#### Title

### Modeling the Galactic Compact Binary Neutron Star Population and Studying the Double Pulsar System

#### Semester

Summer

#### Date of Graduation

2020

#### Document Type

Dissertation

#### Degree Type

PhD

#### College

Eberly College of Arts and Sciences

#### Department

Physics and Astronomy

#### Committee Chair

Maura McLaughlin

#### Committee Member

Sarah Burke-Spolaor

#### Committee Member

Daniel J. Pisano

#### Committee Member

Paul Cassak

#### Committee Member

David Mebane

#### Abstract

Binary neutron star (BNS) systems consisting of at least one neutron star provide an avenue for testing a broad range of physical phenomena ranging from tests of General Relativity to probing magnetospheric physics to understanding the behavior of matter in the densest environments in the Universe. Ultra-compact BNS systems with orbital periods less than few tens of minutes emit gravitational waves with frequencies ~mHz and are detectable by the planned space-based Laser Interferometer Space Antenna (LISA), while merging BNS systems produce a chirping gravitational wave signal that can be detected by the ground-based Laser Interferometer Gravitational-Wave Observatory (LIGO). Thus, BNS systems are the most promising sources for the burgeoning field of multi-messenger astrophysics. In this thesis, we estimate the population of different classes of BNS systems that are visible to gravitational-wave observatories. Given that no ultra-compact BNS systems have been discovered in pulsar radio surveys, we place a 95% confidence upper limit of ~850 and ~1100 ultra-compact neutron star--white dwarf and double neutron star (DNS) systems respectively. We show that among all of the current radio pulsar surveys, the ones at the Arecibo radio telescope have the best chance of detecting an ultra-compact BNS system. We also show that adopting a survey integration time of $t_{\rm int} \sim 1$~min will maximize the signal-to-noise ratio, and thus, the probability of detecting an ultra-compact BNS system. Similarly, we use the sample of nine observed DNS systems to derive a Galactic DNS merger rate of $\mathcal{R}_{\rm MW} = 37^{+24}_{-11}$~Myr$^{-1}$, where the errors represent 90\% confidence intervals. Extrapolating this rate to the observable volume for LIGO, we derive a merger detection rate of $\mathcal{R} = 1.9^{+1.2}_{-0.6} \times \left(D_{\rm r}/100 \ \rm Mpc \right)^3 \rm yr^{-1}$, where $D_{\rm r}$ is the range distance for LIGO. This rate is consistent with that derived using the DNS mergers observed by LIGO. Finally, to illustrate the unique opportunities for science presented by compact DNS systems, we study the J0737--3039 DNS system, also known as the Double Pulsar system. This is the only known DNS system where both of the neutron stars have been observed as pulsars. We measure the sense of rotation of the older millisecond pulsar, pulsar A, in the DNS J0737--3039 system and find that it rotates prograde with respect to its orbit. This is the first direct measurement of the sense of rotation of a pulsar and a direct confirmation of the rotating lighthouse model for pulsars. This result confirms that the spin angular momentum vector is closely aligned with the orbital angular momentum, suggesting that kick of the supernova producing the second born pulsar J0737--3039B was small.

#### Recommended Citation

Pol, Nihan, "Modeling the Galactic Compact Binary Neutron Star Population and Studying the Double Pulsar System" (2020). *Graduate Theses, Dissertations, and Problem Reports*. 7691.

https://researchrepository.wvu.edu/etd/7691

#### Included in

Other Astrophysics and Astronomy Commons, Stars, Interstellar Medium and the Galaxy Commons, Statistical Models Commons