GCN Circular 24168

The LIGO Scientific Collaboration and the Virgo Collaboration report:

We identified the compact binary merger candidate S190425z during
real-time processing of data from LIGO Livingston Observatory (L1) and
Virgo Observatory (V1) at 2019-04-25 08:18:05.017 UTC (GPS time:
1240215503.017). The candidate was found by the GstLAL [1] and PyCBC
Live [2] analysis pipelines.

The signal-to-noise ratio (SNR) was below threshold in V1 so the
candidate was treated as a single-instrument event and no automated
preliminary notice was sent. Nonetheless, the V1 SNR is consistent
with the L1 data given the relative sensitivities of the detectors.
LIGO Hanford Observatory (H1) was offline at the time.

S190425z is an event of interest because its false alarm rate as
estimated by the online analysis is 4.5e-13 Hz, or about one in 7e4
years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S190425z

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

Assuming the candidate is astrophysical in origin, there is strong
evidence for the lighter compact object having a mass < 3 solar masses
(HasNS: >99%). Using the masses and spins inferred from the signal,
there is strong evidence for matter outside the final compact object
(HasRemnant: >99%).

One skymap is available at this time and can be retrieved from the
GraceDB event page:
 * bayestar.fits.gz, an initial localization generated by BAYESTAR [3],
   distributed via GCN notice about 42 minutes after the candidate.

For the bayestar.fits.gz skymap, the 90% credible region is 10183
deg2. Marginalized over the whole sky, the a posteriori luminosity
distance estimate is 155 +/- 45 Mpc (a posteriori mean +/- standard
deviation). The skymap is coarser than usual due to the low
signal-to-noise ratio in V1; the localization is dominated by the L1
antenna pattern.

For further information about analysis methodology and the contents of
this alert, refer to the LIGO/Virgo Public Alerts User Guide
<https://emfollow.docs.ligo.org/userguide/>.

 [1] Messick et al. PRD 95, 042001 (2017)
 [2] Nitz et al. PRD 98, 024050 (2018)
 [3] Singer & Price PRD 93, 024013 (2016)