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Aims. We probe the physical conditions in the core of Arp 299A and try to putconstraints to the nature of its nuclear power source. Methods. We usedHerschel Space Observatory far-infrared and submillimeter observations of H2Oand OH rotational lines in Arp 299A to create a multi-component model of thegalaxy. In doing this, we employed a spherically symmetric radiative transfercode. Results. Nine H2O lines in absorption and eight in emission as well asfour OH doublets in absorption and one in emission, are detected in Arp 299A.No lines of the 18O isotopologues, which have been seen in compact obscurednuclei of other galaxies, are detected. The absorption in the ground state OHdoublet at 119 {\mu}m is found redshifted by ~175 km/s compared to other OH andH2O lines, suggesting a low excitation inflow. We find that at least twocomponents are required in order to account for the excited molecular linespectrum. The inner component has a radius of 20-25 pc, a very high infraredsurface brightness (> 3e13 Lsun/kpc^2), warm dust (Td > 90 K), and a large H2column density (NH2 > 1e24 cm^-2). The outer component is larger (50-100 pc)with slightly cooler dust (70-90 K). In addition, a much more extendedinflowing component is required to also account for the OH doublet at 119{\mu}m. Conclusions. The Compton-thick nature of the core makes it difficult todetermine the nature of the buried power source, but the high surfacebrightness indicates that it is either an active galactic nucleus and/or adense nuclear starburst. The high OH/H2O ratio in the nucleus indicates thation-neutral chemistry induced by X-rays or cosmic-rays is important. Finally wefind a lower limit to the 16O/18O ratio of 400 in the nuclear region, possiblyindicating that the nuclear starburst is in an early evolutionary stage, orthat it is fed through a molecular inflow of, at most, solar metallicity.

Inflowing gas onto a compact obscured nucleus in Arp 299A: Herschel spectroscopic studies of H2O and OH