M. H. Salamon

TeV gamma rays from 3C 279 - A possible probe of origin and intergalactic infrared radiation fields

The gamma-ray spectrum of 3C 279 during 1991 June exhibited a near-perfect power law between 50 MeV and over 5 GeV with a differential spectral index of -(2.02 +/- 0.07). If extrapolated, the gamma-ray spectrum of 3C 279 should be easily detectable with first-generation air Cerenkov detectors operating above about 0.3 TeV provided there is no intergalactic absorption. However, by using model-dependent lower and upper limits for the extragalactic infrared background radiation field, a sharp cutoff of the 3C 279 spectrum is predicted at between about 0.1 and about 1 TeV. The sensitivity of present air Cerenkov detectors is good enough to measure such a cutoff, which would provide the first opportunity to obtain a measurement of the extragalactic background infrared radiation field.

Absorption of High-Energy Gamma Rays by Interactions with Extragalactic Starlight Photons at High Redshifts and the High-Energy Gamma-Ray Background

In this paper, we extend previous work on the absorption of high-energy γ-rays in intergalactic space by calculating the absorption of 10-500 GeV γ-rays at high redshifts. This calculation requires the determination of the high-redshift evolution of the intergalactic starlight photon field, including its spectral energy distribution out to frequencies beyond the Lyman limit. To estimate this evolution, we have followed a recent analysis by Fall, Charlot, & Pei, which reproduces the redshift dependence of the starlight background emissivity obtained by the Canada-France Redshift Survey group. We give our results for the γ-ray opacity as a function of redshift out to a redshift of z = 3. We also give predicted γ-ray spectra for selected blazars and extend our calculations of the extragalactic γ-ray background from blazars to an energy of 500 GeV with absorption effects included. Our results indicate that the extragalactic γ-ray background spectrum from blazars should steepen significantly above 20 GeV, owing to extragalactic absorption. Future observations of a such a steepening would thus provide a test of the blazar origin hypothesis for the γ-ray background radiation. We also note that our absorption calculations can be used to place limits on the redshifts of γ-ray bursts; for example, our calculated opacities indicate that the 1994 February 17 burst observed by EGRET most probably originated at z ≤ ~2.

Photodisintegration of Ultra-High-Energy Cosmic Rays: A New Determination

We present the results of a new calculation of the photodisintegration of ultra-high-energy cosmic-ray (UHCR) nuclei in intergalactic space. The critical interactions for energy loss and photodisintegration of UHCR nuclei occur with photons of the 2.73 K cosmic background radiation (CBR) and with photons of the IR background radiation (IBR). We have reexamined this problem, making use of a new determination of the IBR based on empirical data primarily from IRAS galaxies, which is consistent with direct measurements and upper limits from TeV γ-ray observations. We have also improved the calculation by including the specific threshold energies for the various photodisintegration interactions in our Monte Carlo calculation. With the new smaller IBR flux, the steepness of the Wien side of the now relatively more important CBR makes their inclusion essential for more accurate results. Our results indicate a significant increase in the propagation time of UHCR nuclei of a given energy over previous results. We discuss the possible significance of this for UHCR origin theory.

Predicted Extragalactic TeV Gamma-Ray Sources

We suggest that low-redshift X-ray-selected BL Lacertae objects (XBLs) may be the only extragalactic γ-ray sources observable at TeV energies. We use simple physical considerations involving synchrotron and Compton component spectra for blazars to suggest why the observed TeV sources are XBLs, whereas mostly radio-selected BL objects (RBLs) and flat spectrum radio quasars (FSRQs) are seen at GeV energies. These considerations indicate that the differences between XBLs and RBLs cannot be explained purely as relativistic jet orientation effects. We note that the only extragalactic TeV sources that have been observed are XBLs and that a nearby RBL with a very hard spectrum in the GeV range has not been seen at TeV energies. We also note that of the 14 BL Lac objects observed by EGRET, 12 are RBLs, whereas only two are XBLs. We give a list of nearby XBLs that we consider to be good candidate TeV sources and predict estimated TeV fluxes for these objects.

Limits on the antiproton/proton ratio in the cosmic radiation from 100 MeV to 1580 MeV

A search for antiprotons (p-bars) in the cosmic radiation with energies below 1580 MeV at the top of the atmosphere was performed using the PBAR balloon-borne magnetic spectrometer. No antiprotons were observed in 124,000 proton events. For the energy interval 100-640 MeV, an upper limit is reported to the p-bar/p ratio of 2.8 x 10 to the -5th at the top of the atmosphere, after correcting for instrumental efficiencies and contributions from secondary particles. No antiproton was observed in the energy interval 640-1580 MeV, which yields an upper limit to the p-bar/p ratio of 6.1 x 10. By combining both data sets, the limits on the p-bar/p ratio can be improved to 2.0 x 10 to the -5th. The detector performance and instrumental efficiencies of the individual detector components are discussed. A detail Monte Carlo calculation was used to evaluate the instrumental efficiency for both antiprotons and protons as a function of momentum. 48 refs.

Search for diffuse cosmic gamma rays above 200 TeV

A search for γ-rays in the cosmic radiation above 200 TeV was realised using a two-level array of scintillators. Surface counters measure the size and direction of extensive air showers while counters buried 3 m below the ground are used to measure their muon content in detail. No evidence for an excess number of muon-poor showers are found and it is concluded that γ-rays comprise less than 0.4% of all cosmic rays above 200 TeV and less than 0.05% above 1000 TeV (90% CL)

Can we detect antimatter from other galaxies

Recent developments in particle detection technology now make it possible to use well-established principles of high-order quantum electrodynamics to search for antimatter in the cosmic rays with unprecedented sensitivity. The technique is described and is shown to be superior in both collecting power and resolution to other, more conventional, techniques used in the past. By considering various estimates of the metagalactic cosmic-ray energy density and by taking into account the possible modulation of metagalactic cosmic rays by a galactic wind within the framework of the dynamical halo model, we show that our proposed experiment would be the first to be sensitive to the presence of extragalactic antimatter.

A new method for determining the Hubble constant from sub-TeV gamma-ray observations

The present uncertainty in the Hubble constant leaves unresolved questions regarding the age of the universe and related matters involving the amount and nature of the dark matter in the universe, consistency with the inflationary model of the universe, and the need for a cosmological constant. It is clear that a significantly precise determination of the Hubble constant by as many different methods as possible is crucial to our knowledge and understanding of the character of the universe. We propose here an entirely new method for determining the Hubble constant, based on measuring the extinction of high-energy gamma-rays emitted by detectable gamma-ray emitting blazars at various redshifts.

Mapping the U.H.E. sky in search of point sources

Abstract Techniques are presented for mapping the sky to search for density excesses attributable to point sources. By incorporating event-by-event angular resolution information, a scatter plot of arrival directions becomes the map of a smooth density function. Methods are discussed for quantitatively comparing that observed density function with the expected density function and evaluating the statistical significance of density excesses. Analysis of Flys Eye data provides examples of the techniques.

A coarse-grain search for anisotropy in the arrival directions of cosmic rays above 10 to the 17th eV

The cosmic-ray intensity measured by the Flys Eye reveals no statistically significant anisotropy when averaged over large solid angles. The sky is divided into six lobes of equal solid angle which are centered on the directions of the Galactic coordinate axes. For each of nine logarithmic energy bands, the number of detected cosmic rays in each lobe is compared with the number expected from an isotropic intensity. The excesses and deficits are not large compared to statistical uncertainties in the expected numbers. Harmonic analysis of the right-ascension distribution of cosmic rays is also reported for the different energy bands. 15 refs.