Figure 2.2.3 - Nu-F-nu plot of the Vela pulsar showing the peak emission in the gamma-ray range.
With the results from CGRO, the number of gamma-ray pulsars has risen
from 2 to at least 7, and several surprises have come to light. The new
detections show that pulsars can be remarkably efficient producers of GeV
photons, with 10% or more of the energy in the pulsed emissions for 105-106
year old pulsars. Furthermore, the detection of Geminga as a radio-quiet
pulsar shows that radio and high-energy pulsars are overlapping subsets
of the neutron star population, but with quite different beam patterns.
The gamma-ray observations thus provide a complementary (and apparently
more complete) sample of the young pulsar population. Although Geminga
remains a sample of one, radio-quiet pulsars are expected to represent
a sizable fraction of the unidentified high-energy gamma-ray sources. Thus
studying the gamma-ray sample will greatly advance our understanding of
the neutron star birthrate (and its relationship to the supernova rate).
The site of the pulsar particle acceleration and the gamma radiation is
still under investigation. Theoretical modeling has focused on acceleration
at the polar caps and in vacuum regions in the outer magnetosphere. These
models have advanced to the point where pulse profiles, luminosities, and
spectral variations with pulsar phase can be computed; comparison with
CGRO data on the brightest objects (Crab, Vela, and Geminga) have provided
significant constraints. What these comparisons make clear is that the
GeV emission directly probes the dynamics and geometry of the particle
acceleration region where electron/ positron energies are inferred to exceed
10 TeV. A unique attraction of pulsar modeling arises from the fact that
rotation brings different regions of the acceleration zone into view during
the pulse; with sufficient statistics and energy coverage the rich temporal
structure in pulsar spectra allow a tomographic analysis of physical conditions
in the magnetospheric particle accelerator. Finding more gamma-ray
pulsars, both radio-loud and radio-quiet, will be essential to answering
the outstanding questions. A crucial test of pulsar models will be their
ability to predict which radio pulsars will be detected as gamma-ray pulsars
in the future. When a source is a known radio pulsar, sensitive pulse searches
and measuring high quality phase resolved spectra benefit from very long
exposures. Thus the most important attributes of a future high-energy gamma-ray
mission are large effective area coupled with large field-of-view. When
a source is not identified with a radio pulsar, finding the pulsed emission
directly in the gamma-ray data requires a high count rate. In both cases,
high angular resolution (< 10') at GeV energies will be very important
for isolating sources from the bright galactic background. Further, arcminute
positions in the GeV range will enable powerful searches for counterparts
with imaging X-ray telescopes and ground instruments. Such counterpart
searches offer the best means to trace the origin of the galactic plane
sources. To untangle the physics of the detected pulsars, high quality
phase resolved spectra are crucial. Particularly important are extension
of the sensitive range above 10 GeV where the pulses merge with unpulsed
plerionic emission (and where Compton scattered photons may dominate the
pulsed signal) and below 10 MeV where existing observations require a break
from the flat GeV spectra and important phase variability is expected in
many models.
Objectives: