GLAST

Home page of the Experiment: http://www.pi.infn.it/glast/index.html

The topics GLAST will address include:

A statistically powerful characterization of Active Galactic Nuclei (AGNs).

The power output of AGNs which are currently understood to be super-massive black holes surrounded by accreting matter is enormous ( 1045 erg/s) and time-variable. Furthermore AGN are often ”blazar” class sources, that is they send most of their photon energy to us in the GeV range and above. EGRET has observed about 50 AGNs; extrapolating GLAST should see more than 5000 allowing us to learn much more on how these huge accelerators work. In addition by characterizing the energy distribution as a function of AGN distance we can probe the density of ambient IR and optical light (called the extra galactic background light or EBL) in the early universe (since higher-energy gammas will pair convert with these photons). Probing the ambient EBL density will help us to determine the era of galaxy formation.

Transients.

The high-energy gamma-ray sky is remarkably variable with fluxes changing by more than an order of magnitude in days or even hours. GLAST will be enormously important for the study of these transient phenomena. First with its large effective area, wide field of view and highly efficient duty cycle GLAST will detect transients quickly and can provide an alarm for a wide variety of detectors that either re-orient due to a limited field of view or must rely on statistical cross-correlation due to low event rates. Understanding important phenomena such us AGNs requires a coherent program of complementary measurements (optical, radio, x-ray and ground based > TeV photon detectors, neutrino telescopes etc.) and GLAST will play a key role in tying that program together.

A variety of topics related to supernova remnants and pulsars.

Fermi's seminal work on cosmic-ray acceleration eventually led to the hypothesis that supernova blasts are responsible for the flux of galactic cosmic rays (below 1015 eV). If so the surrounding medium should be swept up by these shocks producing neutral pions and hence gamma rays. Such extended sources should be imaged in gamma rays. GLAST with its substantially better pointing resolution will add valuable information to that already obtained by x-ray and ground-based Cherenkov telescopes. In addition GLAST will be able to test models of high-energy particle acceleration by neutron stars.

Probing for particle dark matter.

It is possible that galactic dark matter is composed of yet-undiscovered particles such as the lightest supersymmetric particle (LSP) in R-parity- conserving SUSY models. If so there are a number of pos-sible processes involving these particles that will yield characteristic gamma ray signature.

Gamma-ray Bursts (GRBs).

GRBs were recently in the news when the Beppo-SAX satellite detected a few transient bursts sufficiently quick to alert other telescopes, which detected fading signals at the location of the bursts, were able to measure their redshift and thereby to determine that those bursts came from a cosmological distance. GRBs may thus be the most energetic phenomena in the universe but the underlying mechanism is still a mystery. EGRET has observed high-energy emission from a few bursts and even observed an 18-GeV photon from one of them 75 minutes later. Are these delayed high-energy emissions a signature feature of gamma-ray bursts? GLAST will answer this question.