2025 Theses Doctoral

Deep Searches for Pulsars in Our Galaxy’s Field and Center

Perez, Karen Isabel

Millisecond pulsars (MSPs) are neutron stars spun up through accretion from a binary companion. Some redbacks, a subclass of MSPs in compact binaries with non-degenerate companions, have been observed to switch between accreting and radio pulsar states. These transitional MSPs (tMSPs) are key in understanding the pulsar recycling process, pulsar evolutionary paths,and interactions with their binary companions. T

his thesis presents a multiwavelength (X-ray and UV/optical) study of 22 confirmed and candidate redback systems using archival observations from the Neil Gehrels Swift Observatory, with the aim of identifying flaring behavior or flux variability indicative of a transition. We report the discovery of a 2.11 ms redback binary pulsar in one candidate system, detected in the radio with the Green Bank Telescope (GBT). PSR J0212+5321 shows evidence of wide-orbit radio eclipses, interstellar scintillation, and dispersion measure delays likely caused by the interaction between the pulsar wind and its companion. We constrain its spin and orbital parameters, demonstrating its consistency with published binary parameters from optical radial velocity spectroscopy and light-curve modeling of the companion star.

We also observed the redback candidate 1FGL J0523.5−2529 in the X-ray and UV/optical, finding that it occasionally produces the most luminous flares observed in any non-accreting spider pulsar to date. The system remains in a non-accreting, quiescent state, suggesting that the flares may originate from the pulsar’s rotational kinetic energy and magnetic reconnection in the shock between the pulsar wind and companion wind. No state transitions were observed in any of the monitored systems. Continued long-term, multi-wavelength monitoring, along with precise timing solutions, will help probe the transition mechanisms of tMSPs and allow for the detection of ?-ray/X-ray/UV pulsations in the event that they switch to a radio-quiet disk-dominated state.

In contrast to the Galactic field where hundreds of MSPs have been found, none have yet been detected within a parsec of the GC despite their expected sizable population due to the region’s high stellar density and star formation rate. This thesis presents one of the most sensitive pulsar surveys to date targeting the inner 1.’4 region of the GC at X-band (8–12 GHz) using the GBT. A comprehensive Fourier-domain periodicity search was performed targeting both canonical pulsars (CPs) and MSPs, using constant and linearly changing acceleration searches to improve sensitivity to compact binaries. Given the DM of the GC magnetar, we assume a weak scattering case, reaching luminosity limits of L_{min}≈0.14 mJy kpc^{2} for CPs and L_{min} ≈0.27 mJy kpc^{2} for MSPs. We identify an interesting 8.19 ms MSP candidate, persistent in time and frequency across a 1-hr scan at a flux density of S_{min} ≈0.007 mJy. We are unable to make a definitive claim about the candidate due to a mixed degree of confidence from its properties, and more broadly, its non-detection in subsequent observations. This reinforces the challenges of GC pulsar detection and suggests that strong interstellar scattering, extreme orbital dynamics, or both may mask the underlying population, further deepening the long-standing “missing pulsar" problem at the heart of our Galaxy.