LIGO/Virgo/KAGRA S231119u

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S231119u during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2023-11-19 07:52:48.095 UTC (GPS time: 1384415586.095). The candidate was found by the CWB [1], GstLAL [2], MBTA [3], and SPIIR [4] analysis pipelines.

S231119u is an event of interest because its false alarm rate, as estimated by the online analysis, is 7.4e-08 Hz, or about one in 5 months. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S231119u

The classification of the GW signal, in order of descending probability, is BBH (95%), Terrestrial (5%), NSBH (<1%), or BNS (<1%).

Assuming the candidate is astrophysical in origin, the probability that the lighter compact object is consistent with a neutron star mass (HasNS) is <1%. [5] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [5] Both HasNS and HasRemnant consider the support of several neutron star equations of state. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassgap) is 4%.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [6], distributed via GCN notice about 27 seconds after the candidate event time.
 * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [6], distributed via GCN notice about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 4240 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 8237 +/- 2603 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/userguide/.

 [1] Klimenko et al. PRD 93, 042004 (2016)
 [2] Tsukada et al. PRD 108, 043004 (2023) and Ewing et al. arXiv:2305.05625
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  (2023)
 [3] Aubin et al. CQG 38, 095004 (2021)
 [4] Chu et al. PRD 105, 024023 (2022)
 [5] Chatterjee et al. ApJ 896, 54 (2020)
 [6] Singer & Price PRD 93, 024013 (2016)

IceCube Collaboration (http://icecube.wisc.edu/) reports:

Searches for track-like muon neutrino events detected by IceCube consistent with the sky localization of gravitational-wave candidate S231119u in a time range of 1000 seconds centered on the alert event time (2023-11-19 07:44:28.000 UTC to 2023-11-19 08:01:08.000 UTC) have been performed [1,2]. During this time period IceCube was collecting good quality data. Two hypothesis tests were conducted. The first search is a maximum likelihood analysis which searches for a generic point-like neutrino source coincident with the given GW skymap.  The second uses a Bayesian approach to quantify the joint GW + neutrino event significance, which assumes a binary merger scenario and accounts for known astrophysical priors, such as GW source distance, in the significance estimate [3].

One track-like event is found in spatial and temporal coincidence with the gravitational-wave candidate S231119u calculated from the map circulated in the 3-Initial notice. This represents an overall p-value of 0.0061 from the generic transient search and an overall p-value of 0.44 for the Bayesian search.  These p-values measure the consistency of the observed track-like events with the known atmospheric backgrounds for this single map (not trials corrected for multiple GW events). The most probable multi-messenger source direction based on the neutrinos and GW skymap is RA 237.39, Dec -35.69 degrees.

The reported p-values can differ due to the estimated distance of the GW candidate. The distance is used as a prior in the Bayesian binary merger search, while it is not taken into account in the generic transient point-like source search. The false alarm rate of these coincidences can be obtained by multiplying the p-values with their corresponding GW trigger rates. Further details are available at https://gcn.nasa.gov/missions/icecube.

Properties of the coincident events are shown below.

 dt(s)    RA(deg)	Dec(deg)    Angular uncertainty(deg)  p-value(generic transient) p-value(Bayesian)
-----------------------------------------------------------------------------------------------------
 -286.38    237.38	-35.65		 0.43			0.006			null


where:
dt = Time of track event minus time of GW trigger (sec)
Angular uncertainty = Angular uncertainty of track event: the radius of a circle 
     representing 90% CL containment by area.
p-value = the p-value for this specific track event from each search.

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@icecube.wisc.edu.

[1] M. G. Aartsen et al 2020 ApJL 898 L10
[2] Abbasi et al. Astrophys.J. 944 (2023) 1, 80
[3] I. Bartos et al. 2019 Phys. Rev. D 100, 083017

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) data around the time of the compact binary merger (CBC) candidate S231119u (GCN Circular 35120
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 ). 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/S231119u

After parameter estimation by RapidPE-RIFT [2], the updated classification of the GW signal, in order of descending probability, is BBH (95%), Terrestrial (5%), NSBH (0%), or BNS (0%).

For the Bilby.offline0.multiorder.fits,0 sky map, the 90% credible region is 5212 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 6597 +/- 2556 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/userguide/.

 [1] Ashton et al. ApJS 241, 27 (2019) and Morisaki et al. arXiv:2307.13380
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  (2023)
 [2] Rose et al. arXiv:2201.05263
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  (2022) and Pankow et al. PRD 92, 023002 (2015)