The High-Altitude Water Cherenkov Gamma-Ray Observatory

The Discovery Potential of TeV Surveys with HAWC

When wavelength bands are explored with unprecendented sensitivity, previously unknown sources and unknown types of sources are discovered. For example, the EGRET catalog contains over 150 previously unidentified sources, HESS has discovered several sources with no known counterparts, the Fermi-LAT saw two huge lobes of gamma-ray emission, and Milagro has detected at least 3 new Galactic sources with no obvious counterpart. The discovery of new classes of objects unobserved at other wavelengths is a major strength of all-sky monitors. These serendipitous discoveries, while not possible to predict a priori, are frequently the most important scientifically. Examples of investigations with discovery potential being done with HAWC are described here.

Cosmic Ray Anisotropy

Analysis of Milagro data indicates a part-per-mille excess in the TeV cosmic-ray flux on angular scales of 10° with nearly 15σ significance. We expect the spatial distribution of cosmic rays to be isotropic, so observing any anisotropy would give us insight into the distribution of cosmic ray sources. Since HAWC can also detect cosmic rays, HAWC can measure the energy spectrum of this anisotropy as well as search for smaller fractional excesses. Such detailed information is needed to explain how charged cosmic rays in the interstellar magnetic field can produce such small scale anisotropy.

Return to top.

Galaxy Clusters

Some fraction of the immense gravitational energy in a cluster of galaxies is predicted to result in shocks that will accelerate electrons and protons up to 1018 eV. Both accretion and merger shocks will accelerate particles, with the former being more efficient at producing the high-energy particles. While individual galaxy clusters can be observed, the unknown history and masses of the clusters makes the prediction of gamma-ray fluxes difficult. There are about 600 galaxy clusters nearer than redshift z=0.1, and the angular extent of the emission is expected to be up to 1° in some cases. Galaxy clusters form within massive dark matter halos, so we can search for Indirect Detection of Dark Mattergamma rays produce from dark matter annihlation or decay. HAWC is observing all clusters in the Northern sky and detecting gamma rays from galaxy clusters would give us new information on the particle physics going on in these systems.

Return to top.

Galactic Pair Halos

Very high energy gamma rays from AGNs are attenuated on their way to Earth through interactions with the extragalactic background light (EBL). However, those interactions are expected to produce high-energy electron and positron pairs. These charged particles would scatter due to the intergalactic magnetic field, causing them to divert from the straight path between the AGN and Earth (recall that photons, which are neutral, are not deflected by magnetic fields and point back to where they came from). Some of the high-energy electrons and positrons will interact with low energy photons (like from the Cosmic Microwave Background) and tranfer their energy producing gamma rays. These secondary gamma rays would have lower energies than the original gamma ray that interacted with the EBL and also point slightly away from the AGN. Therefore we expect AGN to have gamma-ray halos extending to nearly 1° that are produced by pair production of even higher energy gamma rays near the source. The detection of galactic pair halos with HAWC would provide a measure the extragalactic background light at different redshifts and the intergalactic magnetic field, probing the cosmological evolution of the Universe. So far no definitive AGN pair halos have been detected.

Return to top.

Nearby Galaxies

As in our own Galaxy, TeV gamma rays should be produced by cosmic ray interactions with matter in other galaxies. Some galaxies may have an enhanced flux of cosmic rays and gamma rays with respect to our own — for example, the very luminous class of intense star-forming regions known as Starburst galaxies. Also, other galaxies may have different electron and hadronic cosmic ray fluxes from the Milky Way. The large field of view of HAWC allows many potential TeV sources to be studied and compared.

Return to top.

Galactic Center

The center of our Galaxy is a known TeV source, yet the origin of these gamma rays is unknown. While the Galactic center transits over the HAWC site at a zenith angle of 49°, at the edge of our sensitivity, HAWC is still sensitive to the highest-energy gamma rays from the Galactic center. HAWC should be able to extend the spectrum of gamma rays from this region to 100 TeV, and to search for variability in the TeV emission. Since a potential dark matter signal is expected to be steady, any variability or flaring behavior from the Galactic center would rule out dark matter as the dominant origin of the emission. It would also be difficult to explain variable emission with hadronic models of gamma-ray production.

Return to top.

Molecular Clouds

Gamma rays are produced by cosmic ray interactions with matter, which in our Galaxy is concentrated in molecular clouds filled with simple diatomic molecules. Because they are close to Earth, the cosmic-ray flux in these clouds is assumed to be the same as the flux at Earth. If this assumption is correct, the gamma-ray flux can be used to constrain the properties of the molecular clouds, such as the ratio of CO to H2. By conducting a synoptic survey of the sky, HAWC can help identify the angular extent of known molecular clouds and discover new ones.

Return to top.

Compact Binaries

Black holes or neutron stars orbiting a massive star are known to siphon stellar material onto an accretion disk. Shocks produced during accretion are likely to accelerate charged cosmic rays. These kinds of binary systems emit TeV gamma rays, and the TeV emission from several has been observed to be modulated by the orbital period of the sytem. The variability implies a small source region and hence a high optical depth for gamma rays, yet the TeV emission extends to high energies with a hard spectral index.

Over 100 X-ray binaries have been cataloged with orbital periods ranging from hours to years. Daily observations using HAWC are essential to observe all phases of such binary systems in the TeV band.

Return to top.


A binary system that exhibits jet-like behavior is referred to as a micro-quasar and provides a test of jet physics on shorter time and size scales than AGNs. These objects have been known to flare at radio and X-ray wavelengths. At the 2008 International Cosmic Ray Conference, the MAGIC collaboration announced such a TeV flare for the microquasar and black hole Cyg X-1. This flare preceded an X-ray flare, but the statistical significance was weak. Nearly a dozen microquasars are known and HAWC is searching for TeV flares from these objects.

Return to top.

Other Transient Galactic Sources

Surveys of the Galactic plane by sensitive imaging air Cherenkov telescopes (IACTs) can miss transient sources because these telescopes scan their few square degree field of view with observations of short duration. A large fraction of the EGRET unidentified sources at low Galactic latitudes are variable, indicating new classes of gamma-ray emitters which could extend to higher energies.

Return to top.

Solar Energetic Particles

Our Sun is the nearest astrophysical particle accelerator. Solar particles in excess of 10 GeV have been detected by Milagro in association with coronal mass ejections. HAWC is similarly capable of studying energetic solar particles and the dynamics of the inner heliosphere.

Because of the low geomagnetic latitude of Mexico, measuring the tails of the high-energy proton and neutron distributions provide new diagnostic capabilities for investigating coronal shock acceleration. The HAWC measurements can be compared to data from ground-level neutron monitors located only one kilometer away at Sierra Negra in order to extend observations of solar energetic particles to the highest energies.

It is also possible to perform measurements of solar weather with HAWC. Large-scale magnetic structures inside the inner heliosphere modulate the Galactic cosmic ray flux at Earth. Measurements of the cosmic-ray flux and anisotropy with HAWC give detailed information about these phenomena.

Return to top.

Indirect Detection of Dark Matter

Astronomers and physicists have accumulated decades of evidence in favor of the existence of Dark Matter (DM), but the particle nature of the DM is still unknown. Two leading candidates for DM are:

  1. Weakly Interacting Massive Particles, or WIMPs
  2. Axion-like particles

HAWC is being used to search for both WIMPs and axion-like particles. WIMPs are expected to create the gravitational wells of the Milky Way and its satellite galaxies, where they can interact and self-annihilate. If the WIMP mass is above 1 TeV, these interactions can produce TeV gamma-rays. HAWC is sensitive to several high-mass WIMP Models. Therefore, HAWC will either discover gamma-ray evidence for specific WIMP models or place strong limits on the dark matter mass and cross section above 1 TeV.

Axions and Axion-like particles (ALPs) couple to gamma-rays in the presence of magnetic fields. Therefore, gamma ray conversions into ALPs would produce detectable signatures in the otherwise smooth spectra of gamma ray sources like distant AGN. Axions may also be produced with neutron stars and then decay to gamma rays causing the neutron stars to appear as gamma ray point sources.

Gamma rays from high-redshift objects are expected to be attenuated by interactions with extragalactic background light (EBL). However, if TeV gamma rays oscillate into axions in the strong magnetic fields inside an AGN, they can pass unattenuated through the EBL before oscillating back into TeV photons in the magnetic field of the Milky Way. Therefore, the observation of TeV gamma rays from very distant (z~1) AGNs can provide indirect evidence for the existence of axion-like particles.

Return to top.

Lorentz Invariance Violation

The combination of cosmological distances and rapid variability make short-duration transients such as gamma ray bursts a unique laboratory to study the dependence of the speed of light on the energy of the photon.

Theories of quantum gravity predict a time delay Δt for photons of energy E1 and E2 traveling a distance L of

Δt ~ L(E1-E2)/EQG = 40·z·ETeV.

EQG is an energy scale at which Lorentz invariance would be non-negligible, ETeV is the energy in TeV of the highest energy photons detected, and z is the redshift of the GRB.

At the maximum energy allowed by photon absorption on the extragalactic background light, HAWC should be able to probe the scale of the Planck mass (1019 GeV) given a time delay of one second at TeV relative to simultaneous observations in the keV/MeV band.

Recent observations of flaring of Mrk501 (z=0.034) on time scales of one minute with the MAGIC telescopes show evidence of such time delays which could be due to Lorentz invariance. However, a single measurement can only set a stringent limit. Multiple flares or bursts observed from sources at various redshifts would allow differentiation of source effects from a violation of Lorentz invariance.

Return to top.