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تسجيل مستخدم جديدCentaurus A at Ultra-High Energies
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We review the importance of Centaurus A in high energy astrophysics as a nearby object with many of the properties expected of a major source of very high energy cosmic rays and gamma-rays. We examine observational techniques and the results so far obtained in the energy range from 200 GeV to above 100 EeV and attempt to fit those data with expectations of Centaurus A as an astrophysical source from VHE to UHE energies.
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The nearby radio galaxy Centaurus A belongs to a class of Active Galaxies that are very luminous at radio wavelengths. The majority of these galaxies show collimated relativistic outflows known as jets, that extend over hundreds of thousands of parse
cs for the most powerful sources. Accretion of matter onto the central super-massive black hole is believed to fuel these jets and power their emission, with the radio emission being related to the synchrotron radiation of relativistic electrons in magnetic fields. The origin of the extended X-ray emission seen in the kiloparsec-scale jets from these sources is still a matter of debate, although Cen As X-ray emission has been suggested to originate in electron synchrotron processes. The other possible explanation is Inverse Compton (IC) scattering with CMB soft photons. Synchrotron radiation needs ultra-relativistic electrons ($sim50$ TeV), and given their short cooling times, requires some continuous re-acceleration mechanism to be active. IC scattering, on the other hand, does not require very energetic electrons, but requires jets that stay highly relativistic on large scales ($geq$1 Mpc) and that remain well-aligned with the line of sight. Some recent evidence disfavours inverse Compton-CMB models, although other evidence seems to be compatible with them. In principle, the detection of extended gamma-ray emission, directly probing the presence of ultra-relativistic electrons, could distinguish between these options, but instruments have hitherto been unable to resolve the relevant structures. At GeV energies there is also an unusual spectral hardening in Cen A, whose explanation is unclear. Here we report observations of Cen A at TeV energies that resolve its large-scale jet. We interpret the data as evidence for the acceleration of ultra-relativistic electrons in the jet, and favour the synchrotron explanation for the X-rays.
We present the results of a study that simulates trajectories of ultra-high energy cosmic rays from Centaurus A to Earth, for particle rigidities from $E/Z = 2$ EV to 100 EV, i.e., covering the possibility of primary particles as heavy as Fe nuclei w
ith energies exceeding 50 EeV. The Galactic magnetic field is modeled using the recent work of Jansson and Farrar (JF12) which fitted its parameters to match extragalactic Faraday rotation measures and WMAP7 synchrotron emission maps. We include the random component of the GMF using the JF12 3D model for $B_{rm rand}(vec{r})$ and explore the impact of different random realizations, coherence length and other features on cosmic ray deflections. Gross aspects of the arrival direction distribution such as mean deflection and the RMS dispersion depend mainly on rigidity and differ relatively little from one realization to another. However different realizations exhibit non-trivial substructure whose specific features vary considerably from one realization to another, especially for lower rigidities. At the lowest rigidity of 2 EV, the distribution is broad enough that it might be compatible with a scenario in which Cen A is the principle source of all UHECRs. No attempt is made here to formulate a robust test of this possibility, although some challenges to such a scenario are noted.
The fluxes of electrons, positrons, gammas, Cherenkov photons and muons in individual extensive air showers induced by the primary protons and helium, oxygen and iron nuclei at the level of observation have been estimated with help of the code CORSIC
A 6.616. The comparison show that the values of the function Xi**2 per one degree of freedom changes from 1.1 for iron nuclei to 0.9 for primary protons. As this difference is small all readings of detectors of the Vavilov-Cherenkov radiation have been used. At last, readings of underground detectors of muons with energies above 1 GeV have been exploited to make definite conclusion about chemical composition.
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obed radio galaxies such as Centaurus A and Fornax A can explain the data.
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