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

Abstract

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 catalogue of 1959 events previously found in the VVV and extract 21 microlensing parallax candidates. The fitting is done using nested sampling to automatically characterize the multimodal 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 four candidates which have probability >50 per cent that the lens is dark. The strongest candidate corresponds to a nearby (≈0.78 kpc), medium-mass ($1.46^{+1.13}_{-0.71} \ M_{\odot }$) 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 $1.63^{+1.15}_{-0.70} \ M_{\odot }$. 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. We are also demonstrating future possibilities of studying similar events with the Roman Space Telescopeby modelling a mock dataset of Roman photometry and astrometry for an event resembling our strongest candidate.