Active Galactic Nuclei: Sources for ultra high energy cosmic rays?

Monthly Notices of the Royal Astronomical Society, 2008

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 ∼ 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 that energies ∼ 10 20 eV are reached in ∼ 10 6 years for protons. However, our findings reopen the question regarding whether the high-energy cutoff is due solely to propagation effects, or whether it does in fact represent the maximum energy permitted by the acceleration process itself.

Monthly Notices of the Royal Astronomical Society: Letters, 2008

We measure the correlation between the arrival directions of the highest energy cosmic rays detected by the Pierre Auger Observatory with the position of the galaxies in the H I Parkes All-Sky Survey (HIPASS) catalogue, weighted for their H I flux and Auger exposure. The use of this absorption-free catalogue, complete also along the Galactic plane, allows us to use all the Auger events. The correlation is significant, being 86.2 per cent for the entire sample of H I galaxies, and becoming 99 per cent when considering the richest galaxies in H I content or 98 per cent with those lying between 40 and 55 Mpc. We interpret this result as the evidence that spiral galaxies are the hosts of the producers of ultra-high energy cosmic rays and we briefly discuss the classical (i.e. energetic and distant) long gamma-ray burst (GRBs), short GRBs, as well as newly born or late-flaring magnetars as possible sources of the Auger events. With the caveat that these events are still very few, and the theoretical uncertainties are conspicuous, we found that newly born magnetars are the best candidates. If so, they could also be associated with sub-energetic, spectrally soft, nearby, long GRBs. We finally discuss why there is a clustering of Auger events in the direction on the radio galaxy Cen A and an absence of events in the direction of the radio galaxy M87.

2002

Abstract. We argue that clusters of galaxies have an intergalactic medium, which is permeated by strong magnetic fields and also has a contribution of pressure from cosmic rays. These two components of total pressure are probably highly time dependent, and range probably between 1/10 of the gas pressure up to equipartition between gas pressure and the sum of the two other components. Radio galaxies are likely to provide the main source for both magnetic fields and cosmic rays. In this concept it becomes easy to understand the occasional mismatch between the total mass inferred from the assumption of hydrostatic equilibrium derived purely from gas, and the total mass derived from lensing data. We also suggest that the structure and topology of the magnetic field may be highly inhomogeneous- at least over a certain range of scales, and may contain long twisted filaments of strong magnetic fields, as on the Sun. The analogy with the interstellar medium may be fruitful to explore furthe...

Monthly Notices of the Royal Astronomical Society, 2011

We treat of the high-energy astrophysics of the inner ∼200 pc of the Galaxy. Our modelling of this region shows that the supernovae exploding here every few thousand years inject enough power to i) sustain the steady-state, in situ population of cosmic rays (CRs) required to generate the region's non-thermal radio and TeV γ-ray emission; ii) drive a powerful wind that advects non-thermal particles out of the inner GC; iii) supply the low-energy CRs whose Coulombic collisions sustain the temperature and ionization rate of the anomalously warm, envelope H 2 detected throughout the Central Molecular Zone; iv) accelerate the primary electrons which provide the extended, non-thermal radio emission seen over ∼150 pc scales above and below the plane (the Galactic centre lobe); and v) accelerate the primary protons and heavier ions which, advected to very large scales (up to ∼10 kpc), generate the recently-identified WMAP haze and corresponding Fermi haze/bubbles. Our modelling bounds the average magnetic field amplitude in the inner few degrees of the Galaxy to the range 60 < B/µG < 400 (at 2σ confidence) and shows that even TeV CRs likely do not have time to penetrate into the cores of the region's dense molecular clouds before the wind removes them from the region. This latter finding apparently disfavours scenarios in which CRs -in this star-burst-like environment -act to substantially modify the conditions of star-formation. We speculate that the wind we identify plays a crucial role in advecting low-energy positrons from the Galactic nucleus into the bulge, thereby explaining the extended morphology of the 511 keV line emission. We present extensive appendices reviewing the environmental conditions in the GC, deriving the star-formation and supernova rates there, and setting out the extensive prior evidence that exists supporting the notion of a fast outflow from the region. † IIF Marie Curie Fellow ticularly its metallicity ). The Galactic centre (GC) presents our closest view of a Galactic nucleus. The physical conditions pertaining in the GC put it on an intermediate footing between the relative quiescence of the Galactic disk interstellar medium (ISM) and the environs at the centre of luminous starbursts. Still, when viewed from the local perspective, many of the GC ISM parameters are extreme. The GC proffers, then, perhaps our best hope for a detailed understanding of the processes shaping the evolution of the nuclei of spiral galaxies and, in general, regions energised by galaxy mergers or interactions (Melia

Nuclear Physics B-proceedings Supplements, 2001

Using model calculations of the NN and π N interactions at ultrahigh energies, the chemical composition was estimated through the depth of the maximum of the shower development from the data obtained on the Yakutsk array for extensive air showers (EASs) and indicated that protons dominate in the primary radiation at 10 18 -10 19 eV . Similar estimations of the chemical composition from the Fly's Eye EAS array data [2] at an energy of ~10 19 eV demonstrated that the fraction of protons is ~ 90%. Applying an upgraded model of the interaction of particles to an analysis of the maximum of shower development detected on the Fly's Eye array, Wolfendale and Wibig concluded that cosmic rays with energies up to 3 × 10 18 eV are composed of heavy nuclei and that the fraction of heavy nuclei is no less than 50% for energies above 3 × 10 18 eV. Thus, these works exhibit contradictions in estimating the chemical composition of cosmic rays with ultrahigh energies through the maximum of shower development, because such an estimation requires the extrapolation of some characteristics of the NN and π N interactions from low to ultrahigh energies. In this study, the chemical composition of the cosmic rays are alternatively estimated through the observed and expected distributions of the numbers of showers in the galactic latitude.