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Radio and high-energy nonthermal emission from neutron stars -HOPE STUDENTS ONLY

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Project Description:

NASA's flagship in gamma-ray astronomy, the Fermi Gamma-Ray Space Telescope (Fermi) launched in June 2008, has already dramatically improved our understanding of gamma-ray emission from pulsars. Since its launch, Fermi has discovered over 205 new pulsars (as of February 2016) above 100 MeV in photon energy, superseding the gamma-ray pulsar database of six provided by its predecessor, the EGRET instrument aboard the Compton Gamma-Ray Observatory in nine years of observation. Many young pulsars have been discovered through blind searches in gamma rays, with a group of young pulsars which display, as yet, no evidence of a radio signal. In addition, Fermi has identified over 93 new millisecond pulsars most of which have been seen in radio wavelengths where pulsations were discovered through pointed radio observations of Fermi pulsar candidates.

With the wealth of new data from Fermi at hand, we have the first real opportunity in decades to finally try to understand the high-energy emission and acceleration in pulsar magnetospheres. Making full use of this opportunity will require more detailed modeling of different emission mechanisms, gamma-ray luminosity models and geometries, and comparing model predictions with the well-defined trends in the observations should be very productive. In addition to better defining pulsar gamma-ray emission, it will be possible also to constrain the radio emission location and geometry. The large collection of radio-quiet/radio-weak gamma-ray pulsars are defining the viewing angles where we are just crossing the outer edge of the radio beam or missing it altogether.

In conjunction with colleague, Alice Harding, at NASA Goddard, We explore theoretically the emission of gamma rays from the magnetospheres of neutron stars using Monte Carlo simulations. The emission of gamma rays is initiated from the acceleration of charges - electrons, and positrons, along magnetic field lines with a parallel electric field. Two different models describe the location and geometry of the acceleration and subsequent emission process. In the slot gap of the polar cap model the acceleration takes place all along the last open field surface, while the outer gap model describes the acceleration far out in the magnetosphere near the light cylinder where the speed of the co-rotating magnetic field lines approaches the speed of light. In this research project, we also study the radio emission from neutron stars. We have developed a Monte Carlo computer code that simulates the characteristics of both radio and gamma-ray emission predicting the number of radio and gamma-ray pulsars observed by various radio surveys and Fermi. Studies of the correlations of the radio and gamma-ray pulse profiles will provide a framework to differentiate between the competing pulsar models.

A second area of investigation of the program in collaboration with Matthew Baring at Rice University encapsulates a new formulation of the magnetic Compton scattering cross section, which correctly treats spin-dependent effects at the cyclotron resonance. This effort will derive useful analytic formulae for dissemination among the astrophysics community. This new physics offering will be applied to a magnetic Compton upscattering model for the phase-resolved, steep X-ray spectra detected by the INTEGRAL, Chandra, XMM and RXTE observatories in the 1-20 keV band, in the class of high-field pulsars known as magnetars, constituted by Anomalous X-Ray Pulsars and Soft Gamma-Ray Repeaters. The focus of this portion of our research will be aimed at identifying whether resonant Compton upscattering provides the critical ingredient that distinguishes the characteristics of highly-magnetized pulsars from the more abundant, conventional gamma-ray pulsars.

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