Charles D. Dermer

Quasars, Blazars, and Gamma Rays

Before the launch of the Compton Gamma Ray Observatory (CGRO), the only source of >100-megaelectron volt (MeV) gamma radiation known outside our galaxy was the quasar 3C 273. After less than a year of observing, 13 other extragalactic sources have been discovered with the Energetic Gamma Ray Experiment Telescope (EGRET) on CGRO, and it is expected that many more will be found before the full sky survey is complete. All 14 sources show evidence of blazar properties at other wavelengths; these properties include high optical polarization, extreme optical variability, flat-spectrum radio emission associated with a compact core, and apparent superluminal motion. Such properties are thought to be produced by those few, rare extragalactic radio galaxies and quasars that are favorably aligned to permit us to look almost directly down a relativistically outflowing jet of matter expelled from a supermassive black hole. Although the origin of the gamma rays from radio jets is a subject of much controversy, the gamma-ray window probed by CGRO is providing a wealth of knowledge about the central engines of active galactic nuclei and the most energetic processes occurring in nature.

Scattering atmospheres near gamma‐ray burst sources

We show that scattering atmospheres exist in the magnetospheres of luminous compact objects from considerations of resonance effects in magnetic Compton scattering.1 These regions of stable equilibria are formed by a balance between the force of gravity and resonant Compton scattering. We calculate the structure of radiation‐pressure supported atmospheres for neutron stars with different magnetic field strengths and surface temperatures. We propose that the narrow cyclotron absorption lines observed in gamma‐ray burst spectra may be formed by these atmospheres.

Galactic neutron‐star models for gamma‐ray bursts

We examine galactic neutron‐star models for gamma‐ray bursts in light of the recent BATSE results. A population of neutron stars with birth velocities ≳900 kmu2009s−1 could account for the statistics, but special bursting properties are required. The statistics are more simply explained by bursters at cosmological distances.

Pion production in strong magnetic fields: A model for gamma-ray emission from accreting X-ray pulsars

Hadrons accelerated to high energies in the accretion column of a binary X-ray pulsar can produce pions in collisions with accreting protons and ions. The pions decay promptly into charged particles and photons, giving rise to an electromagnetic cascade through pair and photon production processes in the strong magnetic field of the neutron star. We give a simplified analysis of this system for teragauss magnetic fields in the optically-thin limit, assuming that nonlinear interactions between the produced pairs and photons are small, and calculate photon spectra in the MeV range. The calculated spectra are found to depend strongly on energy and angle, and are exponentially truncated at energies determined by the γ — B pair production threshold, which is a function of the magnetic field strength and viewing angle with respect to the magnetic axis.