Massive, Bright and Brand-New Millisecond Pulsars as Probes of Fundamental Physics

Author: ORCID icon
Cromartie, Hannah Thankful, Astronomy - Graduate School of Arts and Sciences, University of Virginia
Ransom, Scott, AS-Astronomy, University of Virginia

Spinning hundreds of times a second and with densities surpassed only by black holes, millisecond pulsars (MSPs) are among the most astonishing astrophysical objects in the Universe. Nearly 40 years of careful study have revealed their efficacy as probes of physical phenomena that could not otherwise be studied by scientists in Earth-based laboratories. This dissertation touches on several disparate aspects of pulsar science, all of which contribute (either directly or indirectly) to the goal of furthering our understanding of fundamental physics. All of the work detailed in this thesis was conducted under the umbrella of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration, the goal of which is to detect the low-frequency stochastic background of gravitational waves from supermassive black hole binaries using an array of precisely timed MSPs called a Pulsar Timing Array (PTA).

Because a PTA's sensitivity to gravitational waves is linearly proportional to the number of MSPs in the array, the discovery of new MSPs with high timing precision is a critical component of NANOGrav's effort to detect gravitational waves. We report the discovery of the first six MSPs in Fermi unassociated Gamma-ray sources discovered with the Arecibo telescope (Cromartie et al. 2016), as well as the 2017 discovery of an additional thirteen with the Arecibo and Green Bank Telescopes (discussed later in the dissertation). Five of the six new Arecibo sources — a disproportionately large fraction — are in highly accelerated binaries with low-mass companions. We therefore explore Arecibo's predisposition to discovering interacting binaries quantitatively. Though the six original Fermi MSPs discovered with Arecibo are not appropriate for PTA inclusion, at least one of the thirteen 2017 discoveries is being provisionally included in the NANOGrav PTA.

Next, we discuss our measurement of the most massive neutron star observed to date. J0740+6620 is a ~2.14 ± 0.09 solar mass (1-sigma confidence interval) MSP in the NANOGrav dataset for which we obtained supplementary, orbital-phase-specific observations with the Green Bank Telescope in order to constrain its mass using the relativistic Shapiro delay (Cromartie et al. 2020). This discovery has significant implications for the extremely poorly understood neutron star equation of state, which describes the behavior of supranuclear-density matter deep within neutron star cores.

We then present the results of an additional Shapiro delay-powered endeavor, this time in order to constrain the mass of the bright Gamma-ray MSP J1231-1411. This source is of particular interest to the Neutron Star Interior Composition Explorer (NICER) mission. Forthcoming modeling of the source's X-ray lightcurve promises to constrain its mass-to-radius ratio, which could further our understanding of the equation of state. An independent measurement of its mass via the radio Shapiro delay would improve the NICER team's modeling of the MSP, and in turn, the project's potential scientific payoff. We conducted a multi-wavelength analysis of timing data, including a new 22-hour campaign over orbital conjunction using the Green Bank Telescope. Both traditional chi-squared minimization fitting and Markov Chain Monte Carlo (MCMC)-based techniques indicate that this source is a low-mass MSP in a highly inclined binary orbit with a low-mass white dwarf. The MCMC trials, which prove the constraining power of our measurement of the white dwarf mass and orbital inclination, are informed by priors based on white dwarf evolutionary models. We also conduct a single-photon MCMC fit to 12 years of Fermi-LAT Gamma-ray data, though the resulting constraints on MSP mass are not as stringent as the (provisional) constraints from radio data.

We conclude by sharing a potpourri of pulsar timing projects that are either in-progress, or that did not merit publication despite being of potential interest to the reader. The thirteen (unpublished) 2017 Fermi MSPs, a survey of highly accelerated "spider" MSP systems for inclusion in the NANOGrav array, and an additional three MSPs for which we conducted targeted Shapiro delay campaigns are all discussed in this final chapter.

PHD (Doctor of Philosophy)
Astronomy, Astrophysics, Radio Astronomy, Pulsars, Gravitational Waves, Equation of State
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