White Paper: Isolated Stellar-Mass Black Holes: Strategy to Improve the Efficiency and Robustness of Detection with Roman

Abstract

Roman telescope provides the best opportunity to detect a large number of Isolated Stellar-Mass Black Holes (ISMBHs) through microlensing. Roman will not only detect long-duration microlensing events caused by ISMBHs, but will also measure the deflections caused by the ISMBHs, which can be used to estimate their masses. Recently, Sajadian and Sahu (2023) studied the efficiency of detecting ISMBHs by Roman through simulation of a large ensemble of such events. They estimated the resulting errors in the physical parameters of the lens objects, including their masses, distances, and proper motions through calculating Fisher and Covariance matrices. Their simulation shows that the 2.3-year time gap between Roman's first three and the last three observing seasons not only lowers the efficiency of detection, but also makes the solutions degenerate. We recommend a small amount of additional observations -- about one hour of observations every 10 to 20 days when the Bulge is observable during the large time gap -- which is equivalent to a total of about one to two additional days of observations with Roman. This small amount of additional observations will greatly improve the efficiency and robustness of detection of ISMBHs, and provide firm estimates of their masses.