  We investigate the possible link between mergers and the enhanced activity of
supermassive black holes (SMBHs) at the centre of galaxies, by comparing the
merger fraction of a local sample (0.003 =< z < 0.03) of active galaxies - 59
active galactic nuclei (AGN) host galaxies selected from the all-sky Swift BAT
(Burst Alert Telescope) survey - with an appropriate control sample (247
sources extracted from the Hyperleda catalogue) that has the same redshift
distribution as the BAT sample. We detect the interacting systems in the two
samples on the basis of non-parametric structural indexes of concentration (C),
asymmetry (A), clumpiness (S), Gini coefficient (G) and second order momentum
of light (M20). In particular, we propose a new morphological criterion, based
on a combination of all these indexes, that improves the identification of
interacting systems. We also present a new software - PyCASSo (Python CAS
Software) - for the automatic computation of the structural indexes. After
correcting for the completeness and reliability of the method, we find that the
fraction of interacting galaxies among the active population, 20(+7/-5) per
cent, exceeds the merger fraction of the control sample, 4(+1.7/-1.2) per cent.
Choosing a mass-matched control sample leads to equivalent results, although
with slightly lower statistical significance. Our findings support the scenario
in which mergers trigger the nuclear activity of supermassive black holes.
