Hope College Physics Department Research Experiences for Undergraduates Summer 2011 Project Summary

Astrophysicists are currently searching for evidence of the existence of dark matter by observing the extragalactic gamma-ray background radiation. Many of them are interested in the especially high position fraction recently observed at high energies by Fermi (Abdo et al.2010) and PAMELA (Adriani et al.2011). Electrons and positron lose much of their energy as they travel through the interstellar medium because of their low mass, which suggests that these particles are from a local (~1 kpc) source. The high positron fraction suggests that these particles came from a primary source, which is a source that creates electron-positron pairs, rather than from a secondary source, where cosmic-ray nuclei knock electrons free from other particles in the interstellar gas. In order to determine whether dark matter annihilation or local astrophysical objects are more likely sources of high-energy electrons, it is important to know how pulsars contribute to the position background. To determine the pulsar contribution to background radiation, we have developed a population synthesis Monte Carlo code (Gonthier et al.2004), Story et al.2007), which has been updated to include a set of sky maps that characterize the emission of millisecond pulsars (MSPs) as a function of the viewing geometry. These updates take into account the relativistic effects of aberration and the distortion of the open field volume of a rapidly rotting dipole field. This code simulates both normal pulsars (NPs) and MSPs and allows us to predict the number of radio and gamma pulsars of each type. Our simulations are normalized to the number of normal radio pulsars detected in a group of ten radio surveys. We adjust the radio luminosity to match the expected isolated neutron star birth rate from supernova type lls of ~2 per century. We are then able to predict the number of gamma-ray pulsars detected by Fermi as well s the millisecond pulsar birth rate. This data can then be used to estimate the local density of pulsars and therefore, their contribution to the diffuse gamma-ray background. While our predictions of radio and Fermi MSPs are consistent with observations, there are clear inconsistencies in the period derivative and period distribution requiring us to better understand the initial assumed distributions. Within these limitations, we can predict the diffuse gamma_ray background. Our preliminary results suggest that the contribution from MSPs is higher than previously understood. Given our predicted local distribution of MSPs, we await the effort of our collaborators to provide the position flux from nearby pulsars.
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