Searching for Periodic Gravitational Waves from Rapidly Spinning Neutron Stars

In 2015, after a three-year upgrade, the LIGO gravitational wave
detectors began a series of observing runs in their “advanced”
configuration, joined in 2017 by Advanced Virgo. In the first two
observing runs (O1 and O2), the LIGO Scientific Collaboration and the
Virgo collaboration detected gravitational waves from ten binary black
hole mergers and one binary neutron star inspiral. In the currently
ongoing third observing run (O3), LIGO and Virgo have reported
numerous potential signals from compact binary coalescences, which are
currently being analyzed in more depth.

In addition to the transient signals already detected from compact
binaries, the LSC and Virgo conduct searches for other types of
gravitational waves, including long-lived nearly periodic signals from
rapidly spinning neutron stars. The methods used to coherently or
semi-coherently integrate these signals over the course of an
observing run depend on the knowledge of the source parameters. These
can range from known pulsars with ephemerides which completely specify
their sky position, rotation frequency and frequency evolution, to
undiscovered neutron stars about which nothing is known. An
intermediate situation is a directed search gravitational waves from a
neutron star in a known sky location, for which the spin frequency and
other parameters are unknown or uncertain.

After presenting an overview of the field, I will focus on
semi-coherent directed methods to search for gravitational waves from
low-mass X-ray binaries, notably Scorpius X-1. Sco X-1 is among the
most promising candidates for detection of periodic gravitational
waves with advanced detectors. Its large X-ray flux is indicative of
accretion of matter from its companion star. This accretion may
provide the asymmetry needed to generate gravitational waves, and if
the spinup torque due to accretion is balanced by loss of angular
momentum in gravitational waves, the resulting GW signal is
potentially detectable with advanced detector sensitivities.