1,557 research outputs found
MeV Pulsars: Modeling Spectra and Polarization
A sub-population of energetic rotation-powered pulsars show high fluxes of
pulsed non-thermal hard X-ray emission. While this MeV pulsar population
includes some radio-loud pulsars like the Crab, a significant number have no
detected radio or GeV emission, a mystery since gamma- ray emission is a common
characteristic of pulsars with high spin-down power. Their steeply rising hard
X-ray spectral energy distributions (SEDs) suggest peaks at 0.1 - 1 MeV but
they have not been detected above 200 keV. Several upcoming and planned
telescopes may shed light on the MeV pulsars. The Neutron star Interior
Composition ExploreR (NICER) will observe pulsars in the 0.2 - 12 keV band and
may discover additional MeV pulsars. Planned telescopes, such as All-Sky
Medium-Energy Gamma-Ray Observatory (AMEGO) and e-ASTROGAM, will detect
emission above 0.2 MeV and polarization in the 0.2 - 10 MeV band. We present a
model for the spectrum and polarization of MeV pulsars where the X-ray emission
comes from electron- positron pairs radiating in the outer magnetosphere and
current sheet. This model predicts that the peak of the SED increases with
surface magnetic field strength if the pairs are produced in polar cap
cascades. For small inclination angles, a range of viewing angles can miss both
the radio pulse and the GeV pulse from particles accelerating near the current
sheet. Characterizing the emission and geometry of MeV pulsars can thus provide
clues to the source of pairs and acceleration in the magnetosphere.Comment: 8 pages, 5 figures, published in Proceedings of Scienc
Gamma-Ray Emission in Dissipative Pulsar Magnetospheres: From Theory to Fermi Observations
We compute the patterns of -ray emission due to curvature radiation
in dissipative pulsar magnetospheres. Our ultimate goal is to construct
macrophysical models that are able to reproduce the observed -ray
light-curve phenomenology recently published in the Second Fermi Pulsar
Catalog. We apply specific forms of Ohm's law on the open field lines using a
broad range for the macroscopic conductivity values that result in solutions
ranging, from near-vacuum to near Force-Free. Using these solutions, we
generate model -ray light curves by calculating realistic trajectories
and Lorentz factors of radiating particles, under the influence of both the
accelerating electric fields and curvature radiation-reaction. We further
constrain our models using the observed dependence of the phase-lags between
the radio and -ray emission on the -ray peak-separation. We
perform a statistical comparison of our model radio-lag vs peak-separation
diagram and the one obtained for the Fermi standard pulsars. We find that for
models of uniform conductivity over the entire open magnetic field line region,
agreement with observations favors higher values of this parameter. We find,
however, significant improvement in fitting the data with models that employ a
hybrid form of conductivity; specifically, infinite conductivity interior to
the light-cylinder and high but finite conductivity on the outside. In these
models the -ray emission is produced in regions near the equatorial
current sheet but modulated by the local physical properties. These models have
radio-lags near the observed values and statistically best reproduce the
observed light-curve phenomenology. Additionally, these models produce GeV
photon cut-off energies.Comment: Accepted for publication in ApJ (revised version, 26 pages, 23
figures
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