GCN Circular 24237

The LIGO Scientific Collaboration and the Virgo Collaboration report:

We identified the compact binary merger candidate S190426c during
real-time processing of data from LIGO Hanford Observatory (H1), LIGO
Livingston Observatory (L1), and Virgo Observatory (V1) at 2019-04-26
15:21:55.337 UTC (GPS time: 1240327333.337). The candidate was found
by the GstLAL [1], MBTAOnline [2], PyCBC Live [3], and SPIIR [4]
analysis pipelines.

S190426c is an event of interest because its false alarm rate, as
estimated by the online analysis, is 1.9e-08 Hz, or about one in 1
year, 7 months. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S190426c

The classification of the GW signal, in order of descending
probability, is BNS (49%), MassGap (24%), Terrestrial (14%), NSBH
(13%), or BBH (<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%).

Two skymaps are available at this time and can be retrieved from the
GraceDB event page:
* bayestar.fits.gz, the preliminary sky localization generated by
  BAYESTAR [5], distributed via GCN notice about 25 minutes after
  the candidate,
* bayestar1.fits.gz, an updated localization distributed via GCN
  notice about an hour after the candidate. This is the preferred
  skymap at this time.

For the bayestar1.fits.gz skymap, the 90% credible region is 1262
deg2. Marginalized over the whole sky, the a posteriori luminosity
distance estimate is 375 +/- 108 Mpc (a posteriori mean +/- standard
deviation).

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/><https://emfollow.docs.ligo.org/userguide/>.

[1] Messick et al. PRD 95, 042001 (2017)
[2] Adams et al. CQG 33, 175012 (2016)
[3] Nitz et al. PRD 98, 024050 (2018)
[4] Qi Chu, PhD Thesis, The University of Western Australia (2017)
[5] Singer & Price PRD 93, 024013 (2016)

[GCN OPS NOTE(30apr19): Per author's request, the double spacing was removed.]