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Astron. Astrophys. 337, L17-L20 (1998)

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Letter to the Editor

EROS 2 intensive observation of the caustic crossing of microlensing event MACHO SMC-98-1 *

The EROS collaboration
C. Afonso 1, C. Alard 11, J.N. Albert 2, J. Andersen 6, R. Ansari 2, É. Aubourg 1, P. Bareyre 1, 4, F. Bauer 1, J.P. Beaulieu 5, A. Bouquet 4, S. Char 7, X. Charlot 1, F. Couchot 2, C. Coutures 1, F. Derue 2, R. Ferlet 5, J.F. Glicenstein 1, B. Goldman 1, 9, 10, A. Gould ** 1, 8, D. Graff 1, 8, M. Gros 1, J. Haissinski 2, J.C. Hamilton 4, D. Hardin 1, J. de Kat 1, A. Kim 4, T. Lasserre 1, É. Lesquoy 1, C. Loup 5, C. Magneville 1, B. Mansoux 2, J.B. Marquette 5, É. Maurice 3, A. Milsztajn 1, M. Moniez 2, N. Palanque-Delabrouille 1, O. Perdereau 2, L. Prévot 3, N. Regnault 2, C. Renault 1, J. Rich 1, M. Spiro 1, A. Vidal-Madjar 5, L. Vigroux 1 and S. Zylberajch 1

1 CEA, DSM, DAPNIA, Centre d'Études de Saclay, F-91191 Gif-sur-Yvette Cedex, France
2 Laboratoire de l'Accélérateur Linéaire, IN2P3 CNRS, Université Paris-Sud, F-91405 Orsay Cedex, France
3 Observatoire de Marseille, 2 pl. Le Verrier, F-13248 Marseille Cedex 04, France
4 Collège de France, Physique Corpusculaire et Cosmologie, IN2P3 CNRS, 11 pl. Marcellin Berthelot, F-75231 Paris Cedex, France
5 Institut d'Astrophysique de Paris, INSU CNRS, 98 bis Boulevard Arago, F-75014 Paris, France
6 Astronomical Observatory, Copenhagen University, Juliane Maries Vej 30, DK-2100 Copenhagen, Denmark
7 Universidad de la Serena, Facultad de Ciencias, Departamento de Fisica, Casilla 554, La Serena, Chile
8 Department of Astronomy, Ohio State University, Columbus, OH 43210, USA
9 Departamento Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
10 European Southern Observatory, Casilla 19001, Santiago 19, Chile
11 DASGAL, 77 avenue de l'Observatoire, F-75014 Paris, France

Received 1 July 1998 / Accepted 16 July 1998


We report on intensive photometric monitoring on 18 June 1998 of MACHO SMC-98-1, a binary-lens microlensing event seen toward the Small Magellanic Cloud (SMC). The observations cover 5.3 hours (UT 5:17-10:37), and show a sharp drop of 1.8 mag during the first 1.8 hours, followed by an abrupt flattening at UT 7:[FORMULA]:02. We interpret the kink at 7:08 as the end of the second caustic crossing (when the source first moved completely outside the caustic).

These results indicate that [FORMULA] at the [FORMULA] level, where µ is the proper motion of the lens (relative to the line of sight to the source), and [FORMULA] is the unknown (and so random) angle of the caustic crossing. Hence, the lens probably does not lie in either the Galactic halo or disk and so is most likely in the SMC itself. Our data can be combined with those of other groups to give more precise constraints on the proper motion (and hence the nature) of the lens.

Key words: Galaxy: halo – Galaxy: kinematics and dynamics – Galaxy: stellar content – Magellanic Clouds – dark matter – gravitational lensing

* Based on observations made at the European Southern Observatory, La Silla, Chile.
** Alfred P. Sloan Foundation Fellow

Send offprint requests to: Nathalie.Delabrouille@cea.fr

© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998