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 } .