| Summary: | Studies of frequency variability can provide an important understanding of physical processes
in the central regions of an AGN (Ulrich et al., 1997). For neutron star X–ray sources, variations
in the intrinsic pulse frequency are believed to reflect changes in the rotation rate of the stellar
crust, produced by torques originating inside and outside the crust (Boynton et al., 1984).
Periodicity analysis therefore plays an important role in astrophysics.
For discretely sampled signals, this analysis is often done using the periodogram,modified periodogram,
and the Lomb–Scargle periodogram. These techniques,with possibly the exception
of the Lomb–Scargle periodogram, are well known in the subject of discrete signal processing.
However, their application to atmospheric Cherenkov ?–ray telescopes have as of yet not been
properly studied.
Atmospheric Cherenkov ?–ray telescopes, particularly H.E.S.S., can be though of as photon
counting devices. This is very different from devices that sample discretely. Only after binning
the data can the data be regarded as discretely sampled. Furthermore, the H.E.S.S. telescope
usually samples in 28 minute intervals followed by 3 minutes of dead time after each interval.
A signal will be simulated/modelled through the use of Monte Carlo simulations. These simulated
signals can consist of white noise, periodic signals, or a mixture of both. Through the
use of these simulations an attempt will be made to find an appropriate bin size, as well as
determining the effect of dead time on the time series and correcting for that effect. The effect
of dead time on the Rayleigh test, when searching low frequency periodicity will also be studied.
An attempt will also be made to explain the discrepancy between the binning of photon
counting events, and discretely sampled signals in terms of discrete signal processing.
Monte Carlo simulations showed that the dead time causes low frequency power to increase
drastically. This increased power could easily be misinterpreted as power–law noise, as well as
resulting in the false positive detection of a further signalwhen applying significance tests. The
proposedmethod for dealingwith this increase in power is to subtract the Fourier transformof
the dead time from that of the signal to be analyzed. This method yielded satisfactory results.
The flaring event of PKS 2155–304 on the nights of 27/28 July and 29/30 July was analyzed by
Aharonian (2007). A reanalysis of this event will be done using the results obtained from the
Monte Carlo simulations. An analysis will also be done on Vela X–1 to determine it as a source
of TeV ?–rays as found by Protheroe et al. (1984),North et al. (1987), Raubenheimer et al. (1989),
and Raubenheimer et al. (1994).
A reanalysis of the flaring event of PKS 2155–304 found that the low frequency power was
very dependent on bin size. For smaller bin sizes the dead time had significant consequences
however, for a bin size of 60 seconds, the results were similar to that found by Aharonian
(2007). The analysis of Vela X–1 however confirmed this object to not be a source of high
energy ?–rays. === Thesis (MSc (Space Physics))--North-West University, Potchefstroom Campus, 2012.
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