  Mrk 501 is one of the brightest blazars at TeV energies and has been
extensively studied since its first VHE detection in 1996. Our goal is to
characterize in detail the source gamma-ray emission, together with the
radio-to-X-ray emission, during the non-flaring (low) activity, which is less
often studied than the occasional flaring (high) activity. We organized a
multiwavelength (MW) campaign on Mrk 501 between March and May 2008. This
multi-instrument effort included the most sensitive VHE gamma-ray instruments
in the northern hemisphere, namely the imaging atmospheric Cherenkov telescopes
MAGIC and VERITAS, as well as Swift, RXTE, the F-GAMMA, GASP-WEBT, and other
collaborations and instruments. Mrk 501 was found to be in a low state of
activity during the campaign, with a VHE flux in the range of 10%-20% of the
Crab nebula flux. Nevertheless, significant flux variations were detected with
various instruments, with a trend of increasing variability with energy. The
broadband spectral energy distribution during the two different emission states
of the campaign can be adequately described within the homogeneous one-zone
synchrotron self-Compton model, with the (slightly) higher state described by
an increase in the electron number density. This agrees with previous studies
of the broadband emission of this source during flaring and non-flaring states.
We report for the first time a tentative X-ray-to-VHE correlation during a low
VHE activity. Although marginally significant, this positive correlation
between X-ray and VHE, which has been reported many times during flaring
activity, suggests that the mechanisms that dominate the X-ray/VHE emission
during non-flaring-activity are not substantially different from those that are
responsible for the emission during flaring activity.
