  This paper is an H band interferometric examination of Arcturus, a star
frequently used as a spatial and spectral calibrator. Using the IOTA 3
telescope interferometer, we performed spectro-interferometric observation
(R~35) of Arcturus. Atmospheric models and prescriptions were fitted to the
data to derive the brightness distribution of the photosphere. Image
reconstruction was also obtained using two software algorithms: Wisard and
Mira. An achromatic power law proved to be a good model of the brightness
distribution, with a limb darkening compatible with the one derived from
atmospheric model simulations using our Marcs model. A Rosseland diameter of
21.05+/-0.21 was derived, corresponding to an effective temperature of
T_eff=4295+/-26 K. No companion was detected from the closure phases, with an
upper limit on the brightness ratio of 8e-4 at 1AU. Dynamic range at such
distance from the photosphere was established at 1.5e-4 (1sigma rms). An upper
limit of 1.7e-3 was also derived for the level of brightness asymmetries
present on the photosphere.
