Search for UHE cosmic sources with a large-area underground muon detector

Summary The newly discovered gamma-ray emission from Cyg-X 3 with energies up to 2·10¹⁶eV gives the first evidence that particle acceleration in galactic gamma-ray sources can reach energies of the order of 10⁴ TeV. We discuss the detectability of this type of sources by an underground muon tracking detector with area of the order of ∼1000 m² and the physical information which could be obtained in this way. Paper presented at the 2° Convegno Nazionale di Fisica Cosmica, held at L'Aquila, 29 May–2 June 1984.     

High-energy gamma-ray sources of cosmological origin

The current generation of instruments in gamma-ray astrophysics launched a new era in the search for a dark matter signal in the high-energy sky. Such searches are said indirect, in the sense that the presence of a dark matter particle is inferred from the detection of products of its pair-annihilation or decay. They have recently started to probe the natural domain of existence for weakly interacting massive particles (WIMPs), the favorite dark matter candidates today. In this article, we review the basic framework for indirect searches and we present a status of current limits obtained with gamma-ray observations. We also devote a section to another possible class of cosmological gamma-ray sources, primordial black holes, also considered as a potential constituent of dark matter.     

Solar cosmic rays

Summary This report is a brief overview of the energetic-particle phenomena related to solar activity discussed at the XXIV International Cosmic-Ray Conference. In the rapporteur's opinion, the most significant results to be highlighted are: 1) the new higher-energy measurements of the solar energetic ion charge state which seem to be consistent with the older measurements at ∼1 MeV/nucleon, and 2) the many reports of extended gamma-ray production during solar flares with multiple episodes of emission implying multiple episodes of particle acceleration. Rapporteur talk given at the XXIV International Cosmic-Ray Conference, Rome, August 28–September 8, 1995.     

Gamma-ray emission from molecular clouds: A probe of cosmic-ray origin and propagation

Cosmic rays up to at least 10¹⁵ eV (PeV) are believed to be emitted by Galactic sources, such as supernova remnants. However, no conclusive evidence of their acceleration has been found yet. A trace of ongoing cosmic-ray acceleration is the gamma-ray emission produced by these highly energetic particles when they scatter off the interstellar medium gas, mainly atomic and molecular hydrogen. Whereas the atomic hydrogen is uniformly distributed in the Galaxy, the molecular hydrogen is usually aggregated in dense clouds, and the gamma-ray emission from such clouds is particularly intense and localised. A multi-frequency approach, which combines the data from the upcoming and future gamma-ray emissions with the data from the submillimeter and millimeter surveys of the molecular hydrogen, is therefore crucial to probe the Galactic cosmic-ray flux. In order to fully exploit this multi-frequency approach, one needs to develop predictions of the expected emission. Here we will discuss the GeV to TeV emission from runaway CRs penetrating molecular clouds close to the young supernova remnant RX J1713-3946 and in molecular clouds illuminated by the background cosmic-ray flux.     

Low-energy gamma-ray sources observed by the MISO telescope

Summary The MISO telescope is a gamma-ray detector operating in the (0.1÷20) MeV energy range. This instrument was flown success-fully from Palestine, Texas (U.S.A.) in May 1977, October 1978, September 1979 and May 1980. During the four flights, the telescope observed a number of galactic and extragalactic sources. The Crab Nebula was detected in 1980 and its spectrum was measured up to 2 MeV. Cygnus X-1 was observed in 1979 and 1980 and was found to be in different hard X-ray states on each occasion: in a superlow state in 1979 and in a low state in 1980. The COS-B high-energy (E>50 MeV) gamma-ray sources CG 135+1 (1978) and CG 195+4 (1978) were also observed by the same instrument. From the region containing CG 135+1 a 5σ excess was measured in the counting rate of the telescope above 120 keV. No statistically significant low-energy gamma-ray flux was detected from CG 195+4. The Seyfert galaxies NCG 4151 (1977, 1979, 1980), and MCG 8-11-11 (1979), the BL LAC object MK 501 (1979) and the ?peculiar? galaxy NGC 1275 (1979) were also searched for gamma-ray emission. The data on NGC 4151 and MCG 8-11-11 are consistent with power law spectra having photon indices α∼1 in the (0.1÷3) MeV energy range. Above this energy, the MISO and SAS-2/COS-B measurements require spectral breaks to α≥3. Upper limits were obtained on the emission from both MK 501 and NGC 1275. Paper presented at the 2° Convegno Nazionale di Fisica Cosmica, held at L'Aquila, 29 May–2 June 1984.     

Ground-based gamma-ray astronomy

Summary This report covers developments in the field of gamma-ray astronomy, essentially in the energy range 300 GeV to 300 TeV, reported at the XXIV International Cosmic-Ray Conference in Rome in 1995. Highlights which receive the main attention are the failure of several experiments to detect TeV photons from several supernova remnants at the level predicted on current models of shock acceleration of cosmic-ray protons, and the detection of a rapidly variable flaring output of TeV photons from two BL Lac objects (Markarian 421 and Markarian 501). Although techniques are now maturing to the extent that different measurements of the flux and spectrum from the Crab Nebula are now converging, unresolved differences remain over the existence of TeV pulses from certain pulsars. However, one well-supported instance of very intense pulsation for an hour from AE Aquarii was presented. The Vela Pulsar was reported as a steady source. Rapporteur talk given at the XXIV International Cosmic-Ray Conference, Rome, August 28–September 8, 1995.     

Highlight results from the Compton gamma-ray observatory

Summary Astronomy is performed nowadays over the entire range of the electromagnetic spectrum from radio to gamma-ray energies. From the so-called ?New Astronomies? which are performed outside the optical window, we have learnt that each spectral range provides specific information, which cannot be obtained by other means. This is also true for gamma-ray astronomy. Gamma radiation represents the most energetic part of the electromagnetic spectrum. Therefore, it provides information about the most energetic processes and phenomena in the Universe. The energy band of space-borne gamma-ray astronomy extends over more than five orders, of magnitude—similar to the band from the radio to optical range. It is thus not surprising that a variety of different objects and phenomena can be studied. Gamma-ray astronomy at present is experiencing a golden age. This is mainly due to the many exciting results from NASA's Compton Gamma Ray Observatory, which has been in orbit for more than 4.5 years. The broad interest in this mission is remarkable. Perhaps its most important achievement is that gamma-ray astronomy has now become an integrated part of astronomy and astrophysics. One has recognized that there are objects in the Universe like radio pulsars and quasars, which do have their peak luminosities at gamma-ray energies. It is practically impossible to understand these objects without knowing their gamma-ray properties. Invited talk given at the XXIV International Cosmic-Ray Conference, Rome, August 28–September 8, 1995.     

Ground-based gamma-ray astronomy: General remarks

Summary The ground-based gamma-ray astronomy has its coming of age currently with 4 sources detected at high significance levels, after experiencing about 30 years history. The energetic gamma rays provide direct means for uncovering higher-energy non-thermal phenomena at various celestial objects. We need more data with improved sensitivities, and multitudes of such efforts are now under way. Paper presented at the Special Session on ground-based gamma-ray astronomy of the XXIV International Cosmic-Ray Conference, Rome, August 28–September 8, 1995.     

Gamma-ray direct observations from space

Summary This paper is devoted to problems of gamma-ray astronomy in the energy range <10¹¹ eV. Measurements of spectra and fluxes in this energy range are carried out by means of direct observations from space. Most of the discussed results have been performed with the four telescopes of the Compton Gamma-Ray Observatory. The main topics of the paper are: diffuse galactic gamma-ray emission, point-like galactic gamma-ray sources, gamma-ray line emission, gamma-ray bursts and active galactic nuclei. Rapporteur talk given at the XXIV International Cosmic-Ray Conference, Rome, August 28–September, 8, 1995.     

Probing physics at extreme energies with cosmic ultra-high energy radiation

The highest energy cosmic rays observed possess macroscopic energies and their origin is likely to be associated with the most energetic processes in the universe. Their existence triggered a flurry of theoretical explanations ranging from conventional shock acceleration to particle physics beyond the standard model (SM) and processes taking place at the earliest moments of our universe. Furthermore, many new experimental activities promise a strong increase of statistics at the highest energies and a combination with γ-ray and neutrino astrophysics will put strong constraints on these theoretical models. We give an overview over this quickly evolving research field with focus on testing new particle physics.     

Large-area atmospheric Cherenkov detectors for high-energy gamma-ray astronomy

Summary This paper describes the development of new ground-based gammaray detectors to explore the energy region between 20 and 200 GeV. This region in energy is interesting because it is currently unexplored by any experiment. The proposed detectors use the atmospheric Cherenkov technique, in which Cherenkov radiation produced in the gamma-ray air showers is detected using mirrors and light-sensitive devices. The important feature of the proposed experiments is the use of large mirror collection areas, which should allow for a significant improvement (i.e. reduction) in energy threshold over existing experiments. Large mirror areas are available for relatively low cost at central tower solar power plants, and there are two groups developing gamma-ray experiments using solar heliostat arrays. This paper summarizes the progress in the design of experiments using this novel approach. Paper presented at the Special Session on ground-based gamma-ray astronomy of the XXIV International Cosmic-Ray Conference, Rome, August 28–September 8, 1995.     

A gamma-ray signature of energetic sources of cosmic-ray nuclei

Astrophysical sources of nuclei are expected to produce a broad spectrum of isotopes, many of which are unstable. An unstable nucleus can beta-decay outside the source into a single-electron ion. Heavy one-electron ions, thus formed, can be excited in their interactions with cosmic microwave background photons, in which case they relax to the ground state with the emission of a gamma ray. Repetitive cycles of excitation and gamma-ray emission can produce an observable feature in the gamma-ray spectrum with a maximum around 8 GeV (for iron). We find that the observed spectrum of Centaurus A is consistent with a substantial flux of nuclei accelerated to 0.1 EeV. A characteristic 5-10 GeV (iron) shoulder in the gamma-ray spectra of various sources can help identify astrophysical accelerators of nuclei or set upper limits on nuclear acceleration.     

Highly Efficient Sources of Wideband and Monochromatic X and Gamma Rays

New physical configurations of x- and gamma-ray sources, in which photons are emitted by circulating electrons in multiple passage through a thin target, are discussed. As examples, sources of bremsstrahlung with an additional acceleration of radiating electrons and sources of monochromatic radiation in a crystalline target are examined. Computer modeling confirms the high efficiency of these configurations that allow the hard photon yield to be significantly increased while retaining the compact dimensions of the facility. This is especially important in the design of various autonomous sources.     

New astronomies with Extensive Air Showers

Summary We discuss some recent results obtained in the fields of VHE-UHE gamma-ray astronomy, search for gamma-ray bursts at high energies, and UHE neutrino astronomy by means of Extensive Air Showers (EAS) arrays. The technical achievements and future possibilities are outlined.     

Upper limits to the radio pulsation of 1E0630+178, indicated as possible X-ray counterpart of the gamma-ray source 2CG195+04

Summary In the framework of an experiment of search for fast pulsars inside the error boxes of the COS-B gamma-ray sources, a special effort has been done to observe with the best possible sensitivity the X-ray source 1E0630+178 believed to be a neutron star on the basis of the high ratioL _x /L _o between X-ray and optical luminosity. The object, which is located inside the error box of the gamma-ray source 2CG195+04 (Geminga) was observed at 318, 430, 1387 and 1414 MHz with long integration times and low sampling rate. The most sensible observation is that at 430 MHz for which a limiting flux threshold of 0.6 mJy has been reached for periods down to 2 ms. The power spectrum does not indicate the presence of any pulsation effect and so is, at slightly higher limiting flux thresholds, for all other frequencies. Paper presented at the 2° Convegno Nazionale di Fisica Cosmica, held at L'Aquila, 29 May–2 June 1984.     

Proper motion of gamma rays from microhalo sources

I discuss the prospects of detecting the smallest dark matter bound structures present in the Milky Way by searching for the proper motion of gamma-ray sources in the upcoming Gamma Ray Large Area Space Telescope all sky map. I show that for dark matter particle candidates that couple to photons the detection of at least one gamma-ray microhalo source with proper motion places a constraint on the couplings and mass of the dark matter particle.     

Optical identification of X- and gamma-ray sources: the use of existing data

Summary We briefly discuss the use of existing astronomical data, mainly deep optical observations performed in order to obtain galaxy and star counts, for the identification of gamma-ray burst sources in particular and of newly discovered X- and gamma-ray sources in general. Use of such data cannot only reduce the amount of required telescope time but also, in comparison with present-day observations, allow the search for variability and proper motion of candidate optical counterparts over a time span of several years. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

Axion-like particles and the propagation of gamma rays over astronomical distances

In this brief review, possible manifestations of mixing between axion-like particles (ALPs) and energetic photons propagating over astronomical distances are considered. We discuss the evidence for the anomalous transparency of the Universe from observations of ensembles of distant gamma-ray sources, present the general formalism for the ALP-photon mixing and explain how this mechanism may remove the anomaly. We present relevant values of ALP parameters and discuss future ways to verify the scenario and to discover the particle in question.     

Development of FPGA-based digital signal processing system for radiation spectroscopy

We have developed an FPGA-based digital signal processing system that performs both online digital signal filtering and pulse-shape analysis for both particle and gamma-ray spectroscopy. Such functionalities were made possible by a state-of-the-art programmable logic device and system architectures employed. The system performance as measured, for example, in the system dead time and accuracy for pulse-height and rise-time determination, was evaluated with standard alpha- and gamma-ray sources using a CsI(Tl) scintillation detector. It is resulted that the present system has shown its potential application to various radiation-related fields such as particle identification, radiography, and radiation imaging.