AGAPEROS: Searches for microlensing in the LMC with the Pixel Method. I. Data treatment and pixel light curves production

TL;DR

This paper introduces the Pixel Method for monitoring pixel fluxes in the LMC to detect microlensing events, demonstrating effective noise reduction and seeing correction techniques on EROS data, enabling studies of unresolved stellar variability.

Contribution

It presents a novel pixel-based analysis technique for microlensing detection in the LMC, with detailed data processing and noise correction methods.

Findings

Achieved a noise level of 1.8% in blue and 1.3% in red super-pixel light curves.

Confirmed that the noise level is about twice the photon noise, supporting previous unresolved star contribution estimates.

Demonstrated efficient correction for seeing variations affecting pixel flux measurements.

Abstract

Recent surveys monitoring millions of light curves of resolved stars in the LMC have discovered several microlensing events. Unresolved stars could however significantly contribute to the microlensing rate towards the LMC. Monitoring pixels, as opposed to individual stars, should be able to detect stellar variability as a variation of the pixel flux. We present a first application of this new type of analysis (Pixel Method) to the LMC Bar. We describe the complete procedure applied to the EROS 91-92 data (one tenth of the existing CCD data set) in order to monitor pixel fluxes. First, geometric and photometric alignments are applied to each images. Averaging the images of each night reduces significantly the noise level. Second, one light curve for each of the 2.1 10^6 pixels is built and pixels are lumped into 3.6"x3.6" super-pixels, one for each elementary pixel. An empirical correction is then applied to account for seeing variations. We find that the final super-pixel light curves fluctuate at a level of 1.8% of the flux in blue and 1.3% in red. We show that this noise level corresponds to about twice the expected photon noise and confirms previous assumptions used for the estimation of the contribution of unresolved stars. We also demonstrate our ability to correct very efficiently for seeing variations affecting each pixel flux. The technical results emphasised here show the efficacy of the Pixel Method and allow us to study luminosity variations due to possible microlensing events and variable stars in two companion papers.