Transient Sources and Extragalactic Cosmic Rays
The origin of extragalactic cosmic rays is still largely unknown. The most likely candidate sources fall into two classes: active galactic nuclei (AGNs) and gamma-ray bursts (GRBs).
Observing Transient Sources with HAWC
While AGNs (but not GRBs) have been observed in the TeV band, many questions surround the nature of TeV emission. For example, how does the TeV emission differ during the AGN quiescent state compared to the flaring state? What fraction of AGNs and GRBs emit GeV or TeV gamma rays? How much energy is available in these sources to accelerate cosmic rays?
Because these sources are transient, continuous observations are needed to answer these questions. This is possible using detectors with a large field of view, such as HAWC and Fermi Large Area Telescope (Fermi LAT). While the Fermi-LAT is optimized to observe GeV gamma rays, its sensitivity above a few hundred GeV is limited by the physical size of the instrument (0.8 m2). The HAWC experiment has the advantage of a much larger effective area at TeV energies, with about 100 m2 already at 100 GeV. These two instuments are complementary, with Fermi monitoring the GeV sky while HAWC is monitoring the TeV sky.
Attenuation by Extragalactic Background Light
Note that both the low- and high-energy thresholds of HAWC are critical for observing extragalactic sources. High-energy gamma rays are attenuated on their way to Earth by e+e- pair production on the extragalactic background light (EBL), the fossil light from stars and galaxies emitted throughout the history of the universe.
At TeV energies, γ-rays are strongly attenuated by the cosmic infrared background. The effect is energy-dependent: as the energy of the γ-rays increases, the universe becomes increasingly "opaque." This attenuation is illustrated in the figure at left, which shows the distance (vs. energy) at which 99% of γ-rays are scattered by the EBL before reaching Earth. Above 10 TeV it becomes quite difficult to observe γ-ray sources even at relatively low redshifts. The attenuation also has the effect of distorting energy spectra from distant sources, since the highest-energy γ-rays are scattered more than the low-energy γ-rays.
Since GRBs are located at cosmological distances from Earth, the highest energies of γ-rays from GRBs are likely to be in the 50 to 300 GeV range, depending on the redshift of the source. For this reason, the low-energy sensitivity of HAWC is essential for observing extragalactic sources at high redshifts.