D.V. Semikoz

Gamma-ray induced cascades and magnetic fields in the intergalactic medium

We present the results of Monte-Carlo simulations of three-dimensional electromagnetic cascade initiated by interactions of the multi-TeV γ-rays with the cosmological infrared/optical photon background in the intergalactic medium. Secondary electrons in the cascade are deflected by the intergalactic magnetic fields before they scatter on CMB photons. This leads to extended 0.1-10 degree scale emission at multi-GeV and TeV energies around extragalactic sources of very-highenergy γ-rays. The morphology of the extended emission depends, in general, on the properties of magnetic fields in the intergalactic medium. Using Monte-Carlo simulated data sets, we demonstrate that the decrease of the size of extended source with the increase of energy allows to measure weak magnetic fields with magnitudes in the range from ≤ 10 G to 10G if they exist in the voids of the Large Scale Structure.

Spectral Distortion of Cosmic Microwave Background Radiation by Scattering on Hot Electrons: Exact Calculations

The spectral distortion of the cosmic microwave background radiation produced by the inverse Compton scattering on hot electrons in clusters of galaxies (thermal Sunyaev-Zeldovich effect) is calculated for arbitrary optical depth and electron temperature. The distortion is found by a numerical solution of the exact Boltzmann equation for the photon distribution function. In the limit of small optical depth and low electron temperature, our results confirm the previous analyses. In the opposite limits, our method is the only one that permits us to make accurate calculations.

Cosmic ray anisotropy as signature for the transition from galactic to extragalactic cosmic rays

We constrain the energy at which the transition from Galactic to extragalactic cosmic rays occurs by computing the anisotropy at Earth of cosmic rays emitted by Galactic sources. Since the diffusion approximation starts to loose its validity for E/Z1016−17 eV, we propagate individual cosmic rays using Galactic magnetic field models and taking into account both their regular and turbulent components. The turbulent field is generated on a nested grid which allows spatial resolution down to fractions of a parsec. Assuming sufficiently frequent Galactic CR sources, the dipole amplitude computed for a mostly light or intermediate primary composition exceeds the dipole bounds measured by the Auger collaboration around E ≈ 1018 eV. Therefore, a transition at the ankle or above would require a heavy composition or a rather extreme Galactic magnetic field with strength 10 μG. Moreover, the fast rising proton contribution suggested by KASCADE-Grande data between 1017 eV and 1018 eV should be of extragalactic origin. In case heavy nuclei dominate the flux at E1018 eV, the transition energy can be close to the ankle, if Galactic CRs are produced by sufficiently frequent transients as e.g. magnetars.

Neutrino oscillations in the early universe: how can large lepton asymmetry be generated?

Abstract The lepton asymmetry that could be generated in the early universe through oscillations of active to sterile neutrinos is calculated (almost) analytically for small mixing angles, sin2θ

Ultrahigh energy nuclei in the galactic magnetic field

Observations are consistent with a significant fraction of heavy nuclei in the cosmic ray flux above a few times 1019 eV. Such nuclei can be deflected considerably in the Galactic magnetic field, with important implications for the search of their sources. We perform detailed simulations of heavy nuclei propagation within recent Galactic magnetic field models. While such models are not yet sufficiently constrained to predict deflection maps in detail, we find general features of the distribution of (de-) magnified flux from sources. Since in most theoretical models sources of heavy nuclei are located in the local large scale structure of galaxies, we show examples of images of several nearby galaxy clusters and of the supergalactic plane. Such general features may be useful to develop efficient methods for source reconstruction from observed ultrahigh energy cosmic ray arrival directions.

Ultrahigh energy nuclei in the turbulent Galactic magnetic field

Abstract In this work we study how the turbulent component of the Galactic magnetic field (GMF) affects the propagation of ultrahigh energy heavy nuclei. We investigate first how the images of individual sources and of the supergalactic plane depend on the properties of the turbulent GMF. Then we present a quantitative study of the impact of the turbulent field on (de-)magnification of source fluxes, due to magnetic lensing effects. We also show that it is impossible to explain the Pierre Auger data assuming that all ultrahigh energy nuclei are coming from Cen A, even in the most favorable case of a strong, extended turbulent field in the Galactic halo.

DEGREE-SCALE GeV “JETS” FROM ACTIVE AND DEAD TeV BLAZARS

We show that images of TeV blazars in the GeV energy band should contain, along with point-like sources, degree-scale jet-like extensions. These GeV extensions are the result of electromagnetic cascades initiated by TeV γ-rays interacting with extragalactic background light and the deflection of the cascade electrons/positrons in extragalactic magnetic fields (EGMF). Using Monte-Carlo simulations, we study the spectral and timing properties of the degree-scale extensions in simulated GeV band images of TeV blazars. We show that the brightness profile of such degree-scale extensions can be used to infer the lightcurve of the primary TeV γ-ray source over the past 10 yr, i.e. over a time scale comparable to the life-time of the parent active galactic nucleus. This implies that the degree-scale jet-like GeV emission could be detected not only near known active TeV blazars, but also from “TeV blazar remnants”, whose central engines were switched off up to ten million years ago. Since the brightness profile of the GeV “jets” depends on the strength and the structure of the EGMF, their observation provides additionally information about the EGMF. Subject headings: gamma rays: galaxies – galaxies: active – galaxies: jets – methods: numerical Introduction. Significant progress in understanding the activity of blazars, i.e. active galaxies with relativistic jets aligned with the line of sight, was achieved with the start of operation of the Fermi telescope. The combination of data from Fermi in the 0.1 – 10 GeV energy band and from ground based γ-ray telescopes like HESS, MAGIC and VERITAS in the 100 GeV – 10 TeV band provides detailed simultaneous spectral and timing information for the most extreme representatives of the blazar population (Abdo et al. 2009). The TeV γ-ray flux from distant blazars is significantly attenuated on the way from the source to the Earth by pair production on the infrared/optical extragalactic background light (EBL) (Kneiske et al. 2004; Stecker et al. 2006; Franceschini et al. 2008; Primack et al. 2008). TeV γ-rays that are absorbed on the way from the primary γ-ray source initiate electromagnetic cascades in the intergalactic space. Charged component of the electromagnetic cascade is deflected by the EGMF. Potentially observable effects of such electromagnetic cascades in the EGMF include the delayed “echoes” of multi-TeV γ-ray flares (Plaga 1995; Murase et al. 2008) and the appearance of extended emission around initially point-like γ-ray sources (Aharonian et al. 1994; Neronov & Semikoz 2007, 2009; Dolag et al. 2009; Elyiv et al. 2009). TeV γ-ray emission from blazars is believed to be relativistically beamed into a narrow cone (jet) with an opening angle Θjet ∼ Γ ∼ 5◦ [Γ/10], where Γ is the bulk Lorentz factor of the γ-ray emitting plasma. Blazars are a special type of γ-ray emitting AGN for which the angle between the line of sight (LOS) and the jet axis, θobs, is θobs . Θjet, see Fig. 1 (Urry et al. 1991). In general, the number of blazars with a given jet-LOS ext 0 O

Anisotropic cosmic ray diffusion and its implications for gamma-ray astronomy

Analyses of TeV-PeV cosmic ray (CR) diffusion around their sources usually assume either isotropic diffusion or anisotropic diffusion due to the regular Galactic magnetic field. We show that none of them are adequate on distances smaller than the maximal scale Lmax ~ 100 pc of fluctuations in the turbulent interstellar magnetic field. As a result, we predict anisotropic gamma-ray emissions around CR proton and electron sources, even for uniform densities of target gas. The centers of extended emission regions may have non-negligible offsets from their sources, leading to risks of misidentification. Gamma-rays from CR filaments have steeper energy spectra than those from surrounding regions. We point out that gamma-ray telescopes can be used in the future as a new way to probe and deduce the parameters of the interstellar magnetic field.

Search for single sources of ultra high energy cosmic rays on the sky

In this paper, we suggest a new way to identify single bright sources of Ultra High Energy Cosmic Rays (UHECR) on the sky, on top of background. We look for doublets of events at the highest energies, E > 6 1019eV, and identify low energy tails, which are deflected by the Galactic Magnetic Field (GMF). For the sources which are detected, we can recover their angular positions on the sky within one degree from the real ones in 68% of cases. The reconstruction of the deflection power of the regular GMF is strongly affected by the value of the turbulent GMF. For typical values of 4μG near the Earth, one can reconstruct the deflection power with 25% precision in 68% of cases.

Unstable massive tau-neutrinos and primordial nucleosynthesis

Abstract The impact of unstable Majorana ντ on primordial nucleosynthesis is considered. The mass and lifetime of ντ are taken in the intervals 0.1–20 MeV and 0.001–400 s respectively. The studied decay modes are ντ → νμ + φ and ντ → νe + φ, where φ is a massless (or light) scalar. Integro-differential kinetic equations are solved numerically without any simplifying assumptions. Our results deviate rather strongly from earlier calculations. Depending on mass, lifetime, and decay channels of the ντ, the number of effective neutrino species (found from 4He), in addition to the 3 standard ones, varies from −2 to +2.5. The abundances of 2H and 7Li are also calculated.