Gamma-ray studies of rapidly rotating neutron stars, or pulsars, can broadly be divided into two categories: pulsed emission arising directly from the neutron star, or secondary emission from energetic particles injectd into the local stellar environment. Of the seven currently known TeV gamma-ray sources three are pulsar powered synchrotron nebulae: the Crab Nebula, PSR B1706-44 and Vela.
EGRET onboard CGRO has detected pulsed emission from seven young rapidly rotating neutron stars (Thompson et al. 1997). VERITAS will bridge the gap between the highest energies accessible from space and the present lowest energies accessible from the ground. In this way, VERITAS will be in the position to address the question of why pulsed emission dominates in the sub GeV domain and unpulsed emission dominates in the sub TeV domain.
Atmospheric Cerenkov telescopes have, somewhat surprisingly, turned out to be the most sensitive probes of activity in jets emerging from Active Galactic Nuclei (AGNs). Whipple Observatory observations of Mrk 421, Mrk 501, 1ES 2344+514 and H1426+428 together with EGRET observations of 3C 279, have become the benchmarks for AGN emission theories.
TeV gamma-ray observations give a unique view of the formation of shocks at irregular intervals every few days near the base of the jet. Flares almost as short as the basic timescale associated with accretion unto a supermassive black hole have been seen, indicating that we may be observing the processes right at the beginning of the jet.
Correlated multi-wavelength observations have been most important in testing the general process of the radiation which is consistent with a synchrotron component extending up to a few keV, followed by the Compton-scattered X-ray to gamma-ray component which probably falls off at energies above 1 TeV (Buckley et al. 1996).
VERITAS will contribute enormously to the understanding of AGNs in several ways. The good flux sensitivity will enable precise measurements of the rise and fall of intensity in flares. The versatility will offer the potential for greater monitoring of AGN activity. Spectral measurements will provide important information on the processes occurring in the jets as well as on the density of the extragalactic infrared photon field through which the TeV photons pass and consequently interact, a fact which limits the visibility of distant AGNs.
Supernova remnants (SNRs) have long been considered the primary sites for the acceleration of cosmic rays up to 10³ TeV. The evidence lending support to this belief is based on several strong arguments. First, supernova blast shock are one of the few galactic sites capable of sustaining the cosmic ray population against loss by escape and nuclear interactions.
Second, shock acceleration models provide a plausible mechanism for converting this explosive energy into accelerated particles with energies ~10² - 10³ TeV. Finally, recent detections of non-thermal X-ray emission in SN 1006 (Koyama et al. 1995), IC 443 (Keohane et al. 1997) and CAS A (Allen et al. 1997) suggests the presence of electrons accelerated to 10 - 100 TeV. However, observations of electrons accelerated by strong shocks only provide indirect evidence of cosmic ray production.
If SNRs do contain significant accelerated proton populations these will inevitably interact with the swept up interstellar medium in the remnants to produce pi° gamma-rays (Drury et al. 1994). One critical test of the shock acceleration model for the origin of cosmic rays is the observation of gamma-rays. VERITAS will be able to make crucial measurements in this area as good angular resolution is required to map the emission region and good energy resolution is required to differentiate contributions to the gamma-ray flux by other mechanisms such as bremsstrahlung and inverse Compton.