Prospects of Finding Detached Black Hole-Star Binaries with TESS

TL;DR

This paper evaluates the potential of TESS photometry to detect black hole companions to stars through self-lensing and phase-curve signals, estimating that hundreds of such systems could be identified, aiding nearby black hole studies.

Contribution

It provides a detailed assessment of TESS's capability to find detached black hole-star binaries using specific photometric signals, with estimates of detection numbers and false positives.

Findings

Self-lensing signals detectable in ~10^5 stars

Phase-curve signals detectable in ~10^6 stars

Potential to identify 10-100 black hole binaries with TESS data

Abstract

We discuss prospects of identifying and characterizing black hole (BH) companions to normal stars on tight but detached orbits, using photometric data from the Transiting Exoplanet Survey Satellite (TESS). We focus on the following two periodic signals from the visible stellar component: (i) in-eclipse brightening of the star due to gravitational microlensing by the BH (self-lensing), and (ii) a combination of ellipsoidal variations due to tidal distortion of the star and relativistic beaming due to its orbital motion (phase-curve variation). We evaluate the detectability of each signal in the light curves of stars in the TESS input catalog, based on a pre-launch noise model of TESS photometry as well as the actual light curves of spotted stars from the prime Kepler mission to gauge the potential impact of stellar activity arising from the tidally spun-up stellar components. We estimate that the self-lensing and phase-curve signals from BH companions, if exist, will be detectable in the light curves of effectively $\mathcal{O}(10^5)$ and $\mathcal{O}(10^6)$ low-mass stars, respectively, taking into account orbital inclination dependence of the signals. These numbers could be large enough to actually detect signals from BHs: simple population models predict some 10 and 100 detectable BHs among these "searchable" stars, although the latter may be associated with a comparable number of false-positives due to stellar variabilities and additional vetting with radial velocity measurements would be essential. Thus the TESS data could serve as a resource to study nearby BHs with stellar companions on shorter-period orbits than will potentially be probed with Gaia.