  High-energy neutrinos could be produced in the interaction of charged cosmic
rays with matter or radiation surrounding astrophysical sources. Even with the
recent detection of extraterrestrial high-energy neutrinos by the IceCube
experiment, no astrophysical neutrino source has yet been discovered. Transient
sources, such as gamma-ray bursts, core-collapse supernovae, or active galactic
nuclei are promising candidates. Multi-messenger programs offer a unique
opportunity to detect these transient sources. By combining the information
provided by the ANTARES neutrino telescope with information coming from other
observatories, the probability of detecting a source is enhanced, allowing the
possibility of identifying a neutrino progenitor from a single detected event.
A method based on optical and X-ray follow-ups of high-energy neutrino alerts
has been developed within the ANTARES collaboration. This program, denoted as
TAToO, triggers a network of robotic optical telescopes (TAROT and ROTSE) and
the Swift-XRT with a delay of only a few seconds after a neutrino detection,
and is therefore well-suited to search for fast transient sources. To identify
an optical or X-ray counterpart to a neutrino signal, the images provided by
the follow-up observations are analysed with dedicated pipelines. A total of 42
alerts with optical and 7 alerts with X-ray images taken with a maximum delay
of 24 hours after the neutrino trigger have been analysed. No optical or X-ray
counterparts associated to the neutrino triggers have been found, and upper
limits on transient source magnitudes have been derived. The probability to
reject the gamma-ray burst origin hypothesis has been computed for each alert.
