K. Mannheim

Implications of Cosmological Gamma-Ray Absorption II. Modification of gamma-ray spectra

Bearing on the model for the time-dependent metagalactic radiation field developed in the first paper of this series, we compute the gamma-ray attenuation due to pair production in photon-photon scattering. Emphasis is on the effects of varying the star formation rate and the fraction of UV radiation assumed to escape from the star forming regions, the latter being important mainly for high-redshift sources. Conversely, we investigate how the metagalactic radiation field can be measured from the gamma-ray pair creation cutoff as a function of redshift, the Fazio-Stecker relation. For three observed TeV-blazars (Mkn501, Mkn421, H1426+428) we study the effects of gamma-ray attenuation on their spectra in detail.

Cosmic ray bound for models of extragalactic neutrino production

We obtain the maximum diffuse neutrino intensity predicted by hadronic photoproduction models of the type which have been applied to the jets of active galactic nuclei or gamma ray bursts. For this, we compare the proton and gamma ray fluxes associated with hadronic photoproduction in extragalactic neutrino sources with the present experimental upper limit on cosmic ray protons and the extragalactic gamma ray background, employing a transport calculation of energetic protons traversing cosmic photon backgrounds. We take into account the effects of the photon spectral shape in the sources on the photoproduction process, cosmological source evolution, the optical depth for cosmic ray ejection, and discuss the possible effects of magnetic fields in the vicinity of the sources. For photohadronic neutrino sources which are optically thin to the emission of neutrons we find that the cosmic ray flux imposes a stronger bound than the extragalactic gamma ray background in the energy range between 10^5 GeV and 10^11 GeV, as previously noted by Waxman & Bahcall (1999). We also determine the maximum contribution from the jets of active galactic nuclei, using constraints set to their neutron opacity by gamma-ray observations. This present upper limit is consistent with the jets of active galactic nuclei producing the extragalactic gamma ray background hadronically, but we point out future observations in the GeV-to-TeV regime could lower this limit. We also briefly discuss the contribution of gamma ray bursts to ultra-high energy cosmic rays as it can be inferred from possible observations or limits on their correlated neutrino fluxes.

High-energy neutrinos from extragalactic jets

Abstract In this paper I calculate the diffuse background of high-energy neutrinos from extragalactic jets emerging from active galactic nuclei (AGN). I assume that radio-quiet AGN have jets that are disrupted by turbulence in the vicinity of an accretion disk surrounding a central black hole and remain invisible in the radio band. In contrast, radio-loud AGN have high Mach number jets which dissipate in a strong shock far away from the AGN. It is assumed that protons are accelerated to ultra-high energies in jets. Photo-production of pions induces neutrino emission and electromagnetic cascades. The cascade spectra are in agreement with the most recent X- and γ-ray observations of AGN; in particular with the lack of γ-ray emission from radio-quiet AGN and the powerful γ-ray emission from flat-spectrum radio-loud AGN. A high energy neutrino background from radio-quiets is predicted assuming that they produce the cosmic diffuse X-ray background. However, the flux limits from the Frejus proton decay experiment are violated by this prediction. On the other hand, recent γ-ray observations make it very plausible that the diffuse γ-ray background at least above 100 MeV is due to radio-loud AGN. A striking similarity exists between the energy fluxes of diffuse γ-rays above 100 MeV and cosmic ray protons above the ankle. This is an independent argument for proton acceleration in radio jets consistent with the explanation of the individual γ-ray spectra by hadronically induced cascades. The corresponding prediction of a neutrino flux at ultra-high energies therefore rests on a firm basis.

Implications of cosmological gamma-ray absorption - I. Evolution of the metagalactic radiation field

Gamma-ray absorption due to -pair creation on cosmological scales depends on the line-of-sight integral of the evolving density of low-energy photons in the Universe, i.e. on the history of the diuse, isotropic radiation eld. Here we present and discuss a semi-empirical model for this metagalactic radiation eld based on stellar light produced and reprocessed in evolving galaxies. With a minimum of parameters and assumptions, the present-day background intensity is obtained from the far-IR to the ultraviolet band. Predicted model intensities are independent of cosmological parameters, since we require that the comoving emissivity, as a function of redshift, agrees with observed values obtained from deep galaxy surveys. The far-infrared background at present predicted from optical galaxy surveys falls short in explaining the observed one, and we show that this decit can be removed by taking into account (ultra)luminous infrared galaxies with a seperate star formation rate. The accuracy and reliability of the model, out to redshifts of z 5, allow a realistic estimate of the attenuation length of GeV-to-TeV gamma-rays and its uncertainty, which will be the focus of a subsequent paper.

Design and Operation of FACT -- The First G-APD Cherenkov Telescope

The First G-APD Cherenkov Telescope (FACT) is designed to detect cosmic gamma-rays with energies from several hundred GeV up to about 10 TeV using the Imaging Atmospheric Cherenkov Technique. In contrast to former or existing telescopes, the camera of the FACT telescope is comprised of solid-state Geiger-mode Avalanche Photodiodes (G-APD) instead of photomultiplier tubes for photo detection. It is the first full-scale device of its kind employing this new technology. The telescope is operated at the Observatorio del Roque de los Muchachos (La Palma, Canary Islands, Spain) since fall 2011. This paper describes in detail the design, construction and operation of the system, including hardware and software aspects. Technical experiences gained after one year of operation are discussed and conclusions with regard to future projects are drawn.

Neutrino production through hadronic cascades in AGN accretion disks.

We consider the production of neutrinos in active galactic nuclei through hadronic cascades. The initial, high-energy nucleons are accelerated in a source above the accretion disk around the central black hole. From the source, the particles diffuse back to the disk and initiate hadronic cascades through pp interactions. The observable output from the caascade is electromagnetic radiation and neutrinos. We use the observed diffuse background x-ray luminosity, which presumably results from this process, to predict the diffuse neutrino flux close to the existing limits from the Frejus experiment. The resulting neutrino spectrum is E −2 down to the GeV region. We discuss modifications of this scenario which reduce the predicted neutrino flux

Coincidence of a high-fluence blazar outburst with a PeV-energy neutrino event

The IceCube neutrino telescope in the South Pole has observed several high-energy neutrinos of undetermined origin. Could the third detected PeV event be from blazar PKS B1424–418?

Multiwavelength Observations of 3C 273 in 1993-1995

We present the results of the multiwavelength campaigns on 3C 273 in 1993-1995. During the observations in late 1993, this quasar showed an increase of its flux for energies ≥100 MeV from about 2.1 × 10-7 photons cm-2 s-1 to approximately 5.6 × 10-7 photons cm-2 s-1 during a radio outburst at 14.5, 22, and 37 GHz. However, no one-to-one correlation of the γ-ray radiation with any frequency could be found. The photon spectral index of the high-energy spectrum changed from Γγ = (3.20 ± 0.54) to Γγ = (2.20 ± 0.22) in the sense that the spectrum flattened when the γ-ray flux increased. Fits of the three most prominent models (synchrotron self-Comptonization, external inverse Comptonization, and the proton-initiated cascade model) for the explanation of the high γ-ray emission of active galactic nuclei were performed to the multiwavelength spectrum of 3C 273. All three models are able to represent the basic features of the multiwavelength spectrum. Although there are some differences, the data are still not decisive enough to discriminate between the models.

The first gamma-ray outburst of a narrow-line Seyfert 1 galaxy: the case of PMN J0948+0022 in 2010 July

We report on a multiwavelength campaign for the radio-loud narrow-line Seyfert 1 (NLS1) galaxy PMN J0948+0022 (z= 0.5846) performed in 2010 July–September and triggered by a high-energy γ-ray outburst observed by the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope. The peak flux in the 0.1–100 GeV energy band exceeded, for the first time in this type of source, the value of ~10^(−6) photon cm^(−2) s^(−1), corresponding to an observed luminosity of ~10^(48) erg s^(−1). Although the source was too close to the Sun position to organize a densely sampled follow-up, it was possible to gather some multiwavelength data that confirmed the state of high activity across the sampled electromagnetic spectrum. The comparison of the spectral energy distribution of the NLS1 PMN J0948+0022 with that of a typical blazar – such as 3C 273 – shows that the power emitted at γ-rays is extreme.

Supersymmetric Dark Matter and the Extragalactic Gamma Ray Background

We trace the origin of the newly determined extragalactic gamma-ray background from EGRET data to an unresolved population of blazars and neutralino annihilation in cold dark matter halos. Using results of high-resolution simulations of cosmic structure formation, we calculate composite spectra and compare with the EGRET data. The resulting best-fit value for the neutralino mass is m(chi) = 515(+110)(-75) GeV (systematic errors approximately 30%).