Barend Visser

AE Aquarii - an emitter of pulsed TeV gamma rays resembling optical emission during flares

During optical flares, strong coherent pulsed emission is detected in the frequency interval ∼29.9-30.0 mHz and at F 0 =30.23 mHz (rotation frequency). Evidence is presented of pulsed VHE emission at these frequencies in all our VHE data at the 99.996% significance level covering 4 years of observation. The total time average VHE luminosity at an average threshold energy of ∼2.4 TeV is (1.5±0.3)×10 32 ergs s −1 , which corresponds to a conversion efficiency of 15%. The significance of these features increased steadily with time during the past four years

Theory and Design of Low-Noise Multipath Amplifiers

The theoretical minimum noise measure of a multipath amplifier (an amplifier that has multiple parallel amplifiers) is achieved by using the optimum source impedance and optimum gain for each amplification path. This optimum source impedance and gain can be calculated by replacing each amplifier with its optimum source impedance. The resulting noise measure is the same as the minimum noise measure of the amplifiers used. The theory is demonstrated by applying it to distributed am plifiers. In an ideal distributed amplifier, the magnitude of the optimum gain of the amplification paths decreases and the phase delay increases the farther the stage is from the input, with the decrease in gain being faster for higher frequencies. The challenge in designing broadband low-noise distributed amplifiers is to achieve optimum gain matching over broad bandwidths. A numerical optimization procedure that separates source impedance and gain matching is proposed and demonstrated by optimizing a 0.5-2-GHz distributed amplifier. An average noise measure of 0.3 dB is achieved, which is only 0.1 dB higher than the minimum noise measure of the amplification stages used. This increase is due to transmission line loss and gain mismatch.

Advanced Plasma Ignition (API): A Simple Corona and Spark Ignition System

Due to stringent emission and fuel consumption requirements, several innovative ignition systems have been proposed to address the shortcomings of current spark ignition systems. Two of the most promising candidates are high-energy spark ignition systems and corona ignition systems. However, both these systems have some fundamental drawbacks. Spark ignition systems are very mature and relatively inexpensive, but the performance of these systems is limited by the small electrode gap, which results in a small ignition volume. Corona systems, on the other hand, have a much larger ignition volume, but have been slow to enter the market due to their complexity and cost.

TeV gamma‐rays from accreting binary systems

Evidence is presented that Vela X‐1 emits steady pulsed emission at the X‐ray period. Another X‐ray binary, Cen X‐3, also show evidence for steady, pulsed emission at the expected X‐ray period, but this emission only occurs in the orbital phase range between 0.5 and 0.75. In both sources the emission occur at a period compatible with the X‐ray period, although slightly shifted. The white dwarf in the cataclysm variable, AE Aqr, shows steady, pulsed emission at the known spin period, and in phase with both the X‐ray and optical emission, indicating the production of gamma‐rays close to the white dwarf.

TeV observations of gamma‐ray pulsar: A test for pulsar models

We present the first results of TeV γ‐ray observations of PSR1706−44, indicating a sharp spectral break between ∼10 GeV and 1000 GeV. Our and other results confirm similar breaks from other γ‐ray pulsars such as Vela, PSR1509−58 and Geminga. Both the polar cap‐ and outer gap models can account for this break, although a much higher TeV flux from Geminga is expected if it is in the post‐Vela phase, given that the gap is controlled by the inverse Compton process. Relatively flat (possibly time variable) spectra from these pulsars are also seen between X‐rays and medium energy γ‐rays, which can also be explained by the outer gap model.

Near simultaneous optical and TeV observations of AE Aquarii

We present results of near simultaneous optical and TeV observations made on Sept. 14, 1990 of the novalike cataclysmic variable AE Aquarii. The optical was in a flaring state resulting in a bolometric luminosity which may have been high enough to give detectable TeV radiation during the TeV observation. During the latter we have seen periodic TeV emission at a frequency redshift of −0.7% relative to the spin frequency which confirms our earlier detection during 1989 at the 99.5% level. Both the optical and TeV did show evidence of a changing frequency with time which appears to be describing a single event. We also show that the shock above the white dwarfs polar cap may be collissionless so that protons may be accelerated to energies above 1 TeV. If the part of the accretion stream from the companion which reprocesses incident X-rays to optical pulsations is magnetized and dense enough, it may trap incident TeV particles for a long enough time to give TeV γ-rays through π° decay with the same period as in optical. The frequency will be constant if the spot of reprocessing is fixed in the binary frame, but shifted if the spot moves.