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

TL;DR

This paper assesses the potential to observe Population III stars and their black hole accretion disks through gravitational lensing caustic transits using upcoming telescopes, based on current brightness constraints and theoretical models.

Contribution

It combines observational limits, stellar evolution models, and lensing predictions to evaluate the detectability of early universe objects with future telescopes.

Findings

Caustic transits could magnify Population III objects by factors of 10^4-10^5.

Monitoring 3-30 clusters over a decade could enable detection of Population III caustic transits.

Black hole accretion disks may produce more observable transits than Population III stars.

Abstract

We summarize panchromatic Extragalactic Background Light data to place upper limits on the integrated near-infrared 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$\simeq$7-17. Theoretical predictions and recent near-infrared power-spectra provide tighter constraints on their sky-signal. We outline the physical properties of zero metallicity Population III stars from MESA stellar evolution models through helium-depletion and of BH accretion disks at z$\gtrsim$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 $\mu$$\simeq$10$^4$-10$^5$, with rise times of hours and decline times of $\lesssim$1 year for cluster transverse velocities of $v_{T}$$\lesssim$1000 km s$^{-1}$. Microlensing by intracluster medium objects can modify transit magnifications, but lengthen visibility times. Depending on BH masses, accretion-disk radii and feeding efficiencies, stellar-mass BH accretion-disk caustic transits could outnumber those from Population III stars. To observe Population III caustic transits directly may require to monitor 3-30 lensing clusters to AB$\lesssim$29 mag over a decade.