Constraints on the emission region of 3C 279 during strong flares in
  2014 and 2015 through VHE gamma-ray observations with H.E.S.S
  The flat spectrum radio quasar 3C 279 is known to exhibit pronounced
variability in the high-energy ($100\,$MeV$<E<100\,$GeV) $\gamma$-ray band,
which is continuously monitored with Fermi-LAT. During two periods of high
activity in April 2014 and June 2015 Target-of-Opportunity observations were
undertaken with H.E.S.S. in the very-high-energy (VHE, $E>100\,$GeV)
$\gamma$-ray domain. While the observation in 2014 provides an upper limit, the
observation in 2015 results in a signal with $8.7\,\sigma$ significance above
an energy threshold of $66\,$GeV. No VHE variability has been detected during
the 2015 observations. The VHE photon spectrum is soft and described by a
power-law index of $4.2\pm 0.3$. The H.E.S.S. data along with a detailed and
contemporaneous multiwavelength data set provide constraints on the physical
parameters of the emission region. The minimum distance of the emission region
from the central black hole is estimated using two plausible geometries of the
broad-line region and three potential intrinsic spectra. The emission region is
confidently placed at $r\gtrsim 1.7\times10^{17}\,$cm from the black hole,
i.e., beyond the assumed distance of the broad-line region. Time-dependent
leptonic and lepto-hadronic one-zone models are used to describe the evolution
of the 2015 flare. Neither model can fully reproduce the observations, despite
testing various parameter sets. Furthermore, the H.E.S.S. data are used to
derive constraints on Lorentz invariance violation given the large redshift of
3C 279.
