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Astron. Astrophys. Suppl. Ser. 134, 377-391

AGAPEROS: Searching for microlensing in the LMC with the pixel method

I. Data treatment and pixel light curves production

A.-L. Melchior1,2,3, C. Afonso4, R. Ansari5, É. Aubourg4, P. Baillon6, P. Bareyre4, F. Bauer4, J.-Ph. Beaulieu7, A. Bouquet2, S. Brehin4, F. Cavalier5, S. Char8, F. Couchot5, C. Coutures4, R. Ferlet7, J. Fernandez8, C. Gaucherel4, Y. Giraud-Héraud2, J.-F. Glicenstein4, B. Goldman4, P. Gondolo2,9, M. Gros4, J. Guibert10, D. Hardin4, J. Kaplan2, J. de Kat4, M. Lachièze-Rey4, B. Laurent4, É. Lesquoy4, Ch. Magneville4, B. Mansoux5, J.-B. Marquette7, E. Maurice11, A. Milsztajn4, M. Moniez5, O. Moreau10, L. Moscoso4, N. Palanque-Delabrouille4, O. Perdereau5, L. Prévôt11, C. Renault4, F. Queinnec4, J. Rich4, M. Spiro4, A. Vidal-Madjar7, L. Vigroux4, and S. Zylberajch4

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1 - Astronomy Unit, Queen Mary and Westfield College, Mile End Road, London E1 4NS, UK
2 - Laboratoire de Physique Corpusculaire et Cosmologie (UMR 7535), Collège de France, 75231 Paris Cedex 05, France
3 - NASA/Fermilab Astrophysics Center, Fermi National Accelerator Laboratory, Batavia, IL 60510-0500, U.S.A.
4 - CEA, DSM, DAPNIA, Centre d'Études de Saclay, 91191 Gif-sur-Yvette Cedex, France
5 - Laboratoire de l'Accélérateur Linéaire, IN2P3 CNRS, Université Paris-Sud, 91405 Orsay Cedex, France
6 - CERN, 1211 Genève 23, Switzerland
7 - Institut d'Astrophysique de Paris, CNRS, 98 bis Boulevard Arago, 75014 Paris, France
8 - Universidad de la Serena, Faculdad de Ciencias, Departemento de Fisica, Casilla 554, La Serena, Chile
9 - Max-Planck-Institut für Physik, Föhringer Ring 6, 80805 München, Germany
10 - Centre d'Analyse des Images de l'INSU, Observatoire de Paris, 61 avenue de l'Observatoire, 75014 Paris, France
11 - Observatoire de Marseille, 2 place Le Verrier, 13248 Marseille Cedex 04, France

Received November 3, 1997; accepted August 5, 1998


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 image. Averaging the images of each night reduces significantly the noise level. Second, one light curve for each of the 2.1 106 pixels is built and pixels are lumped into 3.6$\hbox{$^{\prime\prime}$}\times$ 3.6$\hbox{$^{\prime\prime}$}$ 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.

Key words: methods: data analysis -- techniques: photometric -- Galaxy: halo -- galaxies: Magellanic Clouds -- Cosmology: dark matter -- Cosmology: gravitational lensing

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