  Galaxy-galaxy interactions are predicted to cause gas inflows leading to
enhanced nuclear star formation. In this paper we test the further prediction
that the gas inflows lead to enhanced accretion onto the central supermassive
black hole, triggering activity in the nucleus. Based on a sample of 11,060
SDSS galaxies with a close companion (rp < 80 kpc, Delta V < 200 km/s), we
classify AGN based either on emission line ratios or on spectral classification
as a quasar. The AGN fraction in the close pairs sample is compared to a
control sample of 110,600 mass- and redshift-matched control galaxies with no
nearby companion. We find a clear increase in the AGN fraction in close pairs
of galaxies with projected separations < 40 kpc by up to a factor of 2.5
relative to the control sample (although the enhancement depends on the chosen
S/N cut of the sample). The increase in AGN fraction is strongest in equal mass
galaxy pairings, and weakest in the lower mass component of an unequal mass
pairing. The increased AGN fraction at small separations is accompanied by an
enhancement in the number of `composite' galaxies whose spectra are the result
of photoionization by both AGN and stars. Our results indicate that AGN
activity occurs (at least in some cases) well before final coalescence and
concurrently with ongoing star formation. Finally, we find a marked increase at
small projected separations of the fraction of pairs in which BOTH galaxies
harbour AGN. We demonstrate that the fraction of double AGN exceeds the
expected random fraction, indicating that some pairs undergo correlated nuclear
activity. Taken together with complimentary studies, we favour an
interpretation where interactions trigger AGN, but are not the only cause of
nuclear activity.
