Hard X-ray Time Lags in Accreting Black-Hole Systems

H. Negoro$^1$, W. Brinkmann$^2$, I. Papadakis$^3$, M. Gliozzi$^2$,
and M. Matsuoka$^4$

$^1$ RIKEN (The Institute of Physical and Chemical Research), Japan \newline
$^2$ MPE, Garching, FRG \newline
$^3$ Department of Physics, University of Crete, Heraklion, Greece \newline
$^4$ NASDA, Ibaraki, Japan


It is well known that black hole candidates (BHCs) such Cyg X-1
in the low/hard state and Seyfert galaxies including narrow line 
Seyferts commonly exhibit aperiodic short-term X-ray variability.
The variability has been long studied in terms of power spectral
density or related statistical analysis methods. From those studies, 
their time variations are found to be very similar, as is 
expected in the accretion disk theories.
   The nature of the variability is, however, far from our understanding. 
Also, we do not know if the variations in both the systems really come 
from the same/similar processes.
Then, I have paid my attention to 'shots' in the BHCs and 'flares' 
in the Seyferts to diagnostic the accretion processes.
Average shapes and spectral variations during the shots have been 
revealed in terms of a superposition technique. Similar spectral 
variations, "hard X-ray time-lags", have been also found in some Seyfert 
galaxies.
  These unprecedented information may provide the first direct evidence to 
connect the variability in both the systems.
In the talk, I will show those observational results using "Ginga" and 
"ASCA" satellite data, and discuss the nature of the time variability.