Pulsars at Low Radio Frequencies, Cyclic Spectroscopy, and Pulsar Timing Arrays

Abstract

Pulsars at low radio frequencies $( < 400$ MHz) are ripe with astrophysical applications. For the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) pulsar timing array (PTA), the continual search for and discovery of new pulsars with single-dish telescopes (Arecibo Observatory and the Green Bank Telescope) is an essential part of the project. At Long-Wavelength Array (LWA) frequencies of 10–88 MHz, pulsar signals are highly scattered from the ionized interstellar medium (IISM). However, monitoring IISM effects along the line of sight to each pulsar characterizes the overall noise budget for gravitational wave detection. In some cases the effects of the very low frequency IISM can be mitigated, either through wideband template profile timing or through cyclic spectroscopy. Aside from PTAs, monitoring pulsars at very low frequencies can inform a plethora of topics in pulsar astrophysics: additional neutron star discoveries, frequency-dependent dispersion measures, solar wind science through high-cadence pulsar monitoring campaigns, and giant pulses. An expanded continent-wide LWA-Swarm would assist gravitational wave (GW) detection by resolving pulsar scattering screens and by providing higher sensitivity, leading to improved cyclic spectroscopy IISM deconvolution on more pulsars. Pulsar discoveries can also be made by following up unidentified steep-spectrum point sources in a LWA-Swarm sky survey.