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Guenter Sigl

Publications and source records attributed to Guenter Sigl.

At least 55 records · Page 3Linked to original sources
arXiv2007

Ultrahigh energy cosmic rays as heavy nuclei from cluster accretion shocks

Large-scale accretion shocks around massive clusters of galaxies, generically expected in hierarchical scenarios of cosmological structure formation, are shown to be potential sources of the observed ultrahigh energy cosmic rays (UHECRs) by accelerating a mixture of heavy nuclei including the iron group elements. Current observations can be explained if the source composition at injection for the heavier nuclei is somewhat enhanced from simple expectations for the accreting gas. The proposed picture should be testable by current and upcoming facilities in the near future through characteristic features in the UHECR spectrum, composition and anisotropy. The associated X-ray and gamma-ray signatures are also briefly discussed.

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arXiv2007

Effects of Axion-Photon Mixing on Gamma-Ray Spectra from Magnetized Astrophysical Sources

Astrophysical gamma-ray sources come in a variety of sizes and magnetizations. We deduce general conditions under which gamma-ray spectra from such sources would be significantly affected by axion-photon mixing. We show that, depending on strength and coherence of the magnetic field, axion couplings down to ~ 1/(10**13 GeV) can give rise to significant axion-photon conversions in the environment of accreting massive black holes. Resonances can occur between the axion mass term and the plasma frequency term as well as between the plasma frequency term and the vacuum Cotton-Mouton shift. Both resonances and non-resonant transitions could induce detectable features or even strong suppressions in finite energy intervals of gamma-ray spectra from active galactic nuclei. Such effects can occur at keV to TeV energies for couplings that are currently allowed by all experimental constraints.

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arXiv2007

Ultra High Energy Cosmic ray and gamma-ray signatures of inductive acceleration in AGN jets

The highest energy cosmic rays could be produced by drifts in magnetized, cylindrically collimated, sheared jets of powerful active galaxies (i.e. FR II radiogalaxies; radio loud quasars and high power BL Lacs). We show that in such scenarios proton synchrotron radiation can give rise to detectable photon fluxes at energies ranging from hundreds of keV to tens of MeV.

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arXiv2007

Non-Universal Spectra of Ultra-High Energy Cosmic Ray Primaries and Secondaries in a Structured Universe

Analytical calculations of extra-galactic cosmic ray spectra above ~10^17 eV are often performed assuming continuous source distributions, giving rise to spectra that depend little on the propagation mode, be it rectilinear or diffusive. We perform trajectory simulations for proton primaries in the probably more realistic case of discrete sources with a density of ~10^-5/Mpc^3. We find two considerable non-universal effects that depend on source distributions and magnetic fields: First, the primary extra-galactic cosmic ray flux can become strongly suppressed below a few 10^18 eV due to partial confinement in magnetic fields surrounding sources. Second, the secondary photon to primary cosmic ray flux ratio between ~3x10^18 eV and ~10^20 eV decreases with decreasing source density and increasing magnetization. As a consequence, in acceleration scenarios for the origin of highest energy cosmic rays the fraction of secondary photons may be difficult to detect even for experiments such as Pierre Auger. The cosmogenic neutrino flux does not significantly depend on source density and magnetization.

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arXiv2007

Ultra-High Energy Cosmic Rays and the GeV-TeV Diffuse Gamma-Ray Flux

Ultra-high energy cosmic ray protons accelerated in astrophysical objects produce secondary electromagnetic cascades during propagation in the cosmic microwave and infrared backgrounds. We show that such cascades can contribute between ~1% and ~50% of the GeV-TeV diffuse photon flux measured by the EGRET experiment. The GLAST satellite should have a good chance to discover this flux.

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arXiv2006

High Energy Astroparticle Physics

We give a brief (and highly incomplete) overview of the current experimental and theoretical status of high energy cosmic rays and their secondary gamma-rays and neutrinos. We focus on the role of large scale magnetic fields and on multi-messenger aspects linking these three channels. We also recall that the flavor composition of neutrino fluxes from astrophysical sources contains information on both the source conditions and neutrino physics.

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arXiv2006

Gravitational-wave background from compact objects embedded in AGN accretion disks

We consider a model in which massive stars form in a self-gravitating accretion disk around an active galactic nucleus (AGN). These stars may evolve and collapse to form compact objects on a time scale shorter than the accretion time, thus producing an important family of sources for LISA. Assuming the compact object formation/inspiral rate is proportional to the steady-state gas accretion rate, we use the intrinsic hard X-ray AGN luminosity function to estimate expected event rates and signal strengths. We find that these sources will produce a continuous low-frequency (<~ mHz) background detectable by LISA if more than 1% of the accreted matter is in the form of compact objects. For compact objects with masses >~ 10 solar masses the last stages of the inspiral events should be resolvable above a few mHz, with rates as high as a few hundred per year.

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arXiv2006

Gravitational Waves from Compact Objects Accreting onto Active Galactic Nuclei

We consider a model in which massive stars form in a self-gravitating accretion disk around an active galactic nucleus. These stars may evolve and collapse to form compact objects on a time scale shorter than the accretion time, thus producing an important family of sources for LISA. Assuming the compact object formation/inspiral rate is proportional to the steady-state gas accretion rate, we use the observed extra-galactic X-ray luminosity function to estimate expected event rates and signal strengths. We find that these sources will produce a continuous low-frequency background detectable by LISA if more than >~ 1% of the accreted matter is in the form of compact objects. For compact objects with m >~ 10 Msun, the last stages of the inspiral events should be resolvable above a few mHz, at a rate of ~10-100 per year.

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arXiv2006

Cosmological Gravitational Wave Background from Phase Transitions in Neutron Stars

It has recently been suggested that collapse of neutron stars induced by a phase transition to quark matter can be a considerable source of gravitational waves with kHz frequencies. We demonstrate that if about one percent of all neutron stars undergo this process, the resulting cosmological gravitational wave background would reach about 10^-10 times the critical density. The background would peak at kHz frequencies and could have an observationally significant tail down to Hz frequencies. It would be comparable or higher than other astrophysical backgrounds, for example, from ordinary core collapse supernovae, from r-mode instabilities in rapidly rotating neutron stars, or from magnetars. The scenario is consistent with cosmological backgrounds in neutrinos and photons.

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arXiv2005

Gamma Ray Astronomy with Magnetized Zevatrons

Nearby sources of cosmic rays up to a ZeV(=10^21 eV) could be observed with a multi-messenger approach including secondary gamma-rays and neutrinos. If cosmic rays above ~10^18 eV are produced in magnetized environments such as galaxy clusters, the flux of secondary gamma-rays below ~1 TeV can be enhanced up to several orders of magnitudes compared to unmagnetized sources. A particular source of enhancement are synchrotron and cascade photons from e^+e^- pairs produced by protons from sources with relatively steep injection spectra proportional to E^-2.6. Such sources should be visible at the same time in ultra-high energy cosmic ray experiments and gamma-ray telescopes.

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arXiv2005

Magnetized Sources of Ultra-high Energy Nuclei and Extragalactic Origin of the Ankle

It has recently been suggested that ultra-high energy cosmic rays could have an extragalactic origin down to the "second knee" at ~4x10^{17}eV. In this case the "ankle" or "dip" at ~5x10^{18}eV would be due to pair production of extragalactic protons on the cosmic microwave background which requires an injection spectrum of about E^{-2.6}. It has been pointed out that for injection of a mixed composition of nuclei a harder injection spectrum E^{-2.2} is required to fit the spectra at the highest energies and a galactic component is required in this case to fit the spectrum below the ankle, unless the proton fraction is larger than 85%. Here we perform numerical simulations and find that for sufficiently magnetized sources, observed spectra above 10^{19}eV approach again the case of pure proton injection due to increased path-lengths and more efficient photo-disintegration of nuclei around the sources. This decreases secondary fluxes at a given energy and thus requires injection spectra ~E^{-2.6}, as steep as for pure proton injection. In addition, the ankle may again be sufficiently dominated by protons to be interpreted as a pair production dip.

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arXiv2005

Stochastic Gravitational Wave Background from Cosmological Supernovae

Based on new developments in the understanding of supernovae (SNe) as gravitational-wave (GW) sources we estimate the GW background from all cosmic SNe. For a broad range of frequencies around 1 Hz, this background is crudely comparable to the GW background expected from standard inflationary models. While our estimate remains uncertain within several orders of magnitude, the SN GW background may become detectable by second-generation space-based interferometers such as the proposed Big-Bang Observatory (BBO). By the same token, the SN GWs may become a foreground for searches of the inflationary GWs, in particular for sub-Hz frequencies where the SN background is Gaussian and where the BBO will be most sensitive. SN simulations lasting far beyond the usual cutoff of about 1 second are needed for more robust predictions in the sub-Hz frequency band. An even larger GW background can arise from a hypothetical early population of massive stars, although their GW source strength as well as their abundance are currently poorly understood.

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arXiv2004

Ultrahigh Energy Nuclei Propagation in a Structured, Magnetized Universe

We compare the propagation of iron and proton nuclei above 10^19 eV in a structured Universe with source and magnetic field distributions obtained from a large scale structure simulation and source densities about 10^(-5) Mpc^(-3). All relevant cosmic ray interactions are taken into account, including photo-disintegration and propagation of secondary products. Iron injection predicts spectral shapes different from proton injection which disagree with existing data below about 30 EeV. Injection of light nuclei or protons must therefore contribute at these energies. However, at higher energies, existing data are consistent with injection of pure iron with spectral indices between 2 and 2.4. This allows a significant recovery of the spectrum above roughly 100 EeV, especially in the case of large deflections. Significant auto-correlation and anisotropy, and considerable cosmic variance are also predicted in this energy range. The mean atomic mass A fluctuates considerably between different scenarios. At energies below 60 EeV, if the observed A > 35, magnetic fields must have a negligible effect on propagation. At the highest energies the observed flux will be dominated by only a few sources whose location may be determined by next generation experiments to within 10-20 degrees even if extra-galactic magnetic fields are important.

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arXiv2004

Cosmic Magnetic Fields and Their Influence on Ultra-High Energy Cosmic Ray Propagation

We discuss the influence of large scale cosmic magnetic fields on the propagation of hadronic cosmic rays above 10^19 eV based on large scale structure simulations. Our simulations suggest that rather substantial deflection up to several tens of degrees at 10^20 eV are possible for nucleon primaries. Further, spectra and composition of cosmic rays from individual sources can depend on magnetic fields surrounding these sources in intrinsically unpredictable ways. This is true even if deflection from such individual sources is small. We conclude that the influence of large scale cosmic magnetic fields on ultra-high energy cosmic ray propagation is currently hard to quantify. We discuss possible reasons for discrepant results of simulations by Dolag et al. which predict deflections of at most a few degrees for nucleons. We finally point out that even in these latter simulations a possible heavy component would in general suffer substantial deflection.

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arXiv2004

Ultra-High Energy Cosmic Ray Probes of Large Scale Structure and Magnetic Fields

We study signatures of a structured universe in the multi-pole moments, auto-correlation function, and cluster statistics of ultra-high energy cosmic rays above 10^19 eV. We compare scenarios where the sources are distributed homogeneously or according to the baryon density distribution obtained from a cosmological large scale structure simulation. The influence of extragalactic magnetic fields is studied by comparing the case of negligible fields with fields expected to be produced along large scale shocks with a maximal strength consistent with observations. We confirm that strongly magnetized observers would predict considerable anisotropy on large scales, which is already in conflict with current data. In the best fit scenario only the sources are strongly magnetized, although deflection can still be considerable, of order 20 degrees up to 10^20 eV, and a pronounced GZK cutoff is predicted. We then discuss signatures for future large scale full-sky detectors such as the Pierre Auger and EUSO projects. Auto-correlations are sensitive to the source density only if magnetic fields do not significantly affect propagation. In contrast, for a weakly magnetized observer, degree scale auto-correlations below a certain level indicate magnetized discrete sources. It may be difficult even for next generation experiments to distinguish between structured and unstructured source distributions.

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arXiv2004

Ultra-High Energy Cosmic Ray Nuclei from Individual Magnetized Sources

We investigate the dependence of composition, spectrum and angular distributions of ultra-high energy cosmic rays above 10^19 eV from individual sources on their magnetization. We find that, especially for sources within a few megaparsecs from the observer, observable spectra and composition are severely modified if the source is surrounded by fields of ~ 10^-7 Gauss on scales of a few megaparsecs. Low energy particles diffuse over larger distances during their energy loss time. This leads to considerable hardening of the spectrum up to the energy where the loss distance becomes comparable to the source distance. Magnetized sources thus have very important consequences for observations, even if cosmic rays arrive within a few degrees from the source direction. At the same time, details in spectra and chemical composition may be intrinsically unpredictable because they depend on the unknown magnetic field structure. If primaries are predominantly nuclei of atomic mass A accelerated up to a maximum energy E_max with spectra not much softer than E^-2, secondary protons from photo-disintegration can produce a conspicuous peak in the spectrum at energy ~ E_max/A. A related feature appears in the average mass dependence on energy.

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arXiv2004

Lectures on Astroparticle Physics

These are extended notes of a series of lectures given at the XIth Brazilian School of Cosmology and Gravitation. They provide a selection of topics at the intersection of particle and astrophysics. The first part gives a short introduction to the theory of electroweak interactions, with specific emphasize on neutrinos. In the second part we apply this framework to selected topics in astrophysics and cosmology, namely neutrino oscillations, neutrino hot dark dark matter, and big bang nucleosynthesis. The last part is devoted to ultra high energy cosmic rays and neutrinos where again particle physics aspects are emphasized. The often complementary role of laboratory experiments is also discussed in several examples.

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