  Aims. As a sub-Uranus-mass low-density planet, GJ 3470b has been found to
show a flat featureless transmission spectrum in the infrared and a tentative
Rayleigh scattering slope in the optical. We conducted an optical transmission
spectroscopy project to assess the impacts of stellar activity and to determine
whether or not GJ 3470b hosts a hydrogen-rich gas envelop. Methods. We observed
three transits with the low-resolution OSIRIS spectrograph at the 10.4 m Gran
Telescopio Canarias, and one transit with the high-resolution UVES spectrograph
at the 8.2 m Very Large Telescope. Results. From the high-resolution data, we
find that the difference of the Ca II H+K lines in- and out-of-transit is only
0.67 +/- 0.22%, and determine a magnetic filling factor of about 10-15%. From
the low-resolution data, we present the first optical transmission spectrum in
the 435-755 nm band, which shows a slope consistent with Rayleigh scattering.
Conclusions. After exploring the potential impacts of stellar activity in our
observations, we confirm that Rayleigh scattering in an extended
hydrogen/helium atmosphere is currently the best explanation. Further
high-precision observations that simultaneously cover optical and infrared
bands are required to answer whether or not clouds and hazes exist at
high-altitude.
