Astroparticle Physics Research

Gamma Rays

Gamma-ray photon energies range from 20 keV and up, encompassing the highest energies in the electromagnetic spectrum. Produced in extreme astrophysical environments, celestial gamma-rays are not easily scattered or destroyed offering direct tracers of fundamental physical processes. LHEA scientists and engineers have had extensive involvement in gamma-ray instrument development incorporating scintillator and spark chamber technologies as well as recently developed solid state cadmium zinc telluride array detectors. Past LHEA supported gamma-ray missions include the GRIS balloon experiment and the Compton Observatory's EGRET instrument. Present and future missions include TGRS/WIND, INTEGRAL, Swift, and GLAST. The unique LHEA supported GCN program currently provides rapid world wide electronic notification of transient events related to gamma-ray astronomy.

What shines in the gamma-ray sky? Lab research groups are also involved in theoretical and observational gamma-ray astrophysics in areas including;

Supernovae and Supernova Remnants - Gamma-ray emission lines produced by the decay of radioactive isotopes in the expanding stellar debris cloud detail the dynamics of the expansion and can trace young supernova remnants throughout the galaxy. In addition, gamma-ray emission from supernova remnants may identify them as a source of galactic cosmic rays.

Geminga pulsar light curve Pulsars - Gamma-ray observations of pulsars, rapidly rotating neutron stars, constrain emission models while probing physical laws under the conditions of extreme gravitational and magnetic fields.

Galactic Center - The electron-positron annihilation feature at 0.511 MeV is used to probe sources and conditions in the galactic center region.

EGRET all-sky map above 100 MeV Galactic Diffuse Emission - Produced as energetic cosmic rays illuminate interstellar clouds, diffuse gamma-ray emission can be used to infer the origin and flux of cosmic rays within the galaxy.

Gamma-ray Bursts - The most powerful explosions in the universe, the origin of gamma-ray bursts represents one of the biggest mysteries in modern astrophysics. Clues are sought in the locations, light curves, and spectra of the bursts and their afterglows.

Gamma-ray image of 3c279 Active Galaxies - Gamma-ray flares from active galaxies were first discovered by EGRET in 1992 . Time resolved gamma-ray observations are required to explore the physics and energetics of the flares.

Solar Flares - Gamma-ray spectroscopy of strong solar flares reveals powerful particle accelerators producing nuclear emission and decay lines.

Cosmic Rays

Cosmic rays, particles that have been accelerated to high energies, can originate at the sun, in interplanetary space, or in supernova remnants and other energetic events in the Galaxy and beyond. Although cosmic rays have been studied for many decades, the specific sources, acceleration processes, and their propagation are still not well understood, and are the subject of ongoing research here in LHEA. Cosmic rays are a unique sample of matter from different regions of the universe, as well as providing important probes of the dynamics and evolution of the heliosphere and the Galaxy. Technologies include silicon solid state detectors, silicon strip detectors, and magnet spectrometers with Cerenkov detectors, scintillators, and time-of-flight systems. Current and approved missions are EPACT on WIND, SIS and CRIS on ACE, IMPACT on Stereo and the ISOMAX, Nightglow, and TIGER balloon instruments. Future missions currently under study include ACCESS and OWL.

Specific scientific topics currently being studied include:

Cosmic Ray Composition - The relative isotopic and elemental abundances of cosmic rays provide a fingerprint of their sources. In addition, the abundance of several radioactive isotopes can determine important timescales, such as the lifetime of cosmic rays in the galaxy or the delay between nucleosynthesis and acceleration.

Antimatter - Secondary cosmic ray antiprotons have been observed for many years. Detection of primary antiprotons or heavier antinuclei, as yet unobserved, can test Grand Unified Theories or be evidence of dark matter candidates.

solar flare image Solar Energetic Particles - Current experiments seek to understand the acceleration process in both solar flares and Coronal Mass Ejections (CMEs). They are also seeking to understand why the abundance of 3He can vary by orders of magnitude from flare to flare and why heavy ions are frequently enhanced in 3He-rich events.

Electrons and positrons - These particles are unique in their ability to enable study of charge-sign-dependent solar modulation effects such as drifts in the spherical solar wind cavity. As with the antimatter, detection of primary positrons can give evidence for dark matter candidates or Grand Unified Theories.

diagram of energetic particles in the heliosphere Cosmic Rays in the Heliosphere - Particles are accelerated in the solar system at shocks where co-rotating high-speed solar wind stream interact with slower solar, at shocks driven by CMEs, and at the solar wind termination shock. In addition to the study of acceleration processes, cosmic ray transport in the heliosphere is investigated as a necessary component to the understanding of other cosmic ray observations. Cosmic ray intensity can also provide a diagnostic of solar wind conditions complementary to conventional plasma and magnetic field measurements.

OWL spacecraft viewing configuration High Energy Cosmic Rays - Acceleration of galactic cosmic rays by supernova remnants should be limited to about 10¹⁵ eV, because particles of higher energy cannot be contained, and therefore cannot be accelerated, within the remnants. This is supported by a change in the spectral shape at about this energy, which may indicate the presence of an additional source of cosmic rays, perhaps extragalactic in origin. The limits of supernova shock acceleration and the signature of any new sources are being actively studied.

Ultra High Energy Cosmic Rays - Cosmic rays have been measured with energies as high as 3 x 10²⁰ eV. How subatomic particles obtain these enormous energies is one of the biggest questions in Astrophysics. Energy loss due to interactions with the microwave background should restrict the origin of these particles to within 50 Mpc. Yet no sources are seen within this volume. Because these particles are extremely rare, determining what these particles are and where they come from will push current technological limits, but is one of the major goals for the future.