A multi-wavelength study of the unidentified TeV gamma-ray source HESS J1626-490

Aims. To explore the nature of the unidentified very-high-energy (VHE, E > 100 GeV) gamma-ray source HESS J1626-490, we investigated the region in X-ray, sub-millimeter, and infrared energy bands. Methods. So far only detected with the HESS array of imaging atmospheric Cherenkov telescopes, HESS J1626-490 could not be unambiguously identified with any source seen at lower energies. Therefore, we analyzed data from an archival XMM-Newton observation, pointed towards HESS J1626-490, to classify detected X-ray point sources according to their spectral properties and their near-infrared counterparts from the 2MASS catalog. Furthermore, we characterized in detail the diffuse X-ray emission from a region compatible with the extended VHE signal. To characterize the interstellar medium surrounding HESS J1626-490 we analyzed (CO)-C-12(J = 1-0) molecular line data from the Nanten Galactic plane survey, Hi data from the Southern Galactic Plane Survey (SGPS) and Spitzer data from the GLIMPSE and MIPSGAL surveys. Results. None of the detected X-ray point sources fulfills the energy requirements to be considered as the synchrotron radiation counterpart to the VHE source assuming an inverse-Compton (IC) emission scenario. We did not detect any diffuse X-ray excess emission originating in the region around HESS J1626-490 above the Galactic background and the derived upper limit for the total X-ray flux disfavors a purely leptonic emission scenario for HESS J1626-490. We found a good morphological match between molecular and atomic gas in the -27 km s(-1) to -18 km s(-1) line-of-sight velocity range and HESS J1626-490. The cloud has a mass of 1.8 x 10(4) M-circle dot and is located at a mean kinematic distance of d = 1.8 kpc. Furthermore, we found a density depression in the Hi gas at a similar distance, which is spatially consistent with the SNR G335.2+00.1. We discuss various scenarios for the VHE emission, including the CO molecular cloud being a passive target for cosmic ray protons accelerated by the nearby SNR G335.2+00.1.