X-ray timing properties of Cygnus X-1 and Cygnus X-2.

摘要

Cygnus X-1 and Cygnus X-2 are X-ray sources which are believed to be a black hole and a neutron star, respectively.;We investigate the variability of Cygnus X-1 in the context of shot noise models, and employ a peak detection algorithm to select individual shots. The detected shots are fit to several model templates. The fit shot parameters are found to be distributed.;The cross spectrum of light curves from Cygnus X-1 in different energy bands is studied. Large, frequency dependent time lags are observed, along with high coherence. The high coherence implies that the transfer function between low and high energy variability is uniform. This implies that, if the lags are due to Compton scattering, variations in the seed intensity must originate in a region much smaller than the Comptonizing medium. The frequency dependence of the lags implies that, if they are due to Comptonization, the Comptonizing medium is nonuniform.;The uniformity of the transfer function implies that the observed distribution of shot widths cannot have been acquired through Compton scattering.;The energy spectrum of the radiation reaching us from Cygnus X-1 is found to fluctuate at the shortest timescales observable. This implies rapid changes in the geometry or temperature of some portions of the system, possibly due to dynamical instabilities of the system.;The high counting rates and temporal resolution of the Rossi X-ray Timing Explorer allow more detailed investigation of the Horizontal Branch Oscillations (HBO) in Cygnus X-2 than has been possible previously, including observation of a second harmonic. The relations of the frequencies and widths of these peaks are investigated in order to shed light on their origin and thus the conditions in the region of their formation. The observations support the Magnetospheric Beat Frequency Modulated Accretion model for the origin of the HBO. The data indicate that neither rapid variation in the frequency of, nor short lifetime of, a locally coherent oscillation is sufficient to explain the broadening alone. This argues that the width is due to the simultaneous presence of multiple frequencies, or limitation of the amplitude of oscillations through some nonlinear effect.