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The measurement of the direct-photon spectrum from Au$+$Au collisions at$\sqrt{s_{_{NN}}}=200$ GeV is presented by the PHENIX collaboration using theexternal-photon-conversion technique for 0\%--93\% central collisions in atransverse-momentum ($p_T$) range of 0.8--10 GeV/$c$. An excess of directphotons, above prompt-photon production from hard-scattering processes, isobserved for $p_T<6$ GeV/$c$. Nonprompt direct photons are measured bysubtracting the prompt component, which is estimated as $N_{\rm coll}$-scaleddirect photons from $p$$+$$p$ collisions at 200 GeV, from the direct-photonspectrum. Results are obtained for $0.8<p_T<6.0$ GeV/$c$ and suggest that thespectrum has an increasing inverse slope from ${\approx}0.2$ to 0.4 GeV/$c$with increasing $p_T$, which indicates a possible sensitivity of themeasurement to photons from earlier stages of the evolution of the collision.In addition, like the direct-photon production, the $p_T$-integrated nonpromptdirect-photon yields also follow a power-law scaling behavior as a function ofcollision-system size. The exponent, $\alpha$, for the nonprompt component isfound to be consistent with 1.1 with no apparent $p_T$ dependence.

Nonprompt direct-photon production in Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV