NASA's new 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 117 new pulsars
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. Many of these have been
discovered through blind searches in gamma rays, some of which
display, as yet, no evidence of a radio signal. In addition, Fermi has
identified over 40 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.