Aims. A transient astrophysical event observed in both gravitational wave (GW) and electromagnetic (EM)
channels would yield rich scientific rewards. A first program initiating EM follow-ups to possible transient GW
events has been developed and exercised by the LIGO and Virgo community in association with several
partners. In this paper, we describe and evaluate the methods used to promptly identify and localize GW event
candidates and to request images of targeted sky locations. Methods. During two observing periods (Dec 17
2009 to Jan 8 2010 and Sep 2 to Oct 20 2010), a low-latency analysis pipeline was used to identify GW event
candidates and to reconstruct maps of possible sky locations. A catalog of nearby galaxies and Milky Way
globular clusters was used to select the most promising sky positions to be imaged, and this directional
information was delivered to EM observatories with time lags of about thirty minutes. A Monte Carlo simulation
has been used to evaluate the low-latency GW pipeline's ability to reconstruct source positions correctly.
Results. For signals near the detection threshold, our low-latency algorithms often localized simulated GW burst
signals to tens of square degrees, while neutron star/neutron star inspirals and neutron star/black hole inspirals
were localized to a few hundred square degrees. Localization precision improves for moderately stronger signals.
The correct sky location of signals well above threshold and originating from nearby galaxies may be observed
with ~50% or better probability with a few pointings of wide-field telescopes.