The Science of HAWC

Introduction

What astrophysical sources accelerate cosmic rays? This nearly 100-year-old question is a primary objective for the field of high-energy particle astrophysics. This question is not just important because of its age, but also because of the broad impact of cosmic rays on many scientific fields. Cosmic rays have lead to, and may in the future lead to, a new understanding of particle physics. The accelerators of cosmic rays produce particles of energies far exceeding human capabilities. Black holes and intense gravitational and electromagnetic fields power these accelerators providing unique laboratories that cannot be replicated on Earth. Cosmic rays propagate throughout the Universe and serve as unique probes of the distant universe and dark matter. Cosmic rays influence and probe the dynamics of our Galaxy, where their energy density is comparable to that of starlight and electromagnetic fields. Outside of Physics and Astronomy, Biologists and climate scientists have even explored the effects of cosmic rays on evolution and the Earth's climate. The study of cosmic rays is truly multi-disciplinary.

High-energy gamma-ray observations are an essential probe of cosmic rays, because gamma rays are created by cosmic rays interacting near their origin. The resulting gamma rays travel in straight lines, unperturbed by the Galactic and extra-Galactic magnetic fields, and, unlike the charged cosmic rays, point back to their sources, providing the direction to the cosmic ray accelerator. In addition, the characteristics of the gamma-ray flux variability and spectra constrain the acceleration mechanisms and the environment of the accelerator. The highest energy gamma rays and the shortest timescales of variability provide the strongest constraints on the acceleration mechanisms at work in these sources. These two objectives high energy and transient observations are the primary scientific motivation for HAWC and are described here. In addition, HAWC's wide field of view provides a unique discovery potential. History has shown that astronomical surveys of new wavebands produce unexpected and amazing observations.

HAWC sensitivity compared to other experiments. The solid (dotted) line is 1 (5) years of HAWC exposure vs 50 hours for the IACTs. Note: HAWC sensitivity is for 2π sr vs a single source for IACTs, including the planned CTA.

HAWC Obervations up to 100 TeV Probe Galactic Cosmic Rays

Cosmic rays up to at least 103 TeV, and perhaps as high as 10^6 TeV, are of Galactic origin. The observed spectral break at 10^3 TeV -- the knee -- may be due to the spectrum of cosmic ray sources, the escape of the cosmic rays from the Galaxy, or a combination of the two effects. It has even been speculated that a single nearby source is the cause of the knee. Protons with energies of ~103 TeV colliding with molecular clouds or other matter will produce ~100 TeV gamma rays. These 100 TeV gamma rays will be observable by HAWC from individual point and extended sources as well as from the sea of cosmic rays interacting with matter in the Galactic plane. Supernova remnants (SNRs) have been postulated as the origin of Galactic cosmic rays largely because they have sufficient energy to provide the observed local cosmic ray energy density. SNRs also have sufficiently strong magnetic fields to trap particles long enough to accelerate them up to at least 10^14 eV. TeV gamma rays have been observed from SNRs, but also from other Galactic sources--pulsar winds and compact binaries. Which of these sources accelerate hadronic cosmic rays? What is the total power output of these Galactic accelerators? HAWC's observations up to the highest energies of many sources from different classes are essential to answer these questions. The highest energy observations are key to distinguishing gamma rays produced by electrons from those produced by hadrons. There are observational differences in the TeV gamma-ray spectrum from electron accelerators and proton accelerators accessible to HAWC. Electrons lose their energy more quickly than protons due to synchrotron emission and are therefore more difficult to accelerate to the highest energies. Also, the cross section for inverse Compton scattering decreases at higher energies, resulting in a break in the gamma-ray spectrum by at least 10-50 TeV. Gamma rays from hadronic cascades in the accelerating region, on the other hand, follow the power law spectrum of the particles initiating the cascades up to the highest energies.

Discovering and Understanding Extreme Galactic Accelerators

The flux of gamma rays decreases with increasing energy, thus a large effective area and long integration times are required to detect the highest energy gamma-rays.  HAWC's effective area is comparable to that of IACTs; however, HAWC observes every source in half of the sky for 1500 hours per year.  

Atmospheric Cherenkov telescopes typically observe sources for ~<50 hours and survey observations are ~10 hours.  As seen in Figure 2, at energies above ~6 TeV, the HAWC one year sensitivity is better than the sensitivity of a 50 hour VERITAS or HESS observation of a single source.  An IACT can only spend up to ~200 hours per year observing a single interesting source due to solar and lunar constraints.  However, even with maximum exposure, an IACT still wouldn’t be able to match HAWC sensitivity above 10 TeV.

Consequently, most of the HESS sources in the Galactic plane survey are not detected above 10 TeV. Figure 3 shows the spectra of one of the HESS sources and the capability of HAWC to clearly discern the difference between a continuation of this spectra and an exponential cut off of 40 TeV.  The spectra of the known TeV Galactic sources are hard with an average differential spectrum of index -2.3 as compared to the steeper Crab spectrum of index -2.6.   HESS detects the source in Figure 3 with 0.2 times the Crab flux above 200 GeV, but if the measured spectra continue to higher energies, then this source is as bright as the Crab above 100 TeV. HAWC would detect ~20 gamma rays > 100 TeV per year from such a source in its field of view.

Extended TeV Sources

HESS has observed that most Galactic sources are extended.  Figure 4 shows the sensitivity as a function of source size for HAWC and IACTs. When the source size is much larger than the point spread function, the sensitivity of the detector worsens because the background increases.  The current observations of HESS are clustered around their sensitivity limit, implying the existence of even more extended sources. For example, nearby sources would have a larger angular extent.

Milagro already sees evidence of TeV emission from the nearby pulsar wind nebula, Geminga, with an angular extent of 2.9 degrees, corresponding to a diameter of ~8 pc [ ] (See section 3.22).  HAWC will detect Geminga with a significance > 50 σ and will be able map out the spectrum of this source vs. the distance from the pulsar. The highest energy electrons should lose energy quickly as they propagate away from the source; this is not true for protons.  Therefore, if Geminga is an electron accelerator we will see a clear change in the spectral index of the resultant gamma rays the farther from the source we look.

Diffuse Emission from the Galactic Plane

EGRET and Milagro have shown that the Galactic plane is the brightest feature in the GeV and TeV sky, respectively.  While some of the emission is likely due to unresolved point sources, a large fraction is due to cosmic-ray interactions with the matter in the Galaxy. Gamma-ray observations are the most direct probe of the flux and spectrum of cosmic rays outside our solar neighborhood. Hadronic cosmic rays interacting with matter produce neutral pions that decay to give gamma rays, whereas electrons create high-energy gamma rays through inverse Compton scattering with infrared photons and the cosmic microwave background [ ]. In addition, processes not directly related to cosmic-ray production may also contribute to the diffuse emission. For example, self-annihilating super-symmetric dark matter could play a significant role as an additional emission component with a distinct spectral signature for HAWC to find.

HAWC will map the diffuse emission in the Galaxy at multiple energies to be able to distinguish both the energy and spatial differences between the leptonic and hadronic emission mechanisms.  The HAWC site is closer to the equator and can observe the inner Galaxy all the way to the Galactic center.  HAWC will thus be able to study nearby regions such as Cygnus at a distance of 1-2 kpc as well as the more distant inner Galaxy at ~10 kpc.  The Cygnus region could be dominated by a very few cosmic-ray accelerators whereas the cosmic rays from the inner Galaxy are from a large collection of sources and will reflect the cosmic-ray spectrum after propagation farther from their origins.  These regions are hundreds of square degrees and require the large field of view of HAWC.

The diffuse GeV and TeV gamma-ray flux recorded with EGRET and Milagro respectively are above predictions based upon the assumption that local cosmic rays are representative of those elsewhere in the Galaxy.  In order to match the EGRET data, the cosmic-ray density in the rest of the Galaxy must be two times higher than measured locally[ ].  Increasing the cosmic-ray density enough to match the Milagro data would violate the measured limits on the anti-proton flux.  However, unresolved TeV sources may be contributing to Milagro’s measurement of the flux from the Galactic plane.  For example in the Cygnus region, Milagro detects an excess, MGRO J2031+41, coincident with the largest matter density. This Milagro source is also coincident with TeV J2032+41, but the Milagro source is both brighter by a factor of 3 and more extended than the HEGRA source. Deeper HAWC and VERITAS observations of both the spatial and spectral morphology will determine whether other sources exist in this region and whether the more localized TeV source could be the accelerator of protons which illuminate the entire region. The combination of the diffuse sensitivity of HAWC with the deeper, higher angular resolution IACT follow-up observations provides the most efficient way to map the entire Galactic plane over all angular scales.

HAWC Observations of Transient Sources probe Extragalactic Cosmic Rays

The origin of extragalactic cosmic rays is unknown.  Very few sources are capable of accelerating particles up to 1020 eV.  The source size is either too small or the magnetic field too weak to contain the particles long enough for them to be accelerated to extreme energies.  Two classes of sources are likely candidates—active galactic nuclei (AGN) and gamma-ray bursts (GRBs).  

While the energy in Galactic SNR is well matched to the measured flux of Galactic cosmic rays, it is unclear what sources have sufficient energy to produce the extragalactic cosmic rays.  Both AGN and GRBs are variable and relativistically beamed with unknown opening angles. Do AGN have a quiescent flux or only flares?  What fraction of AGN and GRBs emit GeV or TeV gamma rays?  Because these sources are transient, to accurately estimate the energy available to accelerate cosmic rays it is essential to have unbiased observations of these sources.   

Large field of view detectors with continuous observations, such as HAWC and GLAST, are required to answer these questions.   HAWC will monitor the TeV sky and GLAST will monitor the GeV sky.  Figure 5 illustrates the complementary ability of HAWC to observe shorter time scale variations than GLAST and extend the energy range of observations beyond those of GLAST. If a GRB or an AGN flare is detected by HAWC and GLAST, the overlap will yield observations over 7 orders of magnitude in energy, all in the gamma-ray band.

The low energy sensitivity of HAWC is essential for observing extragalactic sources.  Gamma rays of energy Eγ  emitted at a redshift z are attenuated by pair production on the extragalactic background light and the optical depth is τ ~ z 4/3 (Eγ /90 GeV)3/2  for 0.1<z<2.   So for a source at z = 0.1, 0.5 or 1, the gamma ray flux is reduced by a factor of 1/e = 0.37 at Eγ = 700, 170, or 90 GeV, respectively. HAWC has a large effective area at low energies with ~100 m2 at 100 GeV.  The effective area increases with energy as a power law with index 2.6 as can be understood from basic electromagnetic shower theory or detailed simulations (see Section 5.2). HAWC’s sensitivity for different redshifts is shown in Figure 5 where the y-axis is the flux required prior to pair absorption in order to have a significant HAWC detection.  As also seen in Figure 5, HAWC’s sensitivity in units of energy fluence is better than that of GLAST > 10 GeV even for moderate redshift objects, resulting in many complementary overlapping observations with GLAST.

Active Galactic Nuclei

Active galactic nuclei (AGN) are supermassive black holes (~108 times the mass of the Sun) with luminosities that far outshine the rest of the galaxy in which they are located.  Blazars are a subset of AGN with jets of particles that are pointed towards the Earth, and these objects are highly variable and emit much of their energy in gamma rays. Different classes of blazers exist, and the different observational properties are not yet understood, but are likely to be tied to fundamental properties of these objects.   Gamma rays are produced by particles accelerated in shocks that propagate along the jets.  If the accelerated particles are protons, then gamma rays are most efficiently produced by hadronic cascades originating with a p+γ interaction.  The protons must have energies exceeding ~1018 eV making AGN possible sources of ultra high energy cosmic rays (UHECR).  However, electrons can also be accelerated and will radiate gamma rays via inverse Compton scattering.  In general, rapid variability favors electron acceleration while higher energies favor proton acceleration.
    HAWC’s continuous observation of the TeV sky will detect many flaring AGN at the highest possible energies. Every day, without solar or lunar constraints, HAWC provides unbiased monitoring of all blazers in the Northern sky, resulting in a unique ability to study the properties of the TeV blazar population. The long HAWC observations will determine the average flux as well as the duty factor of flares of different luminosities.  The power at different timescales is indicative of the size of the emission region.  Long periods could be the signature of a precessing jet caused by a binary black hole system. Such binaries are excellent candidates for gravitational wave detection.   HAWC’s improved sensitivity will better measure the fluxes of flares and detect shorter duration flares than Milagro (see Figure 15 for Milagro’s observation of Mrk 421). HAWC will promptly notify IACTs to allow even deeper observations of the flaring states resulting in even shorter transient observations.
    To date TeV emission has been observed from 19 AGN [ ]. EGRET observations showed that 70% of AGN were variable. [ ]  Since energy loss is more rapid with increasing electron energy, the TeV observations are predicted to exhibit even greater variability, which has been the case for Mrk421 and Mrk501. However, few flares have been observed from the newly discovered TeV AGN. The lack of TeV variability may simply be due to the lack of long time scale, continuous observations.   HAWC will provide these long-time-scale continuous observations by observing every AGN in its field of view everyday even when the AGN are up during the day and IACTs can’t look.
    The notable exception to the lack of observed TeV variability is the recent flare observed by HESS of PKS J2155-304 [ ].  This source was monitored with multiple short observations by HESS and was observed to flare to ~50 times its quiescent flux for one hour.  This source is at a redshift of 0.117 and is detected up to ~5 TeV with a differential photon spectral index -3.5, which does not vary with intensity.  Even with such a steep spectrum, HAWC will detect such a one hour flare source with >6σ.

TeV variability constrains both the acceleration process and the environment near the acceleration sites.  Since the variability timescale cannot be shorter than the light travel time across the emitting region, Γtvar > Re/c = (Re/Rs)x(2GM/c3), (where Γ is the bulk Lorentz factor of the emitting region, tvar is the timescale of variability, Re is the size of the emitting region, Rs is the Schwarzschild radius of the blackhole and M is the mass of the black hole), measurements of the fastest variability can probe the bulk Lorentz factor of the emitting region and the size of the emitting region.  For example, in the case of PKS J2155-304, if the emitting region is comparable in size to the Schwarzschild radius of ~20 AU for a 109 solar mass black hole, then the bulk Lorentz factor of the emission region must be ~100.  This would mean that a region the size of our solar system has been accelerated to 99.995% the speed of light by the black hole.  Such Lorentz factors are more typically associated with gamma-ray bursts, and about an order of magnitude larger than those normally associated with AGN. If AGN accelerate electrons that up-scatter synchrotron photons, then the TeV emission should be correlated with x-ray observations.  While several TeV flares follow this pattern, there have been “orphan” TeV flares that are easily detectable by HAWC where there is no commensurate change in the x-ray flux. HAWC naturally provides a mechanism for obtaining many such multi-wavelength datasets and will allow us to study orphan flares and the correlations between TeV gamma ray, x-ray, optical, and neutrino emission in detail. 

Gamma Ray Bursts (GRBs)

Emitting over 1052 ergs in gamma rays, gamma ray bursts are the most energetic phenomena known in the universe.  Like AGN, the emission is thought to be collimated in jets – however the bulk Lorentz factor of the particle flows may be as large as 1000 in GRBs whereas in AGN Lorentz factors are typically believed to be 10-30.  GRBs are transient, lasting from fractions of a second to ~1000 seconds. The duration distribution is bi-modal with the break between short and long bursts at ~2 seconds.  The progenitors of short and long bursts are different. The prevailing model of short bursts is the coalescence of binary neutron star systems and for long bursts the collapse of a supermassive star.  In both cases the energy source is the gravitational potential energy released by the accretion of matter onto a compact object.

The highest energy gamma ray conclusively detected from a GRB is an 18 GeV photon detected by EGRET roughly 90 minutes after the onset of the burst [ ].  In addition, a high-energy component in GRB 941017 that extended to 200 MeV with a spectral index of -1 [ ] has been cited as evidence for proton acceleration in GRBs, with the implication that GRBs accelerate protons to energies above 1018 eV.  However, detection of TeV gamma rays has proven difficult.  The best such evidence comes from the Milagrito detector [ ].  Theoretical considerations argue for the creation of >100 GeV gamma rays in GRBs [ ], [ ], [ ].  Gamma rays are produced by either leptonic or hadronic processes and yield multi-wavelength spectra similar to AGN.  Similar to AGN, GRB measurements of the evolution of the flux at different energies provide strong constraints on the magnetic fields, the circumburst medium, and the bulk Lorentz factors.

HAWC will detect multiple GRBs, if, as predicted, the TeV energy fluence is the same as the sub-MeV fluence.  GRBs occur roughly twice per day throughout the universe, so about 20 GRBs per year are within 20 degrees of HAWC’s zenith.  For this zenith angle cut, HAWC is sensitive to 10 second duration bursts greater than 1x10-5 ergs/cm2 out to z~1 as seen in Figure 6. Approximately ¼ of all bursts are brighter than 1x10-5 ergs/cm2 and dimmer bursts that are more nearby are also detectable.  Therefore, HAWC will detect  several GRBs per year under the assumption of equal fluence from TeV and sub MeV gamma rays.  Even if no TeV emission is detected by HAWC from GRBs, stringent upper limits will be placed that constrain emission models. It is important to note that because the effective are for HAWC at 100 GeV is over 100m2, if GLAST sees a single GRB photon above 100 GeV, HAWC will see hundreds.

Observations at the highest photon energies for a large number of gamma-ray bursts is necessary for a complete understanding of the acceleration processes and energy budgets of these extraordinary phenomena. This is crucial for our understanding of the astrophysics of gamma-ray bursts and also on whether they are the sources of ultra high energy cosmic-rays. Only a wide field of view, high duty factor observatory can make these observations as the key to success is the ability to “catch” a burst (i.e. have it within the field of view of an operating detector).  There is significant complementarity between IACTs and HAWC for the observation of gamma-ray bursts; the IACT’s low duty cycle of 10%, small field of view and the several 10s of second slew time after notification of a GRB, drastically limits the number of GRB that they could observe in the prompt phase. However, HAWC will alert the IACTs of extreme high-energy transient events, so that they can follow with sensitive TeV afterglow observations.

HAWC Survey Observations have Discovery Potential

When new wavelength bands are explored in astronomy, 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, 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 that will be done with HAWC are described here.

Cosmic Ray Anisotropy: Milagro observes an excess on scale of ~10o in the highest energy cosmic rays with nearly 15 σ significance as described in Section 3.4.  HAWC will be able to measure the energy spectrum of this anisotropy as well as search for smaller fractional excesses.  This more detailed information is needed to explain how charged cosmic rays in the interstellar magnetic field can produce such small scale anisotropy.

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 highest 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 ~600 galaxy clusters nearer than z=0.1.  The angular extent of the emission is expected to be up to 1 degree in some cases.  HAWC will observe all nearby clusters and determine which clusters emit VHE gamma rays.
Galactic Pair Halos: AGN will have gamma-ray halos extending to nearly one degree that are produced by pair production of even higher energy gamma rays near the source [ ].  These halos can come from AGN that do not have jets pointed towards Earth.  A HAWC detection of galactic pair halos would measure the extragalactic background light at different redshifts, probing the cosmological evolution of the Universe.
Nearby Galaxies: TeV gamma rays should be produced by cosmic ray interactions with matter in other galaxies just as in the Milky Way.  However, some galaxies may have an enhanced cosmic ray flux, such as starburst galaxies, or different relative electron and hadron fluxes and spectra than our galaxy [ ]. HAWC’s large field of view allows many potential sources to be studied.
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 at only 49 degrees, HAWC will still have sensitivity to detect this source in the highest energy gamma-rays, extending the spectrum to 100 TeV, and to search for variability.   A HAWC observation of variability would rule out a dark matter origin and would be very difficult for hadronic models.
Molecular Clouds: Gamma rays are produced by cosmic ray interactions with matter which is concentrated in molecular clouds.  Because they are nearby the cosmic-ray flux is presumed to be the same as at Earth, so the gamma-ray flux uniquely determines the only free parameter, which is the ratio of CO to molecular hydrogen.  While some of these clouds have large angular extent, smaller but dense clouds may still be undiscovered at high Galactic latitudes [ ].  The sky survey capability of HAWC is required to observe both the large angular extent of known molecular clouds and to discover new ones.
Compact Binaries:    Black holes or neutron stars orbiting a massive star likely accelerate particles by shocks produced in the accretion process. Three binaries have been observed to have TeV emission modulated by the orbital period. 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. HAWC’s daily observations are essential to observe all phases.  For example, HAWC would be able to distinguish the differing models of TeV emission at periastron for PSR B1259-63, as shown in Figure 7, which could not be tested due to the full moon interfering with HESS observations. 
Microquasars:  A binary 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 AGN.  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 will search for TeV flares both coincident and independent of other wavelengths.
Other Transient Galactic Sources:  The surveys of the Galactic plane by Imaging Atmosperic Cherenkov telescopes can miss transient sources because these telescopes scan their few square degree field of view with short duration observations at various locations. 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.
Solar Energetic Particles: Our Sun is the nearest astrophysical particle accelerator.  Solar particles in excess of 10 GeV have been detected by Milagro associated with coronal mass ejections. HAWC provides diagnostic and discovery potential in the area of energetic solar particles and the dynamics of the inner heliosphere.  HAWC with its greater sensitivity will be able to detect the weakest flux of protons.  Because of the low geomagnetic latitude of Mexico, measuring the tails of the high-energy proton and neutron distributions will provide new diagnostic capabilities for investigating coronal shock acceleration.  HAWC will work in concert with ground level neutron monitors only a km away at Sierra Negra, as Milagro has done with the Climax station, in order to extend observations of solar energetic particles to the highest energies.  Solar Weather: Large-scale magnetic structures inside the inner heliosphere modulate the Galactic cosmic ray flux at Earth.  HAWC measurements of the cosmic-ray flux and anisotropy will provide detailed information about these phenomena. Conversely, measurements of time-dependent cosmic-ray anisotropies are telltale signs of approaching coronal mass ejections not visible by other means. 
Indirect Detection of Dark Matter:  Supersymmetric models in high energy physics have provided a candidate particle for the dark matter in the lightest supersymmetric particle, the neutralino.  LHC experiments may be able to determine if this particle exists, but gamma-ray observations are required to know whether this particle is the dark matter. Depending on the mass of the neutralino, space-based detectors such as GLAST or ground-based gamma-ray detectors such as HAWC may be the most sensitive to search for the neutralino signature [ ].  While the Galactic center should have a large concentration of dark matter, there will also be clumps throughout the Galaxy of varying masses.  A nearby clump of sufficient mass could be detectable by HAWC. The spectrum of the gamma-ray flux and the spatial extent of ~ 1 degree provide unique signatures to distinguish nearby dark matter clumps from other gamma-ray sources [ ].  These objects will be bright only at gamma-ray energies and thus can only be found by survey the sky – this is ideal match to the capabilities of HAWC.
Lorentz Invariance:  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=40zETeV sec.  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.  HAWC detections at the maximum energy allowed by the absorption on the extragalactic background light of a one second time delay relative to keV-MeV lightcurve will probe EQG above the Planck mass (1019 GeV). Recent observations of minute time scale flaring of Mrk501 at z=0.034 by MAGIC [ ] show evidence of such time delays which could be due to Lorentz invariance. However, a single measurement can only set a stringent limit. The multiple flares or bursts that HAWC will observe from sources at various redshifts would allow differentiation of source effects from a violation of Lorentz invariance.

Synergy with other High Energy Astrophysics Projects

Multi-wavelength and multi-messenger observations are essential to understanding the gamma-ray sky. HAWC will search the TeV sky in real time for flaring sources and notify the community within seconds of short-duration flares. This capability enables observations at other wavelengths or with more sensitive IACT observations.  For steady sources, HAWC will provide a TeV flux or upper limit for all sources within >2π sr.

GLAST is expected to detect thousands of gamma-ray sources [ ], and many of these will not have obvious counterparts. HAWC will provide a natural extension of the energy reach of GLAST to the TeV scale and beyond for the half of these sources within HAWC’s field of view.

HAWC will discover new TeV sources and monitor known sources.  Follow up IACT observations will reduce the duration of the shortest time scale variability observed, map the spatial morphology, and constrain the spectrum to lower energies. At the highest energies of 10-100 TeV, HAWC will extend the spectra of the IACT observations.

HAWC and IceCube, a TeV-PeV neutrino observatory, will observe the same range of energies and the same Northern hemisphere sky. Because proton cascades produce comparable fluxes of photons and neutrinos at similar energies, HAWC’s sources are excellent IceCube candidates as seen in Figure 8.  HAWC’s observations of flaring sources are thus very useful to select the direction and time interval from which to search for neutrino emission.  Such a selection can improve the sensitivity of IceCube by more than a factor of 2 by reducing the search trials.  Such triggering is particularly probing should HAWC observe an orphan AGN flare since these flares suggest hadron acceleration and neutrino production.

The ultra high energy cosmic ray (UHECR) observatories Auger and HiRes have observed the GZK cutoff in the spectrum 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 magnetic fields. However, some of the UHECRs will interact near their sources and produce gamma rays. HAWC can search for TeV emission from potential classes of sources, such as nearby AGN or galaxy clusters, and with its angular resolution can determine which of these AGN emit TeV gamma rays.  These TeV sources are likely the sources of UHECRs as well.

Last modified: November 08, 2007 15:45:19.