  Transmission spectroscopy can be used to constrain the properties of
exoplanetary atmospheres. During a transit, the light blocked from the
atmosphere of the planet leaves an imprint in the light coming from the star.
This has been shown for many exoplanets with different techniques, with both
photometry and spectroscopy. We aim at testing chromatic line-profile
tomography as a new tool to investigate exoplanetary atmospheres. The signal
imprinted on the cross-correlation function (CCF) by a planet transiting its
star is dependent on the planet-to-star radius ratio. We want to verify if the
precision reachable on the CCF obtained from a subset of the spectral orders of
the HARPS spectrograph is enough to discriminate the radius of a planet at
different wavelengths. Methods. We analyze HARPS archival data of three
transits of HD 189733b. We divide the HARPS spectral range in 7 broadbands,
calculating for each band the ratio between the area of the out-of-transit CCF
and the area of the signal imprinted by the planet on it during the full part
of the transit. We take into account the effect of the limb darkening using the
theoretical coefficients of a linear law. Averaging the results of three
different transits allows us to obtain a good quality broadband transmission
spectrum of HD 189733b, with a precision greater than that of the chromatic RM
effect. Results. We proved chromatic line-profile tomography to be an
interesting way to reveal broadband transmission spectra of exoplanets: our
analysis of the atmosphere of HD 189733b is in agreement with other ground- and
space-based observations. The independent analysis of different transits puts
in evidence the probability that stellar activity plays a role in the extracted
transmission spectrum. Care has thus to be taken when claiming for Rayleigh
scattering in the atmosphere of exoplanets orbiting active stars using only one
transit.
