On the observability of individual Population III stars and their stellar-mass black hole accretion disks through cluster caustic transits

Recent near-IR power-spectra and panchromatic Extragalactic Background Light measurements provide upper limits on the near-IR surface brightness (SB>31 mag/arcsec^2) that may come from Pop III stars and accretion disks around resulting stellar-mass black holes (BHs) in the epoch of First Light (z=7-17). Physical parameters for zero metallicity Pop III stars at z>7 can be estimated from MESA stellar evolution models through helium-depletion, and for BH accretion disks from quasar microlensing results and multicolor accretion models. Second-generation stars can form at higher multiplicity, so that BH accretion disks may be fed by Roche-lobe overflow from lower-mass companions in their AGB stage. The near-IR SB constraints can be used to calculate the number of caustic transits behind lensing clusters that JWST and the 25~39 m ground-based telescopes may detect for both Pop III stars and stellar mass BH accretion disks. Because Pop III stars and stellar mass BH accretion disks have sizes of a few x 10^-11 arcsec at z>7, typical caustic magnifications can be mu=10^4~10^5, with rise times of hours and decline times of < 1 year for cluster transverse velocities of v<1000 km/s. Microlensing by intracluster medium objects can modify transit magnifications, and lengthen visibility times. Depending on BH masses, accretion-disk radii and feeding efficiencies, stellar-mass BH accretion-disk caustic transits could outnumber those from Pop III stars. To observe Pop III caustic transits directly may require monitoring 3~30 lensing clusters to AB< 29 mag over a decade or more. Such a program must be started with JWST in Cycle 1, and -- depending on the role of microlensing in the Intra Cluster Light -- should be continued for decades with the GMT and TMT, where JWST and the ground-based telescopes each will play a unique and strongly complementary role.