  The large gas and dust reservoirs of submm galaxies (SMGs) could potentially
provide ample fuel to trigger an Active Galactic Nucleus (AGN), but previous
studies of the AGN fraction in SMGs have been controversial largely due to the
inhomogeneity and limited angular resolution of the available submillimeter
surveys. Here we set improved constraints on the AGN fraction and X-ray
properties of the SMGs with ALMA and Chandra observations in the Extended
Chandra Deep Field-South (E-CDF-S). This study is the first among similar works
to have unambiguously identified the X-ray counterparts of SMGs; this is
accomplished using the fully submm-identified, statistically reliable SMG
catalog with 99 SMGs from the ALMA LABOCA E-CDF-S Submillimeter Survey (ALESS).
We found 10 X-ray sources associated with SMGs (median redshift z = 2.3), of
which 8 were identified as AGNs using several techniques that enable
cross-checking. The other 2 X-ray detected SMGs have levels of X-ray emission
that can be plausibly explained by their star-formation activity. 6 of the 8
SMG-AGNs are moderately/highly absorbed, with N_H > 10e23 cm-2. An analysis of
the AGN fraction, taking into account the spatial variation of X-ray
sensitivity, yields an AGN fraction of 17+16-6% for AGNs with rest-frame 0.5-8
keV absorption-corrected luminosity >7.8x10e42 erg s-1; we provide estimated
AGN fractions as a function of X-ray flux and luminosity. ALMA's high angular
resolution also enables direct X-ray stacking at the precise positions of SMGs
for the first time, and we found 4 potential SMG-AGNs in our stacking sample.
