High-Energy Astrophysics

The CALorimetric Electron Telescope (CALET) has been collecting cosmic ray and gamma-ray data on the JEM-EF external platform of the International Space Station since mid-October 2015. As of January 2024, more than 4 billion triggered events > 1 GeV and > 1.9 billion events > 10 GeV have been accumulated with a live time fraction of 86%.  The main CALET detector consists of a deep (30 radiation lengths) calorimeter designed to measure the cosmic ray electron spectrum up to 20 TeV (2 × 10^13 eV), where features in the spectrum may indicate the presence of nearby sources or dark matter. As of 2024, the high energy electron spectrum is showing initial indications of nearby sources (e.g., supernova remnants including Vela, the Cygnus Loop, and Monogem). In addition to its primary goal to explore the high energy electron spectrum, CALET can study the cosmic ray hadronic sector by contributing high precision measurements of the energy spectra, relative abundances, and secondary-to-primary ratios of elements from hydrogen to iron. Deviations from a simple power law, as reported by CREAM, PAMELA, and AMS-02 in the spectra of light nuclei, are under study from a few tens of GeV to the multi-TeV region. As an example, a flattening observed in the boron-to-carbon ratio near 1 TeV suggests that the hardening of the ratio could be due to extra material traversed near the source, producing an excess of secondary boron very early in propagation. Finally, CALET is observing X-rays and gamma rays from 7 keV to 10 MeV and 1 GeV up to 10 TeV, providing a capability for high energy counterpart searches for gamma ray bursts in coincidence with LIGO/Virgo  gravitational wave events.

Time-domain astronomy investigates dynamic celestial phenomena, including gamma-ray bursts (GRBs), intense flashes of gamma rays revealing high-energy processes. Multi-messenger astronomy combines high-energy sources with high-energy neutrinos and gravitational waves, offering diverse insights into cosmic events. At LSU we work with experts of gravitational wave detection at LIGO-Livingston facility. Scientists at LSU co-lead the Interplanetary Network which contributes by localizing transient events, enhancing our ability to pinpoint the sources of GRBs across the sky, with the newest instrument launching on board the Psyche spacecraft in Fall of 2023. Future transient missions where LSU plays an important role include the StarBurst mission, launching in 2026. This comprehensive approach advances our understanding of the universe's most energetic and dynamic occurrences. 

Gamma-ray astronomy delves into the high-energy realm, studying celestial sources emitting gamma rays. Instruments like Fermi Gamma-ray Space Telescope, equipped with the Large Area Telescope (LAT) and Gamma-ray Burst Monitor (GBM), play pivotal roles. LAT surveys the sky for high-energy gamma rays, while GBM detects transient bursts. Complementary to Fermi, instruments like the Compton Spectrometer and Imager (COSI) will soon contribute to our understanding of gamma-ray sources. COSI observes gamma rays from nuclear processes, providing insights into cosmic nucleosynthesis. Our team is also involved in development of new concepts for future space-based gamma-ray missions such as APT and AMEGO-X. In particular, our LSU group is collaborating on the construction of the Antarctic Demonstrator for the Advanced Particle-astrophysics Telescope (ADAPT) balloon mission to be launched from Antarctica as a prototype of a future APT Compton Telescope to observe MeV gamma-ray transients and provide sensitivity for thermal WIMP dark matter. The collaborative efforts of these instruments significantly enhance our comprehension of the energetic processes shaping the gamma-ray universe.

High-energy polarimetry is a specialized field in astronomy that focuses on measuring the polarization of high-energy photons, shedding light on the magnetic and geometric properties of cosmic sources. The Imaging X-ray Polarimetry Explorer (IXPE) is a crucial instrument designed to enhance our understanding of polarized X-rays. Complementary to IXPE, the Compton Spectrometer and Imager (COSI) will also contributes to high-energy polarimetry, providing valuable insights into the polarization properties of gamma-ray sources. These instruments collectively enable astronomers to probe the underlying physical processes in extreme environments, offering a more comprehensive understanding of the intricate magnetic and structural characteristics of celestial objects emitting high-energy radiation.

In addition to its primary high energy capabilities, CALorimetric Electron Telescope (CALET) performs a continuous monitoring of the Low Earth Orbit (LEO) radiation environment, including high-energy solar particle (SEP) events, inner Van Allen belt protons in the South-Atlantic anomaly, and relativistic electron precipitation (REP) events. CALET is used to identify and categorize potential REP events out of the CALET database, investigating their relationship with different magnetospheric drivers, such as electromagnetic ion cyclotron (EMIC) waves. We take advantage of magnetically-conjugate plasma-wave measurements from the NASA Van Allen Probes (VAPs) in the equatorial plane or the Arase Japanese satellite in highly-elliptic orbit. We use the large sample of REP events collected by CALET since its launch for statistical studies, exploring the correlation of their properties with solar-wind and geomagnetic conditions.

The TGF and Energetic Thunderstorm Rooftop Array (TETRA) experiment at LSU observed ground-level X-ray/gamma-ray bursts associated with nearby lightning. Satellite observations of intense millisecond flashes of gamma rays (Terrestrial Gamma Flashes, TGFs) produced at the tops of thunderstorms have shown that lightning can accelerate electrons to very high energies, well above tens of MeV. TETRA, with a set of detectors at LSU, in Panama, and in Puerto Rico (locations with high thunderstorm activity) produced a catalog of TGFs produced by lightning near the Earth’s surface and provided data on the detailed characteristics of the energetic radiation and the associated thunderstorms, in particular correlated radiation and meteorological data.