  We calculate the centrality dependence of direct photons produced in Au+Au
collisions at sqrt{s}=200GeV and their transverse momentum spectra within the
Parton-Hadron-String Dynamics (PHSD) transport approach. As sources for
'direct' photons, we incorporate the interactions of quarks and gluons as well
as hadronic interactions (pi+pi->rho+gamma, rho+pi->pi+gamma, meson-meson
bremsstrahlung m+m->m+m+gamma, meson-baryon bremsstrahlung m+B->m+B+gamma), the
decays of phi and a_1 mesons and the photons produced in the initial hard
collisions ('pQCD'). We find that the pT spectra of the 'thermal' photons (i.e.
the direct photons after the pQCD contribution is subtracted) deviate from
exponential distributions and, consequently, observe a strong dependence of the
inverse slope parameter T_eff on the fitting range in pT. On the other hand,
all the obtained 'effective temperatures' are well above the critical
temperature for the deconfinement phase transition even for peripheral
collisions, reflecting primarily a 'blue shift' due to radial collective motion
of hadrons. Our calculations suggest that the channel decomposition of the
observed spectrum changes with centrality with an increasing (dominant)
contribution of hadronic sources for more peripheral reactions. Furthermore,
the thermal photon yield is found to scale roughly with the number of
participant nucleons as N_part^alpha with alpha=1.5, whereas the partonic
contribution scales with an exponent alpha_p=1.75. Additionally, we provide
predictions for the centrality dependence of the direct photon elliptic flow
v_2(pT). The direct photon $v_2$ is seen to be larger in peripheral collisions
compared to the most central ones since the photons from the hot deconfined
matter in the early stages of the collision carry a much smaller elliptic flow
than the final hadrons.

Nuclear Theory