Properties of neutron stars
Because the gravitational force is (relatively speaking) very weak, gravitional waves are only generated by accelerating massive, dense astrophysical objects—so-called ‘compact objects,’ such as neutron stars and black holes. A classic source of compact objects is a supernova (a massive star suffering a violent death after collapsing to the point where Pauli’s exclusion principle becomes important): a neutron star may reside in the ashes of a supernova remnant, often born rotating rapidly about its own axis. If the rotation is measured as electromagnetic pulsations, the neutron star is known as a ‘pulsar’.
If the neutron star rotates with a deviation from axisymmetry (e.g. having a localised ‘hill’), it is able to produce continuous (i.e. quasi-periodic) gravitational radiation. The resulting gravitational wave signal is continuous, with a frequency twice* the rotational frequency.
This is fine for pulsars, with known pulsation rates, but what about the objects that are expected to be neutron stars but haven't detected as pulsars?
Certain young supernova remnants are expected to contain neutron stars at their centres, but have not been positively identified as such. The physical properties inferred from observation of the remnants, including their ages and distances, provide enough constraints on the frequency (and its rates of change) range of the object such that a search for continuous waves is feasible for the object. Such searches for non-pulsing continuous gravitational wave sources is known as a ‘directed’ search. The first target of this type of search is the central compact object in Cassiopeia A, one of the youngest known supernova remnants, at only ~320 years old. The paper describing this type of search method may be found here.
* The gravitational wave frequency is twice the rotation frequency because the neutron star is otherwise transparent to the gravitational waves, so they are radiated ‘above’ and ‘below’ the hill.