  Radial velocity measurements showed evidence that the M dwarf GL 581 might
host a planet, GL 581d, in the so-called "habitable zone" of the star. A study
of Halpha in GL 581 demonstrated that changes in this activity indicator
correlated with radial velocity variations attributed to GL 581d. An exopplanet
that was important for studies of planet habitability may be an artifact of
stellar activity. Previous investigations analyzing the same activity data have
reached different conclusions regarding the existence of GL 581d. We therfore
investigated the Halpha variations for GL 581 to assess the nature of the
radial velocity variations attributed to the possible planet GL 581d. We
performed a Fourier analysis of the published Halpha measurements for GL 581d
using a so-called pre-whitening process to isolate the variations at the
orbital frequency of GL 581d. The frequency analysis yields five significant
frequencies, one of which is associated with the 66.7 d orbital period of the
presumed planet Gl 581d. The Halpha variations at this period show sine-like
variations that are 180 degrees out-of-phase with the radial velocity
variations of GL 581d. This is seen in thefull data set that spans almost 7
years, as well as a subset of the data that had good temporal sampling over 230
days. Furthermore, No significant temporal variations are found in the ratio of
the amplitudes of the Halpha index and radial velocity variations. This
provides additional evidence that the radial velocity signal attributed to GL
581d is in fact due to stellar activity.
