Using Fermi Large Area Telescope Observations to Constrain the Emission and Field Geometries of Young Gamma-ray Pulsars and to Guide Millisecond Pulsar Searches

This thesis has two parts, the first focusing on analysis and modeling of high-energy pulsar emission and the second on pulsar observations. In part 1, I constrain the magnetospheric emission geometry (magnetic inclination α, emission width w, maximum emission radius r, and observer colatitude ζ) by modeling >100 MeV light curves of four bright γ-ray pulsars with geometrical representations of the slot gap and outer gap emission models. I also model the >100 MeV phase resolved spectra, measuring the power law cutoff energy Ec with phase. Assuming curvature radiation reaction (CRR) is the dominant emission process, I use Ec to compute the accelerating electric field strength, E||. The original contributions of this thesis to astrophysical research are the use of the force-free magnetic field solution in light curve modeling, the inclusion of an offset polar cap in the slot gap geometry, and the calculation of E|| from observationally determined quantities (i.e., Ec). The simulations reproduce observed light curve features and accurately match multi-wavelength ζ measurements, but the specific combination of best-fit emission and field geometry varies between pulsars. Perhaps pulsar magnetospheres contain some combination of slot gap and outer gap geometries, whose contributions to the light curve depend on viewing angle. The requirement that, locally, E||/B E|| values imply that the youngest, most energetic pulsar has a near-force-free field, and that CRR and/or narrow acceleration gaps may not be applicable to older pulsars. In part 2, I present discoveries of two radio millisecond pulsars (MSPs) from LAT-guided pulsar searches. I timed the first MSP, resulting in the detection of γ-ray pulsations. The second MSP is in a globular cluster. My initial timing efforts show that it is in a highly eccentric (e ~ 0.95) binary orbit with a massive (>0.7Msun) companion, suggestive of past companion exchanges and an exotic nature of the current companion. Further timing will yield a measurement of the orbital precession rate and the system mass, yielding neutron star mass constraints

Pulsar Emission Geometry and Accelerating Field Strength

The high-quality Fermi LAT observations of gamma-ray pulsars have opened a new window to understanding the generation mechanisms of high-energy emission from these systems, The high statistics allow for careful modeling of the light curve features as well as for phase resolved spectral modeling. We modeled the LAT light curves of the Vela and CTA I pulsars with simulated high-energy light curves generated from geometrical representations of the outer gap and slot gap emission models. within the vacuum retarded dipole and force-free fields. A Markov Chain Monte Carlo maximum likelihood method was used to explore the phase space of the magnetic inclination angle, viewing angle. maximum emission radius, and gap width. We also used the measured spectral cutoff energies to estimate the accelerating parallel electric field dependence on radius. under the assumptions that the high-energy emission is dominated by curvature radiation and the geometry (radius of emission and minimum radius of curvature of the magnetic field lines) is determined by the best fitting light curves for each model. We find that light curves from the vacuum field more closely match the observed light curves and multiwavelength constraints, and that the calculated parallel electric field can place additional constraints on the emission geometr

Gamma-Ray Pulsar Light Curves in Vacuum and Force-Free Geometry

Recent studies have shown that gamma-ray pulsar light curves are very sensitive to the geometry of the pulsar magnetic field. Pulsar magnetic field geometries, such as the retarded vacuum dipole and force-free magnetospheres have distorted polar caps that are offset from the magnetic axis in the direction opposite to rotation. Since this effect is due to the sweepback of field lines near the light cylinder, offset polar caps are a generic property of pulsar magnetospheres and their effects should be included in gamma-ray pulsar light curve modeling. In slot gap models (having two-pole caustic geometry), the offset polar caps cause a strong azimuthal asymmetry of the particle acceleration around the magnetic axis. We have studied the effect of the offset polar caps in both retarded vacuum dipole and force-free geometry on the model high-energy pulse profiles. We find that, compared to the profiles derived from symmetric caps, the flux in the pulse peaks, which are caustics formed along the trailing magnetic field lines, increases significantly relative to the off-peak emission, formed along leading field lines. The enhanced contrast produces improved slot gap model fits to Fermi pulsar light curves like Vela, with vacuum dipole fits being more favorable

Pulsar Pair Cascades in Magnetic Fields with Offset Polar Caps

Neutron star magnetic fields may have polar caps (PC) that are offset from the dipole axis, through field-line sweepback near the light cylinder or non-symmetric currents within the star. The effects of such offsets on electron-positron pair cascades are investigated, using simple models of dipole magnetic fields with small distortions that shift the PCs by different amounts or directions. Using a Monte Carlo pair cascade simulation, we explore the changes in the pair spectrum, multiplicity and energy flux across the PC, as well as the trends in pair flux and pair energy flux with spin-down luminosity, L_{sd}. We also give an estimate of the distribution of heating flux from returning positrons on the PC for different offsets. We find that even modest offsets can produce significant increases in pair multiplicity, especially for pulsars that are near or beyond the pair death lines for centered PCs, primarily because of higher accelerating fields. Pair spectra cover several decades in energy, with the spectral range of millisecond pulsars (MSPs) two orders of magnitude higher than for normal pulsars, and PC offsets allow significant extension of all spectra to lower pair energies. We find that the total PC pair luminosity L_{pair} is proportional to L_{sd}, with L_{pair} ~ 10^{-3} L_{sd} for normal pulsars and L_{pair} ~ 10^{-2} L_{sd} for MSPs. Remarkably, the total PC heating luminosity for even large offsets increases by less than a factor of two, even though the PC area increases by much larger factors, because most of the heating occurs near the magnetic axis.Comment: 41 pages, 17 figures, accepted for publication in Ap