GCN Circular 36236

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

We identified the compact binary merger candidate S240422ed during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2024-04-22 21:35:13.417 UTC (GPS time: 1397856931.417). The candidate was found by the GstLAL [1] and PyCBC Live [2] analysis pipelines.

S240422ed is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.1e-13 Hz, or about one in 1e5 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S240422ed

The classification of the GW signal, in order of descending probability, is NSBH (>99%), Terrestrial (<1%), BBH (<1%), or BNS (<1%).

Noise transients (glitches) were present in LIGO Livingston detector and LIGO Hanford detector data within 10 seconds of the event time, which may affect the parameters or the significance of the candidate. No noise transients were identified in Virgo detector data at the time of the candidate. Additional investigations are ongoing to understand the possible impact of these transients on the results reported in this circular. 

Assuming the candidate is astrophysical in origin, the probability that the lighter compact object is consistent with a neutron star mass (HasNS) is >99%. [3] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is >99%. [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 46%.

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 [4], distributed via GCN notice about 55 seconds after the candidate event time.
 * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [4], 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 well fit by an ellipse with an area of 441 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(08h08m, -23d41m, 18.07d, 7.82d, 109.70d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 214 +/- 64 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] Dal Canton et al. ApJ 923, 254 (2021) doi:10.1088/1361-6382/abe913
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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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