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High-energy particles in jets
A substantial progress in our understanding of matter content and particle processes in jets was made by observations at higher energies, from radio to X-ray domain. These observations revealed brighter spots in jets, called \knots », especially promi-nent in X-ray band (an example is shown in Fig. 2.7 for the jet of M87). The knots show non-thermal emission, with the spectral energy distribution (SED) having typ-ically a broken power law shape. For the case of M87, Marshall et al. (2002) nd that the SED of the knot emission is well represented by a broken power law model, with the spectral index in radio-to-optical band of kn,r-o ’ 0:7, and in X-ray band of kn,x ’ 1:5, and with the break frequency kn,br 1016 Hz. Authors also conclude that the SED is well described with a synchrotron emission model, and estimate the Lorentz factors of particles responsible for the observed emission to be kn 107. The most plausible option for the particles that produce the radiation is electrons, as protons are ine cient at emitting synchrotron emission due to their higher mass. Therefore, knots are lled with either electron-proton, or electron-positron plasma, with maximum energy of electrons around Ee,kn,max 10 TeV.
This result indicates that jets contain high-energy particles, and hence that parti-cle acceleration processes operate inside jets. Let us estimate the synchrotron cooling time-scale (see sub-section 5.1.2 and the Eq. 5.8) for the 10 TeV electrons in the jet. Assuming that the magnetic eld in the knots is slightly stronger than in the interstellar medium of a galaxy, Bkn 10 5 G, one nds te,cool,kn 1011 s. During this time, highest-energy electrons would propagate for distances of 1 kpc along the jet while losing their energy. However, observations show that knots are situ-ated at much further distances of about a few tens of kiloparsecs from the central engine. This contradiction represents one of the long-standing problem of jet physics: why jets extend to distances as long as a few tens, hundreds, or in some cases even thousands of kiloparsecs. The standard explanation is that particles experience con-tinuous acceleration along the path of the jet (e.g. by shocks or turbulence) and are injected locally. An alternative scenario, proposed by Neronov et al. (2002) assumes injection at the base of the jet of a powerful beam of -rays with energies & 1015 eV, producing electron-positron pairs when encountering cosmic microwave background (CMB) photons while propagating through the jet. Such mechanism allows to gener-ate and supply high-energy particles along the entire length of the jet up to distances of hundreds of kiloparsecs.
Energy dissipation
Jets of FR I and FR II galaxies terminate in large-scale lobes (see Fig. 2.2), where particles lose much of their energy through radiation and acceleration of particles of the ambient medium, contributing to the AGN feedback on their environment. The energy carried in the jet ow, in fact, comprises two components: kinetic energy (contained in matter) and electromagnetic energy (Poynting ux). The picture of dissipation of the total energy budget by jets of FR I and FR II galaxies is di erent and not yet fully understood. For jets of FR I galaxies, a signi cant fraction of their kinetic energy is dissipated at the base of the jet in the vicinity of the BH, however the electromagnetic energy is transported much farther, powering the out ow for large distances. To the contrary, jets of FR II galaxies, lose most of their electromagnetic energy close to the BH, but transferring kinetic energy ux to far distances all the way to the lobes (e.g. Blandford et al. (2019)). This is suggested by the di erence in the morphology of radio maps of FR I and FR II galaxies (demonstrated in Fig. 2.2), with the FR I objects having bright core region, and FR II showing prominent emission from the lobes.
The more powerful FR II jets also produce hot-spots, clearly visible in the lobes (an example is shown in the bottom panel of Fig. 2.2). The hot-spots correspond to termination shocks due to the interaction with the ambient medium. Particles are accelerated at the front of these shocks and emit synchrotron radiation, therefore making hot-spots appear very bright.
Relativistic motions in jets
Apart from high-energy particles, observations show that the matter in jets moves, in many cases, with relativistic speeds. The strongest evidence is provided by ob-servation of knots motion with apparent superluminal speed. An example of such observation is presented in the top panel of Fig. 2.8. Knowing the distance to the source, more precisely, angular size distance calculated from the redshift, and angular displacement of the knots over a given time interval, one obtains the linear displace-ment and hence the velocity. The velocities of knots in jets measured in this way were found to be superluminal: for example, for PKS 1510-089 vkn,1510 ’ 22c, for M87 vkn,M87 ’ 5c.
Such superluminal motion stems from relativistic velocities of matter in the jet and geometrical e ects. Fig. 2.8 illustrates a geometrical scheme explaining the emer-gence of the apparent superluminal motion phenomenon. A jet is observed at a small angle with respect to the line of sight. A knot moves with a speed v close to the speed of light along the jet, and an electron inside the knot emits a photon at the moment of time t0, and another photon after some time, at the moment t1. The physical displacement of the knot over the time interval t = t1 t0 as seen by the observer, is a projection of the traveled distance on the direction perpendicular to the line of sight, i.e. x = v t sin . The time interval between the detection by an observer of a photon emitted at point \0″ and at point \1″, is the path di er-ence between these two photons along the line of sight divided by the speed of light:
Figure 2.8: Illustration of apparent superluminal motions in relativistic jets. Top: Observations of superluminal motion of two knots (top and bottom panel) in PKS 1510-089 (z=0.361) performed in radio band by VLBA at 43 GHz. Contours display the intensity level of the total ux, and the color { of the polarized ux. White linear segments indicate the direction of the linear polarization. The rst knot has an apparent velocity of 24 2 c, and the second one of 21:6 0:6 c. The scale of the y-axis is 0.5 pc / 0.1 mas. (adapted from Marscher et al. (2010)). Bottom: a scheme explaining the origin of apparent superluminal motions. (adapted from Courvoisier (2013)) If a relativistic ( ! 1) jet is oriented at a su ciently small angle with respect to the line of sight, one measures an apparent superluminal velocity. The maximal velocity is achieved when = cos , and has a value of vapp,max = v (2.5) where = (1 2) 1=2 is the bulk Lorentz factor of the knot. From this equation for the case of PKS 1510-089 one infers 1510 ’ 22, and for M87 M87 ’ 5, with the angles between the jet axis and the line of sight 1510 ’ 3 , and for M87 M87 ’ 11 . Typical apparent velocities observed in jets of various other AGN are in the vapp 10c range. Therefore, observation of superluminal apparent velocities is a direct indication of relativistic motions of matter in jets with typical bulk Lorentz factors 10.
Blazars
As already mentioned, blazars are FR I and FR II galaxies with the jet very closely aligned with the line of sight. Accidental orientation of the jet (nearly) towards the observer, as well as relativistic motions of plasma in it, produce not only apparent superluminal speed phenomenon, but also a strong emission boosting, discussed just below. We also focus in this section on di erent properties of blazars, including their broad-band SED and emission at high energies.
Doppler boosting
Due to bulk relativistic motion of plasma in the jet, a phenomenon named \Doppler boosting » emerges as a consequence of special relativity e ects, speci cally, relativistic time dilation.
For an emitter moving relativistically with the Lorentz factor at a small angle with respect to the line of sight, the relativistic transformations of physical quantities involve a characteristic Doppler factor , which is so that the time interval is transformed as tobs = tem 1, and the frequency as obs = em, with the sub-script \em » indicating quantities in the emitter’s frame, and \obs » in the observer’s frame.
Emission intensity transformation involves four contributions: (i) relativistic time dilation for the time interval between arrival of two photons, (ii) relativistic de-crease of emission solid angle (aberration), (iii) relativistic Doppler e ect (frequency shift) for each individual photon, and (iv) relativistic transformation of the frequency interval. This results in the following transformation:
Therefore, the Doppler boosting e ect dramatically enhances the ux from the approaching jet, while heavily suppresses the one from the receding jet. This explains why in most cases, the opposite jet is not visible. Also, the non-thermal jet emission exceedingly dominates the observed blazar emission due to the Doppler boosting e ect.
Another relevant e ect of special relativity is the so-called \beaming » (headlight) e ect. The radiation emitted by the relativistically approaching source isotropically in its own frame, is observed as concentrated in a cone with an opening angle of cone 1.
The beaming e ect in particular explains why blazars emit the strongest -ray signal, while radio galaxies are not very bright -ray-emitters. In the above-mentioned spine-sheath jet structure, typically the -ray emission originates from the inner spine, while the most of the radio emission is generated in the outer sheath (e.g. Blandford et al. (2019)). As the spine is faster than the sheath, the observed -ray emission will be much more beamed and will be concentrated in a much narrower cone, than that of the radio ux. As a result, only radio galaxies oriented very closely to the line of sight are brightest -ray-emitters, in this case they are named blazars. Consequently, the SED of blazars spans from radio domain up to -ray band.
Also, bulk motions of emitting matter in blazar jets and the associated relativis-tic time dilation are responsible for shortening the observed variability time-scale: tvar,obs = tvar,source= . In some blazars, it can be as short as a few minutes (for more details, see sub-section 5.3.1).
Blazar emission
As already mentioned, BL Lac objects correspond to aligned FR I galaxies, whereas FSRQs { to aligned FR II galaxies. The main di erence between BL Lac and FSRQ is in their luminosity, with FSRQ being much brighter (typically by a factor of 100).
Short variability of blazars (especially in the -ray band) indicates that the re-gion in which the -ray emission is produced, should be very compact. According to the causality arguments, medium variability time-scale of tvar,obs 1 day, implies a small size of the emitting zone, Rez ctvar,obs =(1 + z) 1016 cm, with typical value of 20. Even shorter variability time-scales of a few minutes, correspond to a size of the emitting region commensurable with the radius of the horizon of a BH with mass 108 M . It is believed that the blazar -ray emission is produced in a compact plasma region, a \blob » lled with relativistic particles, and having higher density and magnetic eld stronger than on average in the large-scale jet. Such a blob is moving at a speed close to the speed of light along the jet axis and presumably represents an ejecta produced in the inner spine of the jet due to magnetohydrody-namical instabilities and other e ects. The exact location of the -ray-emitting site in the jet is not immediately obvious. For the case of an FSRQ, as we will see just below, the observed -ray spectrum can depend on the blob position with respect to the central engine. One of the ways to determine the distance of the -ray emission region from the SMBH is to study the correlation between radio and -ray emission. Such studies nd that -rays are generated in a zone inside the jet, outlying typically up to a few parsecs away from the SMBH (e.g. Marscher et al. (2008)).
The very detection of -ray emission from blazars, and in particular, TeV -ray emission from BL Lac objects, imposes a lower limit on the value of the Doppler factor of the blob, b & 10, otherwise the jet becomes opaque to the -ray photons due to – pair-production e ect (the e ect is explained in sub-section 4.1.5). The \blob-in-jet » model for blazar broad-band emission and relevant emission processes are described more in detail in Section 4.1.
A typical SED of blazar broad-band emission represents a two-bump structure, spanning over 15 or more decades in energy (see an example in Fig. 2.9). The low-energy bump extends from radio band up to UV or soft/hard X-rays, and peaks in the IR, optical, UV or X-ray band, and the higher-energy bump stretches up to GeV or TeV -rays, showing a maximum around MeV to sub-TeV, or even at TeV ener-gies. The very detection of high-energy -rays from blazars provides an additional evidence of the presence of highly energetic particles inside the source. Analogically to the emission of the knots observed in jets of misaligned objects, the origin of the low-energy SED component of blazars is attributed to synchrotron emission generated by high-energy electrons moving in a magnetic eld. A strong evidence in favor of this emission mechanism is the observation of emission polarization in blazars. While there is no doubt about the origin of the low-energy bump, the high-energy component still remains a subject of debate. The most common view is that it is produced by inverse Compton scattering mechanism of soft photons (see sub-section 4.1.4), how-ever, alternative interpretations of the high-energy bump invoking hadronic processes exist (more discussed in sub-section 4.1.1). The dominant photon eld for the inverse Compton process depends on the object type. For the case of a BL Lac with no important soft photon radiation elds except for the synchrotron emission, relativis-tic electrons upscatter the same synchrotron photons that they themselves produce, boosting them in energy up to -ray energies (so-called Synchrotron Self-Compton scenario, SSC). For the case of an FSRQ, due to strong accretion, the dominant target photon eld may be the thermal emission of the accretion disk (e.g. Dermer et al. (1992)), optical-to-UV radiation of the broad line region (BLR) (e.g. Sikora et al. (1994)), or IR radiation of the torus (e.g. Blazejowski et al. (2000)). This scenario is called \external Compton » since the target photons are external to the -ray emitting zone. The target radiation eld for the inverse Compton upscattering process depends on the position of the -ray emitting zone inside the jet, speci cally its distance from the central engine. For the emission region located at distances closer than 0.01 pc from the central black hole, the accretion disk photons dominate the external pho-ton eld, for distances in the domain 0.01 { 0.1 pc, the BLR radiation serves the main target for upscattering, and for distances 0.1 { a few pc, upscattering of the emission of dust torus prevails.
In the radio band, the blazar SED is typically dominated by the synchrotron emission of the large-scale jet, rather than that produced in the blob. Also, in the optical domain, the host galaxy contribution to the observed ux might be quite important.
Table of contents :
1 Introduction
2 Active Galactic Nuclei
2.1 The AGN zoo
2.2 Unied scheme of AGN phenomenon
2.2.1 What denes the observed luminosity?
2.2.2 Radio-loud or radio-quiet?
2.2.3 Orientation eects
2.3 AGN jets
2.3.1 Imaging of jets
2.3.2 Kinematics and structure
2.3.3 High-energy particles in jets
2.3.4 Energy dissipation
2.3.5 Relativistic motions in jets
2.4 Blazars
2.4.1 Doppler boosting
2.4.2 Blazar emission
2.4.3 Blazar sequence
2.4.4 Probing the Universe with blazars
3 Cherenkov gamma-ray astronomy and H.E.S.S. data analysis
3.1 VHE gamma-ray astronomy
3.1.1 Detection of VHE gamma-rays with an IACT
3.1.2 Presently operating IACTs
3.2 H.E.S.S. experiment
3.2.1 Overview
3.2.2 The optical system
3.2.3 The cameras
3.2.4 The trigger system
3.2.5 Data analysis and reconstruction
3.3 Analysis of H.E.S.S. data of 3C 279
3.3.1 The studied source: 3C 279
3.3.2 VHE
3.3.4 Analysis of H.E.S.S. 3C 279 data : June 2018
3.4 Discussion and perspective
4 Modeling of AGN emission: stationary models
4.1 Origin of blazar broad-band emission
4.1.1 Leptonic (SSC and EC) and hadronic models
4.1.2 Blob-in-jet model
4.1.3 Synchrotron emission
4.1.4 Inverse Compton emission
4.1.5 Gamma-gamma pair production
4.1.6 Transformation to observer’s frame
4.2 EBL absorption
5 Modeling of AGN emission: time-dependent approach
5.1 General approach and the kinetic equation
5.1.1 Particle injection
5.1.2 Particle radiative cooling
5.1.3 Particle acceleration
5.1.4 Particle escape
5.2 EMBLEM code
5.2.1 Numerical implementation
5.2.2 Input parameters
5.2.3 Code architecture
5.2.4 Examples of simulated
5.3 Physical modeling of AGN
5.3.1 What causes the observed variability in the VHE-ray regime ?
5.3.2 One-zone models
5.3.3 Multiple-zone models
6 Modeling of a MWL are of Mrk 421
6.1 The studied source: Mrk 421
6.2 Modeling of the low state of the source
6.2.1 Mrk 421 low-state emission: archival data
6.2.2 Mrk 421 low-state emission: physical modeling and discussion
6.3 Observational data set of the February 2010 are of Mrk 421
6.3.1 Archival data
6.3.2 Analysis of Fermi-LAT gamma-ray data
6.4 A general criterion to test one-zone
6.5 Physical modeling of Mrk 421 February 2010
6.5.1 One-zone model
6.5.2 Two-zone model
6.6 Discussion and perspective
7 Preparation of Cherenkov Telescope Array
7.1 CTA project
7.1.1 Overview
7.1.2 Future science with CTA
7.2 Gamma-Ray Cherenkov Telescope: overview and prototyping
7.2.1 Overview
7.2.2 Prototyping
7.3 Gamma-Ray Cherenkov Telescope: performance
7.3.1 Ideal optical performance
7.3.2 Non-ideal optical performance
7.3.3 Modeling of the PSF of the pGCT
7.4 Discussion and perspective
8 Conclusion and perspective
