  In this paper we use a Terahertz (THz) time-domain system to image and
analize the structure of an artwork attributed to Spanish artist Goya painted
in 1771. The Thz images show features that cannot be seen in the optical or
X-ray images that providence evidence of its authenticity, which is validated
by other independent studies. For instance, a feature with a strong resemblance
with one of Goya's known signatures is seen in the THz images that cannot be
detected otherwise. This paper shows the feasibility of using THz imaging in
the field of authentication and characterization of artworks. This may be of
great importance in cases where both the signature and specific features that
are critical for the authentication of an artwork remain hidden to visible,
infrared, or X-ray inspection methods.
