Timothy Dolch

Radio pulsar signals are significantly perturbed by their propagation through the ionized interstellar medium. In addition to the frequency-dependent pulse times of arrival due to dispersion, pulse shapes are also distorted and shifted, having been scattered by the inhomogeneous interstellar plasma which affects pulse arrival times. Understanding the degree to which scattering affects pulsar timing is important for gravitational-wave detection with pulsar timing arrays (PTAs), which depend on the reliability of pulsars as stable clocks with an uncertainty of ~ 100 ns or less over ~ 10 yr or more. Scattering can be described as a convolution of the intrinsic pulse shape with an impulse response function representing the effects of multipath propagation. Cyclic spectroscopy has been applied to pulsar signals to deconvolve the effects of scattering from the original emitted signals, increasing the overall timing precision. We present an analysis of simulated data to test the quality of deconvolution using cyclic spectroscopy over a range of parameters characterizing interstellar scattering and pulsar signal-to-noise ratio (S/N). We show that cyclic spectroscopy is most effective for high S/N and/or highly scattered pulsars. We conclude that cyclic spectroscopy could play an important role in scattering correction to distant populations of highly scattered pulsars not currently included in PTAs. For future telescopes and for current instruments such as the Green Bank Telescope upgraded with the ultrawide bandwidth receiver, cyclic spectroscopy could significantly increase the number of PTA-quality pulsars. As full deconvolution is computationally expensive, other approaches to cyclic spectroscopy that do not require full deconvolution are also discussed.

The Low-Frequency All-Sky Monitor is a radio telescope network consisting of five stations, each with 13 Long-Wavelength Array (LWA) antennas, distributed around the North American continent. The LWA antennas are arranged so as to form a wide-angle drift scan beam, sensitive to bright radio transients, astrophysical and ionospheric, at 10-88 MHz. The multiple stations can reliably verify astrophysical events by simultaneous detection. Adding the fifth station at Hillsdale College in Hillsdale, MI, provides a wider range of declination coverage for the project. Station V, like the previous four stations, was constructed by undergraduate researchers, who continue to operate and manage the telescope. Here we report on Station V's "first light", and discuss future prospects.

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.