Self-lensing binaries in globular clusters -- predictions for ELT

TL;DR

This work provides the first comprehensive assessment of self-lensing prospects in globular clusters, combining MOCCA-simulated GC populations with detailed finite-size lens modelling for white-dwarf lenses and observational forecasts for the ELT/MICADO. The authors predict 1–50 SL sources per cluster with $\mu_{sl}>1$, dominated by WD lenses, and reveal a characteristic bimodal magnitude distribution around $m\approx 24$ and $m\approx 32$ mag (at 10 kpc) with typical event durations $\tau_{eff}\sim 2$ hours. They translate the SL physics into realistic ELT observing strategies, showing that high-cadence, multi-year monitoring of nearby GCs could yield 10–100 well-characterized SL systems after about five years, enabling statistical studies of binary evolution in extreme environments. The results underscore the value of coordinated, high-resolution surveys for probing hidden binary populations and compact object demographics in dense stellar systems, while highlighting current limitations and avenues for refinement in stellar atmosphere modelling, systematic photometry, and cluster parameter space exploration.

Abstract

Self-lensing (SL) represents a powerful technique for detecting compact objects in binary systems through gravitational microlensing effects, when a compact companion transits in front of its luminous partner. We present the first comprehensive study of SL probability within globular cluster (GC) environments, utilizing synthetic stellar populations from MOCCA simulations to predict detection rates for the Extremely Large Telescope (ELT). Our analysis incorporates finite-size lens effects for white dwarf (WD) lenses and the specific observational characteristics of the ELT/MICADO instrument. We find that present-day GCs contain 1-50 SL sources with magnifications $μ_\mathrm{sl} > 1+10^{-8}$, strongly dependent on initial binary fraction, with systems dominated by WD lenses paired with low-mass main-sequence companions. The predicted populations exhibit characteristic bimodal magnitude distributions with peaks at $m \approx 24$ and 32 mag at 10 kpc distance, and typical Einstein ring crossing times of $τ_\mathrm{eff} \sim 2$ hours. ELT observations should achieve detection efficiency of 0.015-10 sources in $\sim150$ nearby GC after a year of observations depending on distance and survey strategy, with nearby clusters ($D \lesssim 10$ kpc) offering the highest yields. Multi-year monitoring campaigns with daily cadence provide order-of-magnitude improvements over single observations through enhanced photometric precision and increased detection probability. Our results demonstrate that coordinated ELT surveys of Galactic GCs represent a viable approach for probing hidden binary populations and compact object demographics in dense stellar environments, with comprehensive programs potentially yielding up to 10-100 well-characterized SL sources after first 5 years of observations suitable for statistical studies of binary evolution in extreme environments.

Figures (10)

• Figure 1: Apparent magnitude distribution of magnified self-lensing systems across 16 simulations, calculated for a reference distance of 10 kpc without extinction correction.
• Figure 2: Distribution of magnifications for self-lensing systems across 16 simulations, expressed as magnitude differences $\Delta m = \mathrel{\mathrm{m}_\mathrm{source}} - \mathrel{\mathrm{m}_\mathrm{source,SL}}$ where positive values indicate brightening due to lensing. Labels as in Figure \ref{['fig:m_distribution']}.
• Figure 3: Distribution of Einstein ring crossing times ($\mathrel{\tau_\textrm{eff}}$) for magnified self-lensing systems across all simulation configurations. Labels as in Figure \ref{['fig:m_distribution']}.
• Figure 4: Apparent magnitude distributions of self-lensing sources in J-band (top) and K$_{\rm S}$-band (bottom) filters for ELT/MICADO observations, calculated at a reference distance of 10 kpc. Vertical dashed lines indicate the adopted magnitude limits of J = 29.15 mag and K$_{\rm S}$ = 28.55 mag corresponding to 3-$\sigma$ detection thresholds. Labels as in Figure \ref{['fig:m_distribution']}.
• Figure 5: Distribution of single-exposure observation probabilities for self-lensing sources across all simulation configurations. Labels as in Figure \ref{['fig:m_distribution']}.