| Abstract: | We report on simultaneous broadband observations of the TeV-emitting blazar
Markarian 501 between 1 April and 10 August 2013, including the first detailed
characterization of the synchrotron peak with Swift and NuSTAR. During the
campaign, the nearby BL Lac object was observed in both a quiescent and an
elevated state. The broadband campaign includes observations with NuSTAR,
MAGIC, VERITAS, the Fermi Large Area Telescope (LAT), Swift X-ray Telescope and
UV Optical Telescope, various ground-based optical instruments, including the
GASP-WEBT program, as well as radio observations by OVRO, Mets"ahovi and the
F-Gamma consortium. Some of the MAGIC observations were affected by a sand
layer from the Saharan desert, and had to be corrected using event-by-event
corrections derived with a LIDAR (LIght Detection And Ranging) facility. This
is the first time that LIDAR information is used to produce a physics result
with Cherenkov Telescope data taken during adverse atmospheric conditions, and
hence sets a precedent for the current and future ground-based gamma-ray
instruments. The NuSTAR instrument provides unprecedented sensitivity in hard
X-rays, showing the source to display a spectral energy distribution between 3
and 79 keV consistent with a log-parabolic spectrum and hard X-ray variability
on hour timescales. None (of the four extended NuSTAR observations) shows
evidence of the onset of inverse-Compton emission at hard X-ray energies. We
apply a single-zone equilibrium synchrotron self-Compton model to five
simultaneous broadband spectral energy distributions. We find that the
synchrotron self-Compton model can reproduce the observed broadband states
through a decrease in the magnetic field strength coinciding with an increase
in the luminosity and hardness of the relativistic leptons responsible for the
high-energy emission. |
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