Application of spectral analysis techniques on the gamma–ray data of Vela X–1 and PKS 2155–304

Abstract

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 y–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 y–ray telescopes have as of yet not been properly studied. Atmospheric Cherenkov y–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 signal when applying significance tests. The proposed method for dealing with this increase in power is to subtract the Fourier transformod 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 y–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 y–rays.