Active Galactic Nuclei
Overview
Active galactic nuclei (AGNs) are supermassive black holes (~108 times the mass of the Sun) with luminosities that far outshine the rest of their host galaxies. In an AGN, the intense gravitational attraction from the central black hole pulls in gas and dust from the host galaxy, forming a rapidly rotating disk of material called an accretion disk. As the enormous gravitational potential energy of the accretion disk is converted to kinetic energy, the gas and dust heat up and are ejected at relativistic velocities from the galactic core.
A classic "edge-on" view of the radio galaxy Cygnus A is shown at left (click here for a closer view). In this image dark regions correspond to intense X-ray emission, and contour lines correspond to radio emission. The X-ray emission is most intense around the galactic core. Streaming away from the core are two oppositely-oriented relativistic jets of gas and dust. As the gas and dust interact with material in intergalactic space, they form turbulent regions where electrons are accelerated and emit radiation into the radio waveband.
AGNs can be viewed "edge-on," like Cygnus A, or "face-on," with the relativistic jets pointed directly at Earth. Such AGNs are called blazars. They tend to have intense and highly variable emission in X-rays and gamma rays. The gamma rays are produced by particles accelerated in shocks that propagate along the jets. If the accelerated particles are protons, then gamma rays can be produced by hadronic cascades originating with a p+γ interaction. The protons may have energies exceeding ~1018 eV, making AGNs possible sources of ultra-high energy cosmic rays. 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.
Observing AGNs with HAWC
By continuously observing the TeV sky with HAWC, we can detect many flaring AGNs at the highest possible energies. The HAWC data provide unbiased monitoring of all blazers in the Northern sky, resulting in a unique ability to study the properties of the TeV blazar population. Long-duration observations with HAWC will determine the average flux as well as the duty factor of flares of different luminosities.
The flux observed from AGNs on different timescales is indicative of the size of the emission region. Long-period emission could be the signature of a precessing jet caused by a binary black hole system. Such binaries are excellent candidates for gravitational wave detection. Because HAWC conducts a synoptic survey of the sky, the HAWC data can be used to send out notices of flaring activity to gamma-ray observatories which conduct deep and highly sensitive "pencil-beam" surveys.
Recent Measurements of Variabile Sources
To date, TeV emission has been observed from about 50 AGNs. Observations with the EGRET satellite showed that 70% of AGNs were variable. Since energy loss increases with electron energy, the TeV observations are predicted to exhibit even greater variability. This has been the case for well-known sources such as Mrk421 and Mrk501. However, few flares have been observed from the newly discovered TeV AGNs. The lack of TeV variability may simply be due to the lack of long-time scale continuous observations. HAWC provides such such observations by continuously observing every AGN in its field of view.
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 (see figure above). 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 with a significance >6σ.
It is important to measure source variability in the TeV band because it constrains both the acceleration process and the environment near the acceleration sites. Since the variability timescale cannot be shorter than the time it takes for light to travel across the emitting region, we can relate the geometry of the source to quantities connected to gamma-ray production. This gives
Γ · tvar > Re/c = (Re/Rs) × (2GM/c3),
where Γ is the Lorentz factor of bulk motion in the gamma-ray emitting region, tvar is the timescale of variability, Re is the size of the emitting region, Rs is the Schwarzschild radius of the black hole, 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 about 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 high Lorentz factors are more typically associated with gamma-ray bursts and would be an order of magnitude larger than those normally associated with AGNs.
If AGNs 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 allows us to study orphan flares and the correlations between TeV gamma-ray, X-ray, optical, and neutrino emission in detail.
