  Measurements of fiducial and differential cross sections are presented for
Higgs boson production in proton-proton collisions at a centre-of-mass energy
of $\sqrt{s}=8$ TeV. The analysis is performed in the $H \rightarrow
\gamma\gamma$ decay channel using 20.3 fb$^{-1}$ of data recorded by the ATLAS
experiment at the CERN Large Hadron Collider. The signal is extracted using a
fit to the diphoton invariant mass spectrum assuming that the width of the
resonance is much smaller than the experimental resolution. The signal yields
are corrected for the effects of detector inefficiency and resolution. The
$pp\rightarrow H \rightarrow \gamma\gamma$ fiducial cross section is measured
to be $43.2 \pm 9.4 (stat) {}^{+3.2}_{-2.9} (syst) \pm 1.2 (lumi)$ fb for a
Higgs boson of mass 125.4 GeV decaying to two isolated photons that have
transverse momentum greater than 35% and 25% of the diphoton invariant mass and
each with absolute pseudorapidity less than 2.37. Four additional fiducial
cross sections and two cross-section limits are presented in phase space
regions that test the theoretical modelling of different Higgs boson production
mechanisms, or are sensitive to physics beyond the Standard Model. Differential
cross sections are also presented, as a function of variables related to the
diphoton kinematics and the jet activity produced in the Higgs boson events.
The observed spectra are statistically limited but broadly in line with the
theoretical expectations.
