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The flat spectrum radio quasar 3C 279 is known to exhibit pronouncedvariability in the high-energy ($100\,$MeV$<E<100\,$GeV) $\gamma$-ray band,which is continuously monitored with Fermi-LAT. During two periods of highactivity in April 2014 and June 2015 Target-of-Opportunity observations wereundertaken 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, theobservation in 2015 results in a signal with $8.7\,\sigma$ significance abovean energy threshold of $66\,$GeV. No VHE variability has been detected duringthe 2015 observations. The VHE photon spectrum is soft and described by apower-law index of $4.2\pm 0.3$. The H.E.S.S. data along with a detailed andcontemporaneous multiwavelength data set provide constraints on the physicalparameters of the emission region. The minimum distance of the emission regionfrom the central black hole is estimated using two plausible geometries of thebroad-line region and three potential intrinsic spectra. The emission region isconfidently 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-dependentleptonic and lepto-hadronic one-zone models are used to describe the evolutionof the 2015 flare. Neither model can fully reproduce the observations, despitetesting various parameter sets. Furthermore, the H.E.S.S. data are used toderive constraints on Lorentz invariance violation given the large redshift of3C 279.

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