GCN Circular 36205

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

We identified the unmodeled transient candidate S240421ar during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2024-04-21 05:29:35.734 UTC (GPS time: 1397712593.734). The candidate was found by the CWB [1] and GstLAL [2] analysis pipelines.

S240421ar is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.6e-08 Hz, or about one in 10 months. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S240421ar

The classification of the GW signal, in order of descending probability, is Terrestrial (59%), BBH (41%), BNS (<1%), or NSBH (<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%. [3] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [3] 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%.

Three 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 [4], distributed via GCN notice about 26 seconds after the candidate event time.
 * cwb.multiorder.fits,0, an initial localization generated by cWB [1], distributed via GCN notice about a minute after the candidate event time.
 * cwb.multiorder.fits,1, an initial localization generated by cWB [1], distributed via GCN notice about 6 minutes after the candidate event time.

The preferred sky map at this time is cwb.multiorder.fits,1. For the cwb.multiorder.fits,1 sky map, the 90% credible region is 5984 deg2.

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
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 [2] 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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 [3] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
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 [4] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013
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