We report a stochastic mechanism of particle acceleration from first principles in an environment having properties like those of Radio Lobes in AGNs. We show that energies $\sim 10^{20}$ eV are reached in $\sim 10^6$ years for protons. Our results reopen the question regarding the nature of the high-energy cutoff in the observed spectrum: whether it is due solely to propagation effects, or whether it is also affected by the maximum energy permitted by the acceleration process itself.
The possibility that rapidly rotating supermassive black holes associated with quasar remnants may provide sites for the acceleration of ultra-high energy cosmic rays (UHECRs) is considered. It is shown that prodigious TeV emission through curvature losses is an important consequence of this mechanism. Given the measured UHECR flux, it is expected that nearby dormant AGNs will be detectable by current Tev experiments. The implications for the Sgr A* source are also briefly considered.
The Pierre Auger Observatory reports that 20 of the 27 highest energy cosmic rays have arrival directions within 3.2 deg of a nearby galaxy in the Veron-Cetty & Veron Catalog of Quasars and Active Galactic Nuclei (12th Ed.), with ~5 of the correlations expected by chance. In this paper we examine the correlated galaxies to gain insight into the possible UHECR sources. We find that 14 of the 21 correlated VCV galaxies are AGNs and we determine their bolometric luminosities. The remaining 7 are primarily star-forming galaxies. The bolometric luminosities of the correlated AGNs are all greater th…
The discoveries of the GZK cutoff with the HiRes and Auger Observatories and the discovery by Auger of clustering of >~60 EeV ultra-high energy cosmic rays (UHECRs) towards nearby <~75 Mpc) AGNs along the supergalactic plane establishes the astrophysical origin of the UHECRs. The likely sources of the UHECRs are gamma-ray bursts and radio-loud AGNs because: (1) they are extragalactic; (2) they are sufficiently powerful; (3) acceleration to ultra-high energies can be achieved in their relativistic ejecta; (4) anomalous X-ray and gamma-ray features can be explained by nonthermal hadron accelerat…
We present the general properties of the Active Galactic Nuclei (AGNs) and discuss the origin and structure of jets that are associated to a fraction of these objects. We then we address the problems of particle acceleration at highly relativistic energies and set limits on the luminosity of AGN jets for being origin of UHECRs.
(abridged) The Pierre Auger Collaboration has reported 27 Ultra-High Energy Cosmic Ray Events (UHECRs) with energies above 56 EeV and well determined arrival directions as of 2007 August 31. They find that the arrival directions are not isotropic, but instead appear correlated with the positions of nearby AGNs. Our aim was to determine the sources of these UHECRs by comparing their arrival directions with more comprehensive source catalogs. Four (eight) of the 27 UHECRs with energy >56EeV detected by the Pierre Auger Observatory have arrival directions within 1.5deg (3.5deg) of the extended (>…
The nearby radio galaxy Centaurus A is poorly studied at high frequencies with conventional radio telescopes because of its very large angular size, but is one of a very few extragalactic objects to be detected and resolved by the Wilkinson Microwave Anisotropy Probe (WMAP). We have used the five-year WMAP data for Cen A to constrain the high-frequency radio spectra of the 10-degree giant lobes and to search for spectral changes as a function of position along the lobes. We show that the high-frequency radio spectra of the northern and southern giant lobes are significantly different: the spec…
In the past year, the HiRes and Auger collaborations have reported the discovery of a high-energy cutoff in the ultra-high energy cosmic-ray (UHECR) spectrum, and an apparent clustering of the highest energy events towards nearby active galactic nuclei (AGNs). Consensus is building that such $\sim 10^{19}$--$10^{20}$ eV particles are accelerated within the radio-bright lobes of these sources, but it is not yet clear how this actually happens. In this paper, we report (to our knowledge) the first treatment of stochastic particle acceleration in such environments from first principles, showing t…
UHECRs are roughly isotropic and attain very large energies, $E \gtrsim 3\times 10^{20} \; eV$. Conventional models fail to explain both facts. I show here that acceleration of UHECRs in GRBs satisfies both observational constraints. Using Mészáros and Rees' (1994) model of GRBs as due to hyperrelativistic shocks, I show that the highest energies that can be attained thusly are $E\simeq 10^{20}\;θ^{-5/3} n_1^{-5/6}\; eV$, explaining the energy of the Bird \etal\/ (1995) event even without beaming. The traditional photopion catastrophe affecting UHECR acceleration in AGNs is circumvented. An or…
We argue that, when compared to AGNs in dynamical terms, microquasars are found in low density, low pressure environments. Using a simple analytic model, we discuss radio lobe dynamics and emission. Dynamical considerations for GRS 1915+105 and GRO J1655-40 show that they are located in ISM densities well below the canonical n_ISM ~ 1 cm^{-3} unless the jets are unusually narrow or much more powerful than currently believed.
The energetics of electromagnetic acceleration of ultra-high-energy cosmic rays (UHECRs) is constrained both by confinement of a particle within an acceleration site and by radiative energy losses of the particle in the confining magnetic fields. We demonstrate that the detection of ~ 3 x 10^{20} eV events is inconsistent with the hypothesis that compact cosmic accelerators with high magnetic fields can be the sources of UHECRs. This rules out the most popular candidates, namely spinning neutron stars, active galactic nuclei (AGNs), and gamma-ray burst blast waves. Galaxy clusters and, perhaps…
The Auger Collaboration reports that the arrival directions of >60 EeV ultra-high energy cosmic rays (UHECRs) cluster along the supergalactic plane and correlate with active galactic nuclei (AGN) within ~100 Mpc. The association of several events with the nearby radio galaxy Centaurus A supports the paradigm that UHECRs are powered by supermassive black-hole engines and accelerated to ultra-high energies in the shocks formed by variable plasma winds in the inner jets of radio galaxies. The GZK horizon length of 75 EeV UHECR protons is ~100 Mpc, so that the Auger results are consistent with an …
A striking concentration of ultra-high energy cosmic ray (UHECR) events observed by the Pierre Auger Observatory around the direction of the nearby radio galaxy Centaurus A revives the idea that radio galaxies may be dominant sources of UHECR. In this paper, we give a brief overview about processes which may accelerate protons and nuclei in radio galaxies, and their relation to jet power, radio morphology and cosmic source density. We argue that, except for the most powerful FR-II radio galaxies, processes in radio lobes are unlikely to explain the origin of UHECR. However, Fermi acceleration …
Since their first observation in 1962, the existence of Ultra High Energy Cosmic Rays (UHECR) remains a mystery in modern astrophysics. Those cosmic rays, with energies well above 50 EeV ($50\times 10^{18}$eV), can hardly be accelerated, even in the most active parts of our universe such as FR-II radio galaxies or AGNs, nor can they travel on distances larger than 100 Mpc. In the following some of the production and acceleration models for UHECR are reviewed and some of the transport issues are exposed. Finally the detection and identification on Earth of those ``cosmic bullets'' are presented…
We have calculated the probability that the clustering of arrival directions of ultra-high energy cosmic rays (UHECRs) is consistent with a finite number of uniformly distributed proton sources. The case of a continuous source distribution is reached only for an unrealisticly high source density, $n_s\gg 10^{-2}/Mpc^3$. Even for densities as large as $n_s=10^{-3}/Mpc^3$, less than half of the observed cluster are on average by chance. For the best-fit value $n_s=(1-3)\times 10^{-5}/Mpc^3$ derived from the AGASA data, the probability that at least one observed cluster is from a true point sourc…
The curvature radiation produced by particles accelerating near the event horizon of a spinning supermassive black hole, threaded by externally supported magnetic field lines is considered. It is shown that light nuclei suffer catastrophic curvature losses that limit the maximum energy they can attain to values well below that imposed by the maximum potential difference induced by the black hole dynamo, unless the curvature radius of magnetic field lines largely exceeds the gravitational radius. It is further shown that the dominant fraction of the rotational energy extracted from the black ho…
The Pierre Auger collaboration has reported a correlation between Ultra-High Energy Cosmic Rays (UHECR) and nearby Active Galactic Nuclei (AGNs) within 75 Mpc. Two of these events fall within 3 degrees from Centaurus A, the nearest AGN, clearly suggesting that this object is a strong UHECR emitter. Here we pursue this hypothesis and forecast the expected rate of ultra-high energy neutrinos in detectors like IceCube. In our baseline model we find a rate of 0.4--0.6 yr^-1 events above a threshold of 100 TeV, the uncertainty of which is mainly related to the poor knowledge of the physical paramet…
Observation of Ultra High Energy Cosmic Rays (UHECR) -whose energy exceeds $10^20$eV- is still a puzzle for modern astrophysics. The transfer of more than 16 Joules to a microscopic particle can hardly be achieved, even in the most powerful cosmic accelerators such as AGN's, GRB's or FR-II radio galaxy lobes. Potential sources must also lie within 100 Mpc of the Earth as the interaction length of protons, nuclei or photons is less than 10Mpc. However no visible counterpart of those sources has been observed. Calling upon new physics such as Topological Defect interactions or Super Massive Reli…
The lobes of radio-loud AGNs are huge reservoirs of energy so even if the central engine turns off, the radio source is still observable for a substantial amount of time. It is quite obvious, however, that the ignition of a new period of activity can occur before the lobes have faded completely. Restarted activity has been observed in several giant radio galaxies but until recently, it has hardly ever been observed in more compact sources. Here we present an early result of a pilot VLBA programme targeted at compact core-dominated triples. It is found that the milliarcsecond structure of the c…
Ultra High Energy Cosmic Rays (UHECRs) hit the Earth's atmosphere with energies exceeding $10^{18}$ eV. This is the same energy as carried by a tennis ball moving at 100 km/h, but concentrated on a sub-atomic particle. UHECRs are so rare (the flux of particles with $E > 10^{20}$ eV is 0.5/km$^2$/century) that only a few such particles have been detected over the past 50 years. Recently, the HiRes and Auger experiments have reported the discovery of a high-energy cut-off in the UHECR spectrum, and Auger has found an apparent clustering of the highest energy events towards nearby active galactic…
We believe that the radiation we receive from Gamma-Ray Bursts (GRBs) and radio loud Active Galacti Nuclei (AGNs) originates from the transformation of bulk relativistic motion into random energy. Mechanisms to produce, collimate and accelerates the jets in these sources are uncertain, and it may be fruitful to compare the characteristics of both class of sources in search of enlightening similarities. I will present some general characteristics of radio loud AGNs and GRBs such as their bulk Lorentz factors and the power of their jets. I will also discuss the way in which the energy in bulk re…
Aims and Methods: We present the results of VLBI observations of nineteen GHz-Peaked-Spectrum (GPS) radio sources at 1.6 GHz. Of them, 15 sources are selected from the Parkes Half Jansky (PHJ) sample (Snellen 2002), 4 others are from our previous observation list. We aimed at imaging the structure of GPS sources, searching for Compact Symmetric Objects (CSOs) and studying the absorption for the convex radio spectra of GPS sources. Results: We obtained total intensity 1.6 GHz VLBI images of 17 sources for the first time. Of them, 80% show mini-double-lobe radio structure, indicating that they a…
Radio core dominance, usually measured by R, the rest frame ratio of core to lobe flux density, has been used as an indicator of Doppler boosting of quasars' radio jets and hence the inclination of the central engine's spin axis to the line of sight. The use of lobe flux density as a means of normalizing the boosted core flux density to the available intrinsic power of the central engine introduces scatter. This is because the emission from the radio lobes depends strongly on the interaction of the jet with the environment at distances beyond several Kpc from the nucleus. Total kinetic power i…
We present results of a multi-epoch intra-night optical monitoring of eleven blazars consisting of six BL Lac objects and five radio core-dominated quasars (CDQs). These densely sampled and sensitive R-band CCD observations, carried out from November 1998 through May 2002 during a total of 47 nights with an average of 6.5 hours/night, have enabled us to detect variability amplitudes as low as $\sim 1%$ on intra-night time scales. A distinction is found for the first time between the intra-night optical variability (INOV) properties of the these two classes of relativistically beamed radio-loud…
In this paper we present an investigation of numerical Monte Carlo simulations of the diffusive shock acceleration in the test particle limit. Very high gamma flow astrophysical plasmas, have been used, from $γ_{up}$ $\sim50$ up to $γ_{up}$ $\sim1000$, which could be relevant to the suggested models of AGNs Jets and their Central Engines as well as the ultra-relativistic shock particle acceleration in Gamma Ray Burst (GRB) fireballs. Particularly the energy gain per shock crossing and the time constant for the above high relativistic processes is numerically calculated. We explicitly find a co…
