. One uncatalogued source is detected within the ECLAIRs error circle, and the source is spatially consistent with the counterpart reported in optical and X-ray bands (Maiolino et al. GCN 41677, An et al. GCN 41679, Götz et al. GCN 41681, Xin et al. GCN 41683, GCN 41684, Pérez-Fournon et al. GCN 41685). Preliminary analysis on this source are automatically conducted, and details are listed as follows. Source 1: EPF_J234711.4-755801 RA (J2000): 356.7979 Dec (J2000): -75.9668 Flux: 2.41 x 10^-12 erg/s/cm2 (observed, 0.5-10 kev) Flux_err: 1.56 x 10^-13 erg/s/cm2 (1 sigma) The position uncertainty of the source is about 10 arcsec in radius (90% C.L. statistical and systematic). Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41688. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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