8 research outputs found
Gamma-ray emission from rotation-powered pulsars
Using a simplified model of cascade pair creation over pulsar polar caps
presented in two previous papers, we investigate the expected gamma-ray output
from pulsars' low altitude particle acceleration and pair creation regions. We
divide pulsars into several categories, based on which mechanism truncates the
particle acceleration off the polar cap, and give estimates for the expected
luminosity of each category.
We find that inverse Compton scattering above the pulsar polar cap provides
the primary gamma rays which initiate the pair cascades in most pulsars. This
reduces the expected -ray luminosity below previous estimates which
assumed curvature gamma ray emission was the dominant initiator of pair
creation in all pulsars.Comment: 10 pages, 5 figures, to be published in Ap
Pair Multiplicities and Pulsar Death
Through a simple model of particle acceleration and pair creation above the
polar caps of rotation-powered pulsars, we calculate the height of the
pair-formation front (PFF) and the dominant photon emission mechanism for the
pulsars in the Princeton catalog. We find that for most low- and moderate-field
pulsars, the height of the pair formation front and the final Lorentz factor of
the primary beam is set by nonresonant inverse Compton scattering (NRICS), in
the Klein-Nishina limit. NRICS is capable of creating pairs over a wide range
of pulsar parameters without invoking a magnetic field more complicated than a
centered dipole, although we still require a reduced radius of curvature for
most millisecond pulsars. For short-period pulsars, the dominant process is
curvature radiation, while for extremely high-field pulsars, it is resonant
inverse Compton scattering (RICS). The dividing point between NRICS dominance
and curvature dominance is very temperature-dependent; large numbers of pulsars
dominated by NRICS at a stellar temperature of K are dominated by
curvature at K. We apply these results to pulsar death-line calculations
and to the issue of particle injection into the Crab Nebula.Comment: 14 pages, 7 figures, to appear in Ap
Pair-production multiplicities in rotation-powered pulsars
We discuss the creation of electron-positron cascades in the context of
pulsar polar cap acceleration models and derive several useful analytic and
semi-analytic results for the spatial extent and energy response of the
cascade. Instead of Monte Carlo simulations, we use an integro- differential
equation which describes the development of the cascade energy spectrum in one
space dimension quite well, when it is compared to existing Monte Carlo models.
We reduce this full equation to a single integral equation, from which we can
derive useful results, such as the energy loss between successive generations
of photons and the spectral index of the response. We find that a simple
analytic formula represents the pair cascade multiplicity quite well, provided
that the magnetic field is below 10^12 Gauss, and that an only slightly more
complex formula matches the numerically-calculated cascade at all other field
strengths. Using these results, we find that cascades triggered by gamma rays
emitted through inverse Compton scattering of thermal photons from the neutron
star's surface, both resonant and non-resonant, are important for the dynamics
of the polar cap region in many pulsars.Comment: to appear in ApJ; 19 pages, 18 figure
Polarization Sweeps in Rotation Powered Pulsars
We re-examine the characteristic polarization angle sweep of rotation-powered
pulsars and calculate the expected deviations from this sweep caused by
aberrational effects and by polar-cap current flow. We find that in addition to
the previously known phase shift of the entire sweep by , aberration shifts the polarization angle itself by . Similarly, current flow above the polar cap
shifts the polarization sweep by , potentially providing a method of directly measuring the magnitude of
the current. The competition between these two effects produces a potentially
observable signature in the polarization angle sweep. Although these effects
may appear similar to orthogonal mode shifts, they are an independent
phenomenon with distinct observational characteristics.Comment: 23 pages, 8 figures; accepted by Ap