TITLE: GCN CIRCULAR
NUMBER: 39502
SUBJECT: GRB 250225B: Fermi GBM Observation
DATE: 25/02/26 22:38:04 GMT
FROM: Matt Godwin <msg0028(a)uah.edu>
M. Godwin (UAH), C. Meegan (UAH), P. Veres (UAH) and R. Hamburg (USRA) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:
"At 19:39:13.83 UT on 25 February 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250225B (trigger 762205158/250225819).
which was also detected by Swift BAT (H. A. Krimm et al. 2025, GCN 39473), Konus-Wind (A. Tsvetkova et al. 2025, GCN 39498), and VLT/FORS2 (Schneider et al. 2025, GCN 39494)
The Fermi GBM on-ground location is consistent with the Swift BAT position.
The angle from the Fermi LAT boresight is 55 degrees.
The GBM light curve consists of one bright peak followed my multiple short peaks with a duration (T90)
of about 335 s. The time-averaged spectrum
from T0-3.6 to T0+340.5 s is best fit by
a power law function with an exponential high-energy cutoff.The power law index is -1.30 +/- 0.05 and the cutoff energy, parameterized as Epeak, is 218 +/- 28 keV.
The event fluence (10-1000 keV) in this time interval is
(2.14 +/- 0.11)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+1.3 s in the 10-1000 keV band is 21.9 +/- 0.3 ph/s/cm^2.
The time-averaged spectrum of the first peak from T0 to T0+3.6 s is best fit by
a power law function with an exponential high-energy cutoff. The power law index is -1.06 +/- 0.02 and the cutoff energy, parameterized as Epeak, is 568 +/- 42 keV. The Band function fits the spectrum equally well, with an Epeak of 547 +/- 46 keV, alpha = -1.05 +/- 0.03 and beta = -2.85 +/-0.67.
The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html
For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"
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TITLE: GCN CIRCULAR
NUMBER: 39501
SUBJECT: GRB 250221A: ATCA Detections and Upper Limits
DATE: 25/02/26 21:23:21 GMT
FROM: agul8829(a)uni.sydney.edu.au
A. Gulati (USyd), G. E. Anderson (Curtin), Claire Morley (Curtin), S. Chastain (UNM), J. K. Leung (UofT/HUJI), A. J. van der Horst (GWU), and L. Rhodes (TSI/McGill) on behalf of the ATCA PanRadio GRB collaboration
We observed long GRB 250221A (Palmer et al., GCN 39396) as part of The Australia Telescope Compact Array (ATCA) "PanRadio GRB" Large Project C3542 (PI: G. Anderson) at 5.5 and 9 GHz on 2025-02-21 (starting 6 minutes post-burst, for 10 hours), and on 2025-02-23 (starting 2.2 days post burst for 4 hours).
No radio sources were detected near the Swift/XRT enhanced position (Beardmore et al., GCN 39404) in the first epoch, with a 3-sigma upper limit of 24 uJy at 9 GHz. We detect a radio counterpart in the second epoch at a position consistent with the Swift/XRT enhanced position, with a flux of 233 +/- 15 uJy at 9 GHz. This value significantly exceeds the 10 GHz VLA detection at 1.95 days (Ricci et al., GCN 39433), possibly due to strong interstellar scintillation near the typical 10 GHz transition frequency.
We thank the CSIRO Space and Astronomy staff for supporting these observations.
We acknowledge the Gomeroi people as the traditional owners of the Observatory site. The Australia Telescope Compact Array is part of the Australia Telescope National Facility (https://ror.org/05qajvd42) which is funded by the Australian Government for operation as a National Facility managed by CSIRO.
View this GCN Circular online at https://gcn.nasa.gov/circulars/39501.
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TITLE: GCN CIRCULAR
NUMBER: 39500
SUBJECT: GRB 250225B: REM NIR upper limits
DATE: 25/02/26 19:12:09 GMT
FROM: Matteo Ferro at INAF-OAB <matteo.ferro(a)inaf.it>
M. Ferro, R. Brivio, P. D'Avanzo, S. Covino, D. Fugazza, S. Campana (INAF-OAB), and L. Izzo (INAF-OACn and DARK/NBI) report on behalf of the REM team:
We observed the field of Swift detected GRB250225B (Williams et al., GCN 39473), also seen by SVOM (Zhang et al., GCN 39493) with the REM 60 cm robotic telescope located at the ESO observatory of La Silla (Chile). The observations were carried in the J, H, and K bands, started on 2025 February 26 at 08:27:46 UT (i.e. 12.8 hr after the burst), and lasted for about 1 hour.
From preliminary inspection inside the BAT error circle, we do not find any counterpart at the position of the reported optical/NIR candidate counterpart (Schneider et al., GCN 39494, Yang et al., GCN 39495) down to the following 3sigma limit:
H > 17.5 (Vega; calibrated against the 2MASS catalogue),
at a mid-time of 13.2 hours after the trigger
J > 17.1 (Vega; calibrated against the 2MASS catalogue)
at a mid-time of 13.4 hours after the trigger.
View this GCN Circular online at https://gcn.nasa.gov/circulars/39500.
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TITLE: GCN CIRCULAR
NUMBER: 39499
SUBJECT: LIGO/Virgo/KAGRA S250223dk and Swift/BAT-GUANO ID 762004910: Updated Sky localization and Coincidence with External Event
DATE: 25/02/26 18:26:53 GMT
FROM: ethan.payne(a)ligo.org
The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration along with the Swift/BAT-GUANO team 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 S250223dk (GCN Circular 39443). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.offline0.multiorder.fits,0, distributed via GCN Notice, is available for retrieval from the GraceDB event page:
https://gracedb.ligo.org/superevents/S250223dk
For the Bilby.offline0.multiorder.fits,0 sky map, the 90% credible region is 18323 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 9589 +/- 6347 Mpc (a posteriori mean +/- standard deviation).
A search performed by the RAVEN pipeline [2] found a temporal coincidence between S250223dk and a sub-threshold Swift/BAT trigger with ID 762004910 (GCN circular 39443). The GRB trigger time is 10.24 seconds before the GW candidate event. The estimated joint false alarm rate for the coincidence using just timing info is 1.9e-07 Hz, or about one in a month. The GRB candidate was found during a joint targeted search between the LIGO/Virgo/KAGRA collaboration and Swift/BAT-GUANO, and has a false alarm rate of 7.4e-05 Hz, or about one in 3 hours. RAVEN has also identified an additional detection from Fermi GBM.
Combined sky maps are also available:
* combined-ext.multiorder.fits,0, an initial localization, distributed via GCN notice about 12 hours after the candidate event time.
* combined-ext.multiorder.fits,1, an initial localization, distributed via GCN notice about 12 hours after the candidate event time.
* combined-ext.multiorder.fits,2, an initial localization, distributed via GCN notice about 13 hours after the candidate event time.
* combined-ext.multiorder.fits,3, an initial localization, distributed via GCN notice about 15 hours after the candidate event time.
* combined-ext.multiorder.fits,5, an updated localization, distributed via GCN notice about 3 days after the candidate event time.
The joint LVK-Swift/BAT localization probability map peaks at
RA = 85.341 deg,
Dec = -47.554 deg.
A circle with a radius of 10 arcmin around this position contains 52% of the integrated joint probability.
For the combined-ext.multiorder.fits,5 sky map, the 90% credible region is 1 deg2. Considering the overlap of the individual sky maps, the estimated joint false alarm rate for the spatial and temporal coincidence is 1.7e-08 Hz, or about one in 2 years. After considering trials factors, this means the joint FAR is larger than the ‘'high significance’' alert threshold and this joint event is no longer considered significant.
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. (2023) arXiv:2307.13380
[2] Urban, A. L. 2016, Ph.D. Thesis https://dc.uwm.edu/etd/1218 and Piotrzkowski, B. J. 2022, Ph.D. Thesis https://dc.uwm.edu/etd/3060
View this GCN Circular online at https://gcn.nasa.gov/circulars/39499.
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TITLE: GCN CIRCULAR
NUMBER: 39498
SUBJECT: Konus-Wind detection of GRB 250225B
DATE: 25/02/26 18:14:00 GMT
FROM: Anna Ridnaia at Ioffe Institute <ridnaia(a)mail.ioffe.ru>
A. Ridnaia, D. Frederiks, A. Lysenko, D. Svinkin,
A. Tsvetkova, M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:
The long-duration GRB 250225B
(Swift-BAT detection: Williams et al., GCN 39473;
SVOM/GRM detection: Zhang et al., GCN 39493)
triggered Konus-Wind at T0=70750.895 s UT (19:39:10.895).
The burst light curve shows a bright initial pulse
which starts at ~T0-2.2 s and has a total duration of ~3.3 s,
followed by two weaker emission episodes centered
around ~T0+60 s and T0+108 s.
The emission is seen up to ~6 MeV.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB250225_T70750/
As observed by Konus-Wind, the burst
had a fluence of 1.54(-0.18,+0.28)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+0.262 s,
of 1.14(-0.24,+0.35)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).
The time-averaged spectrum of the initial pulse
(measured from T0 to T0+8.448 s)
is best fit in the 20 keV - 10 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -1.14(-0.16,+0.18)
and Ep = 634(-181,+393) keV (chi2 = 66/84 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -1.7
(chi2 = 66/83 dof).
The time-averaged spectrum of the second emission episode
(measured from T0+57.600 to T0+65.792 s)
is best fit in the 20 keV - 10 MeV range
by a power law with exponential cutoff model
with alpha = -1.23(-0.21,+0.28)
and Ep = 361(-118,+292) keV (chi2 = 97/99 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -1.7
(chi2 = 95/98 dof).
The time-averaged spectrum of the third emission episode
(measured from T0+106.752 to T0+114.944 s)
is best fit in the 20 keV - 10 MeV range
by a power law with exponential cutoff model
with alpha = -0.91(-0.69,+1.12)
and Ep = 187(-74,+228) keV (chi2 = 110/99 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -1.7
(chi2 = 110/98 dof).
All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.
View this GCN Circular online at https://gcn.nasa.gov/circulars/39498.
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TITLE: GCN CIRCULAR
NUMBER: 39497
SUBJECT: LIGO/Virgo/KAGRA S250223dk: DECam optical detections of Swift X-ray sources
DATE: 25/02/26 17:49:58 GMT
FROM: Antonella Palmese at Carnegie Mellon University <apalmese(a)andrew.cmu.edu>
Antonella Palmese (CMU), Lei Hu (CMU), Xander J. Hall (CMU), Tomás Cabrera (CMU), Igor Andreoni (UNC), Brendan O’Connor (CMU), Keerthi Kunnumkai (CMU), report on behalf of the GW-MMADS team:
The high probability area of the joint LVK/Swift-GUANO alert for the gravitational wave candidate S250223dk (GCN 39443) was observed using the wide-field Dark Energy Camera (DECam) on the 4m Blanco telescope (GCN 39462), and we analyzed the data from observations starting 2025-02-24T03:45:38 and 2025-02-25T00:50:13 (GCN 39468).
We detect a variable source consistent with the location of the Swift XRT source S250223dk_X1 (GCN 39485) at the position of a Quaia (Storey-Fisher et al. 2024) likely quasar source (unWISE source id 0850m470o0042962, RA, dec=85.499,-47.358 [J2000]) with a photometric redshift of 1.4+-0.2, consistent with the distance of the GW alert. Stacked difference imaging with respect to templates from the Dark Energy Survey data reveals a clear source (AT 2025csi) in the griz bands. Our preliminary photometry on the difference imaging results in the following magnitudes:
| MJD | Band | mag |
| ---------- | ---- | -------------- |
| 60730.1567 | g | 20.90 +- 0.01 |
| 60730.1748 | i | 20.38 +- 0.01 |
| 60731.0349 | g | 20.82 +- 0.007 |
| 60731.0530 | i | 20.41 +- 0.009 |
We also inspected archival DECam data at this location and note that this possible quasar showed a steady ~1 mag increase in brightness in all bands between 2013 and 2018. Some archival detections are also present from ATLAS forced photometry over the past two years. The archival detections suggest that the source variability may be unrelated to the GW alert, although we encourage follow-up observations to establish the nature of this object and whether it is currently experiencing flaring activity.
We also detect a transient (AT 2025cpu) in the griz bands consistent with the location of S250223dk_X5 at position RA, dec= 85.51643, -47.70934 [J2000], with the following magnitudes:
| MJD | Band | mag |
| ---------- | ---- | ------------- |
| 60730.1569 | g | 22.71 +- 0.03 |
| 60730.1748 | i | 22.63 +- 0.07 |
| 60731.0349 | g | 22.55 +- 0.03 |
| 60731.0530 | i | 22.74 +- 0.05 |
We note that this source appears as a nuclear transient in a possible galaxy with photometric redshift of 0.797+-0.088 (from the DESI Legacy Survey photometric redshift catalog). The archival DECam observations for this source show variability of up to a magnitude, thus we cannot exclude that these detections are due to AGN variability.
Further analysis of these sources is planned and follow-up observations are encouraged.
We thank the CTIO and NOIRLab staff for supporting observations and data calibration.
View this GCN Circular online at https://gcn.nasa.gov/circulars/39497.
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TITLE: GCN CIRCULAR
NUMBER: 39496
SUBJECT: EP250223a: CrAO ZTSH optical observation
DATE: 25/02/26 17:18:16 GMT
FROM: Alexei Pozanenko at IKI, Moscow <grb.alex(a)gmail.com>
E. Mazaeva (IKI), V. Rumyantsev (CrAO), A. Pozanenko (IKI), N. Pankov
(HSE, IKI), A. Volnova (IKI) report on behalf of IKI GRB-FuN collaboration:
We observed the field of EP250223a (Lian et al., GCN 39429) 2.6-meter ZTSh
telescope of CrAO starting on (UT) 2025-02-25 18:01:15.
The optical counterpart of EP250223a (Hauptmann et al., GCN 39436; Wu et
al., GCN 39439; Pérez-Fournon et al., GCN 39440; Izzo et al., GCN 39441;
Xin et al., GCN 39445; Guo et al., GCN 39447; An et al., GCN 39449; Ducoin
et al., GCN 39453; O’Neill et al., GCN 39455; Aryan et al., GCN 39464;
Saikia et al., GCN 39472; Wang et al., GCN 39476) at the redshift of 2.756
(Levan et al., GCN 39438) is clearly detected.
The preliminary photometry of the optical counterpart is the following:
Date UT start T-T0 Exp. Filter OT Err. UL(3sigma)
(mid, days) (s)
2025-02-25 18:01:15 2.15250 41x120 R 20.62 0.05 22.2
The magnitudes were calibrated using nearby PS1 stars using Lupton
transformations to R-magnitude. No correction has been made for a Galactic
extinction towards the counterpart.
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TITLE: GCN CIRCULAR
NUMBER: 39495
SUBJECT: GRB 250225B: VLT optical and near-infrared candidate counterpart
DATE: 25/02/26 16:31:57 GMT
FROM: Yu-Han Yang at University of Rome Tor Vergata <yyang(a)roma2.infn.it>
Yu-Han Yang (U Rome), Brendan O’Connor (CMU), Eleonora Troja (U Rome), Rosa Becerra (U Rome) and Muskan Yadav (U Rome) report:
We observed the field of GRB 250225B (Williams et al., GCN 39473) with the FORS2 imager on the ESO VLT UT1 (Antu) and the HAWKI imager on the ESO VLT UT4 (Yepun). Observations began at T+13.7 hours and were carried out at an average airmass of about 2.6 in the I filter and in the J filter, respectively.
We detect the source reported by Schneider et al. (GCN 39494) at a preliminary magnitude J~21.4 AB mag calibrated using nearby stars in the 2MASS Catalogue. In comparison with archival imaging, the source appears brighter by ~0.5 mag in both the i- and the J-band, thus confirming the brightening reported by Schneider et al. (GCN 39494).
Although the probability of a chance alignment with the nearby galaxy ESO 340-26 remains small (P_cc<3%; Dichiara et al. 2020), we note that the observed color and magnitude of the candidate counterpart do not match the behavior of the kilonova AT2017gfo, if placed at a distance of 78 Mpc. If confirmed as the GRB afterglow, our observations favor the association with the underlying fainter galaxy, visible in the Legacy Survey (P_cc ~0.8%; Bloom et al. 2002)
We thank the staff at the VLT, for the rapid execution of these observations.
View this GCN Circular online at https://gcn.nasa.gov/circulars/39495.
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TITLE: GCN CIRCULAR
NUMBER: 39495
SUBJECT: GRB 250225A: VLT optical and near-infrared candidate counterpart
DATE: 25/02/26 16:31:57 GMT
FROM: Yu-Han Yang at University of Rome Tor Vergata <yyang(a)roma2.infn.it>
Yu-Han Yang (U Rome), Brendan O’Connor (CMU), Eleonora Troja (U Rome), Rosa Becerra (U Rome) and Muskan Yadav (U Rome) report:
We observed the field of GRB 250225A (Williams et al., GCN 39473) with the FORS2 imager on the ESO VLT UT1 (Antu) and the HAWKI imager on the ESO VLT UT4 (Yepun). Observations began at T+13.7 hours and were carried out at an average airmass of about 2.6 in the I filter and in the J filter, respectively.
We detect the source reported by Schneider et al. (GCN 39494) at a preliminary magnitude J~21.4 AB mag calibrated using nearby stars in the 2MASS Catalogue. In comparison with archival imaging, the source appears brighter by ~0.5 mag in both the i- and the J-band, thus confirming the brightening reported by Schneider et al. (GCN 39494).
Although the probability of a chance alignment with the nearby galaxy ESO 340-26 remains small (P_cc<3%; Dichiara et al. 2020), we note that the observed color and magnitude of the candidate counterpart do not match the behavior of the kilonova AT2017gfo, if placed at a distance of 78 Mpc. If confirmed as the GRB afterglow, our observations favor the association with the underlying fainter galaxy, visible in the Legacy Survey (P_cc ~0.8%; Bloom et al. 2002)
We thank the staff at the VLT, for the rapid execution of these observations.
View this GCN Circular online at https://gcn.nasa.gov/circulars/39495.
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