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Astron. Astrophys. 344, L49-L52 (1999)

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Letter to the Editor

AgapeZ1: a large amplification microlensing event or an odd variable star towards the inner bulge of M31 *

R. Ansari 1, M. Aurière 2, P. Baillon 3, A. Bouquet 4, G. Coupinot 2, Ch. Coutures 5, C. Ghesquière 4, Y. Giraud-Héraud 4, P. Gondolo 6, J. Hecquet 7, J. Kaplan 4, A. Kim 4, Y. LeDu 4, A.L. Melchior 8, M. Moniez 1, J.P. Picat 7 and G. Soucail 7

1 Laboratoire de l'Accélérateur Linéaire, Université Paris-Sud, F-91405 Orsay, France
2 Observatoire Midi-Pyrénées, unité associée au CNRS (UMR 5572), F-62500 Bagnères de Bigorre, France
3 CERN, CH-1211 Genève 23, Switzerland
4 Physique Corpusculaire et Cosmologie, Collège de France, Laboratoire associé au CNRS-IN2P3 (UMR 7550), 11 place Marcelin Berthelot, F-75231 Paris Cedex 05, France
5 SPP/DAPNIA, CEN Saclay, F-91191 Gif-sur-Yvette, France
6 Max-Planck-Institut für Physik (Werner-Heisenberg-Institut), Föhringer Ring 6, D-80805 München, Germany
7 Observatoire Midi-Pyrénées, unité associée au CNRS (UMR 5572), 14 avenue Belin, F-31400 Toulouse, France
8 Astronomy Unit, Queen Mary and Westfield College, Mile End Road, London E1 4NS, UK

Received 17 July 1998 / Accepted 15 February 1999


AgapeZ1 is the brightest and the shortest duration microlensing candidate event found to date in the Agape experiment. It occurred only [FORMULA] from the centre of M31 at [FORMULA] and [FORMULA] (J2000). Our photometry shows that the half intensity duration of the event is 5.3 days and at maximum brightness its magnitude is [FORMULA] ([FORMULA]) with [FORMULA] mag colour. A search on HST archives produced a single resolved star within the projected event position error box. Its magnitude is [FORMULA], and its colour is compatible with that of the event at the [FORMULA] level.

If this event is identified with the HST star, the implied amplification is about 4 magnitudes or 40 in brightness. This would lead to an Einstein crossing time radius of about 60 days. AgapeZ1 could be a bulge/bulge microlensing event involving a binary star.

The photometric properties of the object are inconsistent with classical M31 variable stars such as Miras, classical novae, dwarf-novae, and bumpers. However, we cannot rule out the possibility that AgapeZ1 is in fact an odd variable star.

Key words: Galaxy: halo – cosmology: observations – cosmology: dark matter – cosmology: gravitational lensing

* Based on data collected with the 2 m Bernard Lyot Telescope (TBL) operated by INSU-CNRS and Pic-du-Midi Observatory (USR 5026). The experiment was funded by IN2P3 and INSU of CNRS

Send offprint requests to: M. Aurière (auriere@obs-mip.fr)

© European Southern Observatory (ESO) 1999

Online publication: March 18, 1999