Measurements and Simulations of the Gamma-ray Background with ComPair and Multi-Messenger Analyses of Pulsar Systems

Abstract

Neutron stars are rich testbeds for nuclear physics in environments unable to be reproducedon Earth. A key open question is the form of the neutron star equation of state, which describes the compressibility and deformability of the neutron star bulk material. The equation of state is directly related to observables such as the neutron star mass–radius relationship and the strength of gravitational wave emission from rotating neutron stars, called pulsars. Another open question is the composition of neutron stars’ companions, which could range from rocky planets to ultra- dense remnants of stellar cores. Two novel measurements that would constrain the equation of state and companion composition are enabled by MeV γ-ray observations and presented in this dissertation. The first is a simulation of a pulsar’s GeV γ-ray emission irradiating a range of companion types and re-emitting as 511 keV γ-rays, where the orbital dependence of the 511 keV γ-rays is directly related to companion composition. The second study searches for gravitational waves emitted by pulsars in 2 parts. The first part considers emission following the Vela pulsar glitch on April 29, 2024. The second part simulates possible joint gravitational wave searches with an MeV telescope and a gravitational wave antenna. AMEGO is a proposed MeV γ-ray observatory to address the need for new observations in this energy range. ComPair, a prototype of AMEGO, consists of four detector subsystems and completed a short duration high altitude balloon flight on August 27, 2023. This dissertation includes the development of the detector effects engine, a critical software package to accurately model the instrument, understand its response, and benchmark its performance. I also present the integration, test, and calibration of the ComPair anti-coincidence detector, and demonstrate that it rejects over 99% of likely background charged particle events. This is followed by benchmark- ing the performance of the entire ComPair instrument during the balloon flight with Monte Carlo simulations. This analysis shows that ComPair met all of its goals, including rejecting the cosmic ray background, reconstructing Compton scattering and pair production γ-ray events, and mea- suring the γ-ray background. The dissertation concludes with a consideration of how the initial ComPair instrument’s performance informs the design of ComPair’s next generation.