High Altitude Water Cherenkov Experiment

The origin of the cosmic radiation has been a mystery since its discovery by Victor Hess in 1912. The cosmic-ray energy spectrum extends from a few GeV to above 10²⁰ eV. As yet there is no experimental proof of the transition from Galactic to extragalactic cosmic rays, though it is believed that cosmic rays below about 10^17.5 eV are of Galactic origin. While there is a consensus that supernovae (SN) explosions accelerate cosmic rays up to energies of ~10¹⁵ eV, experimental evidence has been difficult to obtain. The theoretical arguments are based upon the energy released in SN being sufficient to maintain the observed cosmic rays in the Galaxy, and the creation of strong shocks by SN enabling first order Fermi acceleration. Thus the tasks for future experiments are to confirm that supernovae are sites of the acceleration of hadronic cosmic rays up to the knee, and to determine the sources of the Galactic cosmic rays above 10¹⁵ eV.

The diffuse gamma radiation from our Galaxy also probes the origin of cosmic rays. This radiation is due to the interaction of hadronic cosmic rays with interstellar gas and subsequent decay of neutral pions, and the interaction of high-energy electrons with gas and radiation fields (radio, microwave, infrared, optical, UV and magnetic). If the distribution of matter and radiation is known through other measurements, knowledge of the diffuse emission allows one to measure the cosmic-ray flux and spectrum throughout the Galaxy. This information can be used to determine the regions within the Galaxy where particle acceleration has recently occurred.

Over 20 Active Galactic Nuclei (AGN) have been detected in very high energy (VHE) gamma rays and extreme flares of up to 50 times the quiescent flux have been observed. Gamma rays are produced via interactions of the high-energy electrons and/or protons with lower energy photons. There exist several models to explain the source of photons including: synchrotron emission by the same population of electrons, radiation from the accretion disk, and cosmic microwave background photons. Simultaneous observations using multiple wavelengths and multi-messenger approaches are required to distinguish among these models. Monitoring at VHE energies is an efficient mechanism to initiate such observations because the highest energy gamma rays exhibit the most extreme variability and probe the highest energy particles. HAWC will have the sensitivity to detect strong flares, such as those that have been observed from Markarian 421, at greater than 10σ in under 30 minutes.

The Fermi satellite has now observed both long and short gamma-ray bursts that emit multi-GeV gamma rays. No high energy cut off is observed in any of these GRBs, and the highest energy gamma ray observed in the three brightest bursts were emitted (i.e. corrected for the observed redshift) at energies of 70, 60, 94, and 61 GeV in GRBs 080916C, 090510, 090902B, and 090926 respectively. The highest energy gamma-rays require a bulk Lorentz factor of the outflow of nearly 1000 in order to have the rest-frame energies and photon densities be low enough to avoid attenuation by pair production interactions. The Fermi-LAT observations show the most intense GeV emission occurs promptly, and also extends longer than the emission at lower energies. A wide field of view, high duty factor observatory, such as HAWC, is required to observe this prompt emission and determine its extent at high energies especially for a burst such as 090510, in which the prompt emission was less than half a second in duration.

HAWC has the sensitivity to continue these observations into the VHE range. The effective area of HAWC at 100 GeV (~100m²) is more than 100 times that of the Fermi-LAT.

HAWC is a very sensitive detector for TeV cosmic rays. The large number of cosmic rays detected with HAWC forms an undesirable background in the search for gamma-ray sources, but it also permits precise measurements of small deviations from isotropy in the cosmic-ray flux. Over the last few years, cosmic-ray detectors in the northern and southern hemisphere have found anisotropy in the arrival direction distribution of TeV cosmic rays at the per-mille level. Since we expect the arrival directions of charged particles at these energies to be completely scrambled by Galactic magnetic fields, these deviations are surprising and imply that the propagation of cosmic rays from their sources to us is not understood. Mapping the arrival direction distribution of cosmic rays to study the anisotropy with increased sensitivity is a major science goal for HAWC.

High-energy astrophysical observations have the unique potential to explore fundamental physics. However, deriving fundamental physics from the astrophysical observations is complex and requires a deep understanding of the astrophysical sources. The astrophysics background must be understood in order to determine the deviations from this background due to new physics. In some cases, astronomers can help with the understanding of the astrophysical background, such as using supernovae as standard candles to measure dark energy. However, high-energy physicists will have to detect and explain high energy astrophysical phenomena in order to derive the fundamental physics. The HAWC deep survey of the TeV gamma-ray sky will provide an unbiased picture necessary to characterize the properties of the astrophysical sources in order to search for new fundamental physics effects. Examples of HAWC investigations include:

1. Constraining the existence of nearby dark matter. HAWC's unbiased survey of 2π sr of the TeV sky allows searches known and unknown dwarf spheroidal satellites of our galaxy. The number of satellites increases with decreasing mass so there could be very nearby clumps of dark matter, which would therefore have higher gamma-ray fluxes, but might not have optical counterparts. The known dwarf spheroidal galaxies have extents of up to ~1 degree which is well matched to HAWC's angular resolution of <0.5^o. A stacked analysis of these satellites would improve the limit because all will have the same gamma-ray spectra.
2. Testing Lorentz invariance with transient gamma-ray observations. Many quantum gravity theories predict that the speed of light depends upon the energy of the photon as: Δc/c = -(E/M_QGn)ⁿ where n=1 or 2. While M_QG may be the Planck mass (2.4x10¹⁸ GeV), some theories predict much smaller mass scales. For theories where n=1, the Fermi-LAT collaboration has set limits above the Planck mass, and HAWC will have similar sensitivity if a GRB is detected. For theories where n=2, the higher energy sensitivity of HAWC will lead to limits roughly an order of magnitude higher mass scale than is possible with Fermi-LAT.
3. Measuring the attenuation of astrophysical sources due to interactions with the extragalactic background light (EBL). HAWC will enable multiple sources to be observed in various flaring states to understand the intrinsic TeV spectrum. Current constraints on the EBL make a conservative assumption of a very hard intrinsic spectrum and are very close to the maximum allowed from galaxy counts. These observations have led to postulations of the existence of axions in order to reduce the attenuation of TeV emission from EBL.
4. Searching for exotic signals such as massive relic particles, e.g. supersymmetry Q-balls, and tau neutrinos. Special triggers will be developed, allowing HAWC to search for the slow moving and high dE/dx Q-balls and the horizontal air showers produced by tau neutrinos interacting in the nearby mountain.