8 research outputs found
Compact formulae, dynamics and radiation of charged particles under synchro-curvature losses
We consider the fundamental problem of charged particles moving along and
around a curved magnetic field line, revising the synchro-curvature radiation
formulae introduced by Cheng and Zhang (1996). We provide more compact
expressions to evaluate the spectrum emitted by a single particle, identifying
the key parameter that controls the transition between the curvature-dominated
and the synchrotron-dominated regime. This parameter depends on the local
radius of curvature of the magnetic field line, the gyration radius, and the
pitch angle. We numerically solve the equations of motion for the emitting
particle by considering self-consistently the radiative losses, and provide the
radiated spectrum produced by a particle when an electric acceleration is
balanced by its radiative losses, as it is assumed to happen in the outer gaps
of pulsar's magnetospheres. We compute the average spectrum radiated throughout
the particle trajectory finding that the slope of the spectrum before the peak
depends on the location and size of the emission region. We show how this
effect could then lead to a variety of synchro-curvature spectra. Our results
reinforce the idea that the purely synchrotron or curvature losses are, in
general, inadequate to describe the radiative reaction on the particle motion,
and the spectrum of emitted photons. Finally, we discuss the applicability of
these calculations to different astrophysical scenarios.Comment: 9 pages, 5 figures, 2 tables. Accepted for publication in MNRAS main
journal. References update
An assessment of the pulsar outer gap model. II: Implications for the predicted -ray spectra
One of the most important predictions of any gap model for pulsar
magnetospheres is the predicted -ray spectra. In the outer gap model,
the properties of the synchro-curvature radiation are sensitive to many
parameters, whose realistic ranges have been studied in detail in an
accompanying paper. There we demonstrated that the uncertainty in the radius of
curvature, the magnetic field geometry, and the X-ray surface flux may affect
by orders of magnitude the predicted flux and spectral peak in the -ray
regime. Here, we present a systematic, numerical study of the impact of the
different parameters on the particle dynamics along the gap and calculate the
emitted synchro-curvature radiation along the trajectory. By integrating the
emitted radiation along the gap and convolving it with a parametrized particle
distribution, we discuss how the comparison with the wealth of {\em Fermi}-LAT
data can be used to constrain the applicability of the model. The resulting
spectra show very different energy peaks, fluxes and shapes, qualitatively
matching the great variety of the observed {\em Fermi}-LAT pulsars. In
particular, if we see a large fraction of photons emitted from the initial part
of the trajectory, we show that the spectra will be flatter at the low-energy
{\it Fermi}-LAT regime (100 MeV -- 1 GeV). This provides a solution for such
observed flat spectra, while still maintain synchro-curvature radiation as the
origin of these photons.Comment: 9 pages, 6 figures, 1 table. Accepted for pubblication in MNRAS main
journal. References update
An assessment of the pulsar outer gap model. I: Assumptions, uncertainties, and implications for the gap size and the accelerating field
The popular outer gap model of magnetospheric emission from pulsars has been
widely applied to explain the properties observed in -rays. However,
its quantitative predictions rely on a number of approximations and assumptions
that are usually overlooked. Here we examine them, reviewing the main
ingredients entering in the model, evaluating their range of uncertainties.
Usually, in the quantitative applications of the model, key parameters like the
radius of curvature and the energies of the interacting photons are taken to be
a fixed, single value. Instead, here we explore their realistic ranges, and the
impact of these on the consistency of the model itself. We conclude that the
popular evaluation of the trans-field size of the gap as a function of period
and period derivative, is unreliable and affected by a huge dispersion. Last,
the exploration of the possible values for the radius of curvature, the local
magnetic field and other quantities deserve more attention for quantitative
applications of the outer gap model, like the calculation of -ray
spectra, which is the subject of an accompanying paper.Comment: 19 pages, 5 figures, 3 tables. Accepted for pubblication in MNRAS
main journal. References update