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تسجيل مستخدم جديدConcluding Remarks at the Multifrequency Behaviour of High Energy Cosmic Sources -- XIII Workshop -- II
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تأليف
Paolo Padovani
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I summarise the concluding remarks I gave at the Multifrequency Behaviour of High Energy Cosmic Sources - XIII Workshop. That was not a summary talk and was meant to be provocative. I first give what I think the main message of the workshop was, then provide some (biased) highlights, touch upon the upcoming new facilities and the issues of quantity vs. quality and productivity in astronomy, and finally conclude with a look to the future. Astronomers who did not attend the workshop might still find the first two topics appealing and the last two thought-provoking.
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Ultra-high-energy (UHE) cosmic rays (CRs) interact with cosmic background radiation through hadronic processes, and the Universe would become `opaque to UHE CRs of energies $sim$($10^{18}$- $10^{20}$) eV over about several tens of Mpc, setting the Gr
eisen-Zatsepin-Kuzmin (GZK) horizon. We demonstrate that a non-negligible fraction of the UHE CRs arriving on Earth could originate from beyond the GZK horizon when heavy nuclear CRs, and the population and evolution of UHE CR sources are taken into account. We show how the multi-particle CR horizon is modified by different source populations, and discuss how this leads to the natural emergence of an isotropic flux component in the observed UHE CR background. This component would coexist with an anisotropic foreground component contributed by nearby sources within the GZK horizon.
A few simple if not obvious statements at the end of the conference: This was an useful conference. It has shown us the rapidly widening scope of the binary star research which now extends in its applications from the realm of planets to black holes
and binary galactic nuclei. The binary star domain appears to be -- in parallel to searched for extra-solar planets and star formation -- the most active areas of stellar astrophysics. I am glad I attended this conference.
The origin of ultra-high energy cosmic rays (UHECRs) is still unknown. It has recently been proposed that UHECR anisotropies can be attributed to starburst galaxies or active galactic nuclei. We suggest that the latter is more likely and that giant-l
obed radio galaxies such as Centaurus A and Fornax A can explain the data.
We present new measurements of the energy spectra of cosmic-ray (CR) nuclei from the second flight of the balloon-borne experiment Cosmic Ray Energetics And Mass (CREAM). The instrument included different particle detectors to provide redundant charg
e identification and measure the energy of CRs up to several hundred TeV. The measured individual energy spectra of C, O, Ne, Mg, Si, and Fe are presented up to $sim 10^{14}$ eV. The spectral shape looks nearly the same for these primary elements and it can be fitted to an $E^{-2.66 pm 0.04}$ power law in energy. Moreover, a new measurement of the absolute intensity of nitrogen in the 100-800 GeV/$n$ energy range with smaller errors than previous observations, clearly indicates a hardening of the spectrum at high energy. The relative abundance of N/O at the top of the atmosphere is measured to be $0.080 pm 0.025 $(stat.)$ pm 0.025 $(sys.) at $sim $800 GeV/$n$, in good agreement with a recent result from the first CREAM flight.
We reconsider the possibility that gamma-ray bursts (GRBs) are the sources of the ultra-high energy cosmic rays (UHECRs) within the internal shock model, assuming a pure proton composition of the UHECRs. For the first time, we combine the information
from gamma-rays, cosmic rays, prompt neutrinos, and cosmogenic neutrinos quantitatively in a joint cosmic ray production and propagation model, and we show that the information on the cosmic energy budget can be obtained as a consequence. In addition to the neutron model, we consider alternative scenarios for the cosmic ray escape from the GRBs, i.e., that cosmic rays can leak from the sources. We find that the dip model, which describes the ankle in UHECR observations by the pair production dip, is strongly disfavored in combination with the internal shock model because a) unrealistically high baryonic loadings (energy in protons versus energy in electrons/gamma-rays) are needed for the individual GRBs and b) the prompt neutrino flux easily overshoots the corresponding neutrino bound. On the other hand, GRBs may account for the UHECRs in the ankle transition model if cosmic rays leak out from the source at the highest energies. In that case, we demonstrate that future neutrino observations can efficiently test most of the parameter space -- unless the baryonic loading is much larger than previously anticipated.
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