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dc.contributor.advisor Baring, Matthew G
dc.creatorStory, Sarah A
dc.date.accessioned 2016-01-27T17:27:55Z
dc.date.available 2016-01-27T17:27:55Z
dc.date.created 2014-12
dc.date.issued 2014-07-30
dc.date.submitted December 2014
dc.identifier.citation Story, Sarah A. "Pair Creation Transparency in Gamma-Ray Pulsars." (2014) Diss., Rice University. https://hdl.handle.net/1911/88167.
dc.identifier.urihttps://hdl.handle.net/1911/88167
dc.description.abstract Pulsars are rapidly rotating, highly magnetized neutron stars that produce photon pulses in energies from radio to gamma-rays. The population of known gamma-ray pulsars has been increased nearly twenty-fold in the past six years since the launch of the Fermi Gamma-Ray Space Telescope; it now exceeds 145 sources and has defined an important part of Fermi's science legacy. In order to understand the detectability of pulsars in gamma-rays, it is important to consider not only the radiative mechanisms that produce gamma-rays, but the processes that can attenuate photons before they can leave the pulsar magnetosphere. Here I explore two such processes, one-photon magnetic pair creation and two-photon pair creation. Magnetic pair creation has been at the core of radio pulsar paradigms and central to polar cap models of gamma-ray pulsars for over three decades. Among the population characteristics well established for Fermi pulsars is the common occurrence of exponential turnovers in the spectra in the 1-10 GeV range. These turnovers are too gradual to arise from magnetic pair creation in the strong magnetic fields of pulsar inner magnetospheres. By demanding insignificant photon attenuation precipitated by such single-photon pair creation, the energies of these turnovers for Fermi pulsars can be used to compute lower bounds for the typical altitude of GeV band emission. In this thesis, I explore such pair transparency constraints below the turnover energy and update earlier altitude bound determinations that have been deployed in various gamma-ray pulsar papers by the Fermi-LAT collaboration. For low altitude emission locales, general relativistic influences are found to be important, increasing cumulative opacity, shortening the photon attenuation lengths, and also reducing the maximum energy that permits escape of photons from a neutron star magnetosphere. Rotational aberration influences are also explored, and are found to be small at low altitudes, except near the magnetic pole. Our analysis clearly demonstrates that including near-threshold physics in the pair creation rate is essential to deriving accurate attenuation lengths and escape energies. The altitude bounds we compute for Fermi pulsars are typically in the range of 2-7 stellar radii and provide key information on the emission altitude in radio quiet pulsars that do not possess double peaked pulse profiles. The bound for the Crab pulsar is at a much higher altitude, with the detection by the atmospheric Cherenkov telescope MAGIC out to 350-400 GeV implying a lower bound of 310 km to the emission region, i.e., approximately 20% of the light cylinder radius. These results are also extended to the super-critical field domain, where it is found that emission in magnetars originating below around 10 stellar radii will not appear in the Fermi-LAT band. Two-photon pair creation becomes important at high altitudes and for photons produced by curvature radiation from charges flowing downward along magnetic field lines. Because the efficiency of two-photon pair creation does not depend on the local magnetic field strength, it can continue to be active in the weak-field regions far from the neutron star. It is found that two-photon pair creation can strongly attenuate photons emitted from downward-traveling charges except at very high altitudes of emission, but in the absence of rotational aberration, it is unable to produce significant opacity for upward-traveling charges unless unrealistically high neutron star surface temperatures are assumed.
dc.format.mimetype application/pdf
dc.language.iso eng
dc.subjectnon-thermal radiation mechanisms
magnetic fields
neutron stars
pulsars
gamma rays
theory
dc.title Pair Creation Transparency in Gamma-Ray Pulsars
dc.type Thesis
dc.contributor.committeeMember Chan, Anthony A
dc.contributor.committeeMember Damjanovic, Danijela
dc.date.updated 2016-01-27T17:27:55Z
dc.type.material Text
thesis.degree.department Physics and Astronomy
thesis.degree.discipline Natural Sciences
thesis.degree.grantor Rice University
thesis.degree.level Doctoral
thesis.degree.name Doctor of Philosophy


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