GAMMA-RAY EMISSION IN DISSIPATIVE PULSAR MAGNETOSPHERES: FROM THEORY TOFERMIOBSERVATIONS

Abstract

We compute the patterns of γ-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 γ-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 γ-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 phaselags between the radio and γ-ray emission on the γ-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 γ-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.

Gamma-ray pulsars: A gold mine

“…Conversely, in the outer magnetosphere, pair cascades are triggered by two-photon interactions between γ rays and low-energy photons, in particular thermal X rays emerging from the neutron-star surface and/or polar caps heated by the return current. For large pair multiplicities, the efficient screening leads to thin acceleration zones (gaps) and to efficient saturation of the particle Lorentz factors, with values decreasing from 10 8 to 10 6 with the global conductivity of the magnetosphere [19]. Conversely, in pair-starved conditions, the accelerating region partially or entirely fills the open magnetosphere.…”

“…As a result they obtain the distribution of E , allowing particle acceleration and radiation. The dissipative magnetospheres have been used to model the high-energy emission from pulsars [60,19,135], showing that the acceleration and emission is widely distributed over the magnetosphere, concentrating in lobes over the polar cap for small conductivity, but progressively moving to the open field boundary and out to the current sheet beyond the light cylinder for high conductivity. The predicted characteristics of both the γ-ray light curves and phase-resolved spectra generally agree with those of young pulsars detected by Fermi-LAT, in the case of the highest conductivity ( Figure 7).…”

“…However the actual location and mechanism for the emission remain unclear. Global dissipative MHD models find that acceleration of particles near, but not in, the current sheet by electric fields induced by the rotation can produce the observed γ-ray emission through curvature radiation [19,135]. It has also been proposed that reconnection in the current sheet near the light cylinder can energize particles that then radiate via synchrotron emission [157,158] up to possibly 100 GeV energies [159], if Doppler boosting due to particle outflow is taken into account.…”

“…Simulations of dissipative magnetospheres show a clear trend for the emission region to move out from lobes above the polar caps to the current sheet past the light cylinder as one increases the global conductivity of the plasma [19]. This should reflect in the distribution of light curve profiles recorded by Fermi.…”

“…This should reflect in the distribution of light curve profiles recorded by Fermi. Yet, no single geometrical model of the beam origin and shape (see Figure 3) can currently reproduce the whole set of observed light curves [57,19]. Only few trends emerge in the large variety of pulse profiles [49].…”