Active Galactic Nuclei as Potential Sources of Ultra-High Energy Cosmic Rays
Abstract
:1. Introduction
2. Basic Results of Current UHECR Experiments
2.1. Energy Spectrum of UHECRs
2.2. Composition of UHECRs
2.3. Anisotropies in the Arrival Direction of UHECRs
3. On Possible Correlations of UHECRs with Matter and Known Sources
3.1. On Correlations with Catalogs
3.2. On Correlation with Matter
3.3. On Correlation with Nearby Radio Galaxies
3.4. An UHECR Echo from the Past?
4. Radio Galaxies as Prime UHECR Candidate Sources
- Cen A: As the closest active galaxy to Earth ( Mpc), Centaurus A (Cen A) is one of the most prominent radio sources in the southern sky. Its inner region harbours a warped, relatively massive dust and gas disk, most likely the product of a past merger activity between a small, gas-rich spiral galaxy and a larger, elliptical one. There are in fact several lines of evidence suggesting that at least one major merger event occurred some – years ago [49]. Over time, Cen A then most likely underwent several phases of AGN activity [50,51]. Its central engine hosts a black hole of mass ≃ (0.3–0.8) [52], and currently emits a (nuclear) bolometric (photon) luminosity of erg/s [53]. At radio frequencies, Cen A is known for its peculiar morphology including a compact radio core, a sub-pc scale jet and counter-jet, a one-sided kpc-scale jet (∼4 kpc in projection), inner radio lobes as well as giant outer lobes with a length of several hundreds of kiloparsec, e.g., [54]. At high photon energies, gamma-ray observations have revealed GeV emission from the giant lobes as well as TeV emission along its large-scale jet, demonstrating the operation of efficient in situ particle acceleration [55,56,57]. VLBI observations indicate that Cen A is a non-blazar source, with its inner jet misaligned by about (12–45) and characterized by moderate bulk flow speeds ≤ c for the radio region probed [58]. EHT observations have revealed asymmetric (spine-sheath type) jet emission on scales of a few hundred Schwarzschild radii [59]. On larger scales (>100 pc), trans-relativistic jet speeds of ∼0.5–0.7 c have been inferred [60,61]. Estimates for the “current” kinetic jet power are in the range ∼– erg/s [56,62,63].
- M87: As the second closest active galaxy ( Mpc [64]), the Virgo Cluster galaxy M87 (NGC 4486) hosts one of the most massive black holes of [65,66]. Classified as low-excitation FR I source, M87 is thought to be currently accreting in a radiatively inefficient (RIAF) mode [67,68]. Common estimates for its total nuclear (disk and jet) bolometric luminosity in fact do not exceed erg/s by much [53,69,70,71]. In the radio, M87 exhibits an extended morphology including a sub-pc scale jet and counter-jet, a one-sided kpc-scale jet (∼2 kpc in projection) that is also seen in X-rays, inner lobes as well as a large-scale halo of size ∼50 kpc [69,70,72,73,74]. The X-ray emission seen in the large-scale jet of M87 provides evidence for the presence of ultra-relativistic electrons and the operation of efficient in situ acceleration [75]. Both, on sub-parsec as well as on kpc scales evidence has been found for a stratified jet structure compatible with a fast spine and a slower sheath [76,77]. There is evidence, including superluminal motion seen at optical and X-ray energies, that on larger scales (>100 pc) the jet still possesses significantly trans-relativistic (0.7 c) speeds [78,79]. Estimates for the kinetic power of the jet in M87 are somewhat uncertain and range from to erg/s [67,71,73,75]. Being at the center of a galaxy cluster, it has been argued that M87 might be repeatedly fed by accretion of satellite galaxies containing intermediate mass black holes [80]. There is evidence that accretion of a smaller galaxy has caused an important modification of its outer halo in the last Gyr [81]. On the other hand, many X-ray features (e.g., cavities) on halo-scale appear to be a result of recurrent AGN activity (on timescales probably down to ∼ yr) [82,83].
- Fornax A: Located at Mpc [84], Fornax A (NGC 1316) is the brightest member of the Fornax Cluster and a prominent merger remnant. In the optical, it resembles an elongated spheroid-type galaxy. In the radio, an S-shaped nuclear radio jet (∼5 kpc in projection) and giant double lobes spanning ∼350 kpc are seen [85]. Fornax A harbours a supermassive black hole of ≃ [86]. Given its peculiar morphology, Fornax A is thought to be shaped by a major merger about 3 Gyr ago, probably followed by some minor mergers of smaller companions [87,88,89]. There is evidence that the extended radio lobes have been formed by multiple episodes of nuclear activity on timescales of several Myr [90]. Characteristic estimates for the current jet power are in the range erg/s [90,91,92]. While Fornax A is commonly classified as an FR I source, different episodes of activity could explain some challenges as to a unique classification, e.g., the phase that shaped the lobes appears to have been more powerful compared with the activity of the central emission [90].
5. Basics Physics Constraints on UHECR Candidate Sources
6. UHECR Acceleration in AGNs: Sites and Mechanisms
6.1. Cosmic-Ray Acceleration in the Magnetospheres of Rotating Supermassive Black Holes
6.2. Shear Particle Acceleration in the Large-Scale Jets of AGNs
- Non-gradual shear particle acceleration in trans-relativistic FR I type jets: Kimura et al. [134] have reproduced the observed UHECR spectrum in a scenario that considers non-gradual shear particle acceleration in large-scale FR I jets, see Figure 15. The reference model assumes that Galactic-type cosmic rays (with, e.g., CR proton densities comparable to our Galaxy) are picked up, re-cycled and re-accelerated to UHE energies in a mildly relativistic jet (, G) surrounded by a narrow shear layer (). Their Monte Carlo simulations suggest that UHECRs escaping the source, possess a rather hard energy spectrum (). The chemical compositions at UHE energies (relative abundance ratio) is somewhat more complex, as different particle species (i) have different injection thresholds into the acceleration process ( TeV). Achievable maximum energy, EeV, are limited by the jet size (by ) and (via in , cf. Equation (8)) dominated by the diffusion process (assumed turbulence properties) in the cocoon. These numbers are in principle compatible with requirements on source energetics for FR I type sources, cf. also Equation (4). Given an FR I number density of – Mpc (cf. footnote 2) for example, an average source luminosity – erg/s would be needed to account for the observed UHECR luminosity erg/(Mpc yr) [20]. Moreover, possible anisotropy constraints are relaxed, partly due to suitable UHECR spectral shapes, as well as a high source number density (multi-FR I source contributions) with heavy composition. The results shown are, however, sensitive to the chosen cocoon properties (i.e., the assumed cocoon size and turbulence scale) and, in particular, dependent on a narrow velocity transition layer (defining the energy threshold for CR injection). Enlarging to satisfy jet stability constraints [125], for example, is likely to affect the outcome. Nevertheless, these simulations illustrate that non-gradual shear acceleration in an ensemble of FR I’s (and not only FR II’s) could in principle play an important role in UHECR acceleration.
- Non-gradual shear particle acceleration in large-scale FR II type jets: Powerful AGN jets can remain significantly relativistic () on large scales, allowing for a substantial particle energy change per cycle if strong flow gradients are experienced over the mean free path of a particle, cf. Equation (7). In fact, given suitable conditions, significant -type boosts are possible across quasi-narrow shears, in which case only a few cycles (“shots”) would be needed to boost Galactic-type cosmic rays to UHE energies (sometimes referred to as “espresso” acceleration, e.g., ref. [133]). In this context, Mbarek & Caprioli [135,146] have recently studied the recycling of energetic cosmic rays in a relativistic () large-scale jet by following their test particle trajectories in a simulated (PLUTO) 3d-MHD jet environment, assuming a homogeneous ambient medium setup, see, e.g., Figure 16.Given the fact that MHD jet simulations generally come with limited resolution (i.e., here of the order , where is the jet radius), a suitable (sub-grid) prescription needs to be designed to explore the effect of unresolved turbulence on the cosmic-ray transport. In the considered example, a gyro-dependent spatial diffusion coefficient , with the Larmor radius, has been employed (with momentum-diffusion being neglected), exploring cases where , with corresponding to Bohm diffusion, and implying no diffusion/scattering. The results show that incorporation of particle scattering (spatial diffusion) generally allows more particles to reach higher energies by probing the jet spine, making acceleration more efficient and resulting in a hardening of the injection spectrum. These features appear akin to what is expected in gradual- and non-gradual shear particle acceleration. To adequately assess the implications, an extension of the simulation framework to more general conditions will be helpful. As of now, comparison appears limited to, e.g., gyro-dependent pitch-angle diffusion, and rather high-power ( erg/s), significantly magnetized () FR II type jets. Nevertheless, these results already illustrate that for sufficiently large mean free paths multiple energy-boosts can occur in the environment of relativistic jets, allowing to push particle energies beyond what, e.g., stochastic or gradual shear acceleration alone might achieve.
- Gradual shear particle acceleration in relativistic AGN jets: On large scales, the shearing layers around relativistic jets are likely to encompass a sizeable fraction of the jet radius, e.g., [125]. Acceleration of electrons in such shearing flows has been proposed to account for the extended X-ray synchrotron emission that requires the maintenance of ultra-relativistic electrons () on kpc-scales, e.g., refs. [123,126]. In order to compete with diffusive escape, efficient shear acceleration typically requires sufficiently relativistic flow speeds, i.e., bulk Lorentz factors of some few [130,138,139]. At mildly relativistic speeds, intrinsic particle spectra become rather soft and sensitive to the underlying shear flow profile [142]. It seems natural to suppose that the underlying shear acceleration mechanism can be operative not only on electrons but also on cosmic rays. In principle, by modelling their multi-wavelength large-scale jet emission, improved constraints on the parameter space of individual sources can be obtained, thereby allowing a more precise classification [126]. Still, for a first orientation, a Hillas type approach (Section 5) may be chosen, requiring the properties of a potential UHECR source to be such as (i) to allow UHECRs to be confined within the shear, (ii) to enable particle acceleration to overcome radiative cooling (e.g., ) and (iii) to operate within the lifetime () of the system. Figure 17 shows an example, adjusted to (mildly relativistic) large-scale AGN jets, assuming a linearly decreasing flow profile with on the jet axis. A shear-width to jet length ratio has been employed.Figure 17 suggests that shear acceleration of protons to energies ∼ eV can in principle be achieved for a quite plausible range of parameters, cf. also refs. [123,126,138,139] The vertical (black) line, for example, denotes the regime for a Cen A type source (G, cf. [57]), where the presence of a shear width of several tens of parsec would be sufficient. However, proton acceleration to ∼ eV in mildly relativistic shearing flows would seem to be less likely (requiring large, extended shears and higher magnetic fields). Since higher energies are possible either, in faster flows or for heavier particles, this would then suggest that current Cen A type (FR I) sources are contributors of heavy UHECRs, while ∼100 EeV protons would have to be associated with an earlier, more powerful (faster jet, FR II type) stage. This could possibly be linked to an evolutionary scenario where during the lifetime of a radio galaxy (at least for some of the nearby), basic characteristics could change FR class from FR II to FR I. In a simple leaky-box approach for gradual shear acceleration, UHECRs escaping their source, can have a relatively hard spectrum, approaching [130]. The spectrum is expected to be more complex, though, if non-gradual shear acceleration becomes operative toward higher energies.
6.3. Shock Acceleration of UHECRs in AGN Jets
7. Conclusions and Perspectives
Funding
Acknowledgments
Conflicts of Interest
1 | This so-called GZK horizon is longest ( Mpc) for 100 EeV protons and heavy nuclei (e.g., iron), but can be much shorter for intermediate masses (e.g., Mpc for helium and Mpc for silicon). |
2 |
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Rieger, F.M. Active Galactic Nuclei as Potential Sources of Ultra-High Energy Cosmic Rays. Universe 2022, 8, 607. https://doi.org/10.3390/universe8110607
Rieger FM. Active Galactic Nuclei as Potential Sources of Ultra-High Energy Cosmic Rays. Universe. 2022; 8(11):607. https://doi.org/10.3390/universe8110607
Chicago/Turabian StyleRieger, Frank M. 2022. "Active Galactic Nuclei as Potential Sources of Ultra-High Energy Cosmic Rays" Universe 8, no. 11: 607. https://doi.org/10.3390/universe8110607