  Two-particle long-range azimuthal correlations are measured in photonuclear
collisions using 1.7 nb$^{-1}$ of 5.02 TeV Pb+Pb collision data collected by
the ATLAS experiment at the LHC. Candidate events are selected using a
dedicated high-multiplicity photonuclear event trigger, a combination of
information from the zero-degree calorimeters and forward calorimeters, and
from pseudorapidity gaps constructed using calorimeter energy clusters and
charged-particle tracks. Distributions of event properties are compared between
data and Monte Carlo simulations of photonuclear processes. Two-particle
correlation functions are formed using charged-particle tracks in the selected
events, and a template-fitting method is employed to subtract the non-flow
contribution to the correlation. Significant nonzero values of the second- and
third-order flow coefficients are observed and presented as a function of
charged-particle multiplicity and transverse momentum. The results are compared
with flow coefficients obtained in proton-proton and proton-lead collisions in
similar multiplicity ranges, and with theoretical expectations. The unique
initial conditions present in this measurement provide a new way to probe the
origin of the collective signatures previously observed only in hadronic
collisions.
