Floyd W. Stecker

EGRET observations of the extragalactic gamma-ray emission

The all-sky survey in high-energy gamma rays (E > 30 MeV) carried out by EGRET aboard the Compton Gamma Ray Observatory provides a unique opportunity to examine in detail the diffuse gamma-ray emission. The observed diffuse emission has a Galactic component arising from cosmic-ray interactions with the local interstellar gas and radiation, as well as an almost uniformly distributed component that is generally believed to originate outside the Galaxy. Through a careful study and removal of the Galactic diffuse emission, the flux, spectrum, and uniformity of the extragalactic emission are deduced. The analysis indicates that the extragalactic emission is well described by a power-law photon spectrum with an index of -(2.10 ± 0.03) in the 30 MeV to 100 GeV energy range. No large-scale spatial anisotropy or changes in the energy spectrum are observed in the deduced extragalactic emission. The most likely explanation for the origin of this extragalactic high-energy gamma-ray emission is that it arises primarily from unresolved gamma-ray-emitting blazars.

Intergalactic Photon Spectra from the Far-IR to the UV Lyman Limit for 0 < z < 6 and the Optical Depth of the Universe to High-Energy Gamma Rays

We calculate the intergalactic photon density as a function of both energy and redshift for 0 < z < 6 for photon energies from.003 eV to the Lyman limit cutoff at 13.6 eV in a ΛCDM universe with ΩΛ = 0.7 and Ωm = 0.3. The basic features of our backward-evolution model for galaxies were developed in earlier papers by Malkan & Stecker. With a few improvements, we find that this evolutionary model gives predictions of new deep number counts from Spitzer, as well as a calculation of the spectral energy distribution of the diffuse infrared background, which are in good agreement with the data. We then use our calculated intergalactic photon densities to extend previous work on the absorption of high-energy γ-rays in intergalactic space owing to interactions with low-energy photons and the 2.7 K cosmic microwave background radiation. We calculate the optical depth of the universe, τ, for γ-rays having energies from 4 GeV to 100 TeV emitted by sources at redshifts from 0 to 5. We also give an analytic fit with numerical coefficients for approximating τ(Eγ, z). As an example of the application of our results, we calculate the absorbed spectrum of the blazar PKS 2155-304 at z = 0.117 and compare it with the spectrum observed by the HESS air Cerenkov γ-ray telescope array.

New tests of Lorentz invariance following from observations of the highest energy cosmic gamma-rays

We use the recent reanalysis of multi-TeV -ray observations of Mrk 501 to constrain the Lorentz invariance breaking parameter involving the maximum electron velocity. Our limit is two orders of magnitude better than that obtained from the maximum observed cosmic ray electron energy.

New Limits on Planck Scale Lorentz Violation in QED

Constraints on possible Lorentz symmetry violation (LV) of order E/M(Planck) for electrons and photons in the framework of effective field theory (EFT) are discussed. Using (i) the report of polarized MeV emission from GRB021206 and (ii) the absence of vacuum Cerenkov radiation from synchrotron electrons in the Crab Nebula, we improve previous bounds by 10(-10) and 10(-2), respectively. We also show that the LV parameters for positrons and electrons are different, discuss electron helicity decay, and investigate how prior constraints are modified by the relations between LV parameters implied by EFT.

BLAZAR GAMMA-RAYS, SHOCK ACCELERATION, AND THE EXTRAGALACTIC BACKGROUND LIGHT

The observed spectra of blazars, their intrinsic emission, and the underlying populations of radiating particles are intimately related. The use of these sources as probes of the extragalactic infrared background, a prospect propelled by recent advances in TeV-band telescopes, soon to be augmented by observations by NASAs upcoming Gamma-Ray Large Area Space Telescope, has been a topic of great recent interest. Here, it is demonstrated that if particles in blazar jets are accelerated at relativistic shocks, then γ-ray spectra with indices less than 1.5 can be produced. This, in turn, loosens the upper limits on the near-infrared extragalactic background radiation previously proposed. We also show evidence hinting that TeV blazars with flatter spectra have higher intrinsic TeV γ-ray luminosities, and we indicate that there may be a correlation of flatness and luminosity with redshift.

An Empirically Based Model for Predicting Infrared Luminosity Functions, Deep Infrared Galaxy Counts, and the Diffuse Infrared Background

We predict luminosity functions and number counts for extragalactic infrared sources at various wavelengths using the framework of our empirically based model. Comparisons of our galaxy count results with existing data indicate that either galaxy luminosity evolution is not much stronger than Q = 3.1, where L ∝ (1 + z)Q, or that this evolution does not continue beyond a redshift of 2. However, a derivation of the far-infrared background from COBE Diffuse Infrared Background Experiment data suggests a stronger evolution for the far-infrared emission, with Q > 4 in the redshift range between 0 and 1. We discuss several interpretations of these results and also discuss how future observations can reconcile this apparent conflict. We also make predictions of the redshift distributions of extragalactic infrared sources at selected flux levels, which can be tested by planned detectors. Finally, we predict the fluxes at which various future surveys will become confusion-limited.

Note on high-energy neutrinos from active galactic nuclei cores

Taking into account new physics and astronomy developments I give a revised high energy neutrino flux for the AGN core model of Stecker, Done, Salamon and Sommers

PeV Neutrinos Observed by IceCube from Cores of Active Galactic Nuclei

Recently, the Ice Cube collaboration has reported the first observation of cosmic 2 PeV energy neutrinos giving a signal ∼ 3σ above the atmospheric background [1]. More recently 18 events ∼4σ above the expected atmospheric background were reported with energies above 100 TeV. These neutrinos are likely to be of cosmic origin; their angular distribution is consistent with isotropy. The average spectral index for these neutrinos was approximately -2 over the energy range between ∼ 100 TeV and 2 PeV [2]. In 1991 we proposed a model suggesting that very high energy neutrinos could be produced in the cores of active galaxies (AGN) such as Seyfert galaxies [3]. Using that model, we gave estimates of the flux and spectrum of high energy neutrinos to be expected. In light of subsequent AGN observations and the discovery of neutrino oscillations the flux estimates for this model were revised downward [5], although the shape of the predicted neutrino spectrum remained unchanged. The new estimate was obtained by lowering the flux shown in the Figure in Ref. [4] by a factor of 20. This rescaling gives a value for the νμ flux at 100 TeV of E ν Φ(Eν) ∼ 10 −8 GeV cmssr and a flux of ∼ 5.6× 10 GeV cmssr at ∼ 1 PeV. The peak flux in these units occurs at an energy ∼1 PeV In our model protons are accelerated by shocks in the cores of AGN in the vicinity of the black hole accretion disk (see, e.g., Ref. [6]). Being trapped by the magnetic field, they lose energy dramatically by interactions with the dense photon field of the ”big blue bump” of thermal emission from the accretion disk (see, e.g., Ref. [7]) which is optically thick to protons [3]. The primary interactions are those from photomeson production. The primary neutrino producing channel, which occurs near threshold, is

Components of the Extragalactic Gamma Ray Background

We present new theoretical estimates of the relative contributions of unresolved blazars and star-forming galaxies to the extragalactic ?-ray background (EGB) and discuss constraints on the contributions from alternative mechanisms such as dark matter annihilation and truly diffuse ?-ray production. We find that the Fermi source count data do not rule out a scenario in which the EGB is dominated by emission from unresolved blazars, though unresolved star-forming galaxies may also contribute significantly to the background, within order-of-magnitude uncertainties. In addition, we find that the spectrum of the unresolved star-forming galaxy contribution cannot explain the EGB spectrum found by EGRET at energies between 50 and 200 MeV, whereas the spectrum of unresolved flat spectrum radio quasars, when accounting for the energy-dependent effects of source confusion, could be consistent with the combined spectrum of the low-energy EGRET EGB measurements and the Fermi-Large Area Telescope EGB measurements.

Constraints on Lorentz invariance violation from Fermi -Large Area Telescope observations of gamma-ray bursts

We analyze the MeV/GeV emission from four bright gamma-ray bursts (GRBs) observed by the Fermi Large Area Telescope to produce robust, stringent constraints on a dependence of the speed of light in vacuo on the photon energy (vacuum dispersion), a form of Lorentz invariance violation (LIV) allowed by some quantum gravity (QG) theories. First, we use three different and complementary techniques to constrain the total degree of dispersion observed in the data. Additionally, using a maximally conservative set of assumptions on possible source-intrinsic, spectral-evolution effects, we constrain any vacuum dispersion solely attributed to LIV. We then derive limits on the QG energy scale (the energy scale where LIV-inducing QG effects become strong,