Supernova remnants:

SNRs are widely believed to be the sources of hadronic cosmic rays up to energies of approximately $Z\times 10^{14}$eV, where Z is the nuclear charge of the particle. This belief is supported by the fact that supernovae are the only galactic objects which appear to be capable of supplying the energy required for cosmic-ray generation. Also, the fairly well understood process of first-order Fermi acceleration (e.g., [Blandford & Ostriker 1978]) predicts the generation of high energy cosmic rays in supernova shock fronts and leads to power law energy spectra matching those inferred from cosmic-ray observations (e.g., [Swordy et al. 1990]). The existence of energetic electrons is well-known from observations of synchrotron emission at radio and X-ray energies (e.g., [Koyama et al. 1995]). Recently, the detection of TeV gamma-rays from the shell-type SNRs SN1006, RXJ1713.7-3946 and CassiopeiaA has been reported ([Weekes 1999]). Unlike at X-ray and radio wavelengths, the TeV emission mechanism in these objects is unknown (e.g., [Aharonian & Atoyan 1999]; [Baring et al. 1999]). In addition to hadronic cosmic rays, electrons in SNRs may produce gamma-rays via non-thermal bremsstrahlung and inverse Compton scattering of low energy photons. When combined with X-ray and radio observations, the VHE gamma-ray observations provide a means of resolving these various contributions and providing information about SNR shell environments such as the maximum particle energy and magnetic field. Both are important but unknown parameters in shock acceleration theories.

Thus, a clear indication for the acceleration of hadronic particles in SNRs is still missing. The evidence for such processes would be gamma-ray spectra characteristic of $\pi^0$ decay subsequent to nuclear interactions in the SNR. The gamma-ray energies should range from $\sim$10MeV up to ten's of TeV ($\sim$1/10 of the maximum proton energy), and the energy spectrum should follow the energy spectrum of cosmic rays in the source (thought to be a power law spectrum with index $\sim$-2.1). While EGRET has detected signals from several regions of the sky consistent with the positions of shell-type SNRs ([Sturner & Dermer 1995]; [Esposito et al. 1996]; [Lamb & Macomb 1997]; [Jaffe et al. 1997]), its limited angular resolution makes definitive identification difficult. In addition, background from the diffuse Galactic gamma-ray emission complicates spectral measurements.

  

Figure 6: Predicted gamma-ray spectra in the shell-type SNR IC443. The region between the solid lines depicts the spectra from hadronic interactions for a range of model parameters (adapted from [Buckley et al. 1998]). The region between the dashed lines is the predicted inverse-Compton spectra for B between 20$\mu$G and 50$\mu$G and the range of flux normalizations and electron spectra allowed by the X-ray observations. EGRET data points (filled circles) and the current upper limits (open circles) from Whipple and the Cygnus air shower array are shown. The sensitivity of VERITAS for a 50 hour observation of this object is indicated by the thick curve.

The Whipple Collaboration has searched for gamma-rays from several shell-type SNRs, but no emission has been detected ([Buckley et al. 1998]). Figure 6 shows the EGRET measurements and Whipple upper limits for the SNR IC443. These results already eliminate much of the allowed parameter space for gamma-ray emission from these objects (from hadrons and electrons), and begin to raise some questions about the validity of current models for the objects studied or for the propagation of cosmic rays.

As shown in Figure 6, there are predictions for strong gamma-ray emission from shell-type SNRs by hadron and electron interactions. Model fits to EGRET and Whipple data (e.g., [Gaisser et al. 1998]) indicate that if the emission detected by EGRET is from the SNR, the combination of EGRET spectra and Whipple upper limits imply contributions from inverse Compton and bremsstrahlung scattering of electrons as well as hadronic interactions. They also require a relatively steep spectrum compared with the E^-2.1expected from direct measurements. Resolving these various components will require accurate measurements over a large range of energies. VERITAS and GLAST will provide excellent sensitivity and energy resolution over more than six orders of magnitude in energy for the study of these objects. In addition, the excellent angular resolution of VERITAS will allow detailed mapping of the emission regions in the SNRs. For a typical SNR luminosity and angular extent, VERITAS should be able to detect objects within 4kpc of Earth according to one popular model of gamma-ray production by hadronic interactions ([Drury et al. 1994]). Approximately, twenty shell-type SNRs with known distances lie within this distance range, permitting VERITAS to investigate which characteristics of SNR are necessary for them to be sites of particle acceleration.