LIGO/Virgo/KAGRA S241230bd

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

We identified the compact binary merger candidate S241230bd during real-time processing of data from LIGO Livingston Observatory (L1) and Virgo Observatory (V1) at 2024-12-30 08:45:04.540 UTC (GPS time: 1419583522.540). The candidate was found by the GstLAL [1] analysis pipeline.

S241230bd is an event of interest because its false alarm rate, as estimated by the online analysis, is 6.2e-14 Hz, or about one in 1e6 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S241230bd

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 the lighter compact object is consistent with a neutron star mass (HasNS) is <1%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [2] 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 <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 [3], distributed via GCN notice about 25 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [3], 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 10806 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2719 +/- 776 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] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004
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  and Ewing et al. (2023) arXiv:2305.05625
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 [2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
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 [3] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013
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The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Livingston Observatory (L1) and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S241230bd (GCN Circular 38722
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 ). 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/S241230bd

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 11129 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2811 +/- 842 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
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  and Morisaki et al. (2023) arXiv:2307.13380
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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 75.9% of the GW localization probability ([Bilby.multiorder.fits](https://gracedb.ligo.org/api/superevents/S241230bd/files/Bilby.multiorder.fits)) at merger time. A fraction 32.05% 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
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 ], and spectral shape from GRB170817A [arXiv:1710.05446
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 ]) and for four time bins.
In units of 10^-7 erg/s/cm^2:

|time_bin (s) |soft  |normal|hard  |GRB170817
|-|-|-|-|-|
|0.256        |10.80 |7.25  |6.44  |8.08  
|1.024        |5.50  |3.69  |3.28  |4.11  
|4.096        |2.96  |1.99  |1.77  |2.21  
|16.384       |1.84  |1.24  |1.10  |1.38  


The upper limits as function of sky position are plotted here, alongside the GW localization:
https://zenodo.org/records/14589498

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/