for comparison with the structure functions.

The structure functions for all four observations of 3C 273 are shown in the first four panels of Figure \[sfminus1\]. The observation with the 3.3 ks quasi-period (upper left, Figure \[sfminus1\]) is dominated by whitish noise around 3000s, as inferred from its flat slope; however, the SF is unsuited to quantifying the autocorrelation coefficient precisely. Recall that the wavelet analysis finds an autocorrelation coefficient of $\alpha = 0.14$, relatively small, and consistent with a flattish structure function. We note that this observation also has the greatest excess of such noise above the photon noise, compared to the other three observations, consistent with this being a unique time series out of all those analyzed.

Discussion
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Mass Estimates of 3C 273
------------------------

There are several mass estimates for 3C 273 obtained from different methods. One method is reverberation mapping whereby one uses the time lag of the emission-line light curve with respect to the continuum light curve to determine the light crossing size of the broad line region (BLR) and then assumes Keplerian conditions in the broad line region gas motion (i.e. $M_{BH}=v^{2}R_{BLR}/G$) [@pet00].

Reverberation mapping results based on the optical continuum (i.e. Balmer lines) 