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 \times 10 ^ { 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 .