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