The γ-ray sky seen at X-ray energies - II. The Swift hunt of Fermi BL Lac objects among unidentified γ-ray sourcesE. J. Marchesini, A. Paggi, F. Massaro, N. Masetti, R. D’Abrusco and I. AndruchowA&A, 638 (2020) A128DOI: https://doi.org/10.1051/0004-6361/201936928
