Temporal and Spectral Variations of the Superposed Shot as Causes of Power Spectral Densities and Hard X-Ray Time Lags of Cygnus X-1

Details Temporal and Spectral Variations of the Superposed Shot as Causes of Power Spectral Densities and Hard X-Ray Time Lags of Cygnus X-1 Temporal and Spectral Variations of the Superposed Shot as Causes of Power Spectral Densities and Hard X-Ray Time Lags of Cygnus X-1

Jun 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001apj...554..528n&link_type=abstract

The Astrophysical Journal, Volume 554, Issue 1, pp. 528-533.

Physics

19

Accretion, Accretion Disks, Black Hole Physics, Stars: Individual (Cygnus X-1), X-Rays: Stars

Scientific paper

We examined the temporal and spectral variations of Cyg X-1 in the hard state by average shot properties obtained by superposing a number of large shots by aligning their peaks. The average shot shows an almost symmetric time profile and a softer energy spectrum than the average spectrum. The energy spectrum also exhibits rapid hardening around the peak intensity, followed by complex changes. Importantly, not only power spectral density (PSD) but also hard X-ray time lags (cross spectrum) of the average shot can give a good representation of those of the entire light curves (all the data) at frequencies below a few Hz. In other words, low-frequency parts of the PSD and time-lag diagrams simply reflect the temporal and spectral structures of such large shots. Higher frequency parts of the diagrams, in contrast, seem to be related to smaller time variations with shorter time constants. Our findings provide unprecedent, severe constraints on theoretical models for variability; namely, it is not enough to reproduce the observed PSDs and/or time lags, since neither has relative phase information between different frequencies, but successful models should also account for the complex spectral variations during the shot as described above as well as the symmetric shot profile. No models proposed so far can satisfy all these requirements. The presence of at least two different timescales characterizing the shot profile and the rapid hardening seemingly indicate two (or more) physical processes being involved, e.g., disturbance propagation and magnetic flares.

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