Continuum Power Spectrum Components in X-Ray Sources: Detailed Modelling   and Search for Coherent Periodicities
  This paper summarises two recently developed techniques in power spectral analysis and their application to a sample of X-ray light curves of accreting collapsed objects in active galactic nuclei and X-ray binaries. The first technique is designed to carry out detailed model fitting of continuum power spectrum components arising from noise variability by using maximum likelihood methods. The technique is applied to the light curves of a number of highly variable AGNs observed with EXOSAT. Substantially steeper logarithmic power spectrum slopes are obtained than previously estimated with standard methods. The second technique was devised in order to reveal coherent periodicities in the presence of ``coloured" (i.e. non--white) noise variability components from the source. To this aim the power spectra are searched for significant narrow peaks superposed on the ``coloured" continuum components. We present the results of a search for an orbital modulation in the light curves of a sample of 25 low mass X--ray binaries (LMXRBs), for which the orbital period is either unknown or detected only at optical wavelengths. This led to the discovery of a significant X--ray orbital modulation at the few percent level in the burster MXB1636-539. 
L. Stella, E. Arlandi, G. Tagliaferri and G.L. Israel