TITLE: GCN CIRCULAR
NUMBER: 42360
SUBJECT: Fermi trigger No 782500909: Global MASTER-Net observations report
DATE: 25/10/18 18:01:02 GMT
FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru>
V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko,
G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A.Sosnovskij (CrAO),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory)
MASTER-SAAO robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in South Africa (South African Astronomical Observatory) started inspect of the Fermi GRB251018.72 (trigger No 782500909,16h 23m 38.40s , -85d 57m 00.0s, R=3.1) errorbox 721 sec after notice time and 754 sec after trigger time at 2025-10-18 17:34:19 UT, with upper limit up to 16.1 mag. Observations started at twilight. The observations began at zenith distance = 56 deg. The sun altitude is -9.4 deg.
The galactic latitude b = -25 deg., longitude l = 307 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3017672
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
785 | 2025-10-18 17:34:19 | MASTER-SAAO | (16h 29m 10.44s , -85d 56m 26.8s) | C | 60 | 15.2 |
864 | 2025-10-18 17:35:38 | MASTER-SAAO | (16h 29m 10.67s , -85d 56m 25.2s) | C | 60 | 16.1 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
View this GCN Circular online at https://gcn.nasa.gov/circulars/42360.
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TITLE: GCN CIRCULAR
NUMBER: 42359
SUBJECT: GRB 251016A: HXMT/HE detection
DATE: 25/10/18 16:40:50 GMT
FROM: xueyuan zao <zaoxueyuan(a)gmail.com>
Xue-Yuan Zao, Chen-Wei Wang, Zheng-Hang Yu, Cheng-Kui Li, Shao-Lin Xiong, and Chao Zheng report on behalf of the Insight-HXMT team:
At 2025-10-16T14:59:19.750 (T0), Insight-HXMT/HE detected the short burst GRB 251016A, which is also detected by SVOM/ECLAIRs (Basa et al., GCN 42308), FERMI-GBM (Fermi GBM team, GCN 42309), CALET-GBM (Kawakubo et al., GCN 42321), Glowbug (Woolf et al., GCN 42313) and Astro Sat CZTI (Arya et al., GCN 42327).
The Insight-HXMT/HE light curve mainly consists of a single pulse with a T90 of 0.34 +/-0.06 s. The total counts from this burst is 2297 counts.
The HXMT/HE light curve can be found here:
https://www.bursthub.cn//admin/static/hxmtgrb251016A.png
All measurements above are made with the CsI detectors operating in the regular mode with the energy range of about 30-1000 keV (deposited energy). Only gamma-rays with energy greater than about 200 keV can penetrate the spacecraft and leave signals in the CsI detectors installed inside of the telescope.
Insight-HXMT is the first Chinese space X-ray telescope, which was funded jointly by the China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS). More information about it could be found at: http://www.hxmt.org.
View this GCN Circular online at https://gcn.nasa.gov/circulars/42359.
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TITLE: GCN CIRCULAR
NUMBER: 42357
SUBJECT: LIGO/Virgo/KAGRA S251018bi: Identification of a GW compact binary merger candidate
DATE: 25/10/18 16:05:11 GMT
FROM: shoki.iwaguchi(a)ligo.org
The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:
We identified the compact binary merger candidate S251018bi during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-10-18 15:25:47.021 UTC (GPS time: 1444836365.021). 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.
S251018bi 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/S251018bi
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 (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 [9], distributed via GCN and SCiMMA notices about 23 seconds after the candidate event time.
* bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [10], 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,0. For the bayestar.multiorder.fits,0 sky map, the 90% credible region is 1463 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 5450 +/- 1489 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] Chatterjee et al. MLST 5, 045030 (2024) doi:10.1088/2632-2153/ad8982
[10] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013
View this GCN Circular online at https://gcn.nasa.gov/circulars/42357.
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TITLE: GCN CIRCULAR
NUMBER: 42356
SUBJECT: GRB 251016A: Swift-XRT afterglow detection
DATE: 25/10/18 13:49:20 GMT
FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk>
P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), E. Ambrosi
(INAF-IASFPA) , S. Lanava (PSU), S. Dichiara (PSU), J.A. Kennea (PSU),
J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore
(U. Leicester) and P.A. Evans (U. Leicester) reports on behalf of the
Swift-XRT team:
Swift-XRT has performed follow-up observations of GRB 251016A. We
searched for X-ray sources in 3.5 ks of Photon Counting (PC) mode
data. The total exposure at the position of the afterglow (see below)
is 5.6 ks, obtained between T0+6.6 ks and T0+107.3 ks.
Thirty-two uncatalogued X-ray sources are detected, of which one
("Source 1") is fading with >3-sigma significance, and is therefore
likely the GRB afterglow. Using 1190 s of PC mode data and 2 UVOT
images, we find an enhanced XRT position (using the XRT-UVOT alignment
and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec =
266.30279, -39.56008 which is equivalent to:
RA (J2000): 17h 45m 12.67s
Dec(J2000): -39d 33' 36.3"
with an uncertainty of 3.2 arcsec (radius, 90% confidence).
The light curve can be modelled with a power-law decay with a decay
index of alpha=4.53 (+0.08, -3.08).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.4 (+1.0, -0.6). The
best-fitting absorption column is 4.8 (+8.8, -0.9) x 10^21 cm^-2,
consistent with the Galactic value of 3.9 x 10^21 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum is 6.4 x 10^-11 (8.0 x
10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 4.8 (+8.8, -0.9) x 10^21 cm^-2
Galactic foreground: 3.9 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index: 1.4 (+1.0, -0.6)
The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/03000141.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/SVOM/SVOM_FIELD00042.
This circular is an official product of the Swift-XRT team.
View this GCN Circular online at https://gcn.nasa.gov/circulars/42356.
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TITLE: GCN CIRCULAR
NUMBER: 42355
SUBJECT: GRB 251017A: Swift/UVOT Detection
DATE: 25/10/18 12:06:08 GMT
FROM: s.shilling(a)lancaster.ac.uk
S. P. R. Shilling (Lancaster U.) and R. Gupta (NASA GSFC) report on behalf of the
Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 251017A
98 s after the BAT trigger (Gupta et al., GCN Circ. 42322).
A source consistent with the enhanced XRT position (Goad et al., GCN Circ. 42326)
is detected in the initial UVOT exposures.
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
white 121 270 147 20.09 +/- 0.22
v 98 1234 109 >18.8
b 588 5138 83 >19.6
u 333 5126 501 >20.3
w1 712 4921 255 >18.5
w2 811 1384 58 >20.4
The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.108 in the direction of the burst
(Schlegel et al. 1998).
View this GCN Circular online at https://gcn.nasa.gov/circulars/42355.
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TITLE: GCN CIRCULAR
NUMBER: 42354
SUBJECT: GRB 251016A: Swift/UVOT Detection
DATE: 25/10/18 12:05:09 GMT
FROM: s.shilling(a)lancaster.ac.uk
S.P.R. Shilling (Lancaster U.) reports on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 251016A,
detected by SVOM/ECLAIRs (Basa et al., GCN Circ. 42308) and Fermi/GBM
(Fermi GBM team, GCN Circ. 42309), 6.66 ks after the SVOM/ECLAIRs trigger.
A source is detected in the optical by the Swift/UVOT at a position that is
consistent with the likely X-ray counterpart detected by the Swift/XRT
(Source 1: Evans et al., GCN Circ. 42311).
Preliminary detections and upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the images are:
Filter T_start(ks) T_stop(ks) Exp(s) Mag
u 6.66 12.6 491.7 19.13 +/- 0.26
b 6.91 12.7 296.0 18.70 +/- 0.18
v 7.42 7.67 245.8 17.52 +/- 0.15
w1 12.1 12.4 245.8 >18.93
w2 7.17 7.42 245.8 >19.05
m2 7.68 7.85 168.3 >18.66
The magnitudes reported here are not corrected for Galactic extinction.
View this GCN Circular online at https://gcn.nasa.gov/circulars/42354.
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TITLE: GCN CIRCULAR
NUMBER: 42353
SUBJECT: GRB 251018A: Swift/UVOT Detection
DATE: 25/10/18 11:57:31 GMT
FROM: Paul Kuin at MSSL <npkuin(a)gmail.com>
N.P.M. Kuin (MSSL-UCL) and R. Gupta (NASA GSFC) report on behalf of the
Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 251018A 84 s
after the BAT trigger (Gupta et al., GCN Circ. 42343). A source consistent
with the XRT position is detected in the initial UVOT exposures.
Preliminary detections 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
white 84 128 44 17.98 +/- 0.08
The magnitudes in the table are not corrected for the Galactic extinction due
to the reddening of E(B-V) = 0.101 in the direction of the burst (Schlegel
et al. 1998).
View this GCN Circular online at https://gcn.nasa.gov/circulars/42353.
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TITLE: GCN CIRCULAR
NUMBER: 42352
SUBJECT: IceCube-251018A - IceCube observation of a high-energy neutrino candidate track-like event
DATE: 25/10/18 11:11:12 GMT
FROM: A. Zegarelli at Ruhr University Bochum <azegarelli(a)icecube.wisc.edu>
The IceCube Collaboration (http://icecube.wisc.edu/) reports:
On 25-10-18 at 05:05:42.97 UT IceCube detected a track-like event with a high probability of being of astrophysical origin.
The event was selected by the ICECUBE_Astrotrack_GOLD alert stream.
The average astrophysical neutrino purity for Gold alerts is 50%.
This alert has an estimated false alarm rate of 0.2566 events per year due to atmospheric
backgrounds.
The IceCube detector was in a normal operating state at the time of detection.
After the initial automated alert (https://gcn.gsfc.nasa.gov/notices_amon_g_b/141495_22018019.amon), more sophisticated reconstruction algorithms have been applied offline, with the direction refined to:
Date: 25-10-18
Time: 05:05:42.97 UT
RA: 321.11 (+3.09/-2.94 deg 90% PSF containment) J2000
Dec: 41.31 (+2.46/-3.51 deg 90% PSF containment) J2000
Two gamma-ray sources listed in the Fermi 4FGL-DR4 catalog are located within the 90% uncertainty region of the event, both with no clear multi-wavelength counterpart and classification: 4FGL J2118.3+4055 and 4FGL J2120.5+433, situated 1.3 deg and 2.3 deg away from the best-fit direction of our alert, respectively.
We encourage follow-up observations by both ground- and space-based instruments to help identify a possible astrophysical counterpart to the candidate neutrino.
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
View this GCN Circular online at https://gcn.nasa.gov/circulars/42352.
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TITLE: GCN CIRCULAR
NUMBER: 42351
SUBJECT: IceCube-251018A - IceCube observation of a high-energy neutrino candidate track-like event
DATE: 25/10/18 11:10:28 GMT
FROM: A. Zegarelli at Ruhr University Bochum <azegarelli(a)icecube.wisc.edu>
The IceCube Collaboration (http://icecube.wisc.edu/) reports:
On 25-10-18 at 05:05:42.97 UT IceCube detected a track-like event with a high probability of being of astrophysical origin.
The event was selected by the ICECUBE_Astrotrack_GOLD alert stream.
The average astrophysical neutrino purity for Gold alerts is 50%.
This alert has an estimated false alarm rate of 0.2566 events per year due to atmospheric
backgrounds.
The IceCube detector was in a normal operating state at the time of detection.
After the initial automated alert (https://gcn.gsfc.nasa.gov/notices_amon_g_b/141495_22018019.amon), more sophisticated reconstruction algorithms have been applied offline, with the direction refined to:
Date: 25-10-18
Time: 05:05:42.97 UT
RA: 321.11 (+3.09/-2.94 deg 90% PSF containment) J2000
Dec: 41.31 (+2.46/-3.51 deg 90% PSF containment) J2000
Two gamma-ray sources listed in the Fermi 4FGL-DR4 catalog are located within the 90% uncertainty region of the event, both with no clear multi-wavelength counterpart and classification: 4FGL J2118.3+4055 and 4FGL J2120.5+433, situated 1.3 deg and 2.3 deg away from the best-fit direction of our alert, respectively.
We encourage follow-up observations by both ground- and space-based instruments to help identify a possible astrophysical counterpart to the candidate neutrino.
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
View this GCN Circular online at https://gcn.nasa.gov/circulars/42351.
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