GCN Circular 24069

The LIGO Scientific Collaboration and the Virgo Collaboration report:

We identified the compact binary merger candidate S190408an during
real-time processing of data from LIGO Hanford Observatory (H1), LIGO
Livingston Observatory (L1), and Virgo Observatory (V1) at 2019-04-08
18:18:02.288 UTC (GPS time: 1238782700.288). The candidate was found by
the GstLAL [1], SPIIR [2], CWB [3], MBTAOnline [4], and PyCBC Live [5]
analysis pipelines.

S190408an is a candidate of interest because its false alarm rate, as
determined by the online analysis, is less than one in 100 years. The
candidate's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S190408an

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

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 evidence against matter outside the final compact object(HasRemnant: 12%).
We believe that the latter quantity (HasRemnant) may be overestimated; we
are reviewing it and will provide an update when available.

One skymap is available at this time and can be retrieved from the GraceDB
candidate page:
 * bayestar.fits.gz, a preliminary localization generated by BAYESTAR [6],
   distributed via GCN notice about 34 minutes after the candidate.

For the bayestar.fits.gz skymap, the 90% credible region is 387 deg2.
Marginalized over the whole sky, the luminosity distance estimate is
1473 +/- 358 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] Messick et al. PRD 95, 042001 (2017)
 [2] Hooper et al. PRD 86, 024012 (2012)
 [3] Klimenko et al. PRD 93, 042004 (2016)
 [4] Adams et al. CQG 33, 175012 (2016)
 [5] Nitz et al. PRD 98, 024050 (2018)
 [6] Singer & Price PRD 93, 024013 (2016)