Dark lenses through the dust: parallax microlensing events in the VVV

Abstract

We use near-infrared photometry and astrometry from the VISTA Variables in the Via Lactea (VVV) survey to analyse microlensing events containing annual microlensing parallax information. These events are located in highly extincted and low-latitude regions of the Galactic bulge typically off-limits to optical microlensing surveys. We fit a catalog of 1959 events previously found in the VVV and extract 21 microlensing parallax candidates. The fitting is done using nested sampling to automatically characterise the multi-modal and degenerate posterior distributions of the annual microlensing parallax signal. We compute the probability density in lens mass-distance using the source proper motion and a Galactic model of disc and bulge deflectors. By comparing the expected flux from a main sequence lens to the baseline magnitude and blending parameter, we identify 4 candidates which have probability > 50 % that the lens is dark. The strongest candidate corresponds to a nearby (≈ 0.78 kpc), medium-mass (1.46 +1.13 −0.71 𝑀 ) dark remnant as lens. In the next strongest, the lens is located at heliocentric distance ≈ 5.3 kpc. It is a dark remnant with a mass of 1.63 +1.15 −0.70 𝑀 . Both of those candidates are most likely neutron stars, though possibly high-mass white dwarfs. The last two events may also be caused by dark remnants, though we are unable to rule out other possibilities because of limitations in the data.

First semi-empirical test of the white dwarf mass-radius relationship using a single white dwarf via astrometric microlensing

“…A semi-empirical test of the MRR for a single and isolated white dwarf has yet to be performed. Astrometric microlensing events offer unique opportunities to measure the mass of isolated objects (e.g., Miralda-Escude 1996; Kains et al 2017;Rybicki et al 2018;Dong et al 2019;Sahu et al 2022;Lam et al 2022;Kaczmarek et al 2022). When a foreground lens with mass 𝑀 L aligns closely enough with a more distant background source, the gravitational field of the lens deflects the light of the background source forming a major and minor image of the source.…”