Very deep images of the innermost regions of the beta Pictoris debris disc at L'

Context. Very few debris discs have been imaged in scattered light at wavelengths beyond 3 mum because the thermal emission from both the sky and the telescope is generally too strong with respect to the faint emission of a debris disc. We present here the first analysis of a high angular resolution image of the disc of beta Pictoris at 3.8 mum. <BR /> Aims: Our primary objective is to probe the innermost parts of the beta Pictoris debris disc and describe its morphology. We performed extensive forward modelling to correct for the biases induced by angular differential imaging on extended objects and derive the physical parameters of the disc. <BR /> Methods: This work relies on a new analysis of seven archival data sets of beta Pictoris observed with the NaCo instrument at the Very Large Telescope in the L' band, including observations made with the Annular Groove Phase Mask vortex coronagraph in 2013. The data analysis consists of angular differential imaging associated with disc forward modelling to correct for the biases induced by that technique. The disc model is subtracted from the data and the reduction performed again in order to minimize the residuals in the final image. <BR /> Results: The disc is detected above a 5sigma level between 0.4'' and 3.8''. The two extensions have a similar brightness within error bars. We confirm an asymmetry previously observed at larger distances from the star and at shorter wavelengths: the isophotes are more widely spaced