Active Galactic Nuclei

Active galactic nuclei (AGN) are galaxies with an unresolved central nucleus that outshines the rest of the galaxy and for which the emission is often dominated by nonthermal processes. Their emission is widely believed to be powered by accretion onto a supermassive black hole. The only AGN detected at gamma-ray energies are blazars: radio-loud, flat spectrum sources characterized by highly variable and primarily non-thermal emission at most wavelengths. Flat spectrum radio quasars and BL Lacertae objects (BL Lacs) are members of the blazar class of AGN. The rapid variability and radio properties of these objects imply that they have relativistic jets whose axes make small angles with respect to the observer's line of sight. The gamma-ray emission from these objects is also most naturally explained as beamed radiation from the jets which is Doppler boosted in energy and luminosity. Currently, no model for the production of gamma-rays in the AGN is generally accepted. The two most popular classes are those in which high energy electrons (known to be present in the jets from radio, optical, and X-ray observations) produce the gamma-rays by inverse Compton scattering of low energy photons and those in which very high energy protons produce the gamma-rays by initiating cascades within the jets. These two processes are illustrated schematically in Figure 2.

  

Figure 2: Artist's conception of the nucleus of an active galaxy. Two possible scenarios for the production of gamma-rays in a relativistic jet are indicated (from [Buckley 1998]).

Whipple observations of Markarian 421 (Mrk 421; Figure 3) and Mrk 501 have revealed extremely variable VHE emission (Figure 4; [Quinn et al. 1999]; [Gaidos et al. 1996]). Variability on a 15 minute time-scale observed in Mrk 421 ([Gaidos et al. 1996]) implies a compact emission region of dimension $R \mathrel{\hbox{\rlap{\hbox{\lower4pt\hbox{$\sim$ }}}\hbox{$<$ }}}10^{-4}$ parsec (pc) which is only an order of magnitude larger than the radius of the event horizon of a 10⁸ solar mass black hole. The emission region might exist at a somewhat greater distance from the central object (e.g., at a shock front propagating out along the jet), but this emission still originates well inside the $\sim$1pc region directly imaged by current radio telescopes. Correlations observed between X-rays and VHE gamma-rays (e.g., Figure 4; [Macomb et al. 1995]; [Buckley et al. 1996]; [Catanese et al. 1997]) are most easily explained as emission at both wavelengths being produced by the same population of electrons. However, models which produce gamma-rays primarily through proton interactions ([Mannheim 1993]) can also explain the observations. The combined low energy and VHE data provide quantitative constraints on jet parameters such as the magnetic field strength and Doppler factor. Whipple and EGRET observations have also led to reformulations of blazar unification models based on intrinsic source luminosity rather than solely on the effects of orientation differences ([Ghisellini et al. 1998]; [Georganopoulos & Marscher 1998]).

  

Figure 3: The VHE gamma-ray emission from Mrk421 (from [Buckley et al. 1996]). The solid contours are 2$\sigma$ excess levels (the second contour is the 99.7% confidence region for the source location). The dashed ellipse is the 95% confidence interval determined by EGRET. The cross indicates the position of Mrk421.

Despite these gains, we have much to learn about these enigmatic objects. VERITAS can help answer many of the unknowns concerning blazars, particularly when combined with lower energy observations by GLAST, X-ray and optical telescopes. We will approach the study of AGN from two perspectives: detailed studies where we conduct long observations of individual objects to reveal the physics operating in the AGN, and broad studies where we detect more VHE sources to illuminate patterns among various objects.

  

Figure 4: Upper panel: Observations of Mrk 501 in VHE gamma-rays (top) and X-rays (bottom) taken during 1997 April 2-20 (April 2 corresponds to MJD 50540). Figure adapted from Catanese (1999). Lower panel: Average daily gamma-ray rates observed with Whipple for Mrk 501 in 1997. Figure from Quinn et al. (1998).