Parallax in microlensing toward the Magellanic Clouds: Effect on detection efficiency and detectability

Aims. We study the effect of the parallax on the search for microlensing events on very long timescales towards the Magellanic Clouds caused by dark massive compact objects within the past MACHO and EROS, the on-going MOA and OGLE, and the future LSST surveys. We quantify what neglecting this effect means in the classical event selection process and also quantify the parallax detectability without the help of follow-up observations. Methods. We defined the distance between true events affected by parallax and the closest events without parallax. This distance was used to estimate the probability of missing the preselection of events because of parallax for any survey characterised by its time sampling and photometric performance. We also defined another distance to quantify the detectability of the parallax effect in order to trigger complementary observations. Results. We find that the preselection of year-long timescale events is marginally affected by parallax for all surveys if the criteria are reasonably tight. We also show that the parallax should be detectable in most of the events found by the LSST survey without follow-up observations