On the Observability of Individual Population III Stars and Their Stellar-mass Black Hole Accretion Disks through Cluster Caustic Transits

We summarize panchromatic Extragalactic Background Light data to place upper limits on the integrated near-IR surface brightness (SB) that may come from Population III stars and possible accretion disks around their stellar-mass black holes (BHs) in the epoch of First Light, broadly taken from z=7-17.We outline the physical properties of zero-metallicity Population III stars from MESA stellar evolution models through helium depletion and of BH accretion disks at z>7. We assume that second-generation non-zero-metallicity stars can form at higher multiplicity, so that BH accretion disks may be fed by Roche-lobe overflow from lower-mass companions.We use these near-infrared SB constraints to calculate the number of caustic transits behind lensing clusters that the James Webb Space Telescope and the next-generation ground-based telescopes may observe for both Population III stars and their BH accretion disks. Typical caustic magnifications can be 10^4-10^5x, with rise times of hours and decline times of z~<1 year for cluster transverse velocities of v_T<~1000 km/s.Microlensing by intracluster-medium objects can modify transit magnifications but lengthen visibility times. Depending on BH masses, accretion-disk radii, and feeding efficiencies, stellar-mass BH accretion-disk caustic transits could outnumber those from Population III stars. To observe Population III caustic transits directly may require monitoring 3-30 lensing clusters to AB<29 mag over a decade (see Windhorst et al. 2018, ApJS, 234, 41; astro-ph/1801.03584).This work was supported by NASA JWST Interdisciplinary Scientist grants NAG5-12460, NX14AN10G, and 80NSSC18K0200, NASA Theoretical and Computational Astrophysics Networks grant NNX14AB53G, NSF Software Infrastructure for Sustained Innovation grant 1339600, NSF Physics Frontier Center JINA-CEE grant PHY-1430152, Australian Research Council projects AYA2015-64508-P, AYA2012-39475-C02-01, and Ministerio de Economia y Competitividad of Spain Consolider Project CSD2010-00064.