Observations of Supernova Remnants and Pulsar Wind Nebulae: A VERITAS Key Science Project

The relics of supernova explosions, from the highvelocity blast waves to the rapidly spinning, highly magnetic compact stars produced in some events, play a crucial role in high-energy astrophysics. In addition to seeding the Galaxy with metal-rich stellar ejecta, the high Mach-number shocks in supernova remnants (SNRs) are almost certainly the principal source of cosmic-ray acceleration to energies approaching the knee of the cosmic-ray spectrum. However, while the supernova birthrate and overall energetics appear adequate to yield the observed energy density of cosmic rays, it remains unclear exactly how the acceleration occurs. Radio observations of SNRs demonstrate clearly that electrons have been accelerated up to ∼ GeV energies, but it is only in the past decade that X-ray observations have demonstrated the presence of electrons with energies up to ∼ 100 TeV. However, it is the ions that dominate both the energy density of the cosmic rays and the dynamical evolution of the SNR shocks. Therefore, it is of fundamental importance that, to date, there is only ambiguous evidence of ion acceleration in SNR shocks. Detections of very-high-energy (VHE, E > 100 GeV) gamma rays from SNRs offer the opportunity for direct detection of the ion component through the decay of neutral pions that have been produced in energetic protonproton collisions; these collisions should be particularly evident in the vicinity of dense molecular clouds [9]. By combining the TeV spectrum with that in the radio and X-ray bands, this process can be compared with inverse-Compton scattering models to determine whether the TeV emission is actually associated with the electrons or the ions. In either case, strong constraints can be placed on the acceleration process (see, e.g., [4]).

For supernova events that result from the collapse of massive stars, the relic neutron stars that are typically formed are sources of extremely energetic radiation as well. Particles accelerated in the pulsar magnetosphere stream outward, accompanied by Poynting flux from the rotating magnetic field, and filling a bubble whose expansion is restricted by the surrounding ejecta. As the energetic particle wind meets this restricted flow, a wind termination shock forms, at which additional acceleration occurs. Curiously, while models for particle acceleration in the magnetosphere predict a wind in which the particles carry only ∼ 10 -4 of the energy flux, the spectrum and dynamics of the downstream wind nebula require a particle-dominated wind. The composition of the wind changes drastically between the pulsar light cylinder and the region downstream of the termination shock, but how this happens is not yet understood. The broadband spectrum of the particles in the nebula strongly constrains the process by which this conversion occurs. For all pulsar wind nebulae (PWNe), there is a change in spectral slope between the radio and X-ray bands. Whether this change is due to a simple break due to synchrotron aging, or to a more complicated electron spectrum is not known for most PWNe. Yet, at least for those with somewhat low magnetic fields, emission in the TeV band holds crucial information. This is because the electrons that produce synchrotron radiation in the ultraviolet band, which probes the region below the steeper X-ray spectrum (but is virtually always unseen because of Galactic absorption), also produce TeV gamma rays through inverse-Compton scattering of the microwave background. Observations of PWNe at TeV energies thus offer an opportunity to investigate a key portion of the spectrum and place constraints on the acceleration and evolution of particles in these energetic systems.

VERITAS defined four Key Science Projects for its first two years of operations, in the areas of Active Galactic Nuclei, the search for dark matter, a survey of the Cygnus region of the Galactic plane, and the study of supernova remnants and pulsar wind nebulae. These were recognized as areas addressing high-priority science questions, questions for which specific targets or observing strategies and a substantial investment of observing time would be required. This paper summarizes the state of the VERITAS program of dedicated observations of SNRs and PWNe prior to the ICRC. It complements the VERITAS survey of the Cygnus region, which contains a number of other interesting SNRs and PWNe such as γCygni and CTB 87. This survey spans Galactic longitude 67 < l < 82 and latitude -1 < b < 4 and is discussed elsewhere in these proceedings [25].

VERITAS [15] consists of four 12-m telescopes located at an altitude of 1268 m a.s.l. at the Fred Lawrence Whipple Observatory in southern Arizona, USA (31°40’ 30" N, 110°57’ 07" W). Each telescope is equipped with a 499-pixel camera of 3.5°field of view. The array, completed in the spring of 2007, is sensitive to a point source of 1% of the steady Crab Nebula flux above 300 GeV at 5 σ in less than 50 hours at 20°zenith angle.

Candidate SN

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