Probing the innermost region of the AU Microscopii debris disc

Context. AU Mic is a young and nearby M-dwarf star harbouring a circumstellar debris disc and one recently discovered planet on an eight-day orbit. Large-scale structures within the disc were also discovered and are moving outwards at high velocity. Aims. We aim to study this system with the highest spatial resolution in order to probe the innermost regions and to search for additional low-mass companions or set detection limits. Methods. The star was observed with two different high-angular resolution techniques probing complementary spatial scales. We obtained new K-s-band sparse aperture masking observations with VLT/SPHERE, which we combined with data from VLT/NACO, VETI/PIONIER and VLTI/GRAVITY. Results. We did not detect additional close companions within the separation range 0.02-7 au from the parent star. We determined magnitude upper limits for companions of H similar to 9.8 mag within 0.02-0.5 au, K-s similar to 11.2 mag within 0.4-2.4 au, and L' similar to 10.7 mag within 0.7-7 au. Using theoretical isochrones, we converted these magnitudes into upper limits on the mass of similar to 17 M-jup, similar to 12 M-jup, and similar to 9 M-jup, respectively. The PIONIER observations also allowed us to determine the angular diameter of AU Mic, theta(LD) = 0.825 +/- 0.033(stat) +/- 0.038(sys) mas, which converts to a linear radius R = 0.862 +/- 0.052R(circle dot) when combined with the Gaia parallax. Conclusions. We did not detect the newly discovered planets orbiting AU Mic (M < 0.2 M-jup), but we derived upper limit masses for the innermost region of AU Mic. We do not have any detection with a significance beyond 3 sigma, the most significant signal with PIONIER being 2.9 sigma and that with SPHERE being 1.6 sigma. We applied the pyMESS2 code to estimate the detection probability of companions by combining radial velocities, multi-band SPHERE imaging, and our interferometric detection maps. We show that 99% of the companions down to similar to 0.5 M-jup can be detected within 0.02 au or 1 M-jup down to 0.2 au. The low-mass planets orbiting at less than or similar to 0.11 au (less than or similar to 11 mas) from the star will not be directly detectable with the current adaptive optics (AO) and interferometric instruments because of its close orbit and very high contrast (similar to 10(-10) K). It will also be below the angular resolution and contrast limit of the next Extremely Large Telescope Infrared (ELT IR) imaging instruments.