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[vsnet-grb-info 41163] EP251212a: KAIT optical upper limit
by GCN Circulars 13 Dec '25

13 Dec '25

TITLE: GCN CIRCULAR NUMBER: 43083 SUBJECT: EP251212a: KAIT optical upper limit DATE: 25/12/13 09:04:48 GMT FROM: Weikang Zheng at UC Berkeley <weikang(a)berkeley.edu> WeiKang Zheng (UCB), Xuhui Han (NAOC), Pinpin Zhang (NAOC) and Alexei V. Filippenko (UCB) report on behalf of the KAIT GRB team: The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at Lick Observatory, observed the field of EP251212a (Yang et al., GCN 43076; Salunke et al., GCN 43082) starting at 11:09, Dec 12 UT, about 4.95 hours after the burst. A set of 38x60s clear (roughly R) filter images were obtained. In our coadd image, we did not detect any new source within the EP-FXT error circle (Yang et al., GCN 43081) down to a limiting mag of 21.2 mag (Vega) at a mid-time of 5.57 hours after the burst. Our result is consistent with the upper limit reported from COLIBRÍ observations (Ducoin et al., GCN 43077). View this GCN Circular online at https://gcn.nasa.gov/circulars/43083. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41162] EP251212a / GRB 251212A: AstroSat CZTI detection
by GCN Circulars 13 Dec '25

13 Dec '25

TITLE: GCN CIRCULAR NUMBER: 43082 SUBJECT: EP251212a / GRB 251212A: AstroSat CZTI detection DATE: 25/12/13 07:55:15 GMT FROM: Gaurav Waratkar at Caltech <gauravwaratkar(a)iitb.ac.in> S. Salunke (IUCAA), M. Tembhurnikar (IUCAA), Harsha K. H. (IUCAA), A. Arya (IITB), A. Goyal (IITB), G. Waratkar (Caltech/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 GRB 251212A which was also detected by Konus-Wind (via IPN Notices), about 20 s after the trigger time of EP251212a detected by Einstein Probe / WXT (Yang et. al., GCN Circ. 43076). The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2025-12-12 06:13:13.50 UTC. The measured peak count rate associated with the burst is 64 (+20, -21) counts/s above the background in the data of one quadrant (out of four), with a total of 136 (+55, -58) counts. The local mean background count rate was 83 (+2, -2) counts/s. Using cumulative rates, we measure a T90 of 5.6 (+0.6, -3.8) s. The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2025-12-12 06:13:13.22 UTC. The measured peak count rate associated with the burst is 475 (+72, -74) counts/s above the background in the combined data of all quadrants, with a total of 1524 (+245, -269) counts. The local mean background count rate was 1568 (+7, -9) counts/s. We measure a T90 of 5.7 (+0.9, -1.9) 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/43082. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41161] EP251212a: EP-FXT follow-up observation
by GCN Circulars 13 Dec '25

13 Dec '25

TITLE: GCN CIRCULAR NUMBER: 43081 SUBJECT: EP251212a: EP-FXT follow-up observation DATE: 25/12/13 04:25:21 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> J. Yang (ZZU), Y. H. Jiang (NJU), Z. X. Li, G. L. Huang, J. Y. Cao (IHEP), S. Y. Fu (HUST), Z. X. Ling (NAOC) on behalf of the Einstein Probe (EP) team: EP-FXT performed a follow-up observation of EP251212a (GCN 43076) at 2025-12-12T13:44:09 (UTC), about 7.5 hours after the WXT detection, with an exposure time of 2.7 ks. FXT detected an uncatalogued source within the WXT error circle at R.A. = 134.0471, DEC = -23.2992 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The FXT X-ray spectrum of this uncatalogued source can be fitted with an absorbed powerlaw with a hydrogen column density of 1.1 x 10^21 cm^-2 and a photon index of 1.7. The derived unabsorbed 0.5-10 keV flux is approximately 1.54 x 10^-12 erg/s/cm^2. 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/43081. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41160] GRB 251211B: AstroSat CZTI detection
by GCN Circulars 13 Dec '25

13 Dec '25

TITLE: GCN CIRCULAR NUMBER: 43080 SUBJECT: GRB 251211B: AstroSat CZTI detection DATE: 25/12/13 04:13:33 GMT FROM: Utkarsh Pathak at IIT Bombay <utkarshpathak.07(a)gmail.com> A. Arya (IITB), A. Goyal (IITB), U. Pathak (IITB), M. Tembhurnikar (IUCAA), S. Salunke (IUCAA), Harsha K. H. (IUCAA), G. Waratkar (Caltech/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 GRB 251211B, which was also detected by Fermi/GBM (Fermi GBM Team, GCN Circ. 43072). The source was faintly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2025-12-11 20:32:04.31 UTC. The measured peak count rate associated with the burst is 176 (+60, -53) counts/s above the background in the combined data of all quadrants, with a total of 367 (+222, -238) counts. The local mean background count rate was 1180 (+8, -8) counts/s. Due to the intrinsic 1 s binning of veto data and the relatively faint detection, we cannot reliably estimate a T90 for this burst. 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/43080. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41159] IceCube-251210A: No Candidate Transients from the Zwicky Transient Facility
by GCN Circulars 12 Dec '25

12 Dec '25

TITLE: GCN CIRCULAR NUMBER: 43079 SUBJECT: IceCube-251210A: No Candidate Transients from the Zwicky Transient Facility DATE: 25/12/12 18:25:09 GMT FROM: Jannis Necker at DESY <jannis.necker(a)desy.de> Robert Stein (JSI), Jannis Necker (Leiden University), Akshay Eranhalodi (DESY) and Anna Franckowiak (Ruhr University Bochum) report On behalf of the Zwicky Transient Facility (ZTF) and Global Relay of Observatories Watching Transients Happen (GROWTH) collaborations: As part of the ZTF neutrino follow up program (Stein et al. 2023), we observed the localization region of the neutrino event IceCube-251210A (Zegarelli et. al, GCN 43064) with the Palomar 48-inch telescope, equipped with the 47 square degree ZTF camera (Bellm et al. 2019, Graham et al. 2019). We started observations in the g- and r-band beginning at 2025-12-11 02:31 UTC, approximately 15.4 hours after event time. We covered 97.7% (1.6 sq deg) of the reported localization region. This estimate accounts for chip gaps. Each exposure was 300s with a typical depth of 21.0 mag. The images were processed in real-time through the ZTF reduction and image subtraction pipelines at IPAC to search for potential counterparts (Masci et al. 2019). AMPEL (Nordin et al. 2019, Stein et al. 2021) was used to search the alerts database for candidates. We reject stellar sources (Tachibana and Miller 2018) and moving objects, and apply machine learning algorithms (Mahabal et al. 2019). We are left with the following candidate: +--------------------------------------------------------------------------------+ | ZTF Name | IAU Name | RA (deg) | DEC (deg) | Filter | Mag | MagErr | +--------------------------------------------------------------------------------+ | ZTF25acgniiw | ------- | 035.5190217 | +20.3633434 | r | 20.55 | 0.09 | +--------------------------------------------------------------------------------+ ZTF25acgniiw was first detected on 2025-11-24. Its position is coincident with a quasar listed in MILLIQUAS (Flesch et al., 2023). Because there is no sign of contemporaneous flaring activity, ZTF25acgniiw is likely not associated with IceCube-251210A. This is in agreement with previous reports (Becerra et al. 2025, GCN 43071). ZTF and GROWTH are worldwide collaborations comprising Caltech, USA; IPAC, USA; WIS, Israel; OKC, Sweden; JSI/UMd, USA; DESY, Germany; TANGO, Taiwan; UW Milwaukee, USA; LANL, USA; TCD, Ireland; IN2P3, France. GROWTH acknowledges generous support of the NSF under PIRE Grant No 1545949. Alert distribution service provided by DIRAC@UW (Patterson et al. 2019). Alert database searches are done by AMPEL (Nordin et al. 2019). Alert filtering is performed with the nuztf (Stein et al. 2021, https://github.com/desy-multimessenger/nuztf ). View this GCN Circular online at https://gcn.nasa.gov/circulars/43079. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41158] GRB 251209A: NuSTAR detection of prompt emission
by GCN Circulars 12 Dec '25

12 Dec '25

TITLE: GCN CIRCULAR NUMBER: 43078 SUBJECT: GRB 251209A: NuSTAR detection of prompt emission DATE: 25/12/12 18:15:43 GMT FROM: Gaurav Waratkar at Caltech <gauravwaratkar(a)iitb.ac.in> G. Waratkar (Caltech) and B. Grefenstette (Caltech) report on behalf of the NuSTAR Search for INteresting Gamma-ray Signals (SINGS) working group: The NuSTAR SINGS working group reports the detection of prompt emission from the long-duration GRB 251209A in both the NuSTAR CsI anti-coincidence shields. This GRB was identified through a blind search using the CsI shield rates. Details of the search algorithm will be described in a future paper. The NuSTAR SINGS algorithm triggered at 2025-12-09 13:24:26 (with a resolution ~5-seconds). This is consistent with the detections of GRB 251209A by Fermi/GBM (Fermi GBM Team, GCN Circ. 43048) and AstroSat/CZTI (Tembhurnikar et al., GCN Circ. 43066). The NusTAR trigger is consistent with triggering on the longer-duration “main burst” as seen in the Fermi-GBM lightcurves. The NuSTAR CsI shield data are recorded at 1 Hz. We detect two peaks lasting for ~30-s, consistent with the detections of Fermi/GBM & AstroSat/CZTI. A fainter, narrower initial burst at 2025-12-09 13:24:03 is also seen by eye in the NuSTAR lightcurves but did not trigger the automated algorithm. The peak count rate is ~3000-cps over a baseline rate of ~1,000-cps during this time period. The Fermi/GBM localization (GCN Circ. 43048) at RA = 214.2, Dec = 43.1 implies an offset from the NuSTAR boresight of 92-deg (i.e. from the side of the instrument) and an offset from the geocenter of 120-deg. Lightcurves and analysis for this GRB can be found here: https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2025/251209A Information on NuSTAR SINGS can be found here: https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/ NuSTAR is a NASA Small Explorer mission led by Caltech and managed by JPL for NASA's Science Mission Directorate in Washington. View this GCN Circular online at https://gcn.nasa.gov/circulars/43078. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41157] EP251212a : COLIBRÍ optical upper limit
by GCN Circulars 12 Dec '25

12 Dec '25

TITLE: GCN CIRCULAR NUMBER: 43077 SUBJECT: EP251212a : COLIBRÍ optical upper limit DATE: 25/12/12 14:49:35 GMT FROM: J.-G. Ducoin at CPPM <ducoin(a)cppm.in2p3.fr> Jean-Grégoire Ducoin (CPPM), Enrique Moreno Méndez (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Nikos Mandarakas (LAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM): We imaged the field of the EP251212a (Yang et al., GCN Circ. 43076) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-12-12T11:08:54.118 to 11:27:56.049 UTC (from 4.94 to 5.25 hours after the trigger) and obtained 960 seconds of simultaneous exposure in the r and z filters. The data were reduced, coadded and analyzed with the COLIBRÍ pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and was not corrected for Galactic extinction. In the stacked image, we do not detect any new source at the WXT uncertainty region (Yang et al., GCN Circ. 43076) down to the following 5-sigma limit: r > 22.59 z > 21.41 We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams. COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico. View this GCN Circular online at https://gcn.nasa.gov/circulars/43077. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41156] EP251212a: Einstein Probe detection of an X-ray transient
by GCN Circulars 12 Dec '25

12 Dec '25

TITLE: GCN CIRCULAR NUMBER: 43076 SUBJECT: EP251212a: Einstein Probe detection of an X-ray transient DATE: 25/12/12 12:39:39 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> J. Yang (ZZU), Y. H. Jiang (NJU), Z. X. Li, G. L. Huang, J. Y. Cao (IHEP), S. Y. Fu (HUST), Z. X. Ling (NAOC) on behalf of the Einstein Probe (EP) team: We report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP251212a. The source did not trigger the WXT on-board trigger unit. A GCN Notice was sent manually (trigger ID: 11916652735). The ground analysis of the WXT data shows that the event started at T0=2025-12-12T06:12:52 (UTC) and lasted for around 100 seconds. The WXT position of the source is R.A. = 134.041 deg, DEC = -23.279 deg (J2000) with an uncertainty of 2.4 arcmin in radius (90% C.L. statistical and systematic). The average WXT 0.5 - 4 keV spectrum can be fitted with an absorbed power law with a hydrogen column density of 3.6 x 10^21 cm^-2 and a photon index of 1.1. The derived average unabsorbed 0.5-4 keV flux is 2.0 x 10^-9 erg/s/cm^2. The follow-up observation with the Follow-up X-ray Telescope (FXT) has been scheduled. Further information will be updated when the FXT telemetry data are received. 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/43076. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41155] Fermi-LAT Gamma-ray Observations of IceCube-251210A
by GCN Circulars 12 Dec '25

12 Dec '25

TITLE: GCN CIRCULAR NUMBER: 43075 SUBJECT: Fermi-LAT Gamma-ray Observations of IceCube-251210A DATE: 25/12/12 09:04:29 GMT FROM: Leo Pfeiffer at University of Würzburg <pfeiffer.leo(a)gmail.com> L. Pfeiffer (Univ. of Wuerzburg), S. Buson (DESY, Univ. of Wuerzburg) and S. Garrappa (Weizmann Institute of Science) on behalf of the Fermi-LAT collaboration: We report an analysis of observations of the vicinity of the high-energy IC251210A neutrino event (GCN 43064) with all-sky survey data from the Large Area Telescope (LAT), on board the Fermi Gamma-ray Space Telescope. The IceCube event was detected on 25-12-10 at 11:06:14.40 UTC (T0) with J2000 position RA = 35.29 (+0.68, -0.69) deg, Decl. = 20.34 (+0.64, -0.71) deg 90% PSF containment (J2000). No cataloged gamma-ray sources are found within the 90% IC251210A localization error (4FGL-DR4; The Fourth Fermi-LAT catalog Data Release 4, The Fermi-LAT collaboration 2023, arXiv:2307.12546). We searched for the existence of intermediate (days to years) timescale emission from a new gamma-ray transient source. Preliminary analysis indicates no significant (>5sigma) new excess emission (> 100 MeV) within the IC251210A 90% confidence localization. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC251210A best-fit position, the >100 MeV flux upper limit (95% confidence) is <2.2e-10 ph cm^-2 s^-1 for ~17-years (2008-08-04 / T0), <1.5e-08 (<9.1e-08) ph cm^-2 s^-1 for a 1-month (1-day) integration time before T0. Since Fermi normally operates in an all-sky scanning mode, regular monitoring of this source will continue. For this analysis, the Fermi-LAT contact person is L. Pfeiffer (leonard.pfeiffer at uni-wuerzburg.de). The Fermi LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden. View this GCN Circular online at https://gcn.nasa.gov/circulars/43075. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41154] IceCube-251210A: Upper limits from a search for additional neutrino events in IceCube
by GCN Circulars 12 Dec '25

12 Dec '25

TITLE: GCN CIRCULAR NUMBER: 43074 SUBJECT: IceCube-251210A: Upper limits from a search for additional neutrino events in IceCube DATE: 25/12/12 05:49:23 GMT FROM: Sam Hori at IceCube/U Wisc-Madison <sahori(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-251210A (https://gcn.nasa.gov/circulars/43064) in a time range of 1000 seconds centered on the alert event time (2025-12-10 10:57:54.429 UTC to 2025-12-10 11:14:34.429 UTC) during which IceCube was collecting good quality data. Excluding the event that prompted the alert, zero track-like events are found within the 90% containment region of IceCube-251210A. We report a p-value of 1.00 in this time window. IceCube’s sensitivity to neutrino point sources with an E^-2.5 spectrum, expressed as E^2 dN/dE evaluated at 1 TeV, is 1.4e-01 GeV cm^-2 within the 90% spatial containment region of IceCube-251210A 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 9e+04 GeV. A subsequent search was performed including 2 days of data centered on the alert event time (2025-12-09 11:06:14.429 UTC to 2025-12-11 11:06:14.429 UTC). In this case, we report a p-value of 1.00, consistent with no significant excess of track events. IceCube’s sensitivity to neutrino point sources with an E^-2.5 spectrum, expressed as E^2 dN/dE evaluated at 1 TeV, is 1.7e-01 GeV cm^-2 within the 90% spatial containment region of IceCube-251210A 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/43074. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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