Tracking continuous gravitational waves from a neutron star at once and twice the spin frequency with a hidden Markov model

Understand

Searches for continuous gravitational waves from rapidly spinning neutron stars normally assume that the star rotates about one of its principal axes of moment of inertia, and hence the gravitational radiation emits only at twice the spin frequency of the star, $2f_*$.

• The superfluid interior of a star pinned to the crust along an axis nonaligned with any of its principal axes allows the star to emit gravitational waves at both $f_*$ and $2f_*$, even without free precession, a phenomenon not clearly observed in known pulsars.
• The dual-harmonic emission mechanism motivates searches combining the two frequency components of a signal to improve signal-to-noise ratio.
• We describe an economical, semicoherent, dual-harmonic search method, combined with a maximum likelihood coherent matched filter, F-statistic, and improved from an existing hidden Markov model (HMM) tracking scheme to track two frequency components simultaneously.