Five-year Monitorings of TeV Blazars with $ASCA$ and $RXTE$

J.Kataoka$^1$, T.Takahashi$^2$, S.Wagner$^3$, P.Edwards$^2$, 
and G.Madejski$^4$

$^1$Kyoto University, Sakyo-ku, Kyoto, Japan\\
$^2$ISAS, Sagamihara, Kanagawa, Japan\\
$^3$Landessternwarte Heidelberg, K\H{o}nigstuhl, Heidelberg, Germany\\
$^4$Stanford Linear Accelerator Center, USA\\


We study the temporal/spectral variability of two extragalactic TeV sources, 
Mrk 421 and Mrk 501, based on 5-year observations with the $ASCA$ and $RXTE$ 
satellites. We found that the peak of the synchrotron emission exists just 
in the X-ray band and its position shifted from lower to higher energy when 
the source became brighter. Relationship between the peak energy  and peak 
luminosity showed quite different behavior in the two sources; Mrk 421 
showed very little change in the peak position (0.5$-$2 keV), while Mrk 501 
revealed the largest shift ever observed in blazars (1$-$100 keV).  
We analyze these X-ray data with the flux changes in TeV band, which are 
obtained from 35  $truly$ simultateneous observations.  
Very different spectral evolution of both objects indicates some 
differences in the electron acceleration mechanism at work during the 
flares. We argue that the flux variability of Mrk 421 is associated with 
an increase in the number of electrons, while the flare of Mrk 501 is 
mostly due to the large changes in maximum energy of electrons. 
We also discuss the characteristic temporal variability of these TeV sources,
and implications for the X-ray emitting site in the jet.