GCN Circular 24141

The LIGO Scientific Collaboration and the Virgo Collaboration report:

We identified the compact binary merger candidate S190421ar during
real-time processing of data from LIGO Hanford Observatory (H1) and
LIGO Livingston Observatory (L1) at 2019-04-21 21:38:56.251 UTC (GPS
time: 1239917954.251). The candidate was found by the PyCBC Live [1],
CWB [2], GstLAL [3], MBTAOnline [4], and spiir [5] analysis pipelines.

S190421ar is an event of interest because its false alarm rate, as
determined by the online analysis, is 1.5e-08 Hz, or about one in 2
years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S190421ar

The classification of the GW signal, in order of descending
probability, is Terrestrial (96%), BBH (4%), BNS (<1%), NSBH (<1%), or
MassGap (<1%). We think that BBH probability may be underestimated and
the Terrestrial probability may be overestimated; we are reviewing it
and will provide an update when available.

Assuming the candidate is astrophysical in origin, there is strong
evidence against the lighter compact object having a mass < 3 solar
masses (HasNS: <1%). Using the masses and spins inferred from the
signal, there is strong evidence against matter outside the final
compact object (HasRemnant: <1%).

One skymap is available at this time and can be retrieved from the
GraceDB event page:
 ��* bayestar.fits.gz, an updated localization generated by BAYESTAR
[6], distributed via GCN notice about 18 hours after the candidate

For the bayestar.fits.gz skymap, the 90% credible region is 1917 deg2.
Marginalized over the whole sky, the a posteriori luminosity distance
estimate is 2281 +/- 697 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/>.

 ��[1] Nitz et al. PRD 98, 024050 (2018)
 ��[2] Klimenko et al. PRD 93, 042004 (2016)
 ��[3] Messick et al. PRD 95, 042001 (2017)
 ��[4] Adams et al. CQG 33, 175012 (2016)
 ��[5] Hooper et al. PRD 86, 024012 (2012)
 ��[6] Singer & Price PRD 93, 024013 (2016)