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The nature of a substantial percentage (about one fifth) of hard X-raysources discovered with the BAT instrument onboard the Neil Gehrels SwiftObservatory (hereafter Swift) is unknown because of the lack of an identifiedlonger-wavelength counterpart. Without such follow-up, an X-ray catalogue is oflimited astrophysical value: we therefore embarked, since 2009, on a long-termproject to uncover the optical properties of sources identified by Swift byusing a large suite of ground-based telescopes and instruments. In this work,we continue our programme of characterization of unidentified or poorly studiedhard X-ray sources by presenting the results of an optical spectroscopiccampaign aimed at pinpointing and classifying the optical counterparts of 35hard X-ray sources taken from the 70-month BAT catalogue. (...) With the use ofoptical spectra taken at six different telescopes we were able to identify themain spectral characteristics (continuum type, redshift, and emission orabsorption lines) of the observed objects, and determined their nature. Weidentify and characterize a total of 41 optical candidate counterpartscorresponding to 35 hard X-ray sources given that, because of positionaluncertainties, multiple lower energy counterparts can sometimes be associatedwith higher energy detections. We discuss which ones are the actual (or atleast most likely) counterparts based on our observational results. Inparticular, 31 sources in our sample are active galactic nuclei: 16 areclassified as Type 1 (with broad and narrow emission lines) and 13 areclassified as Type 2 (with narrow emission lines only); two more are BLLac-type objects. We also identify one LINER, one starburst, and 3 ellipticalgalaxies. The remaining 5 objects are galactic sources: we identify 4 of themas cataclysmic variables, whereas one is a low mass X-ray binary.

Optical spectroscopic classification of 35 hard X-ray sources from the Swift-BAT 70-month catalogue