  Context. Based on their overwhelming dominance among associated Fermi gamma
ray catalogue sources, it is expected that a large fraction of the unidentified
Fermi objects are blazars. Through crossmatching between the positions of
unidentified gamma ray sources from the First Fermi Catalog of gamma ray
sources emitting above 10 GeV (1FHL) and the ROSAT and Swift XRT catalogues of
X ray objects and between pointed XRT observations, a sample of 36 potential
associations was found in previous works with less than 15 arcsec of positional
offset. One third of them have recently been classified; the remainder, though
believed to belong to the blazar class, still lack spectroscopic
classifications. Aims. We study the optical spectrum of the putative
counterparts of these unidentified gamma ray sources in order to find their
redshifts and to determine their nature and main spectral characteristics.
Methods. An observational campaign was carried out on the putative counterparts
of 13 1FHL sources using medium resolution optical spectroscopy from the
Osservatorio Astronomico di Bologna in Loiano, Italy; the Telescopio Nazionale
Galileo and the Nordic Optical Telescope, both in the Canary Islands, Spain;
and the Observatorio Astronomico Nacional San Pedro Martir in Baja California,
Mexico. Results. We were able to classify 14 new objects based on their
continuum shapes and spectral features. Conclusions. Twelve new blazars were
found, along with one new quasar and one new narrow line Seyfert 1 (NLS1) to be
potentially associated with the 1FHL sources of our sample. Redshifts or lower
limits were obtained when possible alongside central black hole mass and
luminosity estimates for the NLS1 and the quasar.
