Decoding the X-ray Properties of Pre-Reionization Era Sources

TL;DR

This paper investigates how variations in black hole properties and accretion disk spectra influence the 21-cm signal during the early universe, highlighting potential confounding factors in interpreting upcoming measurements.

Contribution

It introduces a novel radiative transfer approach to quantify how black hole mass and disk spectrum uncertainties affect the 21-cm signal, emphasizing their significance.

Findings

Black hole mass variations impact the 21-cm signal amplitude by 10-20 mK.

Changes in disk spectrum due to Compton scattering shift the signal's minimum redshift by ~0.5.

Spectral uncertainties can alter the 21-cm signal amplitude by up to 50 mK.

Abstract

Evolution in the X-ray luminosity -- star formation rate ($L_X$-SFR) relation could provide the first evidence of a top-heavy stellar initial mass function in the early universe, as the abundance of high-mass stars and binary systems are both expected to increase with decreasing metallicity. The sky-averaged (global) 21-cm signal has the potential to test this prediction via constraints on the thermal history of the intergalactic medium, since X-rays can most easily escape galaxies and heat gas on large scales. A significant complication in the interpretation of upcoming 21-cm measurements is the unknown spectrum of accreting black holes at high-$z$, which depends on the mass of accreting objects and poorly constrained processes such as how accretion disk photons are processed by the disk atmosphere and host galaxy interstellar medium. Using a novel approach to solving the cosmological radiative transfer equation (RTE), we show that reasonable changes in the characteristic BH mass affects the amplitude of the 21-cm signal's minimum at the $\sim 10-20$ mK level --- comparable to errors induced by commonly used approximations to the RTE --- while modifications to the intrinsic disk spectrum due to Compton scattering (bound-free absorption) can shift the position of the minimum of the global signal by $\Delta z \approx 0.5$ ($\Delta z \approx 2$), and modify its amplitude by up to $\approx 10$ mK ($\approx 50$ mK) for a given accretion history. Such deviations are larger than the uncertainties expected of current global 21-cm signal extraction algorithms, and could easily be confused with evolution in the $L_X$-SFR relation.