We present state-of-the-art calculations of viscous photon emission from nuclear collisions at the BNL Relativistic Heavy Ion Collider (RHIC) and the CERN Large Hadron Collider (LHC). Fluctuating initial density profiles are evolved with event-by-event viscous hydrodynamics. Momentum spectra of thermal photons radiated by these explosively expanding fireballs and their $p_T$-differential anisotropic flow coefficients $v_n\left(p_T\right)$ are computed, both with and without accounting for viscous corrections to the standard thermal emission rates. Viscous corrections to the rates are found to have a larger effect on the $v_n$ coefficients than the viscous suppression of hydrodynamic flow anisotropies. The benefits of taking the ratio of elliptic to triangular flow, $v_2/v_3$, are discussed, and the space-time regions that contribute dominantly to the photon flow harmonics are identified. The directed flow $v_1$ of thermal photons is predicted for the energies currently available at the RHIC and the LHC.

• Received 9 August 2013
• Revised 19 December 2014

DOI:https://doi.org/10.1103/PhysRevC.91.024908