OGLE 2002-BLG-069: Determination of mass and distance of the binary lens

OGLE 2002-BLG-069: Determination of mass and distance of the binary lens

First posted 4-Feb-2005

As can be seen in the depicted light curve, the event OGLE-2002-BLG-069 involves a pair of fold-caustic passages which produce characteristic peaks of well-defined shape. With its 2002 season telescope network involving the Danish 1.54m and the ESO/MPG 2.2m at ESO LaSilla (Chile), the Canopus 1.0m near Hobart (Tasmania), the Perth 0.6m at Bickley (Western Australia) and the SAAO 1.0m at Sutherland (South Africa), PLANET managed not only to obtain a good coverage of the caustic exit, but also of a fair part of the caustic entry. Together with data obtained by OGLE with their 1.3m telescope at Las Campanas (Chile), this allowed to obtain the model parameters of the underlying binary lens, up to a well-known ambiguity between close and wide binary lenses, where we find for the instantaneous angular separation d in units of the angular Einstein radius θE and the mass ratio q between the binary components either (d, q) = (0.46, 0.58) or (d, q) = (3.68,0.17) with significantly different event time-scales tE = 105 d or tE = 287 d, respectively, being the time in which the source moves by θE with respect to the lens.

The plot to the left shows the complete lightcurve with I-band data from OGLE, Tasmania, Perth, and SAAO, as well as R-band data from the ESO 2.2m and the Danish 1.54m at ESO LaSilla.

High-resolution spectra obtained with the UVES spectrograph mounted on the Kuyen unit telescope of the ESO VLT (Paranal, Chile) during several phases of the caustic exit provided excellent data for determining the type of the source star which turned out to be a G5III Bulge giant. Our knowledge about the physical size and distance of the source star gave us a measurement of the proper motion between lens and source together with the measured duration of the caustic passage which is sensitive to the source size. For the close-binary lens model, annual parallax effects caused by the Earth's orbital motion provide a reliable measurement on πLSE, where πLS is the relative lens-source parallax. Combining this with the proper motion measurement and the event time-scale, yields the lens mass and distance as M = (0.51 ± 0.15) Msun and DL = (2.9 ± 0.4) kpc, which is compatible with the limit for the lens mass arising from the observed blend ratio. In contrast, we find an upper limit for for πLSE with our wide-binary lens model, yielding a mass M ≥ (126 ± 22) Msun and a distance DL ≥ (9.0 ± 2.3) kpc, which is in contradiction with the lens being dark, unless one assumes an unlikely black-hole binary.

The corresponding paper (D. Kubas et al. 2005) has been accepted by Astronomy & Astrophysics. A preprint is also available as astro-ph/0502018.

More details about the source star and our spectral observations can be found in a previous paper (A. Cassan et al. 2004, A&A 419, L1), also available as preprint astro-ph/0401071. A thoroughful analysis of the properties of the source star using the spectral measurements obtained with UVES is underway and the results will be presented in a forthcoming paper (J.-P. Beaulieu et al. 2005, in preparation).

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