Physics with Milagro
High-energy gamma-ray astronomy uniquely probes non-thermal, energetic acceleration processes in the universe. Gamma rays allow us to view the sources of relativistic charged particles. The list of established gamma-ray sources includes active galaxies, supernova remnants, and gamma ray bursts (GRB). Gamma rays are also produced when high-energy cosmic rays interact with matter in our galaxy. Other potential sources include more esoteric objects such as evaporating primordial black holes, topological defects, and dark matter particle annihilation and decay.

Cosmic gamma-rays with energies up to 10-100 GeV can be directly detected with satellite-based detectors, such as EGRET or GLAST. EGRET gave us a new view of the sky at high energies. Among the many important discoveries was the discovery of gamma emission from a new class of extra-galactic objects, high-energy gamma-ray AGN, blazars, that emit a significant fraction of their energy at gamma-ray energies. At very high energies (VHE), the gamma-ray flux from even the brightest source is too low to be measured in the relatively small detectors that can be placed on satellites: thus earth-based techniques are used. High-energy gamma rays interact high in the atmosphere, producing a cascade of particles, called an extensive air shower (EAS). Ground-based gamma-ray telescopes detect the products of an EAS that survive to ground level, either the Cherenkov light produced in the atmosphere by the shower particles (by Atmospheric Cherenkov Telescopes or ACTs) or the shower particles that reach ground level (by EAS-arrays).

The excellent angular resolution and sensitivity of ACTs make them ideal to study steady VHE emission as well as short-term flaring from known sources. However, ACTs can only be used on clear, dark nights, and have a relatively narrow field of view, ~3 deg. Thus they are not well suited to perform an all-sky survey, to monitor on a daily basis a known source for episodic emission, or to search for emission from a source at an unknown direction (such as from a GRB), or an a still undiscovered category of gamma-ray source. On the other hand, an EAS-array can operate 24 hours per day, regardless of weather, and can observe the entire overhead sky; an EAS-array is able to observe every source in its field of view every day of the year. Previous EAS scintillation arrays have been sensitive to showers above 10's of TeV while Milagro is the first EAS detector which has a peak sensitivity near 1 TeV. The vastly superior aperature and exposure of the EAS-arrays makes them ideal instruments for the study and potentially discovery of GRBs in VHE gamma-rays.

VHE gamma rays are a natural byproduct of most GRB production models and are often predicted to have comparable fluence at TeV and MeV scales , . This is due to the fact that the MeV emission from GRBs is likely due to synchrotron radiation produced by highly accelerated charged particles within the strong magnetic field of a jet with bulk Lorentz factors exceeding 100. In such an environment, the inverse Compton mechanism for transferring energy from charged particles to gamma rays is likely to complement synchrotron radiation and produce a second VHE component of GRB emission with fluence possibly peaked at 1 TeV or beyond. Whether or not the inverse Compton mechanism contributes minimally or even dominates the energy production depends on the environment of the particle acceleration and the gamma-ray production. VHE measurements may be critical to the understanding of gamma-ray production in GRBs similar to the manner in the TeV measurements by ACTs have resolved the degeneracy between magnetic field and electron energy in blazars.

Detection of a VHE component of gamma-ray emission would also provide the most sensitive measurement to date for the constancy of the speed of light as a function of energy. Some quantum gravity theories predict a breakdown of Lorentz invariance observable as an energy dependent speed of light, c --> c' = c + E/ alpha . Such an effect is best detected through the observation of high energy gamma rays traveling over cosmological distances. The effect of the breakdown would be temporal dispersion of short duration VHE gamma-ray signals. Currently the best limits on alpha, come from a TeV flare from a nearby blazar . The flare duration was 15 minutes with an energy scale of ~2TeV and originated at a relatively nearby blazar with a redshift of 0.03. Measurement of a short duration VHE GRB signal at a redshift z=0.1, a duration of 1s and with similar gamma-ray energies would provide a laboratory to measure alpha with a sensitivity 3 orders of magnitude better.