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

[vsnet-grb-info 39773] GRB 250902B: CALET Gamma-Ray Burst Monitor detection
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41707 SUBJECT: GRB 250902B: CALET Gamma-Ray Burst Monitor detection DATE: 25/09/04 18:04:18 GMT FROM: Yuta Kawakubo at Aoyama Gakuin University <kawakubo(a)phys.aoyama.ac.jp> N. Cannady (GSFC/UMBC), A. Yoshida, T. Sakamoto, S. Sugita, Y. Kawakubo (AGU), K. Yamaoka (Nagoya U), S. Nakahira (JAXA), Y. Asaoka (ICRR), S. Torii, Y. Akaike, K. Kobayashi (Waseda U), Y. Shimizu, T. Tamura (Kanagawa U), M. L. Cherry (LSU), S. Ricciarini (U of Florence), P. S. Marrocchesi (U of Siena), and the CALET collaboration: The long GRB 250902B (Fermi GBM Final Real-time Localization: Fermi GBM team, GCN Circ. 41660; SVOM/GRM observation: Wang et al., GCN Circ. 41668; AstroSat CZTI detection: Tembhurnikar et al., GCN Circ. 41678) triggered the CALET Gamma-ray Burst Monitor (CGBM) at 18:45:35.15 UTC on 2 September 2025 (https://cgbm.calet.jp/cgbm_trigger/flight/1440873547/index.html) The burst signal was seen by all CGBM detectors. Because of a problem with the ground alert processing script, the GCN notice was not distributed automatically for this event. The burst light curve shows a multi-peaked structure that starts at T-2.4 sec, peaks at T+9.6 sec, and ends at T+17.4 sec. The T90 and T50 durations measured by the SGM data are 17.4 +/- 0.7 sec and 8.8 +/- 0.6 sec (40-1000 keV), respectively. The ground-processed light curve is available at https://cgbm.calet.jp/cgbm_trigger/ground/1440873547/ The CALET data used in this analysis are provided by the Waseda CALET Operation Center located at Waseda University. View this GCN Circular online at https://gcn.nasa.gov/circulars/41707. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39772] GRB 250904A: Prompt enhanced Swift-XRT position
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41706 SUBJECT: GRB 250904A: Prompt enhanced Swift-XRT position DATE: 25/09/04 18:01:38 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team: Using 343 s of promptly downlinked XRT event data for GRB 250904A, we find an enhanced XRT position of the afterglow: RA, Dec: 84.38029, 3.58466 which is equivalent to: RA (J2000) = 05 37 31.27 Dec (J2000) = +03 35 04.8 with an uncertainty of 2.2 arcseconds (radius, 90% confidence). Analysis of the promptly available data is online at http://www.swift.ac.uk/sper/1347179. Position enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans et al. (2009, MNRAS, 397, 1177). This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41706. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39771] EP250827b: Gemini Spectroscopic Confirmation of AT2025wkm as a Ic-BL Supernova
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41705 SUBJECT: EP250827b: Gemini Spectroscopic Confirmation of AT2025wkm as a Ic-BL Supernova DATE: 25/09/04 17:53:57 GMT FROM: Brendan O'Connor at Carnegie Mellon University <boconno2(a)andrew.cmu.edu> Brendan O’Connor (CMU), Xander J. Hall (CMU), Antonella Palmese (CMU), Daniel Gruen (LMU), Gokul Srinivasaragavan (UMD), Anna Y. Q. Ho (Cornell): We observed the optical transient AT2025wkm, associated with EP250827b (Schroeder et al., GCN 41635), using GMOS mounted on Gemini-North (PI: O’Connor). Observations started on 2025-09-02 at 13:36:55 UT (6.3 d) and 2025-09-04 at 10:58:00 UT (8.2 d). At each epoch, we obtained 2x600 s exposures using the B480 grating covering 3950-8210 AA. The spectra show a steep blueward slope (likely due to shock cooling) and display broad features that are consistent with ejecta moving at a high velocity (see also Corcoran et al., GCN 41670 and Ho et al., GCN 41639). We identify blueshifted Fe II absorption features in both spectra, indicative of ejecta velocities ~ 40,000 km/s. The Fe II absorption features in combination with the lack of broad Hydrogen and Helium features associated with the transient itself allow us to confirm the classification of a Ic-BL supernova as initially reported by Corcoran et al. (GCN 41670). In addition, we identify numerous narrow emission lines, including the [O II] doublet, multiple [Ne III] lines, the entire Balmer series between Hbeta and Halpha, He I lines, [N II], [S II], and [Ar III], that identify the host galaxy as highly star forming. Further observations are planned and encouraged. We thank the staff of the Gemini Observatory, in particular Brian Lemaux, Eunchong Kim, Teo Mocnik, and Aleksandar Cikota for their assistance in planning and obtaining these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41705. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39770] GRB 250904A: Swift detection of a burst
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41704 SUBJECT: GRB 250904A: Swift detection of a burst DATE: 25/09/04 17:49:36 GMT FROM: David Palmer at LANL <palmer(a)lanl.gov> C. Salvaggio (INAF-OAB), R. Brivio (INAF-OAB), S. Dichiara (PSU), R. Gupta (NASA GSFC), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), K. L. Page (U Leicester), D. M. Palmer (LANL) and T. Sbarrato (INAF-OAB) report on behalf of the Neil Gehrels Swift Observatory Team: At 17:37:03 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250904A (trigger=1347179). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 84.358, +3.584 which is RA(J2000) = 05h 37m 26s Dec(J2000) = +03d 35' 04" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single complex peak structure with a duration of about 50 sec. The peak count rate was ~2000 counts/sec (15-350 keV), at ~8 sec after the trigger. The XRT began observing the field at 17:38:25.4 UT, 82.4 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source located at RA, Dec 84.38001, 3.58520 which is equivalent to: RA(J2000) = 05h 37m 31.20s Dec(J2000) = +03d 35' 06.7" with an uncertainty of 3.7 arcseconds (radius, 90% containment). This location is 79 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. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 3.34 x 10^21 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 85 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.292. 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/41704. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39769] GRB 250902A: CALET Gamma-Ray Burst Monitor detection
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41703 SUBJECT: GRB 250902A: CALET Gamma-Ray Burst Monitor detection DATE: 25/09/04 16:50:35 GMT FROM: Yuta Kawakubo at Aoyama Gakuin University <kawakubo(a)phys.aoyama.ac.jp> T. Tamura (Kanagawa U), A. Yoshida, T. Sakamoto, S. Sugita, Y. Kawakubo (AGU), K. Yamaoka (Nagoya U), S. Nakahira (JAXA), Y. Asaoka (ICRR), S. Torii, Y. Akaike, K. Kobayashi (Waseda U), Y. Shimizu (Kanagawa U), N. Cannady (GSFC/UMBC), M. L. Cherry (LSU), S. Ricciarini (U of Florence), P. S. Marrocchesi (U of Siena), and the CALET collaboration: The long GRB 250902A (Fermi GBM Final Real-time Localization: Fermi GBM team, GCN Circ. 41648; SVOM/GRM observation: Wang et al., GCN Circ. 41653; Fermi GBM Detection: Bala et al., GCN Circ. 41662; GECAM-B detection: Zheng et al., GCN Circ. 41687; VZLUSAT-2 detection: Ripa et al., GCN Circ. 41701) triggered the CALET Gamma-ray Burst Monitor (CGBM) at 01:30:01.56 UTC on 2 September 2025 (https://cgbm.calet.jp/cgbm_trigger/flight/1440811415/index.html) The burst signal was seen by all CGBM detectors. Because of a problem with the ground alert processing script, the GCN notice was not distributed automatically for this event. The burst light curve shows a multi-peaked structure that starts at T-0.6 sec, peaks at T+2.9 sec, and ends at T+6.8 sec. The T90 and T50 durations measured by the SGM data are 6.1 +/- 0.3 sec and 2.5 +/- 0.3 sec (40-1000 keV), respectively. The ground-processed light curve is available at https://cgbm.calet.jp/cgbm_trigger/ground/1440811415/ The CALET data used in this analysis are provided by the Waseda CALET Operation Center located at Waseda University. View this GCN Circular online at https://gcn.nasa.gov/circulars/41703. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39768] LIGO/Virgo/KAGRA S250904ae: Updated Sky localization
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41702 SUBJECT: LIGO/Virgo/KAGRA S250904ae: Updated Sky localization DATE: 25/09/04 16:44:30 GMT FROM: Michael J. Williams at University of Portsmouth <michael.williams(a)ligo.org> The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250904ae (GCN Circular 41689). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page: https://gracedb.ligo.org/superevents/S250904ae For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 931 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 3545 +/- 913 Mpc (a posteriori mean +/- standard deviation). For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/. [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040 View this GCN Circular online at https://gcn.nasa.gov/circulars/41702. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39767] GRB 250902A: VZLUSAT-2 detection
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41701 SUBJECT: GRB 250902A: VZLUSAT-2 detection DATE: 25/09/04 15:29:58 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 250902A (Fermi/GBM detection GCN 41662; NuSTAR detection trigger time 2025-09-02 01:29:54 UTC; SVOM detection GCN 41653; GECAM-B detection GCN 41687) 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 unit no. 1. The detection was confirmed at the peak time 2025-09-02 01:30:02 UTC. The T90 duration is 8 s and the significance during T90 reaches 29 sigma. The light curve obtained by VZLUSAT-2 is available here: https://vzlusat2.konkoly.hu/static/share/GRB250902A_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/41701. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39766] LIGO/Virgo/KAGRA S250904cv: Identification of a GW compact binary merger candidate
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41700 SUBJECT: LIGO/Virgo/KAGRA S250904cv: Identification of a GW compact binary merger candidate DATE: 25/09/04 14:40:32 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 S250904cv during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2025-09-04 13:49:52.532 UTC (GPS time: 1441029010.532). The candidate was found by the Aframe [1], cWB [2], cWB BBH [3], GstLAL [4], MBTA [5], PyCBC Live [6], and SPIIR [7] analysis pipelines. S250904cv 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/S250904cv 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%. [8] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [8] 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 [9], distributed via GCN and SCiMMA notices about 2 minutes after the candidate event time. * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [9], distributed via GCN and SCiMMA notices about 7 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 1237 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2414 +/- 670 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] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023 [8] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe [9] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013 View this GCN Circular online at https://gcn.nasa.gov/circulars/41700. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39765] GRB 250903A: Swift-XRT observations
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41699 SUBJECT: GRB 250903A: Swift-XRT observations DATE: 25/09/04 13:38:46 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 SVOM/ECLAIRs-detected source GRB 250903A, collecting 2.4 ks of Photon Counting (PC) mode data between T0+12 ks and T0+24 ks after the trigger. We have detected 2 sources inside the ECLAIRs error region. These have been automatically classified as follows: * 0 likely counterparts * 0 candidate counterparts * 2 uncatalogued X-ray sources * 0 known X-ray sources Uncatalogued X-ray sources -------------------------- Source 1 (SWIFT J234712.1-755757): ================================== RA (J2000.0): 356.8008 = 23 47 12.19 Dec (J2000.0): -75.9660 = -75 57 57.6 Error: 4.3 (arcsec, radius, 90% confidence). Detect flag: GOOD Distance: 2.7 arcmin from the SVOM/ECLAIRs position. Mean rate: 0.0387 [+0.0049, -0.0047] ct s^-1 Mean flux: (1.33 [+0.17, -0.16])e-12 erg cm^-2 s^-1 Peak rate: 0.052 +/- 0.014 ct s^-1 Peak flux: (1.80 +/- 0.47)e-12 erg cm^-2 s^-1 ECF: 3.42e-11 erg cm^-2 ct^-1 assuming NH=4.68e+20 cm^-2, gamma=1.95 determined from a spectral fit. XMM UL: 2.0e-12 erg cm^-2 s^-1, (0.3-10 keV) so the source is not above this 3-sigma upper limit. The source may be fading, at the 0.8-sigma level. Source 2 (SWIFT J234819.4-755214): ================================== RA (J2000.0): 357.0812 = 23 48 19.49 Dec (J2000.0): -75.8707 = -75 52 14.5 Error: 6.4 (arcsec, radius, 90% confidence). Detect flag: GOOD Distance: 4.8 arcmin from the SVOM/ECLAIRs position. Mean rate: (8.6 [+2.9, -2.4])e-3 ct s^-1 Mean flux: (1.06 [+0.36, -0.30])e-11 erg cm^-2 s^-1 Peak rate: (8.6 [+2.9, -2.4])e-3 ct s^-1 Peak flux: (1.06 [+0.36, -0.30])e-11 erg cm^-2 s^-1 ECF: 1.23e-09 erg cm^-2 ct^-1 assuming NH=4.41e+20 cm^-2, gamma=-2.35 determined from a spectral fit. XMM UL: 1.9e-12 erg cm^-2 s^-1, (0.3-10 keV) so the source is 2.7-sigma above this 3-sigma upper limit. There is no evidence for fading. A SIMBAD object `LEDA 242511' is 4.1" 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. Source 1 position is consistent with the optical counterpart discovered by TRT (GCN 41679). 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/SVOM/SVOM_FIELD00034. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41699. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39764] EP250903a: Swift-XRT observations
by GCN Circulars 04 Sep '25

04 Sep '25

TITLE: GCN CIRCULAR NUMBER: 41698 SUBJECT: EP250903a: Swift-XRT observations DATE: 25/09/04 13:34:42 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 EP250903a, collecting 3.7 ks of Photon Counting (PC) mode data between T0+1.3 ks and T0+35 ks after the trigger. We have detected 1 source inside the EP/WXT error region. This has been automatically classified as follows: * 0 likely counterparts * 0 candidate counterparts * 1 uncatalogued X-ray source * 0 known X-ray sources Uncatalogued X-ray sources -------------------------- Source 2 (SWIFT J223627.0-495113): ================================== RA (J2000.0): 339.1126 = 22 36 27.02 Dec (J2000.0): -49.8538 = -49 51 13.7 Error: 7.1 (arcsec, radius, 90% confidence). Detect flag: GOOD Distance: 37 arcsec from the Einstein Probe/WXT position. Mean rate: (3.8 [+1.4, -1.2])e-3 ct s^-1 Mean flux: (2.20 [+0.83, -0.67])e-13 erg cm^-2 s^-1 Peak rate: (3.8 [+1.4, -1.2])e-3 ct s^-1 Peak flux: (2.20 [+0.83, -0.67])e-13 erg cm^-2 s^-1 ECF: 5.78e-11 erg cm^-2 ct^-1 assuming NH=1.17e+20 cm^-2, gamma=1.15 determined from a spectral fit. XMM UL: 1.9e-12 erg cm^-2 s^-1, (0.3-10 keV) so the source is not above this 3-sigma upper limit. There is no evidence for fading. There are 2 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 position of Source 2 is consistent with the optical counterpart detected by LCO (GCN 41680) and SVOM/VT (GCN 41683). 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_FIELD00068. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41698. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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