GCN Circular 24377

The LIGO Scientific Collaboration and the Virgo Collaboration report:

We identified the compact binary merger candidate S190503bf during
real-time processing of data from LIGO Hanford Observatory (H1), LIGO
Livingston Observatory (L1), and Virgo Observatory (V1) at 2019-05-03
18:54:04.294 UTC (GPS time: 1240944862.294). The candidate was found
by the GstLAL [1], CWB [2], MBTAOnline [3], PyCBC Live [4], and SPIIR
[5] analysis pipelines.

S190503bf is an event of interest because its false alarm rate, as
estimated by the online analysis, is 1.6e-09 Hz, or about one in 19
years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S190503bf

The classification of the GW signal, in order of descending
probability, is BBH (96%), MassGap (3%), NSBH (<1%), Terrestrial
(<1%), or BNS (<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 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 36 minutes after the candidate

For the bayestar.fits.gz skymap, the 90% credible region is 448 deg2.
Marginalized over the whole sky, the a posteriori luminosity distance
estimate is 421 +/- 105 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] Klimenko et al. PRD 93, 042004 (2016)
[3] Adams et al. CQG 33, 175012 (2016)
[4] Nitz et al. PRD 98, 024050 (2018)
[5] Qi Chu, PhD Thesis, The University of Western Australia (2017)
[6] Singer & Price PRD 93, 024013 (2016)

[GCN OPS NOTE(05may19): Per author's request, the redundant "SUBJECT:"
string was removed from the SUBJECT-line.