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

[vsnet-grb-info 39504] LIGO/Virgo/KAGRA S250818k: Upper limits from Fermi-GBM Observations
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41441 SUBJECT: LIGO/Virgo/KAGRA S250818k: Upper limits from Fermi-GBM Observations DATE: 25/08/20 11:04:15 GMT FROM: Cuán de Barra at UCD <cuan.debarra(a)ucdconnect.ie> For S250818k (GCN 41437, LVK Collaboration) and using the updated bayestar skymap, Fermi-GBM was observing 51.0% of the localization probability at event time. There was no Fermi-GBM onboard trigger around the event time of the LIGO/Virgo/KAGRA (LVK) detection of GW trigger S250818k . An automated, blind search for short gamma-ray bursts below the onboard triggering threshold in Fermi-GBM also identified no counterpart candidates. The GBM targeted search, the most sensitive, coherent search for GRB-like signals, was run from +/-30 s around merger time, and also identified no counterpart candidates. Part of the LVK localization region is behind the Earth for Fermi, located at an RA=66.9, Dec=-25.2 with a radius of 68.0 degrees. We therefore set upper limits on impulsive gamma-ray emission for the GW localization region visible to Fermi at merger time. Using the representative soft, normal, and hard GRB-like templates described in arXiv:1612.02395, we set the following 3 sigma flux upper limits over 10-1000 keV, weighted by GW localization probability (in units of 10^-7 erg/s/cm^2): Timescale Soft Normal Hard ------------------------------------ 0.128 s: 1.5 2.5 4.3 1.024 s: 0.66 0.87 1.5 8.192 s: 0.22 0.29 0.45 Assuming the median luminosity distance of 259.4 Mpc from the GW detection, we estimate the following intrinsic luminosity upper limits over the 1 keV-10 MeV energy range (in units of 10^50 erg/s): Timescale Soft Normal Hard ------------------------------------ 0.128s: 0.018 0.029 0.081 1.024s: 0.008 0.010 0.028 8.192s: 0.003 0.003 0.008 View this GCN Circular online at https://gcn.nasa.gov/circulars/41441. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

1 0

[vsnet-grb-info 39503] LIGO/Virgo/KAGRA S250818k: Updated Sky localization and EM Bright Classification
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41440 SUBJECT: LIGO/Virgo/KAGRA S250818k: Updated Sky localization and EM Bright Classification DATE: 25/08/20 10:12:16 GMT FROM: Soichiro Morisaki at U. of Tokyo <soichiro.morisaki(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 S250818k (GCN Circular 41437). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.offline0.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page: https://gracedb.ligo.org/superevents/S250818k Based on posterior support from parameter estimation [1], under the assumption that the candidate S250818k is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass above one solar mass (HasNS) is 80%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is 80%. [2] HasRemnant is assumed to be zero when the heavier component mass is below 1 solar mass. 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 7%. The probability that the lighter compact object is below 1 solar mass (HasSSM) is >99%. The source chirp mass falls with highest probability in the bin (0.1, 0.87) solar masses after parameter estimation [1], assuming the candidate is astrophysical in origin. For the Bilby.offline0.multiorder.fits,0 sky map, the 90% credible region is 949 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 237 +/- 62 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 [2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe View this GCN Circular online at https://gcn.nasa.gov/circulars/41440. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

1 0

[vsnet-grb-info 39502] LIGO/Virgo/KAGRA S250818k: Pan-STARRS pre-detection limits for AT2025ulz
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41439 SUBJECT: LIGO/Virgo/KAGRA S250818k: Pan-STARRS pre-detection limits for AT2025ulz DATE: 25/08/20 09:52:22 GMT FROM: Matt Nicholl at Queens University Belfast <matt.nicholl(a)qub.ac.uk> M. Nicholl, D. R. Young, A. Aamer, C. R. Angus, M. D. Fulton, D. Magill, M. McCollum, T. Moore, S. Sim, J. Weston, X. Sheng (QUB), S. J. Smartt, K.W. Smith, J. Gillanders, S. Srivastav, H. Stevance, F. Stoppa, J. Tweddle (Oxford), L. Shingles (GSI/QUB), P. Ramsden (Birmingham/QUB), K. C. Chambers, M. E. Huber, A. S. B. Schultz, T. de Boer, J. Fairlamb, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, G, Paek, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii), T.-W. Chen (NCU), A. Rest (STScI), C. Stubbs (Harvard): We report pre-discovery limits on the source AT2025ulz, reported by Stein et al. (GCN 41414) and possibly associated with the sub-threshold gravitational wave candidate S20250818k (The LIGO-Virgo-Kagra Collaboration, GCN 41437), obtained with the Pan-STARRS telescope system (Chambers et al., 2016, ArXiv e-prints, 1612.05560). The Pan-STARRS system comprises of two 1.8m telescope units located at the summit of Haleakala on the Hawaiian island of Maui, employing an SDSS-like filter system denoted as grizy, and a broad w-filter, which is a composite of the gri-filters. In our primary NASA mission for Near-Earth Object discovery, we scan the visible night sky North of -50 degrees declination to magnitude depths m~22, weather and Moon permitting. Examining recent data obtained during normal Pan-STARRS survey coverage (see Fulton et al. 2025, MNRAS, 542, 541) at the location of AT2025ulz, we report the following pre-discovery upper limits and time since the S20250818k trigger (t-t0) Date t-t0 (days) Magnitude Band 60904.233 -0.82 <19.7 y 60895.314 -9.7 <21.0 i We note that the y-band non-detection was obtained only ~0.8 days before the GW signal. This indicates that there was no bright, red transient at this position less than a day prior. The images were processed with the Pan-STARRS pipeline. After astrometric and photometric calibration, reference images were subtracted from the target images (Magnier et al., 2020a, ApJS, 251, 3; Magnier et al., 2020b, ApJS, 251, 6; Waters et al., 2020, ApJS, 251, 4). Further Pan-STARRS grizy imaging of this source, and coverage of the S20250818k skymap (northern banana) is underway in the i-band with PS1 and PS2 jointly. The discoveries from this program are a byproduct of the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) NEO survey observations. Operation of the Pan-STARRS1 and Pan-STARRS2 telescopes is primarily supported by the National Aeronautics and Space Administration under Grant No. NNX12AR65G and Grant No. NNX14AM74G issued through the SSO Near-Earth Object Observations Program. Data are processed at Queen's University Belfast enabled through the STFC grants ST/P000312/1 and ST/T000198/1. View this GCN Circular online at https://gcn.nasa.gov/circulars/41439. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

1 0

[vsnet-grb-info 39501] GRB 250818A: AstroSat CZTI detection
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41438 SUBJECT: GRB 250818A: AstroSat CZTI detection DATE: 25/08/20 09:19:25 GMT FROM: Gaurav Waratkar at IIT Bombay <gauravwaratkar(a)iitb.ac.in> Harsha K. H. (IUCAA), Anuraag Arya (IITB), G. Waratkar (IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration: Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a long-duration GRB 250818A which was also detected by Fermi/GBM (Fermi GBM Team, GCN Circ. 41402), and Swift/BAT (Cenko et. al., GCN Circ. 41403). The source was clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2025-08-18 01:32:39.98 UTC. The measured peak count rate associated with the burst is 214 (+77, -0) counts/s above the background in the combined data of all quadrants, with a total of 6518 (+769, -832) counts. The local mean background count rate was 1187 (+3, -3) counts/s. We measure a T90 of 89 (+7, -5) s from the cumulative Veto light curve. CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project. CZTI GRB detections are reported regularly on the payload site at: http://astrosat.iucaa.in/czti/?q=grb View this GCN Circular online at https://gcn.nasa.gov/circulars/41438. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

1 0

[vsnet-grb-info 39500] LIGO/Virgo/KAGRA S250818k: Properties of the low-significance GW compact binary merger candidate potentially associated with AT 2025ulz
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41437 SUBJECT: LIGO/Virgo/KAGRA S250818k: Properties of the low-significance GW compact binary merger candidate potentially associated with AT 2025ulz DATE: 25/08/20 06:04:02 GMT FROM: gwangeon.seong(a)ligo.org The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration (LVK) report: We identified the compact binary merger candidate S250818k during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-08-18 01:20:06.030 UTC (GPS time: 1439515224.030). The candidate was found by the GstLAL SSM [1] and PyCBC Live [2] analysis pipelines. Two LVK Preliminary GCN Notices were issued within 5 minutes after the candidate was identified. Based on the analysis of gravitational-wave (GW) data alone, this candidate does not meet our criteria for a high-significance public alert as its false alarm rate is estimated by the online analysis to be 6.8e-08 Hz or about one in 5 months. However, we are issuing this Circular to confirm the properties of the GW candidate because of its potential association with the optical transient ZTF25abjmnps / AT 2025ulz (Stein et al, GCN 41414) which has been confirmed and reported to be fading and reddening rapidly (Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436). The GW candidate’s properties can be found at this URL: https://gracedb.ligo.org/superevents/S250818k The classification of the GW candidate, in order of descending probability, is Terrestrial (71%), BNS (29%), NSBH (<1%), or BBH (<1%). Initial data quality checks found that there is no evidence for presence of a glitch or other data quality issues. 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 >99%. [3] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is >99%. [3] 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 (0.1, 0.87) solar masses, assuming the candidate is astrophysical in origin. Three 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 [4], distributed via GCN and SCiMMA notices about 28 seconds after the candidate event time. * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [4], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time. * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [4], distributed via GCN and SCiMMA notices about 2 days after the candidate event time. The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 786 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 259 +/- 74 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] Hanna et al. (2024) arXiv 2412.10951 [2] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a [3] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe [4] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013 View this GCN Circular online at https://gcn.nasa.gov/circulars/41437. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

1 0

[vsnet-grb-info 39499] LIGO/Virgo/KAGRA S250818k: Keck I LRIS spectroscopy of ZTF25abjmnps (AT2025ulz)
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41436 SUBJECT: LIGO/Virgo/KAGRA S250818k: Keck I LRIS spectroscopy of ZTF25abjmnps (AT2025ulz) DATE: 25/08/20 05:23:52 GMT FROM: Mansi Kasliwal at Caltech <mansikasliwal(a)gmail.com> Viraj Karambelkar (Caltech), Mansi M. Kasliwal (Caltech) and Xander J. Hall (CMU) report on behalf of the larger ZTF and GROWTH collaborations We observed ZTF25abjmnps (AT2025ulz; Stein et al. GCN 41414) starting UTC 2025-08-19 06:42 with the LRIS spectrograph on Keck I. We derive a redshift of 0.0848 based on host galaxy emission lines - this is consistent with the 3D GW localization for S250818k. Subtracting host galaxy light, we see a blue continuum excess from the transient, and no tell-tale supernova-like features. The data in-hand thus far appears consistent with a kilonova. However, continued monitoring (such as Busmann et al. GCN 41421, Hall et al. GCN 41433) is encouraged to rule out any future re-brightening from a faint and fast supernova Type IIb (e.g., Barna et al. 2025, arXiv:2506.15900). Panchromatic follow-up is ongoing and encouraged. View this GCN Circular online at https://gcn.nasa.gov/circulars/41436. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

1 0

[vsnet-grb-info 39498] GRB 250818B: Swift/UVOT Detection
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41435 SUBJECT: GRB 250818B: Swift/UVOT Detection DATE: 25/08/20 04:23:17 GMT FROM: Mike Siegel at PSU/Swift MOC <mhs18(a)psu.edu> M. H. Siegel (PSU) reports on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 250818B 1.7 ks after the SVOM/ECLAIRs trigger (Wang et al., GCN Circ. 41405). An uncatalogued source consistent with the XRT position (Farro et al., GCN Circ. 41407) and the optical transient (Kumar et al., GCN Circ. 41406; Yao et al., GCN Circ. 41409; Zheng et al., GCN Circ. 41417; Bendtsen et al., GCN Circ. 41426; Moskvitin et al., GCN Circ. 41428; An et al., GCN Circ. 41430; Kumar et al., GCN Circ. 41431) is detected in the UVOT exposures. The preliminary UVOT position is: RA (J2000) = 03:04:13.45 = 46.05602 (deg.) Dec (J2000) = -03:07:31.2 = -3.12532 (deg.) with an estimated uncertainty of 0.42 arc sec. (radius, 90% confidence). Preliminary detections and 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag u 1723 6610 1544 18.25+/-0.06 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.063 in the direction of the burst (Schlegel et al. 1998). View this GCN Circular online at https://gcn.nasa.gov/circulars/41435. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

1 0

[vsnet-grb-info 39497] Fermi Gamma-ray Burst Monitor trigger 777342746/250820022 is not a GRB
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41434 SUBJECT: Fermi Gamma-ray Burst Monitor trigger 777342746/250820022 is not a GRB DATE: 25/08/20 01:39:55 GMT FROM: oindabimukherjee(a)gmail.com O. Mukherjee (USRA) reports on behalf of the Fermi Gamma-ray Burst Monitor Team: "The Fermi Gamma-ray Burst Monitor (GBM) trigger 777342746/250820022 at 00:32:21.59 UT on 20 August 2025, tentatively classified as a GRB, is in fact not due to a GRB. This trigger is likely due to local particles." View this GCN Circular online at https://gcn.nasa.gov/circulars/41434. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

1 0

[vsnet-grb-info 39496] LIGO/Virgo/KAGRA S250818k: FTW Fast reddening of AT 2025ulz
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41433 SUBJECT: LIGO/Virgo/KAGRA S250818k: FTW Fast reddening of AT 2025ulz DATE: 25/08/20 01:24:57 GMT FROM: xjh(a)andrew.cmu.edu Xander J. Hall (Carnegie Mellon U.), Malte Busmann (LMU), Daniel Gruen (LMU), Brendan O’Connor (Carnegie Mellon U.), and Antonella Palmese (Carnegie Mellon U.) report: We observed the source AT 2025ulz reported by Stein et al. (GCN 41414) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the g, r, i, z, and J bands starting at 2025-08-19T20:09:46 for 22 x 180 s. We took 11 observations in the giJ configuration and 11 in the rzJ configuration. In the difference imaging with templates from the Legacy Survey, we detect AT 2025ulz at g = (22.08 +/- 0.09) AB mag, r = (21.83 +/- 0.06) AB mag. We note significant reddening and fading from our previous report (GCN 41421), with the g band dropping 0.83 AB mag in 24.6 hours and r band now being brighter than g. The magnitudes are calibrated against the PS1 catalog and not corrected for Galactic extinction. Further analysis is underway. We thank Michael Schmidt from the Wendelstein Observatory staff for obtaining these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/41433. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

1 0

[vsnet-grb-info 39495] GRB 250820A: Fermi GBM Final Real-time Localization
by GCN Circulars 20 Aug '25

20 Aug '25

TITLE: GCN CIRCULAR NUMBER: 41432 SUBJECT: GRB 250820A: Fermi GBM Final Real-time Localization DATE: 25/08/20 00:42:49 GMT FROM: Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply(a)GIOC.nsstc.nasa.gov> The Fermi GBM team reports the detection of a likely LONG GRB At 00:32:21 UT on 20 Aug 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250820A (trigger 777342746.588176 / 250820022). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 28.0, Dec = -70.8 (J2000 degrees, equivalent to J2000 01h 52m, -70d 47'), with a statistical uncertainty of 1.1 degrees. The angle from the Fermi LAT boresight is 169.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250820022/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250820022/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250820022/… View this GCN Circular online at https://gcn.nasa.gov/circulars/41432. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

1 0

2025

2024

2023

2022

2021