Context :

Aims : This paper is an H band interferometric examination of Arcturus , a star frequently used as a spatial and spectral calibrator .

Methods : Using the IOTA 3 telescope interferometer , we performed spectro-interferometric observation ( R \approx 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 .

Results : 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 \pm 0.21 was derived , corresponding to an effective temperature of T _ { eff } = 4295 \pm 26 K. No companion was detected from the closure phases , with an upper limit on the brightness ratio of 8 \times 10 ^ { -4 } at 1AU .
Dynamic range at such distance from the photosphere was established at 1.5 \times 10 ^ { -4 } ( 1 \sigma rms ) .
An upper limit of 1.7 \times 10 ^ { -3 } was also derived for the level of brightness asymmetries present on the photosphere .

Conclusions :