One Small Step for $Roman$; One Giant Leap for Black Holes

TL;DR

The paper discusses optimizing the Roman microlensing survey schedule to improve black hole detection and characterization, addressing current limitations and proposing adjustments that enhance scientific outcomes with minimal impact on other measurements.

Contribution

It proposes a small schedule adjustment for the Roman mission to reduce detection blind spots and improve black hole orbit characterization without significantly degrading proper-motion measurements.

Findings

Schedule adjustment reduces detection blind spots near P=3.5 yr.

Combining Roman data with radial-velocity follow-up enables full orbit recovery.

Detection of black hole binaries with periods up to 95 years is feasible through astrometric acceleration.

Abstract

The $Roman$ microlensing program can detect and fully characterize black holes (BHs) that are in orbit with about 30 million solar-type and evolved stars with periods up to the mission lifetime $P<T=5$ yr, and semi-major axes $a>0.2$au, i.e., $P> 10$ d $(M/M_\odot)^{-1/2}$, where $M$ is the BH mass. For BH companions of about 150 million later (fainter) main-sequence stars, the threshold of detection is $a>0.2$ au $\times 10^{(H_{\rm Vega}-18.5)/5}$. The present $Roman$ scheduling creates a "blind spot" near periods of $P=3.5$ yr due to a 2.3-year gap in the data. It also compromises the characterization of BHs in eccentric orbits with periods $P>3$ yr and peribothra within a year of the mission midpoint. I show that one can greatly ameliorate these issues by making a small adjustment to the $Roman$ observing schedule. The present schedule aims to optimize proper-motion measurements, but the adjustment proposed here would degrade these by only 4%-9%. For many cases of $P>90$ d BHs, there will be discrete and/or continuous degeneracies. For G-dwarf and evolved sources, it will be straightforward to resolve these by radial-velocity (RV) follow-up observations, but such observations will be more taxing for fainter sources. Many BH-binaries in orbits of 5 yr $<P<10$ yr will be reliably identified as such from the $Roman$ data, but will lack precise orbits. Nevertheless, the full orbital solutions can be recovered by combining $Roman$ astrometry with RV followup observations. BH binaries with periods 10 yr $<P<$ 95 yr $(M/10 M_\odot)^{1/4}$ can be detected from their astrometric acceleration, but massive multi-fiber RV monitoring would be needed to distinguish them from the astrophysical background due to stellar binaries.