Constraining Black Hole Natal Kicks with Astrometric Microlensing

Multiple pieces of evidence suggest that neutron stars receive large kicks when formed from the remnant of a collapsing star. However, the evidence for whether black holes (BHs) receive natal kicks is less clear, reliant on weak constraints from the analysis of BH X-ray binaries and massive runaway and walkaway stars. Here we show, for the first time, that recent microlensing detections offer a new method for measuring the kicks BHs receive at birth. When a BH is identified through both photometric and astrometric microlensing and when the lensed star has a known distance and proper motion, the mass, distance, and proper motion of the BH can be determined. We study the runaway velocities for components of eccentric binaries disrupted during a supernova (SN), finding the peculiar velocity correlates strongly with the kick a BH received at birth, typically within 20%, even when the natal kick is smaller than the orbital velocity. Therefore, by measuring the peculiar velocity of a BH or other compact object that formed from a binary which disrupted during core collapse, we are in effect measuring the natal kick that object received. We focus on MOA-2011-BLG-191/OGLE-2011-BLG-0462, an isolated, single BH detected by microlensing, and consider a range of possible formation scenarios, including its formation from the disruption of a binary during a SN event. We determine that MOA-2011-BLG-191/OGLE-2011-BLG-0462 has a Milky Way orbit consistent with a thick-disk population, but if it was formed within the kinematic thin disk it received a natal kick ≳100 km s-1.