Simultaneous Multiwavelength observation of Mkn 501 in a low state in
  2006
  We present results of the multiwavelength campaign on the TeV blazar Mkn 501
performed in 2006 July, including MAGIC for the VHE gamma-ray band and Suzaku
for the X-ray band. A VHE gamma-ray signal was clearly detected with an average
flux above 200 GeV of ~20 % of the Crab Nebula flux, which indicates a low
state of source activity in this energy range. No significant variability has
been found during the campaign. The VHE gamma-ray spectrum can be described by
a simple power-law from 80 GeV to 2 TeV with a photon index of 2.8+/-0.1, which
corresponds to one of the steepest photon indices observed in this energy range
so far for this object. The X-ray spectrum covers a wide range from 0.6 to 40
keV, and is well described by a broken power law, with photon indices of
2.257+/-0.004 and 2.420+/-0.012 below and above the break energy of 3.24+/-0.13
keV. No apparent high-energy cut off is seen above the break energy. Although
an increase of the flux of about 50 % is observed in the X-ray band within the
observation, the data indicate a consistently low state of activity for this
source. Time-resolved spectra show an evidence for spectral hardening with a
flux level. A homogeneous one-zone synchrotron self-Compton (SSC) model can
adequately describe the SED from the X-ray to the VHE gamma-ray bands with a
magnetic field intensity B=0.313 G and a Doppler beaming factor delta = 20,
which are similar to the values in the past multiwavelength campaigns in high
states. Based on our SSC parameters derived for the low state, we are able to
reproduce the SED of the high state by just changing the Lorentz factor of the
electrons corresponding to the break energy in the primary electron spectrum.
This suggests that the variation of the injected electron population in the jet
is responsible for the observed low-high state variation of the SED.
