G. van Urk

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

Very high energy gamma rays from isolated radio pulsars : a survey

During the period 1985-1989 a group of 18 isolated radio pulsars (IRPs) was observed with the Nooitgedacht TeV γ-ray telescope. These pulsars were selected from a priority list which was compiled by taking both the astrophysical and observational constraints for their observation into account. All IRPs were analyzed in the same way. Of the 18 IRPs observed only one (PSR 1509-58) gave a positive signal over an extended period (1985-1988). No evidence for pulsed emission was seen during the period between 1989 and 1991, however. Two other IRPs (PSR 1823-13 and PSR 1821-24) gave interesting results which merit further observations

The arrival time distribution of muons in extensive air showers

The longitudinal structure of the muon disc has been studied up to distances of 60 m from the shower axis by measuring the relative arrival times between single muons in extensive air showers with particle numbers of 1*105 to 2*107. A rather constant thickness of the shower disc was found up to 60 m from the shower axis. The longitudinal distribution up to 30 m from the shower axis is not in agreement with the results from a shower model with a fairly high multiplicity rule for secondary particle formation in hadron interactions. It is highly improbable that this discrepancy is due to inaccuracies in the measurements and contamination by the hadron and electron components. Heavy primaries also do not explain the results. The conclusion is that many more nucleons may be formed in hadron interactions than the model predicts or that massive particles may be formed in the first few collisions of shower development. Further, there is no indication that this discrepancy has any connection with the knee in the primary energy spectrum, which would be expected if a change in the interaction mechanism occurred in this energy region.

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.