In addition to jet models, another potential region for TeV γ-ray emission in radio galaxies and other misaligned AGN is in the vicinity of the central black hole. Rieger & Aharonian (2008b) argue that the most likely site for very high energy γ-ray production in M87 is close to the event horizon of the central black hole at a radius r ∼ rs. If this is the case then modelling the emission
in this manner can be used to show a link between accretion-disc physics and jet-formation theory, as the mechanism for particle acceleration described could provide the energetic seed particles required for efficient Fermi acceleration on larger scales.
Rieger and Aharonian’s model relies on a rigidly rotating, dipolar magnetosphere in the vicinity of the black hole to accelerate particles to high energies. This magnetosphere is generated in a
magnetohydrodynamic jet-disc framework as magnetic flux is dragged inwards and amplified by dynamo action in the inner accretion disc. Efficient particle acceleration occurs close to the light surface, rL = c/Ω ∼ (5 − 10)rs (where Ω = angular velocity, assumed to be constant), on radial
scales that are small compared to the curvature of the magnetic field so that the magnetic field can be modelled as a simple monopole-like structure without too great a loss of accuracy. The AGN- black hole environment is plasma-rich and any electric field component parallel to the magnetic field is expected to be screened off at r < rL, so efficient gap-type particle acceleration, as seen in
pulsars, is unlikely and inertial effects become the main driving mechanism.
Using Hamiltonian mechanics, and assuming that the charged particle corotates with the field as in bead-on-wire motion, Rieger and Aharonian (2008b) find that as the radius of rotation of the particle approaches rL, its Lorentz factor, γ, must increase dramatically. As the Lorentz factor of
the particle increases, radiative energy losses also increase, the bead-on-wire approximation breaks down, and the field lines bend with increasing inertia so a maximum obtainable Lorentz factor will be reached. The maximum achievable Lorentz factor can be constrained by the validity of the bead-on-wire approximation, requiring that the characteristic acceleration timescale derived from the Hamiltonian must be greater than the relativistic gyrofrequency, leading to:
γmaxBB ≤ 1 ˜ m16 qBrL 2m0c2 23 (5.29) where ˜m = 1/(γ2
0[1 − r20/rL2]2) and hence is determined by the initial conditions of the particle.
This is equivalent to the requirement that the Coriolis force must not exceed the Lorentz force. Using parameters consistent with observations of M87, B(rL) ∼ 10 G, rL∼ 5×1015cm, Rieger and
Aharonian (2008b) found that γmaxBB ∼ 5 × 108 for electrons, and because γmaxBB ∝ m −2/3
0 (Equation
5.29), centrifugal acceleration is not an efficient mechanism for protons, indicating that interactions of accelerated proton with the ambient photon field are negligible. Radiative energy losses due to inverse-Compton interactions with the ambient photon field close to rL(in relatively low luminosity
sources) are not expected to play a large enough role to provide any stronger constraints and hence γBB
max is likely to be a sensible estimate of the electron Lorentz factors present.
The TeV emission from a misaligned AGN in this model is thus assumed to arise via inverse- Compton scattering of photons from an advection-dominated accretion disc by the centrifugally accelerated electrons. At energies << 5 TeV, assuming a power-law electron distribution, the interaction between the highly energetic electrons from the acceleration process and a comptonised photon field originating from the accretion disc results in a power-law evolution for the inverse- Compton spectrum, independent of the incident photon spectrum. At TeV energies, however, the
inverse-Compton spectrum becomes sensitive to the seed-photon distribution from the accretion disc, where the comptonisation of soft cyclosynchrotron photons adds to the spectrum a power-law tail with spectral index αc ∼ 1.2. This leads to the very high energy spectrum following a power
law with spectral index α ≈ αc.
The model presented shows good agreement with the spectrum observed in M87 and is able to reproduce rapid variability as detected from the object. Rieger & Aharonian (2008b) argue that if the particle acceleration and TeV emission do indeed originate close to the central black hole in misaligned AGN, these effects would be difficult to discern in more luminous objects that are more closely aligned to the line of sight, as the emission would be swamped by relativistically beamed emission from the jet. A particularly interesting consequence of the model is that detection at TeV energies could be expected, even for very highly misaligned AGN, something that is extremely unlikely in the case of any of the jet models. Without the detection of such objects, however, it is difficult to distinguish between jet models and the core-emission model. Attempts to model the multiwavelength spectrum of M87 using a core-emission model have had difficulties reproducing flared emission from the object (Hilburn & Liang, 2012). Another potential issue is that the region closest to the black hole is expected to have a high photon density, effectively rendering it opaque to any TeV emission in the region as a result of photon-photon annihilation (Cheung, Harris & Stawarz, 2007); however this view is contested by Rieger and Aharonian (2008b), who argue that this region in M87 is transparent to 10 TeV photons on scales of 5rs - 13rs, consistent with the
scenario they present.
5.7
Conclusions
To explain the SED of blazars, relatively simple single-zone models, such as that discussed in Section 5.3, can be used; however, such models have great difficulty reproducing the very high energy spectra of AGN observed at greater angles. To explain the emission from such objects, more complicated scenarios such as those presented in this chapter must be envisaged. Three of these are inhomogeneous-jet SSC models and rely on either a structured jet or multiple smaller structures within the jet to reproduce the detected SED; the remaining model posits that TeV emission originates close to the central black hole and is an EC model in which the seed photons for the inverse-Compton emission originate in the accretion disc. Unfortunately, currently available data do not allow for unambiguous rejection of any of the models. Moreover, in all cases, there are issues that need to be resolved.
characteristics is the multiblob model proposed by Lenain et al. (2008). As it is merely an extension of previously presented blob-in-jet models, it requires few extra assumptions when compared to them. It posits that nonthermal emission from AGN originates in a number of relativistic blobs of plasma close to the Alfv´en surface, moving at slightly different angles to the line of sight. The model has shown success in explaining the spectrum of M87 at very high energies, including data both from the low state of the object and from the flare detected in 2005.
The decelerating-jet model of Georganopoulos & Kazanas (2003) has been used to successfully explain the TeV emission from M87 during its low state; however, owing to the requirement of relatively large length scales, this model has great difficulty explaining the rapid variability observed during the flaring from the object in 2005. Additionally, the model appears to have difficulty reproducing the TeV spectrum observed from Centaurus A, leading to potential problems with AGN unification.
The spine-layer model presented in Tavecchio & Ghisellini (2008) proposes that the jets of AGN consist of a central fast-moving spine, surrounded by a slower-moving layer. Unfortunately, the model requires 18 free parameters to successfully describe the jet, making it difficult to derive meaningful constraints. Additionally, this model has great difficulty explaining the hard TeV spectrum observed during the flare of M87 in 2005.
The final model presented here is the core-emission model proposed by Rieger & Aharonian (2008b), in which the nonthermal emission from misaligned AGN originates close to the central black hole. In this model, the seed photons for inverse-Compton scattering are provided by the accretion disc, and the electrons are centrifugally accelerated in the vicinity of the black hole. A potential problem for the model is explaining how the TeV photons escape the intense photon field expected in the proposed emission region, although the authors claim that this would not be a problem for photons of energy 10 TeV. Additionally the authors themselves acknowledge that the model is relatively simplistic and that further work would be required to develop it fully.
Chapter 6
Modelling the Emission from
Fermi -LAT Selected Misaligned
AGN
6.1
Introduction
The detection of three misaligned AGN by current-generation IACTs has opened up a new class of TeV γ-ray source for study and over the coming years the construction of the Cherenkov Telescope Array (CTA), discussed in Section 6.4, will hopefully lead to the detection of more such objects. To identify likely targets, it is useful to model the SEDs of misaligned AGN detected at high energies, using currently available data to place constraints on the physical parameters. By considering projected CTA response curves and the SEDs generated, it should be possible to make reasonable predictions relating to the detection or non-detection of the objects modelled.
As discussed in the previous chapter, modelling the observed TeV emission of the currently detected misaligned AGN has required a shift away from relatively simple single-zone models towards more complicated multizone models. Of the models discussed, the most successful thus far has been the multiblob model of Lenain et al. (2008), which has been able to reproduce the TeV emission from both M87 (Lenain et al., 2008) and Centaurus A (Lenain et al., 2009). This model will be used in this chapter to produce the SEDs of Fermi -LAT-selected misaligned AGN.
Object R.A. Dec. Redshift Class Catalogue (J2000.0) (J2000.0) z Radio Optical 3C 78/NGC 1218 03h 08m 26.2s +04◦ 06’ 39” 0.029 FRI G 3CR 3C 84/NGC 1275 03h 19m 48.1s +41◦ 30’ 42” 0.018 FRI G 3CR 3C 111 04h 18m 21.3s +38◦ 01’ 36” 0.049 FRII BLRG 3CRR 3C 120 04h 33m 11.1s +05◦ 21’ 16” 0.033 FRI BLRG 3CR PKS 0625-354 06h 27m 06.7s -35◦ 29’ 15” 0.055 FRIa G MS4 3C 207 08h 40m 47.6s +13◦ 12’ 24” 0.681 FRII SSRQ 3CRR PKS 0943-76 09h 43m 23.9s -76◦ 20’ 11” 0.27 FRII G MS4 M87/3C 274 12h 30m 49.4s +12◦ 23’ 28” 0.004 FRI G 3CRR Cen A 13h 25m 27.6s -43◦ 01’ 09” 0.0009b FRI G MS4 NGC 6251 16h 32m 32.0s +82◦ 32’ 16” 0.024 FRI G 3CRR 3C 380 18h 29m 31.8s +48◦ 44’ 46” 0.692 FRII/CSS SSRQ 3CRR Table 6.1: Fermi -detected misaligned AGN. Notesa PKS 0625-354 shows some BL Lac object
characteristics in the optical band (Wills et al., 2004); b Distance to Cen A is assumed to be
3.8 Mpc (Harris et al. 2010). Taken from Abdo et al. (2010c).