  We report on a multi-wavelength study of the compact object candidate 1RXS
J141256.0+792204 (Calvera). Calvera was observed in the X-rays with XMM/EPIC
twice for a total exposure time of ~50 ks. The source spectrum is thermal and
well reproduced by a two component model composed of either two hydrogen
atmosphere models, or two blackbodies (kT_1~ 55/150 eV, kT_2~ 80/250 eV,
respectively, as measured at infinity). Evidence was found for an absorption
feature at ~0.65 keV; no power-law high-energy tail is statistically required.
Using pn and MOS data we discovered pulsations in the X-ray emission at a
period P=59.2 ms. The detection is highly significant (> 11 sigma), and
unambiguously confirms the neutron star nature of Calvera. The pulse profile is
nearly sinusoidal, with a pulsed fraction of ~18%. We looked for the timing
signature of Calvera in the Fermi Large Area Telescope (LAT) database and found
a significant (~5 sigma) pulsed signal at a period coincident with the X-ray
value. The gamma-ray timing analysis yielded a tight upper limit on the period
derivative, dP/dt < 5E-18 s/s (dE_rot/dt <1E33 erg/s, B<5E10 G for magneto-
dipolar spin-down). Radio searches at 1.36 GHz with the 100-m Effelsberg radio
telescope yielded negative results, with a deep upper limit on the pulsed flux
of 0.05 mJy. Diffuse, soft (< 1 keV) X-ray emission about 13' west of the
Calvera position is present both in our pointed observations and in archive
ROSAT all-sky survey images, but is unlikely associated with the X-ray pulsar.
Its spectrum is compatible with an old supernova remnant (SNR); no evidence for
diffuse emission in the radio and optical bands was found. The most likely
interpretations are that Calvera is either a central compact object escaped
from a SNR or a mildly recycled pulsar; in both cases the source would be the
first ever member of the class detected at gamma-ray energies.
