LIGO/Virgo/KAGRA S251105aj
GCN Circular 42819
Subject
LIGO/Virgo/KAGRA S251105aj: SVOM/VT optical observations and preliminary results on SVOM J2320.0-2901
Date
2025-11-24T15:06:20Z (4 days ago)
From
Yinuo Ma <mayn@bao.ac.cn>
Via
Web form
Y. N. Ma, H. L. Li, L. P. Xin, Z. H. Yao, Y. L. Qiu, C. Wu, X. H. Han, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA), H. Yang, O. Godet, J.-L. Atteia, M. Brunet, S. Guillot (IRAP), M. Pillas (IAP) report on behalf of the SVOM/VT team.
SVOM/VT performed a Target of Opportunity observation of the high-energy transient SVOM J2320.0-2901 detected by SVOM/ECLAIRs (Yang et al., GCN 42797), right after S251105aj reported by LIGO/Virgo/KAGRA (The LIGO-Virgo-Kagra Collaboration, GCN 42587; GCN 42616). The observation began at 2025-11-16T00:47:45 UT, 10.5 days after SVOM/ECLAIRs detection, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
The VT field of view covered 96% of the 90% C.L. ECLAIRs error region, but no uncataloged sources were detected within the error box of ECL. VT also covered three EP/FXT sources (source#1: RA, Dec = 349.8643, -29.0602 deg; source#2: 350.1419, -29.0148 deg; source#3: 349.9737, -28.8522 deg) reported by Guillot et al. (GCN 42818). None of the known sources became apparent brighter within errorbox of three FXT candidates compared with Legacy Survey DR10. The 3 sigma upper limit was VT_R > 23.6 mag and VT_B > 23.8 mag with the exposure time of 65*70 sec.
Further analysis is ongoing.
The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.
GCN Circular 42818
Subject
LIGO/Virgo/KAGRA S251105aj: EP-FXT follow-up observations and preliminary results on SVOM J2320.0-2901
Date
2025-11-24T15:05:57Z (4 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
S. Guillot, H. Yang, O. Godet, J.-L. Atteia (IRAP), M. Pillas (IAP), L. P. Xin, Y. N. Ma, H. W. Pan, T. Zhao (NAOC), Y. J. Yi, A. Li (BNU), R. X. Hu (WHU), D. Z. Du (ZJU), C. Y. Wang (THU) on behalf of the Einstein Probe (EP) team:
EP-FXT performed a Target of Opportunity observation of the high-energy transient SVOM J2320.0-2901 detected by SVOM/ECLAIRs (Yang et al., GCN 42797), in a search for possible counterparts of the compact binary merger candidate S251105aj (The LIGO-Virgo-Kagra Collaboration, GCN 42587). The observation was conducted at 2025-11-15T21:39:22 UTC with an exposure of 5 ks, about 10.36 days after the GW trigger and the detection time of SVOM J2320.0-2901. EP-FXT covered the full area of the 90% C.L. ECLAIRs error region and detected six sources above a SNR of 4.5 within 15 arcminutes from SVOM J2320.0-2901. Their information is summarized below (RA/Dec in degrees with 10" error radius (90% C.L.); fluxes are the observed ones in 0.5–10 keV):
# | Name | RA | Dec | Flux (erg/s/cm^2) | SNR
—----------------------------------------------------------------------------------------------------
1 | EPF_J231927.4-290337 | 349.8643 | -29.0602 | 1.4e-13 | 4.7
2 | EPF_J232034.0-290053 | 350.1419 | -29.0148 | 2.2e-13 | 10.0
3 | EPF_J231953.7-285108 | 349.9737 | -28.8522 | 1.7e-13 | 5.7
4 | EPF_J231906.2-290806 | 349.7760 | -29.1351 | 3.4e-13 | 5.8
5 | EPF_J232051.4-285356 | 350.2140 | -28.8990 | 2.4e-13 | 8.4
6 | EPF_J232108.1-290032 | 350.2839 | -29.0088 | 2.6e-13 | 7.9
We cross-matched all six FXT detections against the SIMBAD, NED, and Vizier catalogues, using a 20″ matching radius (for a galaxy at 2000 Mpc, this corresponds to an offset of ~200 kpc). Five FXT detections (#2,3,4,5,6) are matched to five optical quasars correspondingly, with their redshifts in a range of 0.721 to 1.641, higher than z=0.374 estimated for the GW event. The remaining source (#1) is matched to a source from the KiDSDR4 QSOs photometric redshifts catalog (Nakoneczny et al., 2021), which is classified as a quasar with 68% probability and a photometric redshift of 0.5 +/- 1.2 based on machine learning estimates.
We conducted SVOM/VT on this field with the results reported in GCN 42819.
Further FXT observations of these candidates will be conducted.
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).
GCN Circular 42797
Subject
LIGO/Virgo/KAGRA S251105aj: SVOM/ECLAIRs detection of a high-energy transient SVOM J2320.0-2901 through targeted search
Date
2025-11-22T10:00:14Z (7 days ago)
Edited On
2025-11-22T21:08:52Z (6 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
H. Yang, O. Godet, J.-L. Atteia, M. Brunet, S. Guillot (IRAP), M. Pillas (IAP), L. P. Xin, Y. N. Ma (NAOC)
Using the event-by-event data downloaded through the X-band ground stations, we report on the identification of a faint SVOM/ECLAIRs transient SVOM J2320.0-2901 detected at 2025-11-05T13:00:48.34 UTC (T0), 88.4 s after the compact binary merger candidate S251105aj (The LIGO-Virgo-Kagra Collaboration, GCN 42587). This transient was detected through an offline targeted search for possible counterparts of the GW event S251105aj.
SVOM J2320.0-2901 was detected with a timescale of 82 s and below 60 keV, with a best signal-to-noise ratio of 6.1 starting at T0.
The localization of SVOM J2320.0-2901 is RA, Dec = 349.998, -29.022 degrees:
RA (J2000) = 23h20m00s
Dec (J2000) = -29d01m19s
with a 90% C.L. radius of 13.9 arcmin (including a systematic error of 6 arcmin added in quadrature). This position is about 2 degrees outside the 99% area contour of the Bilby.multiorder.fits skymap of S251105aj. At the time of the GW trigger, ECLAIRs was covering 88% of the skymap 99% area.
The time-averaged spectrum from T0 to T0+82 s in the energy range of 5-60 keV is well fitted by a blackbody (BB) model, with a measured temperature of 7.8 (+1.5/-1.2) keV. With this model, the total 4-120 keV flux is (3.6+/-0.8)e-9 erg/cm^2/s. Assuming a distance of 2071 Mpc (z=0.374) for the source, this corresponds to a luminosity of 1.8e48 erg/s and a BB emission radius of 1.9e6 cm.
A broken power-law model is also tested, providing a comparable fit, though with less well-constrained parameters of photon index 1 around -0.5, break energy of 22 (+21/-10) keV, and photon index 2 around 2.9.
At this stage, the nature of SVOM J2320.0-2901 is unknown.
We conducted SVOM/VT and EP/FXT follow-up observations on this field. The results will be published in dedicated circulars.
All the quoted errors are at the 68% confidence level.
The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC.
The SVOM/ECLAIRs point of contact for this source is: Hui Yang (IRAP) (hui.yang@irap.omp.eu).
GCN Circular 42616
Subject
LIGO/Virgo/KAGRA S251105aj: Updated Sky localization
Date
2025-11-07T17:23:04Z (21 days ago)
From
Lorenzo Pompili at Max Planck Institute for Gravitational Physics <lorenzo.pompili@aei.mpg.de>
Via
Web form
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 S251105aj (GCN Circular 42587). 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/S251105aj
For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 376 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2483 +/- 735 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
GCN Circular 42587
Subject
LIGO/Virgo/KAGRA S251105aj: Identification of a GW compact binary merger candidate
Date
2025-11-05T13:43:35Z (23 days ago)
From
Jeremias Sobrinho <jeremiasbezerra.sobrinho@ligo.org>
Via
Web form
The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:
We identified the compact binary merger candidate S251105aj during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-11-05 12:59:19.959 UTC (GPS time: 1446382777.959). 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.
S251105aj is an event of interest because its false alarm rate, as estimated by the online analysis, is 8e-13 Hz, or about one in 1e5 years. The event's properties can be found at this URL:
https://gracedb.ligo.org/superevents/S251105aj
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:
* bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [9], distributed via GCN and SCiMMA notices about 27 seconds after the candidate event time.
* bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [9], 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,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is 721 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2071 +/- 545 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