  Searches for new resonances decaying into two photons in the ATLAS experiment
at the CERN Large Hadron Collider are described. The analysis is based on
proton--proton collision data corresponding to an integrated luminosity of 3.2
fb$^{-1}$ at $\sqrt{s} = 13$ TeV recorded in 2015. Two searches are performed,
one targeted at a spin-2 particle of mass larger than 500 GeV, using
Randall--Sundrum graviton states as a benchmark model, and one optimized for a
spin-0 particle of mass larger than 200 GeV. Varying both the mass and the
decay width, the most significant deviation from the background-only hypothesis
is observed at a diphoton invariant mass around 750 GeV with local
significances of 3.8 and 3.9 standard deviations in the searches optimized for
a spin-2 and spin-0 particle, respectively. The global significances are
estimated to be 2.1 standard deviations for both analyses. The consistency
between the data collected at 13 TeV and 8 TeV is also evaluated. Limits on the
production cross section times branching ratio to two photons for the two
resonance types are reported.
