Technological advances in radio telescopes and X-ray instruments over the last 20 years have greatly increased the number of known supernova remnants (SNRs) and led to a better determination of their properties. In particular, more SNRs now have reasonably determined distances. However, many of these distances were determined kinematically using old rotation curves (based on R ⊙ = 10 kpc and V ⊙ = 250 km/s). A more modern rotation curve (based on R ⊙ = 8.5 kpc and V ⊙ = 220 km/s) is used to verify or recalculate the distances to these remnants. We use a sample of 36 shell SNRs (37 including Cas A) with known distances to derive a new radio surface brightness-to-diameter (Σ − D) relation. The slopes derived here (β = −2.64 including Cas A, β = −2.38 without Cas A) are significantly flatter than those derived in previous studies. An independent test of the accuracy of the Σ − D relation was performed by using the extragalactic SNRs in the Large and Small Magellanic Clouds. The limitations of the Σ − D relation and the assumptions necessary for its use are discussed. A revised Galactic distribution of SNRs is presented based on the revised distances as well as those calculated from this Σ − D relation. A scaling method is employed to compensate for observational selection effects by computing scale factors based on individual telescope survey sensitivities, angular resolutions and sky coverage. The radial distribution of the surface density of shell SNRs, corrected for selection effects, is presented and compared to previous works.

We perform radio pulsar population synthesis to study the evolution of the pulsar population. In doing this, we continue our earlier work on this subject (Bhattacharya et al. 1992). We have extended our work by 1) calculating orbits in the whole galaxy (the simulation of the observations however is still limited to the solar neighborhood), 2) using the high birth velocities of radio pulsars from Lyne & Lorimer (1994) and 3) implementing the new model by Taylor & Cordes (1993) of the distribution of free electrons in the galaxy. A full report of this work will be given in Hartman et al. (1996).

The Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) is a new ground-based atmospheric Cherenkov telescope for gamma-ray astronomy. STACEE uses the large mirror area of a solar heliostat facility to achieve a low energy threshold. A prototype experiment which uses 32 heliostat mirrors with a total mirror area of ∼ 1200 m 2 has been constructed. This prototype, called STACEE-32, was used to search for high energy gamma-ray emission from the Crab Nebula and Pulsar. Observations taken between November 1998 and February 1999 yield a strong statistical excess of gamma-like events from the Crab, with a significance of +6.75σ in 43 hours of on-source observing time. No evidence for pulsed emission from the Crab Pulsar was found, and the upper limit on the pulsed fraction of the observed excess was < 5.5% at the 90% confidence level. A subset of the data was used to determine the integral flux of gamma rays from the Crab. We report an energy threshold of E th = 190 ± 60 GeV, and a measured integral flux of I(E > E th ) = (2.2 ± 0.6 ± 0.2) × 10 −10 photons cm −2 s −1 . The observed flux is in agreement with a continuation to lower energies of the power law spectrum seen at TeV energies.

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Abstract. It has been suggested that young pulsars, with ages less than a million years, are possible counterparts of some of the Galactic unidentified gamma-ray sources detected by the EGRET instrument on-board the Compton Gamma Ray Observatory. In this work, we compare different aspects of the EGRET unidentified (EUI) source distribution in the Galactic plane with those of the pulsar distribution. An EUI source Log N-Log S analysis is presented and compared with the Galactic radio pulsar Log N-Log S distribution. A number of systematic effects that could introduce errors to the EGRET Log N-Log S relation are discussed. A two-point angular correlation analysis of the EUI sources and Galactic pulsars is performed. We find that the global distribution properties of the plane EUI sources do not follow those of the young pulsar population. We conclude that even though a fraction of the EUI sources may have young pulsar counterparts, the majority of them follow a distribution that is similar to the molecular cloud distribution in nearby spiral arms.