Energy spectrum of extragalactic gamma-ray sources

The result of Monte Carlo electron photon cascade calculations for propagation of gamma rays through regions of extragalactic space containing no magnetic field are given. These calculations then provide upper limits to the expected flux from extragalactic sources. Since gamma rays in the 10 to the 14th power eV to 10 to the 17th power eV energy range are of interest, interactions of electrons and photons with the 3 K microwave background radiation are considered. To obtain an upper limit to the expected gamma ray flux from sources, the intergalactic field is assumed to be so low that it can be ignored. Interactions with photons of the near-infrared background radiation are not considered here although these will have important implications for gamma rays below 10 to the 14th power eV if the near infrared background radiation is universal. Interaction lengths of electrons and photons in the microwave background radiation at a temperature of 2.96 K were calculated and are given

Neutrino Emission from HBLs and LBLs

The Synchrotron Proton Blazar model is a promising model to explain high energy emission from gamma-ray loud BL Lac objects like Mkn 421. In contrast to leptonic models, the hadronic explanation of gamma-ray emission predicts ultrahigh energy neutrinos. The predicted neutrino spectra from a typical High-energy cutoff BL Lac Object (HBL) and a Low-energy cutoff BL Lac Object (LBL) are presented. We find that cooling due to muon synchrotron radiation causes a cutoff of the neutrino spectrum at $\sim 10^{18}$ eV, with the exception of $\nu_\mu$ from kaon decay which may extend to higher energies if meson production takes place in the secondary resonance region of the cross section. The impact of the neutrino output from both source populations to the diffuse neutrino background is discussed.Comment: 4 pages, 3 figures, to appear in: Proc. 27th Int. Cosmic Ray Conf., Hamburg/German

The physical parameters of Markarian 501 during flaring activity

We determine the physical parameters (magnetic field and Doppler factor) of the homogeneous synchrotron self-Compton model allowed by the observed X-ray to gamma-ray spectra and variability of Markarian~501 during the 15-16 April 1997 flaring activity. We find that magnetic fields between 0.07 G and 0.6 G and Doppler factors between 12 and 36 could fit (depending on observed variability time scale) these observations. We take account of photon-photon pair production interactions of gamma-ray photons occurring both inside the emission region and during propagation to Earth and find these to be extremely important in correctly determining the allowed model parameters. Previous estimates of the allowed parameter space have neglected this effect. Future multi-wavelength campaigns during strong flaring activity, including observations from optical to TeV gamma-rays, should enable the physical parameters to be further constrained.Comment: 18 pages, 5 figures, minor changes, additional reference, accepted for publication in MNRA

Radiation force on relativistic jets in active galactic nuclei

Radiative deceleration of relativistic jets in active galactic nuclei as the result of inverse Compton scattering of soft photons from accretion discs is discussed. The Klein-Nishina (KN) cross section is used in the calculation of the radiation force due to inverse Compton scattering. Our result shows that deceleration due to scattering in the KN regime is important only for jets starting with a bulk Lorentz factor larger than 1000. When the bulk Lorentz factor satisfies this condition, particles scattering in the Thomson regime contribute positively to the radiation force (acceleration), but those particles scattering in the KN regime are dominant and the overall effect is deceleration. In the KN limit, the drag due to Compton scattering, though less severe than in the Thomson limit, strongly constrains the bulk Lorentz factor. Most of the power from the deceleration goes into radiation and hence the ability of the jet to transport significant power (in particle kinetic energy) out of the subparsec region is severely limited. The deceleration efficiency decreases significantly if the jet contains protons and the proton to electron number density ratio satisfies the condition $n_p/n_{e0}>2\gamma_{\rm min}/\mu_p$ where $\gamma_{\rm min}$ is the minimum Lorentz factor of relativistic electrons (or positrons) in the jet frame and $\mu_p$ is the proton to electron mass ratio.Comment: 10 pages including 8 figures; accepted for publication in MNRA

Gamma rays from interactions of stars with AGN jets

We have developed a model for gamma ray emission in jets of active galactic nuclei in which particle acceleration takes place at a shock in the relativistic jet plasma due to a massive star in the central region of the host galaxy moving through the jet. The gamma rays are produced in a pair-Compton cascade in the radiation field of the star initiated by accelerated electrons. Our model may account for the observed GeV to TeV gamma ray spectrum and variability of Markarian 421 and other blazars detected by the EGRET instrument on the Compton Gamma Ray Observatory.Comment: 5 pages, 3 figures, latex (uses mn.sty), submitted to MNRA

Ultrahigh-Energy Photons as a Probe of Nearby Transient Ultrahigh-Energy Cosmic-Ray Sources and Possible Lorentz-Invariance Violation

Detecting neutrinos and photons is crucial to identifying the sources of ultrahigh-energy cosmic rays (UHECRs), especially for transient sources. We focus on ultrahigh-energy gamma-ray emission from transient sources such as gamma-ray bursts, since >EeV gamma rays can be more direct evidence of UHECRs than PeV neutrinos and GeV-TeV gamma rays. We demonstrate that coincident detections of about 1-100 events can be expected by current and future UHECR detectors such as Auger and JEM-EUSO, and the detection probability can be higher than that of neutrinos for nearby transient sources at <50-100 Mpc. They may be useful for constraining the uncertain cosmic radio background as well as knowing the source properties and maximum energy of UHECRs. They can also give us more than 10^4 times stronger limits on the Lorentz-invariance violation than current constraints.Comment: 4 pages, 3 figures, replaced to match the published version (PRL, 103, 081102