LIGO/Virgo/KAGRA S250331o
GCN Circular 39953
Subject
LIGO/Virgo/KAGRA S250331o: Identification of a GW compact binary merger candidate
Date
2025-03-31T02:06:54Z (2 months ago)
From
Gyoik Kim at Yonsei University <gyoik_kim@yonsei.ac.kr>
Via
Web form
The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:
We identified the compact binary merger candidate S250331o during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-03-31 01:34:48.438 UTC (GPS time: 1427420106.438). The candidate was found by the cWB [1], cWB BBH [2], GstLAL [3], PyCBC Live [4], and SPIIR [5] analysis pipelines.
S250331o 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/S250331o
After parameter estimation by RapidPE-RIFT [6], the classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), or BNS (<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%. [7] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [7] 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%.
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 [8], distributed via GCN notice about 23 seconds after the candidate event time.
* bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [8], distributed via GCN notice 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 well fit by an ellipse with an area of 27 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):
icrs; ellipse(02h07m, +31d24m, 4.57d, 1.87d, 116.86d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 582 +/- 120 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] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
[2] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
[3] 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
[4] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
[5] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023
[6] Rose et al. (2022) arXiv:2201.05263 and Pankow et al. PRD 92, 023002 (2015) doi:10.1103/PhysRevD.92.023002
[7] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
[8] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013
GCN Circular 39975
Subject
LIGO/Virgo/KAGRA S250331o: NED Galaxies in the 3-Initial Localization Volume
Date
2025-03-31T15:48:32Z (2 months ago)
From
David Cook at Caltech/IPAC-NED <dcook@ipac.caltech.edu>
Via
Web form
David O. Cook (Caltech/IPAC), Rick Ebert (Caltech/IPAC), George Helou (Caltech/IPAC), Joseph M. Mazzarella (Caltech/IPAC), Marion Schmitz (Caltech/IPAC), and Leo Singer (NASA/GSFC)
On behalf of the NASA/IPAC Extragalactic Database (NED) Team.
We spatially cross-matched the LVK S250331o-3-Initial sky localization with the NED Local Volume Sample (NED-LVS; Cook et al. 2023), which is a subset of NED with a redshift or redshift-independent distance less than 1000 Mpc. We find 582 galaxies within the 90% containment volume, and we list here the top 20 galaxies sorted by the joint probability of the 3D localization and the WISE W1 luminosity (an observable proxy for stellar mass). For the full or top 20 list of galaxies in the 90% volume go either to the NED Gravitational Wave Followup service at https://ned.ipac.caltech.edu/uri/NED::GWF/ or click on the following links:
Full List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250331o/3
Top 20 List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250331o/3/20
The NED-GWF service provides downloadable galaxy lists and visualizations for candidate host galaxies. For each GW alert, these products are automatically generated and made available within minutes to expedite efficient electromagnetic follow-up observations. The NED top 20 list is sorted by the joint probability of the 3D localization and the WISE W1 luminosity, but users can sort on additional pre-computed prioritization metrics (star formation rate, P_3D * P_SFR; and specific star formation rate, P_3D * P_sSFR; etc.) which are available via downloading the entire galaxy list inside the event's probability volume.
| objname| ra| dec|objtype| DistMpc|DistMpc_unc| m_NUV| m_NUV_unc| m_Ks| m_Ks_unc| m_W1| m_W1_unc| P_3D|P_3D_LumW1|
|-------------------------|--------------|--------------|-------|-----------|-----------|------------|------------|------------|------------|------------|------------|--------|----------|
|WISEA J020352.96+300418.6| 30.97068| 30.07186| G| 450.96| null| null| null| 13.417| 0.135| 10.147| 0.006|2.21e-06| 3.99e-08|
|WISEA J020524.84+300356.4| 31.35354| 30.06568| G| 653.76| null| null| null| 13.537| 0.143| 12.061| 0.009|1.69e-06| 1.10e-08|
| 3C 059 NED01| 31.75919| 29.51278| G| 510.64| 2.89| 18.553| 0.047| 12.531| 0.068| 11.452| 0.006|1.47e-06| 1.02e-08|
|WISEA J020815.86+330236.7| 32.06612| 33.04354| G| 528.82| null| null| null| 12.409| 0.098| 12.307| 0.010|1.99e-06| 6.80e-09|
|WISEA J020728.78+301402.8| 31.86992| 30.23411| G| 488.46| null| 22.228| 0.345| 12.899| 0.118| 12.205| 0.008|2.12e-06| 6.76e-09|
|WISEA J021035.33+303608.5| 32.64721| 30.60238| G| 569.79| null| null| null| 12.895| 0.117| 12.315| 0.013|1.64e-06| 6.45e-09|
|WISEA J020735.81+301139.9| 31.89924| 30.19442| G| 697.62| null| null| null| 13.258| 0.120| 12.206| 0.008|9.86e-07| 6.42e-09|
|WISEA J020447.91+305422.6| 31.19964| 30.90630| G| 421.78| null| null| null| 12.281| 0.097| 11.835| 0.007|1.92e-06| 6.41e-09|
|WISEA J021036.53+303612.4| 32.65223| 30.60347| G| 607.46| null| null| null| 13.762| 0.178| 12.435| 0.008|1.55e-06| 6.19e-09|
|WISEA J020031.78+285022.0| 30.13243| 28.83945| G| 548.45| null| null| null| 13.274| 0.142| 12.152| 0.008|1.32e-06| 5.57e-09|
|WISEA J020247.16+295941.4| 30.69650| 29.99484| G| 557.31| null| 20.629| 0.118| 13.505| 0.133| 12.951| 0.014|2.62e-06| 5.47e-09|
|WISEA J020311.55+300127.5| 30.79816| 30.02432| G| 512.31| 0.08| null| null| 13.071| 0.101| 12.808| 0.010|2.61e-06| 5.24e-09|
|WISEA J021613.00+340524.3| 34.05421| 34.09010| G| 677.00| null| null| null| 13.485| 0.160| 11.789| 0.007|5.73e-07| 5.16e-09|
|WISEA J020959.78+325524.0| 32.49910| 32.92334| G| 589.92| 0.10| 21.761| 0.336| 13.738| 0.179| 13.027| 0.012|2.33e-06| 5.10e-09|
|WISEA J020619.97+294730.7| 31.58324| 29.79188| G| 507.55| 0.07| null| null| 12.533| 0.104| 12.599| 0.020|2.12e-06| 5.09e-09|
|WISEA J020633.75+303828.7| 31.64066| 30.64131| G| 429.37| null| null| null| 12.181| 0.101| 12.041| 0.008|1.73e-06| 4.95e-09|
| VI Zw 139| 30.40470| 30.82735| G| 509.90| 0.06| null| null| 13.148| 0.120| 12.551| 0.009|1.91e-06| 4.80e-09|
|WISEA J020659.62+303130.8| 31.74842| 30.52523| G| 462.52| null| 21.179| 0.192| 13.405| 0.144| 12.477| 0.009|2.02e-06| 4.49e-09|
|WISEA J020919.83+314709.5| 32.33265| 31.78598| G| 656.06| 0.12| null| null| 14.209| 0.268| 13.409| 0.016|2.34e-06| 4.46e-09|
|WISEA J020626.94+302913.7| 31.61227| 30.48715| G| 672.91| 0.08| null| null| 13.829| 0.091| 13.152| 0.013|1.69e-06| 4.27e-09|
Table 1: Top 20 galaxies in NED-LVS that fall in the 90% probability volume for S250331o sorted by the joint probability of 3D position and WISE W1 luminosity (P_3D * P_LumW1). Galaxy is the NED preferred name. RA and Dec are the Equatorial coordinates in degrees (J2000). Objtype is the object type of the galaxy candidate. Distance is the distance to the galaxy in Mpc. m_NUV and mErr_NUV are the apparent magnitude and error from GALEX. m_Ks and mErr_Ks are the apparent magnitude and error from 2MASS. m_W1 and mErr_W1 are the apparent magnitude and error from AllWISE. P_3D is the probability that the galaxy is in the volume given the distance of GW event. P_3D_LumW1 is the joint probability within the volume weighted by the WISE1 luminosity of the galaxy (P_3D * P_LumW1).
GCN Circular 39986
Subject
LIGO/Virgo/KAGRA S250331o: Updated Sky localization
Date
2025-03-31T19:50:46Z (2 months ago)
From
Michael J. Williams at University of Portsmouth <michael.williams@ligo.org>
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 S250331o (GCN Circular 39953). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN Notice, is available for retrieval from the GraceDB event page:
https://gracedb.ligo.org/superevents/S250331o
For the Bilby.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 18 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):
icrs; ellipse(02h07m, +31d03m, 3.98d, 1.46d, 114.57d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 657 +/- 91 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 40109
Subject
LIGO/Virgo/KAGRA S250331o: Upper limits from Swift/BAT-GUANO
Date
2025-04-07T19:04:57Z (2 months ago)
From
Maia Williams at PSU <mjw6837@psu.edu>
Via
Web form
Maia Williams (PSU), Samuele Ronchini (PSU), Gayathri Raman (PSU), James DeLaunay (PSU), Tyler Parsotan (NASA GSFC), Jamie A. Kennea (PSU), Aaron Tohuvavohu (Caltech) report:
Swift/BAT was observing 100.0% of the GW localization probability ([Bilby.multiorder.fits](https://gracedb.ligo.org/api/superevents/S250331o/files/Bilby.multiorder.fits)) at merger time. A fraction 96.02% of the GW localization posterior is contained inside the BAT coded FoV.
The LVK notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; [Tohuvavohu et al. 2020, ApJ, 900, 1](https://iopscience.iop.org/article/10.3847/1538-4357/aba94f)).
Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.
Using the NITRATES analysis ([DeLaunay + Tohuvavohu 2022, ApJ, 941, 169](https://iopscience.iop.org/article/10.3847/1538-4357/ac9d38)), we searched for emission on 8 timescales from 0.128s to 16.384s in the interval [-20,+20] seconds around the merger time. We find no evidence for a signal, and derive the following upper limits.
We quote the 5-sigma flux upper limits in the 15-350 keV band, weighted over the GW localization, for four spectral templates (soft, normal, and hard GRB-like templates described in [arXiv:1612.02395], and spectral shape from GRB170817A [arXiv:1710.05446]) and for four time bins.
In units of 10^-7 erg/s/cm^2:
|time_bin (s) |soft |normal|hard |GRB170817
|-|-|-|-|-|
|0.256 |3.32 |3.49 |3.33 |3.61
|1.024 |1.69 |1.78 |1.70 |1.84
|4.096 |0.91 |0.95 |0.91 |0.99
|16.384 |0.55 |0.58 |0.56 |0.60
The upper limits as function of sky position are plotted here, alongside the GW localization:
https://zenodo.org/records/15169889
The solid and dashed lines indicate the 90% and 50% GW contour levels, respectively. The corresponding fits file is also included.
GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches.
A live reporting of Swift/BAT event data recovered by GUANO can be found at: https://www.swift.psu.edu/guano/