Evolution of the accretion disk around the supermassive black hole of
  NGC 7213
  We present observations of the double-peaked broad H$\alpha$ profile emitted
by the active nucleus of NGC 7213 using the the Gemini South Telescope in 13
epochs between 2011 September 27 and 2013 July 23. This is the first time that
the double-peaked line profile of this nucleus -- typical of gas emission from
the outer parts of an accretion disk surrounding a supermassive black hole
(SMBH) -- is reported to vary. From the analysis of the line profiles we find
two variability timescales: (1) the shortest one, between 7 and 28 days, is
consistent with the light travel time between the ionizing source and the part
of the disk emitting the line; and (2) a longer one of $\gtrsim 3$ months
corresponding to variations in the relative intensity of the blue and red sides
of the profile, which can be identified with the dynamical timescale of this
outer part of the the accretion disk. We modeled the line profiles as due to
emission from a region between $\approx$ 300 and 3000 gravitational radii of a
relativistic, Keplerian accretion disk surrounding the SMBH. Superposed on the
disk emissivity, the model includes an asymmetric feature in the shape of a
spiral arm with a rotation period of $\approx$ 21 months, which reproduces the
variations in the relative intensity of the blue and red sides of the profile.
Besides these variations, the $rms$ variation profile reveals the presence of
another variable component in the broad line, with smaller velocity width
W$_{68}$ (the width of the profile corresponding to 68$\%$ of the flux) of
$\sim 2100$ km s$^{-1}$.
