Multi-Frequency Observations of
a TeV blazar, Mkn421, from a big campaign in 1998
T. Takahashi, J. Kataoka
We present results of the recent multiband campaign of a TeV blazar,
Mkn421. Judging by the results of previous campaigns, correlations of
inter-band variability of Mkn 421 have proven to provide our best
opportunity to understand the high energy emission from blazar jets.
In particular, the rapid intra-day variability seen in both X-ray and
TeV energy bands gives us clues to study the physics of blazar jets.
However, the sparse sampling of the previous campaigns has prevented
us from obtaining definitive conclusions. With this in mind, we
performed an unprecedented seven-day continuous observations with
ASCA, coordinated with EUVE, RXTE and SAX in April 1998. At
the same time, TeV detectors (CAT, HEGRA, Whipple), optical
telescopes, and 22 GHz radio antennae attempted to observe the source
In the campaign, we detected a historical high activity of the
source. The 2 - 10 keV flux in the beginning of the observation was
1.2×10-10erg cm-2s-1 and increased up to
5.0×10-10 erg cm-2s-1 at the maximum. More than
10 flares are clearly seen superimposed on the general increasing
trend. The doubling time scale of each flare is about 0.5 days. The
continuous light curve of 7 days implies that the source actually
flares daily, and perhaps more often. ASCA data clearly reveal
spectral variability. The 2 - 7 keV energy index ranges from 1.4 to
Importantly, we discovered that the amplitudes of soft X-ray lag,
which allows further constraints on the parameters of the emitting
region, varied flare by flare. Hard X-ray lag does exist in some
flares. The comparison of the data from ASCA, EUVE and RXTE
indicates that the variability amplitudes in the LE (synchrotron)
component are larger at higher photon energies. We clearly detected
the correlation between TeV flux and X-ray flux from truly
File translated from TEX by TTH, version 2.32.
On 16 Jul 1999, 09:19.