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

Evolution of the accretion disk around the supermassive black hole of NGC 7213