Multi-Messenger Impact of HAWC
Multi-wavelength and multi-messenger observations are essential to understanding not only the gamma-ray sky, but also many other astrophysical phenomena. HAWC searches the TeV sky in real time for flaring sources and provides rapid notifications of flares. This early-warning system is designed to enable observations at other wavelengths or with more sensitive TeV instruments. For steady sources, HAWC provides a TeV flux or upper limit for all sources in a search region of 8 sr, or about two-thirds of the sky. In addition to quickly reporting flares, HAWC also provides a synoptic survey of the TeV sky and produces all-sky maps of the observed TeV gamma-ray sources.
HAWC is sharing data with (and following up on triggers from) many different experiments across the particle astrophysics community. These include:
- Imaging Air Cherenkov Telescopes (IACTs): VERITAS, MAGIC, HESS, FACT
- Neutrino Telescopes: IceCube, ANTARES
- X-ray Telescopes: SWIFT
- GeV Gamma-Ray Missions: Fermi-LAT
- Gravitational Wave Observatories: LIGO
The Fermi-LAT detector is measuring thousands of GeV gamma-ray sources, and many of these do not have obvious counterparts. HAWC provides a natural extension of the energy reach of Fermi-LAT to the TeV scale for the sources within the HAWC field of view.
Because it is a wide-field survey, HAWC is able to discover new TeV sources and monitor known sources. Follow-up IACT observations can do a deep search for source variability on very short time scales, map the morphology of the sources, and constrain the spectra at lower energies than HAWC. At the high energies above 10 TeV, HAWC data can be used to extend the spectra measured by IACTs.
HAWC and IceCube, a TeV-PeV neutrino observatory, observe the same range of energies and the same part of the sky in the northern hemisphere. Because cosmic ray proton cascades produce comparable fluxes of photons and neutrinos at similar energies, the sources detected by HAWC provide excellent search candidates for neutrino emission by IceCube. Just like gamma rays, neutrinos are not deflected by cosmic magnetic fields and therefore point back to where they came from. By providing target souces, the HAWC catalog improves the sensitivity of TeV neutrino observatories such as IceCube and ANTARES (as well as future observatories such as KM3Net) by more than a factor of two.
HAWC is being used to search for electromagnetic counterparts to gravitational waves (GWs) observed by LIGO, VIRGO, and other gravitational wave detectors. The large field of view and high duty cycle provided by HAWC makes the detector well-suited to search for TeV counterparts to GW signals, particularly because the current generation of GW observatories provide poor localization of GW events.
Finally, the ultra high energy cosmic ray (UHECR) observatories Auger, HiRes, and Telescope Array have observed the GZK cutoff in the spectrum. The most likely explanation is that those UHECR interact and lose energy on their way to Earth, which implies that the highest energy cosmic ray origins are within ~100 Mpc of earth. An anisotropy in the UHECRs might be detectable by Auger but would be blurred by the deflection of charged cosmic rays in magnetic fields. However, some of the UHECRs will interact near their sources and produce gamma rays, allowing us to use gamma rays to investigate the origin of high-energy cosmic rays. HAWC is searching for TeV emission from potential classes of cosmic ray sources such as nearby AGNs or galaxy clusters, and with its angular resolution can determine which of these AGNs emit TeV gamma rays. These TeV sources are likely the sources of UHECRs as well.