  Significant human and observational resources have been dedicated to
electromagnetic followup of gravitational-wave events detected by Advanced LIGO
and Virgo. As the sensitivity of LIGO and Virgo improves, the rate of sources
detected will increase. Margalit & Metzger (2019; arXiv:1904.11995) have
suggested that it may be necessary to prioritize observations of future events.
Optimal prioritization requires a rapid measurement of a gravitational-wave
event's masses and spins, as these can determine the nature of any
electromagnetic emission. We extend the relative binning method of Zackay et
al. (2018; arXiv:1806.08792) to a coherent detector-network statistic. We show
that the method can be seeded from the output of a matched-filter search and
used in a Bayesian parameter measurement framework to produce marginalized
posterior probability densities for the source's parameters within 20 minutes
of detection on 32 CPU cores. We demonstrate that this algorithm produces
unbiased estimates of the parameters with the same accuracy as running
parameter estimation using the standard gravitational-wave likelihood. We
encourage the adoption of this method in future LIGO-Virgo observing runs to
allow fast dissemination of the parameters of detected events so that the
observing community can make best use of its resources.
