Astrophysics with GLAST
Blazars and Active Galactic Nuclei
EGRET discovered that blazar-class active galactic nuclei (AGNs) are bright and variable sources of high-energy gamma rays. In fact, the bulk of the luminosity for many blazars is emitted in GLAST's energy range. The emission is believed to be powered by accretion onto supermassive black holes at the cores of distant galaxies. GLAST will increase the number of known AGN gamma-ray sources from about 70 to thousands. Moreover, it will effectively be an all-sky monitor for AGN flares, scanning the full sky every ninety minutes. It will greatly decrease the minimum time scale for detection of variability, and will offer near-real-time alerts for spacecraft and ground-based observatories operating at other wavelengths. Using EGRET, AGN flares were measured to vary on the shortest time scales -- eight hours -- that were able to be determined with statistical significance.
Unidentified Sources
GLAST will enable identification of the EGRET sources for which no counterparts are known at other wavelengths by providing much smaller error boxes. More than 60% of the EGRET sources are unidentified. Considering their distribution on the sky, less than one third of these are extragalactic (probably blazar AGNs), with the rest most likely within the Milky Way. Recent work suggests that many of these unidentified sources are associated with the nearby Gould Belt of star-forming regions that surrounds the solar neighborhood. Apparently-steady sources are likely to be radio-quiet pulsars and GLAST will be able to directly search for periods in sources at least down to EGRET's flux limit. Transient sources within the Milky Way are poorly understood, and may represent interactions of individual pulsars or neutron star binaries with the ambient interstellar medium. Some of the unidentified EGRET sources may be associated with recently discovered Galactic microquasars. GLAST will be able to explore these source classes in detail.
New Particle Physics
The large area and low instrumental background of GLAST will also allow searches for decays of exotic particles in the early Universe and for annihilations of postulated weakly-interacting massive particles (WIMPs) in the halo of the Milky Way. Much of the isotropic background detected by EGRET will be resolved by GLAST into discrete AGN sources. A truly diffuse, cosmic residual would be a tremendous discovery and could relate to particle decay in the early Universe. Recent theoretical work suggests that annihilation emission from the lightest supersymmetric particle, a candidate Galactic halo WIMP, could be detectable with GLAST. The signature would be spatially diffuse, narrow line emission peaked toward the Galactic center.
Extragalactic Background Light
The sensitivity of GLAST at high energies will also permit study of the extragalactic background light by measurement of the attenuation of AGN spectra at high energies. This attenuation is from pair production with photons in the background light primarily produced by young stars at visible to ultraviolet wavelengths. Owing to the large size of the AGN catalog that GLAST will amass, intrinsic spectra of AGNs will be distinguishable from the effects of attenuation. The measured attenuation as a function of AGN redshift will relate directly to the star formation history of the Universe.
Gamma-Ray Bursts
GLAST will continue the recent revolution of gamma-ray burst understanding by measuring spectra from keV to GeV energies and by tracking afterglows. With its high-energy response and very short deadtime, GLAST will offer unique capabilities for the high-energy study of bursts that will not be superseded by any planned mission. GLAST will make definitive measurements of the high-energy behavior of bursts that EGRET could not. Time-resolved spectral measurements with GLAST, combining data from LAT and GBM, will permit determination of the minimum Lorentz factors and baryon fractions for the emitting regions, and distinguish between internal and external shocks as the mechanism for gamma-ray production, and may also permit gamma-ray-only distance determinations. The LAT and the GBM will detect more than 200 bursts per year and provide near-real-time location information to other observatories for afterglow searches. GLAST will have the capability to slew autonomously toward bursts to monitor for delayed emission with the LAT.
Pulsars
GLAST will discover many gamma-ray pulsars, potentially 250 or more, and will provide definitive spectral measurements that will distinguish between the two primary models proposed to explain particle acceleration and gamma-ray generation: the outer gap and polar cap models. From observations made with gamma-ray experiments through the CGRO era, seven gamma-ray pulsars are known. GLAST will be able to search for periodicities directly in all EGRET unidentified sources. Because the gamma-ray beams of pulsars are apparently broader than their radio beams, many radio-quiet, Geminga-like pulsars likely remain to be discovered.
Cosmic Rays and Interstellar Emission
GLAST will spatially resolve some supernova remnants and precisely measure their spectra, and may determine whether remnants are sources of cosmic-ray nuclei. Cosmic rays produce the pervasive diffuse gamma-ray emission in the Milky Way via their collisions with interstellar nuclei and photons. GLAST will also be able to detect the diffuse emission from a number of local group and starburst galaxies, and map the emission within the largest of these, for the first time. Spatial and spectral studies of the gamma-ray emission will permit the distributions of cosmic-ray protons and electrons to be measured separately and will test cosmic-ray production and diffusion theories.
Solar Flares
GLAST will have unique high-energy capability for study of solar flares. EGRET discovered that the sun is a source of GeV gamma rays. GLAST will be able to determine where the acceleration takes place, and whether protons are accelerated along with the electrons. The large effective area and small deadtime of GLAST will enable the required detailed studies of spectral evolution and localization of flares. GLAST will be the only mission observing high-energy photons from solar flares during Cycle 24.
Complementarity with Ground-Based Gamma-Ray Telescopes
GLAST in orbit will complement the capabilities of the next-generation atmospheric Cherenkov and shower gamma-ray telescopes that are planned, under construction, or beginning operation, such as ARGO, CANGAROO III, CELESTE, HESS, MAGIC, MILAGRO, STACEE, and VERITAS. These ground-based telescopes detect the Cherenkov light or air-shower particles from cascading interactions of very high-energy gamma rays in the upper atmosphere. They have very large effective collecting areas (> 100 m2), but small fields of view (~2 degrees, with the exception of MILAGRO) and limited duty cycles relative to GLAST. GLAST will monitor the whole sky on timescales of hours and will provide alerts when flaring AGNs are detected. Some of the next-generation Cherenkov telescopes will have sensitivities extending down to 50�eV and below, providing a broad useful range of overlap with GLAST.