arXiv subjects

Karl Mannheim

Publications and source records attributed to Karl Mannheim.

70 records · Page 4Linked to original sources

arXiv1999

The relation of extragalactic cosmic ray and neutrino fluxes: the logic of the upper bound debate

In a recent paper, Waxman and Bahcall (PRD 59, 023002) claimed that the present data on ultra-high energy cosmic rays imply a model-independent upper bound on extragalactic neutrino fluxes of 2*10^-8 GeV cm^-2 s^-1 ster^-1, independent of neutrino energy. Mannheim, Protheroe and Rachen (astro-ph/9812398) have repeated this calculation and confirmed the WB-bound, within a factor of 2, only at a very limited energy range of E_ν\sim 10^(16-18) eV, while at other energies the neutrino flux is mainly limited by the extragalactic gamma ray background to a level about two orders of magnitude higher than the WB bound. In this paper we present a simple, (almost) no-math approach to the problem, and discuss under which astrophysical assumptions the WB-bound and the MPR-bound, respectively, apply. Then we discuss to which respect these assumptions apply to presently discussed models of extragalactic neutrino production. We note that, averaged over the observed luminosity function, blazars are sufficiently opaque to ultra-high energy neutrons that there is no conflict of the predicted neutrino fluxes with the cosmic ray data, and that these models are rather constrained by their contributions to the extragalactic gamma ray background. At present, no modifications are implied to the predicted neutrino events from these models in active or planned neutrino detectors.

astro-ph↗

arXiv1999

Frontiers in High-Energy Astroparticle Physics

With the discovery of evidence for neutrino mass, a vivid gamma ray sky at multi-TeV energies, and cosmic ray particles with unexpectedly high energies, astroparticle physics currently runs through an era of rapid progress and moving frontiers. The non-vanishing neutrino mass establishes one smooth component of dark matter which does not, however, supply a critical mass to the Universe. Other dark matter particles are likely to be very massive and should produce high-energy gamma rays, neutrinos, and protons in annihilations or decays. The search for exotic relics with new gamma ray telescopes, extensive air shower arrays, and underwater/-ice neutrino telescopes is a fascinating challenge, but requires to understand the astrophysical background radiations at high energies. Among the high-energy sources in the Universe, radio-loud active galactic nuclei seem to be the most powerful accounting for at least a sizable fraction of the extragalactic gamma ray flux. They could also supply the bulk of the observed cosmic rays at ultrahigh energies and produce interesting event rates in neutrino telescopes aiming at the kubic kilometer scale such as AMANDA and ANTARES. It is proposed that the extragalactic neutrino beam can be used to search for tau lepton appearance thus allowing for a proof of the neutrino oscillation hypothesis. Furthermore, a new method for probing the era of star formation at high redshifts using gamma rays is presented which requires new-generation gamma ray telescopes operating in the 10-100 GeV regime such as MAGIC and GLAST.

astro-ph↗

arXiv1999

Neutrino Oscillations and Blazars

Three independent predictions follow from postulating the existence of protons co-accelerated with electrons in extragalactic jets (i) multi-TeV gamma ray emission from nearby blazars, (ii) extragalactic cosmic ray protons up to 10^20 eV, and (iii) extragalactic neutrinos up to 5 10^18 eV. Recent gamma ray observations of Mrk 421 and Mrk 501 employing the air-Cerenkov technique are consistent with the predicted gamma ray spectrum, if one corrects for pair attenuation on the infrared background. Prediction (ii) is consistent with cosmic ray data, if one requires that jets are responsible for at least a sizable fraction of the extragalactic gamma ray background. With kubic kilometer neutrino telescopes, it will be possible to test (iii), although the muon event rates are rather low. Neutrino oscillations can increase the event rate by inducing tau-cascades removing the Earth shadowing effect.

astro-ph↗

arXiv1998

Possible Production of High-Energy Gamma Rays from Proton Acceleration in the Extragalactic Radio Source Markarian 501

The active galaxy Markarian 501 was discovered with air-Cerenkov telescopes at photon energies of 10 tera-electron volts. Such high energies may indicate that the gamma rays from Markarian 501 are due to the acceleration of protons rather than electrons. Furthermore, the observed absence of gamma ray attenuation due to electron-positron pair production in collisions with cosmic infrared photons implies a limit of 2 to 4 nanowatt per squaremeter per steradian for the energy flux of an extragalactic infrared radiation background at a wavelength of 25 micrometers. This limit provides important clues on the epoch of galaxy formation.

astro-ph↗

arXiv1997

AGN Models: High-Energy Emission

Astrophysical models for the high-energy emission of blazars are reviewed. Blazars ejecting relativistic radio jets at small angles to the line-of-sight are the only type of active galactic nuclei (AGN) discovered above 100 MeV. The gamma-rays apparently originate in the jets which explains the absence of gamma-ray pair creation attenuation. The bulk Lorentz factor of the radio jets is much smaller than the Lorentz factors of the emitting particles requiring in situ particle acceleration such as Fermi acceleration at shocks. The mounting evidence for a correlation between the optical and gamma-ray emission argues for the same accelerated electrons emitting the polarized optical synchrotron photons to be responsible for the high-energy emission. Alternatively, the gamma-rays could be due to electrons of a secondary origin related to the energy losses of protons accelerated at the same shocks as the synchrotron emitting electrons. In this case blazars would produce an observable flux of high-energy neutrinos. Unified schemes for AGN predict a circum-nuclear warm dust torus attenuating gamma-rays above ~300 GeV emitted from within a central sphere of radius ~2 10^4 Schwarzschild radii which rules out external Compton scattering models as the origin of the TeV gamma-rays from Mrk421 and Mrk501.

astro-ph↗

arXiv1996

Hadronic gamma-ray emission models

The discovery of rapid and simultaneous TeV/optical (UV, X-ray) variability provides strong evidence against the interaction between a thermal radiation field produced by an accretion flow and relativistic electrons in a jet as the origin of the gamma-rays in BL Lacertae objects. Synchrotron-self-Compton scattering (SSC) or proton-initiated cascades (PIC) remain viable mechanisms. The mechanisms predict very different magnetic field strengths and upper cut-off energies in the gamma-ray sources, i.e. the field strengths in SSC models are more than two orders of magnitude lower than in PIC models and the PIC spectra do not cut-off at TeV.

astro-ph↗

arXiv1996

The Gamma Ray Burst Rate at High Photon Energies

Some gamma-ray burst (GRB) spectra exhibit high energy tails with the highest photon energy detected at 18 GeV. The spectral slope of the high-energy tails is sufficiently flat in nu F_nu to consider the possibility of their detection at still higher energies. We calculate how many bursts can reasonably be expected above a given energy threshold for a cosmological distribution of bursts satisfying the observed apparent brightness distribution. The crucial point is that the gamma-ray absorption by pair production in the intergalactic diffuse radiation field eliminates bursts from beyond the gamma-ray horizon tau ~ 1, thus drastically reducing the number of bursts at high energies. Our results are consistent with the non-detection of bursts by current experiments in the 100 GeV to 100 TeV energy range. For the earth-bound detector array MILAGRO, we predict a maximal GRB rate of ~ 10 events per year. The Whipple Observatory can detect, under favorable conditions, ~1 event per year. The event rate for the HEGRA array is ~ 0.01 per year. Detection of significantly higher rates of bursts would severely challenge cosmological burst scenarios.

astro-ph↗

arXiv1996

Search for Supergalactic Anisotropies in the 3B Catalog

The angular distribution of GRBs is isotropic, while the brightness distribution of bursts shows a reduced number of faint events. These observations favor a cosmological burst origin. If GRBs are indeed at cosmological distances and if they trace luminous matter, we must eventually find an anisotropic distribution of bright bursts. If a significant number of bursts originate at redshifts less than z~1, the concentration of nearby galaxies towards the supergalactic plane is pronounced enough that we could discover the corresponding clustering of burst locations. We used the 3B catalog to search for a pattern visible in supergalactic coordinates. No compelling evidence for anisotropies was found. The absence of anisotropies in SG coordinates implies a minimum sampling distance of 200h^-1 Mpc.

astro-ph↗

arXiv1995

Gamma Rays from Compact Objects

A review on some recent developments in gamma ray astrophysics with particular emphasis on the role of relativistic bulk motion in nonthermal gamma ray sources, radiation processes in AGN jets, high-energy neutrino astronomy and the use of gamma rays in the energy range 10 GeV - 50 TeV as a probe of the diffuse IR-UV radiation field at cosmic distances.

astro-ph↗

arXiv1995

ROSAT PSPC observations of six PG quasars and PHL 1657

We report results from the spectral analysis of pointed ROSAT PSPC observations of six PG quasars and the weak-bump quasar PHL 1657. Disregarding other frequency bands, the PSPC data are represented best by simple power-law source spectra ($dN/dE \propto E^Γ$) with a slope $Γ= -2.5\pm 0.4$ and do not show evidence for a more complex structure. However, within the limits given by statistics and systematical errors, a superposition of two power-law spectra, which allows a connection to the observed UV fluxes and to the generally flatter hard-X-ray spectra observed by EXOSAT and GINGA, is found to be indistinguishable from a simple power-law in the PSPC energy band. For the PG quasars, a direct connection to the observed UV flux is obtained with a steep soft-X-ray slope $Γ_{sx}= -3.1\pm 0.3$, fixing the hard-X-ray slope. For PHL 1657, the soft-X-ray spectrum seen by ROSAT is much steeper than the GINGA spectrum, rising towards the EUV in $νF_ν$, and it can not be extrapolated down to meet the exceptionally weak UV flux. A Wien-shaped thermal UV/soft-X-ray bump with a temperature of $\sim $50\,eV connects the UV and soft-X-ray spectra in all cases, but gives only a poor representation of the high signal-to-noise PSPC spectra. We discuss implications of our results for models of the X-ray emission in quasars.

astro-ph↗

arXiv1995

TEV GAMMA-RAYS FROM PROTON BLAZARS

Proton acceleration in nearby blazars can be diagnosed measuring their intense TeV $γ$-ray emission. Flux predictions for 1101+384 (Mrk421) and 1219+285 (ON231), both strong EGRET sources (0.1-10 GeV), are obtained from model spectra of unsaturated synchrotron pair cascades fitted to publicly available multifrequency data. An experimental effort to confirm the predicted emission in the range 1-10 TeV would be of great importance for the problems of the origin of cosmic rays, the era of galaxy formation and the cosmological distance scale.

astro-ph↗

arXiv1994

The UV-Drag on Hadronic Hot Jets as the Origin of X-Ray Irradiation in AGN

Consequences of anisotropic cascade radiation in AGN are investigated. The anisotropic cascades are produced by protons accelerated in a nuclear jet. The protons are assumed to cool in the UV photon field of an accretion disk by photo-production of secondary particles. Most cascade photons are produced in the direction of the accretion disk leading to strong high-energy irradiation and weak emission of observable MeV photons. Nuclear, turbulent jets rapidly disrupt and remain radio-faint, so that the scenario could hold the clue to the understanding of the X-ray spectra of radio-quiet AGN.

astro-ph↗

arXiv1993

On the Difference between Radio Loud and Radio Quiet AGN

Nuclear jets containing relativistic ``hot'' particles close to the central engine cool dramatically by producing high energy radiation. The radiative dissipation is similar to the famous Compton drag acting upon ``cold'' thermal particles in a relativistic bulk flow. Highly relativistic protons induce anisotropic showers raining electromagnetic power down onto the putative accretion disk. Thus, the radiative signature of hot hadronic jets is x-ray irradiation of cold thermal matter. The synchrotron radio emission of the accelerated electrons is self-absorbed due to the strong magnetic fields close to the magnetic nozzle.

astro-ph↗

arXiv1993

The Galactic Center radio jet

Recent observations of the radio and NIR source Sgr A* reinforce the interpretation of the Galactic Center as a scaled down version of an AGN. The discovery of an elongated structure at 43 GHz and increasing evidence for the presence of an accretion disk surrounding a Black Hole lead us to assume that both, an accretion disk and a jet, are present in the Galactic Center and are physically linked. We model the radio emission of Sgr A* successfully with a Blandford & K"onigl type jet and analyze the energetics of the coupled jet-disk system in Sgr A* where jet and disk are parametrized in terms of the accretion power. With this method we are able to confirm independently the lower limit of the Sgr A* accretion rate \dot M >> 10^-8.5 M_sun found previously. Moreover, using the limits imposed by observational data, we show that within such a jet-disk model, the total jet power Q_jet is of comparable order as the radiated disk luminosity L_disk. A jet model together with the assumption of an 10^6 M_sun Black Hole also qualitatively explains the submm excess and the lack of non-thermal IR radiation. The small size of the visible part of the jet (< 1 mas) is due to the low accretion rate of Sgr A*. << >>

astro-ph↗

arXiv1993

Gamma Rays and Neutrinos from a Powerful Cosmic Accelerator

Recent measurements of gamma rays from the powerful quasar 3C273 by the Compton Gamma Ray Observatory show that a powerful cosmic accelerator must be operating. In this paper the evidence for proton acceleration is collected with the result that (i) the gamma ray spectrum should flatten slightly above a few GeV and that (ii) high energy neutrinos from the decay of photomesons are difficult to observe, although the power carried by these particles is as large as the gamma ray power. However, the power is concentrated at energies in the EeV range, whereas in the TeV-PeV range neutrinos from pp and p$α$ collisions dominate due to their steeper spectrum. Consequently, the flux of cosmic neutrinos from flat spectrum radio sources such as 3C273 in the energy range relevant for proposed underwater or underice detectors could be much lower than inferred from assuming the same spectrum for gamma rays and neutrinos.

astro-ph↗

arXiv1993

The Proton Blazar

Considering shock-accelerated protons in addition to electrons in a synchrotron radio jet naturally produces the observed X- through gamma ray continuum emission of flat-spectrum radio-loud AGN, whereas the corresponding shock-accelerated electrons produce the infrared through optical continuum. All of these emission components are rapidly variable on short time scales which can be accounted for by a common origin of the blazar emission in a relativistic sub-parsec scale jet. Moreover, neutrino fluxes are estimated which may be observable with the HiRes experiment and the impact on the ISM of the host galaxy of neutrons escaping from the nuclear jet is investigated.

astro-ph↗