Principales símbolos que utilizamos en un diagrama de flujo.

Now that we have become familiar with the observational properties of blazars, the discussion can progress to an overview of some of the models that have been posited to explain how the observed radiation and variability are produced. In Section 1.1.1 we discussed the AGN unification model, which explains the different classes of AGN we observe as resulting from different orientations of the same basic central engine to the observer’s line of sight. However, to explain the observed properties of blazars we must understand how the different components of the central engine, such as the accretion disk and relativistic jet, work together to create the observed properties.

Referring back to our discussion of SEDs in Section 1.2.4, we recall that the emissions from radio to UV wavelengths comprise what is called the Synchrotron Peak. The name of this feature comes from the near certainty that the radiation produced therein results from the spiraling of elec- trons around the magnetic field lines of the jet. The primary evidence for this is the measurement of significant polarization in all of the bands in the synchrotron “hump”, as well as an often significant correlation between the polarimetric properties between these bands.

The second, higher-energy peak in blazar SEDs is known as the inverse Compton peak, due to (most likely) high-energy charged particles up-scattering photons to x-ray and gamma-ray en- ergies. However, the physics and conditions governing the emissions in this part of the SED are less clear. For example, there remains some controversy concerning the origin of the seed photons for the process. The current leading theory is that the seed photons come from the aforementioned synchrotron process, giving rise to the so-called synchrotron self-Compton (SSC) model (Marscher and Travis, 1996). However, external sources for these photons cannot be ruled out, such as the accretion disk, accretion corona (hot gas above/below the plane of the disk), torroidal region, or broad-line clouds (Hartman et al., 1996). The nature of the energetic particles responsible for up-scattering the seed photons is also an open question. Currently, leptonic models (dominated by electrons & positrons) are most often used to explain the emissions, though hadronic models which incorporate protons in the relativistic jet flow have had some success in explaining the gamma-ray

Figure 1.13: Model of a Quasar

Scale diagram illustrating the spatial relationship between different parts of a blazar. From the Boston University blazar research group’s webpage at http://www.bu.edu/blazars/research.html emission of some blazars, particularly those emitting at TeV energies (e.g. Böttcher et al., 2013). Though some interesting and important questions concerning their details still remain to be answered, the synchrotron emission and inverse Compton models sufficiently explain the origin of the radio to gamma-ray emission. Even so, we are still without an explanation for the extreme variability exhibited by blazars. While many theoretical scenarios have been proposed to explain these observations, the most widely accepted theory posits a shock propagating down the jet as the preferred mechanism (Marscher et al., 2010). The general workings of this model will now be discussed, along with supporting observational results.

We’ll begin with a general overview of the process of generating a shock and propagating it down the relativistic jet. The reader is encouraged to refer to Figure 1.13 to get a sense of the spatial relationship between the different components of an AGN with a relativistic jet. The process starts with an instability in the accretion disk. The instability leads to a collapse of the inner disk, causing the matter therein to fall towards the event horizon of the SMBH. Some of this matter will pass beyond the event horizon, while some is incorporated into the relativistic jet. This sudden influx of material accelerates down the jet as a shock, spiraling along the lines of the helical magnetic field

in what is known as the acceleration and collimation zone (ACZ). The magnetic field lines in this part of the jet are tightly wound due to their origin in the rotating accretion disk and act as a sort of nozzle for the material streaming down the jet. As the shock passes into and out of the observers line of sight due to its spiral path, the observer will note alternating increases and decreases of the broadband flux due to relativistic beaming (Rosen, 1990). Eventually, the moving shock will encounter a standing shock at the radio core (as seen by long baseline radio interferometry), where the moving shock will undergo another outburst of emission before dispersing and continuing its propagation down the jet. It can be deduced from Figure 1.13 that the moving shock should trigger broadband flares at different times in different bands as it travels down the jet, with the relative timing of each flare governed by the relative distance between the part of the jet most associated with that particular waveband. Figure 1.14 illustrates how large flares in blazars often manifest across different, widely separated bandpasses. Additionally, the timescales of flares (i.e. how long it takes to increase/decrease flux by a significant degree), can yield information about the size of the region emitting that radiation.

With this illustration of the shock-in-jet model in mind, we can now explore some of the supporting evidence for it, starting with the collapse of the inner accretion disk and emergence of the shock. Observations of galactic x-ray binaries (GXRBs), sometimes called “microquasars”, occasionally reveal marked decreases in their x-ray flux shortly before emitting a new radio blob in their jets (Mirabel and Rodríguez, 1998). Since most of the x-ray flux in these systems is thought to come from the inner part of the accretion disk near a neutron star or black hole, it is thought that the dip in x-ray flux is the result of a disruption in that part of the disk. This leads to a sudden consumption of much of the involved disk material by the SMBH, while some is ejected from the system as a new radio blob along the jet (Belloni, 2001).

In order to link this behavior to the central engines of AGN and the SMBHs within them, the galaxy 3C 120 was selected for an intense multiwavelength campaign. This galaxy was chosen for it similarities to GXRB systems, specifically in its x-ray spectrum and x-ray variability: hard and soft x-rays were believed to originate from the hot accretion disk corona and inner accretion

Figure 1.14: Multiwavelength variability: gamma-rays to radio

Multiwavelength data from a recent study by Jorstad et al. (2013) revealing correlated flares in the (from top to bottom) gamma-ray, x-ray, UV, optical, and radio regimes.

disk, respectively - much like a Seyfert galaxy. However, unlike most Seyfert galaxies, 3C 120 was known to possess a radio jet. With the use of long baseline radio interferometry and RXTE monitoring, Marscher et al. (2002) were able to discern that over a 3-year period 3C 120 displayed the same reduction in x-ray flux which preceded the ejection of a radio blob (as seen in GXRB systems) along 3C 120’s jet, thus providing a hard link between activity in the accretion disk and the relativistic jet in an AGN.

In order to explore the production of broadband variability, we refer to the data presented in Figure 1.15, presented by Marscher et al. (2008). The optical data shows two distinct flares

Figure 1.15: Correlated broadband variability and polarimetric changes

Multiwavelength and polarimetry data showing simultaneous flaring across multiple bandpasses, with the polarimetry revealing the rotation of shock as it travels down the jet in the object PKS 1510-089 (Marscher et al., 2008). The right-hand side of the figure is a zoomed-in version of the

region of interest indicated by the dotted lines in the left-hand side of the figure.

(between the vertical dotted lines), which correspond to 1) a shock in the inner jet orienting itself to the observer’s line of sight, and 2) the same shock emitting a burst of radiation as it passes through a standing shock at the radio core. Since the region of jet closer the the SMBH is opaque to radio wavelengths we only see the second flare in that band, as the traveling shock leaves the ACZ. Additionally, a clear rotation in EVPA is seen during the first flare (Figure 1.15, right side, second panel from bottom), which is clear evidence of the shock following a spiral path while in the inner jet, due to the helical magnetic field lines.