Context. Not only is gravitational microlensing a successful tool for
discovering distant exoplanets, but it also enables characterization of
the lens and source stars involved in the lensing event.
Aims: In
high-magnification events, the lens caustic may cross over the source
disk, which allows determination of the angular size of the source and
measurement of its limb darkening.
Methods: When such
extended-source effects appear close to maximum magnification, the
resulting light curve differs from the characteristic Paczyński
point-source curve. The exact shape of the light curve close to the peak
depends on the limb darkening of the source. Dense photometric coverage
permits measurement of the respective limb-darkening coefficients.
Results: In the case of the microlensing event OGLE 2008-BLG-290, the
K giant source star reached a peak magnification at about 100. Thirteen
different telescopes have covered this event in eight different
photometric bands. Subsequent light-curve analysis yielded measurements
of linear limb-darkening coefficients of the source in six photometric
bands. The best-measured coefficients lead to an estimate of the source
effective temperature of about 4700+100-200 K.
However, the photometric estimate from colour-magnitude diagrams favours
a cooler temperature of 4200 ± 100 K.
Conclusions: Because
the limb-darkening measurements, at least in the CTIO/SMARTS2 V_s- and
I_s-bands, are among the most accurate obtained, the above disagreement
needs to be understood. A solution is proposed, which may apply to
previous events where such a discrepancy also appeared.
