Difference Image Analysis of the OGLE-II bulge data. II. Microlensing events

TL;DR

This paper reports the discovery and analysis of microlensing events in OGLE-II data using Difference Image Analysis, presenting a large sample of candidate events and an automated selection method to improve detection accuracy.

Contribution

It introduces an automated procedure for selecting microlensing events from OGLE-II data, enhancing detection reliability and providing a substantial dataset for further analysis.

Findings

520 microlensing candidate events identified

Automated selection method unambiguously selects up to 237 best events

Large dataset includes transients, artifacts, and variable stars

Abstract

We present a sample of microlensing events discovered in the Difference Image Analysis (DIA) of the OGLE-II images collected during 3 observing seasons, 1997--1999. 4424 light curves pass our criteria on the presence of a brightening episode on top of a constant baseline. Among those, 512 candidate microlensing events were selected visually. We designed an automated procedure, which unambiguously selects up to 237 best events. Including 8 candidate events recovered by other means, a total of 520 light curves are presented in this work. In addition to microlensing events, the larger sample contains certain types of transients, but is also strongly contaminated by artifacts. All 4424 light curves in the weakly filtered group are available electronically, with the intent of showing the gray zone between microlensing events and variable stars, as well as artifacts, to some extent inevitable in massive data reductions. We welcome suggestions for improving the selection process before the full analysis of complete 4 seasons of the OGLE-II bulge data. Selection criteria for binary events can also be investigated with our extended sample.