A New Search Technique for Short Orbital Period Binary Pulsars

We describe a new and efficient technique, which we call sideband or phase-modulation searching, that allows one to detect short-period binary pulsars in observations longer than the orbital period. The orbital motion of the pulsar during long observations effectively modulates the phase of the pulsar signal, causing sidebands to appear around the pulsar spin frequency and its harmonics in the Fourier transform. For the majority of binary radio pulsars or low-mass X-ray binaries (LMXBs), large numbers of sidebands are present, allowing efficient searches using Fourier transforms of short portions of the original power spectrum. Analysis of the complex amplitudes and phases of the sidebands can provide enough information to solve for the Keplerian orbital parameters. This technique is particularly applicable to radio pulsar searches in globular clusters and searches for coherent X-ray pulsations from LMXBs and is complementary to more standard "acceleration" searches.