How Robust are the EGRET AGN gamma-ray identifications?

Summary Several AGNs have been proposed as identifications of gamma-ray sources discovered by CGRO. Using the NED database, we show that these AGNs are not the only extragalactic objects within the error boxes of the sources. Therefore, an in-depth study is needed on a case-by-case basis to access the most probable counterparts of the gamma-ray sources. A comparison with the COS-B database is also presented. 28Paper presented at the 6th Cosmic Physics National Conference, Palermo, 3–7 November 1992.     

FIGARO IV: Large-area balloon-borne telescope to study rapid time variabilities in the gamma-ray sources at energies above 50 MeV

Summary We present a new γ-ray telescope based on the Limited Streamer Tube technology, used as tracking chambers to detect photons above 100 MeV. This technique allows to obtain very large sensitive areas (16 m² in our experiment), together with a good angular resolution for payloads embarcable in high-altitude balloon flights. The capability to collect a large signal in a short exposure time makes the telescope particularly suitable and competitive with respect to satellite-based detectors for studying both periodic and random time variabilities on galactic and extragalactic γ-ray sources. Paper presented at the 6th Cosmic Physics National Conference, Palermo, 3–7 November 1992.     

Angular resolution of an array of tracking-chamber detectors for use in high-energy gamma-ray astronomy

Summary We describe a method of analysis for the reconstruction of the arrival direction of a high-energy photon impacting on the top of the atmosphere by using the directions of the particles on the shower front as measured by a ground-based array of tracking chambers uniformly distributed on a square surface; as an example, an array with enclosed surface of (120×120) m² has been considered. Estimates on the accuracy of reconstruction are given, after application of the method to computer-simulated shower directions as a function of the number of ?smapling? detectors and of their accuracy in the measurement of the ?local? shower direction. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

VHE gamma-ray sources and implications

Summary This paper highlights the status of very high-energy (VHE) gamma-ray astronomy with emphasis on the astrophysical significance of the discovery of TeV γ-radiation from four celestial objects, Crab Nebula, PSR B1706-44, Mrk 421 and Mrk 501, detected at very high confidence levels using the atmospheric Cherenkov imaging technique. 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 programme for X-ray/optical identification of Soft-γ-ray sources observed by the SIGMA telescope

Summary Anad hoc programme aimed at the search for the optical counterparts of the hard-X-ray/soft-γ-ray sources discovered and/or observed by the SIGMA telescope is described and some of the results, obtained so far, are presented. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

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.     

Searches for gamma-quanta from local sources by a wide-angle imaging telescope

Summary A new Cherenkov imaging telescope SHALON equipped with a very high-definition camera (144 pixels, imaging 7.2⁰×7.2⁰) takes data since 1993 at a mountain altitude of 3338 m. We discuss some results of the observations of the indicated gamma-ray sources and the discrimination methods between gamma-rays and protons. Selection of showers produced by gamma-quanta from a background of showers produced by protons has been made according to the following criteria: 1) alpha<20⁰; 2) length/width>1.6 for γ; 3) Cherenkov light intensity in pixels with max light to the light in the light pixels near around is >0.2 for γ and <0.6 for p; 4) Cherenkov light intensity in pixels with max light to all light in imaging excluding in centre is >0.8 for γ and <0.8 for p; 5) distance for γ is <3.5 pixels.I _Crab=(0.95±0.18)·10⁻¹² cm⁻²c⁻¹.I _Mark421=(0.98±0.36)·10⁻¹² cm⁻²c⁻¹. 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.     

The discovery of proper motion of Geminga's optical counterpart

Summary After reviewing the evidence that the X/γ-ray source 1E0630+ 178/Geminga is a relatively young, isolated neutron star similar to the Vela Pulsar, we present our recent discovery of the proper motion of its faint optical counterpart. This measurement confirms the proposed optical identification, and suggests a distance of the order of 100 pc, consistent with the upper limits derived from the Geminga high-energy emission. The hypothetical nearby supernova event which originated Geminga about 350 000 years ago is also briefly discussed. Paper presented at the 6th Cosmic Physics National Conference, Palermo, 3–7 November 1992.     

An analysis of the morphological and spectral behaviour of the gamma-ray bursts

Summary We investigate by a statistical analysis possible correlations among the observational parameters of the γ-ray bursts: the rise time, the decay time and spectral hardness. We discuss the physical constraints implied by the results. To speed up publication, the proofs were not sent to the authors and were supervised by the Scientific Committee.     

Cygnus X-3: Six years of observations atE0≥50 TeV from the Plateau Rosa array

Summary Cygnus X-3 has been observed from 1982 to 1987 at primary energyE ₀≃10¹⁴ eV from the Plateau Rosa array. The absolute excess in the counting rate from the source region amounts to 0.15%, mostly concentrated in 1982 and 1986. The effect is observed in two classes of events although with small significance (2 and 1.4 s.d.). The 1986 data are consistent with observations of other arrays. The study of the 4.8 h time periodicity over a long observation time does not show any significant structure (2.5 s.d. excess at phase 0.2÷0.5). In 1982 and 1986 the light curves show respectively a wide excess between phases 0.60 and 0.85 and a 2.7 s.d. excess at phase 0.2÷0.25. The analysis of daily excesses from the source shows a good agreement with the expectations from pure statistical fluctuations, and no significant evidence of burst activity. To speed up publication, the proofs were not sent to the authors and were supervised by the Scientific Committee.