(Abridged) We present a detailed analysis of week-long simultaneous observations of the blazar Mrk421 at 2-60 keV X-rays (RXTE) and TeV gamma-rays (Whipple and HEGRA) in 2001. The unprecedented quality of this dataset enables us to establish firmly t
he existence of the correlation between the TeV and X-ray luminosities, and to start unveiling some of its more detailed characteristics, in particular its energy dependence, and time variability. The source shows strong, highly correlated variations in X-ray and gamma-ray. No evidence of X-ray/gamma-ray interband lag is found on the full week dataset (<3 ks). However, a detailed analysis of the March 19 flare reveals that data are not consistent with the peak of the outburst in the 2-4 keV X-ray and TeV band being simultaneous. We estimate a 2.1+/-0.7 ks TeV lag. The amplitudes of the X-ray and gamma-ray variations are also highly correlated, and the TeV luminosity increases more than linearly w.r.t. the X-ray one. The strong correlation supports the standard model in which a unique electrons population produces the X-rays by synchrotron radiation and the gamma-ray component by inverse Compton scattering. However, for the individual best observed flares the gamma-ray flux scales approximately quadratically w.r.t. the X-ray flux, posing a serious challenge to emission models for TeV blazars. Rather special conditions and/or fine tuning of the temporal evolution of the physical parameters of the emission region are required in order to reproduce the quadratic correlation.
We estimate the limiting angular resolution and detection area for an array of 3 large-aperture Imaging Atmospheric Cherenkov Telescopes. We consider an idealized IACT system in order to understand the limitations imposed by the intrinsic nature of t
he atmospheric showers and geometry of the detector configuration. The idealization includes the assumptions of a perfect optical system and the absence of the night sky background with the goal of finding the optimum camera geometry and array configuration independent of detailed assumptions about the telescope design. The showers are simulated using the ALTAI code for the altitude of 2700 m corresponding to one of possible future sites for a new northern-hemisphere array. The optimal design depends on the target energy range; for each energy we vary both the cell length (telescope spacing) and the image processing parameters in order to maximize the signal-to-noise ratio. We then present the resulting values of the detection area and the angular resolution for this energy dependent optimization. We discuss the dependence of these quantities on the field of view of the telescopes and pixel size of the camera.
