1999A&A...350...17D

The BL Lac object Mkn 501 went into a very high state of activity during 1997, both in VHE γ-rays and X-rays. We present here results from observations at energies above 250 GeV carried out between March and October 1997 with the CAT Cerenkov imaging Telescope. The average differential spectrum between 330 GeV and 13 TeV shows significant curvature and is well represented by φ₀ E_TeV^–(α+βlog10_ETeV_)^, with: φ₀=5.19±0.13^stat±0.12^sys–MC+1.66_–1.04^sys–atmx10^–11cm^–2/s/TeV, α=2.24±0.04^stat±0.05^sys, and β=0.50±0.07^stat (negligible systematics). The TeV spectral energy distribution of Mkn 501 clearly peaks in the range 500 GeV-1 TeV. Investigation of spectral variations shows a significant hardness-intensity correlation with no measurable effect on the curvature. This can be described as an increase of the peak TeV emission energy with intensity. Simultaneous and quasi-simultaneous CAT VHE γ-ray and BeppoSAX hard X-ray detections for the highest recorded flare on 16^th April and for lower-activity states of the same period show correlated variability with a higher luminosity in X-rays than in γ-rays. The observed spectral energy distribution and the correlated variability between X-rays and γ-rays, both in amplitude and in hardening of spectra, favour a two-component emission scheme where the low and high energy components are attributed to synchrotron and inverse Compton (IC) radiation, respectively.