The analysis and interpretation of VHE (Eg > 1 TeV gamma ray measurements
De Jager, Ocker Cornelis
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It is well known that Very High Energy Gamma kay Astronomy (VHEGRA) is facing a dilemma in the sense that all reported sources are very weak emitters of VHE 1-rays. The status of most sources are also questionable on statistical grounds, while very few sources have been confirmed. This dilemma can be solved by improving the sensitivities of telescopes. In this study it is however reasoned that some light can be shed on this dilemma by treating the data .in a more consistent way. Since one is dealing with a strong stochastic component of background cosmic radiation, the data should be analysed by means of sound statistical techniques. The analysis of low counting statistics are treated, with the accent on periodic analysis. Existing statistical tests for uniformity on a circle are reviewed and it is concluded that they are only usable if one has some a priori information about the form of the light curve. A 'new'' test (the Hm-test) is developed to identify sources for which the form of the light curve is unknown. It is also shown how one can overestimate ·the significance of a signal if a search is conducted within one independent Fourier spacing: With the Rayleigh test one can report a 'probability for uniformity' which is a factor of three too small, while this factor can be up to twenty for tests like z210 or Pearson's x2 -test with twenty bins. It is also shown how a x-ray light curve can be estimated from the data (phases) alone without making any ad hoc assumptions. It is a fact that such a light curve estimator will converge to the true unknown light curve if the sample size increases to infinity and if the phases are independently and identically distributed. The MeV light curve of the Vela pulsar (using the COS-B data) is estimated as an example. It is also shown how the signal strength of a periodic source can be estimated in some cases. The isolated pulsar PSR 1509-58 is identified by means of the Hm-test at a confidence level of 99.9':.. The light curve is a triple sinusoid and it is shown that the observed radiation from night to night is steady and coherent. The data on PSR 1802-23 are reanalysed, taking the effect of oversampling within one independent Fourier spacing into account. Indications of steady emission is found at a confidence level of 98.6%. Existing models for r-ray emission from isolated pulsars are investigated with the aim of predicting the VHE r-ray luminosity. It is found that only millisecond pulsars can produce observable VHE polar cap r-rays while an outer gap near the light cylinder can provide observable VHE r-rays from pulsars like Crab, Vela and PSR 1509-58. The outer gap model provides consistent results in the sense that the predicted MeV and TeV luminosities agree with COS-B and VHE observations respectively From the existing theory of outer gaps it is found that transients above 1 TeV should occur more often than transients below 3 GeV. It is also shown from this theory that a pulsar with the tentatively identified parameters of PSR 1802-23 is an ideal VHE emitter. Furthermore, certain pulsars can be MeV quiet but TeV loud. Finally, there may be one or more pulsars in the Southern Sky which could cause the observed count rate to double - thus solving the VHEGRA dilemma.