place the mass of the central black hole in 3C 273 at $2.35^{+0.37}_{-0.33}\times 10^8$ M$_\sun$ [@kas00]. In a different study, @pian use $Hubble$ $Space$ $Telescope$ UV luminosities to find the broad line region size. To do so, they derive a relationship between $R_{BLR}$ and UV luminosity using the empirical relationship found by @kas00 between $R_{BLR}$ and the optical luminosity. @pian obtain a mass of $4.0^{+2}_{-2}\times 10^{8}M_\sun$ for 3C 273, consistent with the @kas00 value within errors. In another study, @paltani look at the strongest broad emission UV lines (Ly$\alpha$ and C IV ) in archival $International$ $Ultraviolet$ $Explorer$ observations and obtain a mass of $6.59^{+1.86}_{-0.9}\times10^9M_\sun$ for the central supermassive black hole in 3C 273.

There are also mass estimates for 3C 273 that do not come from reverberation mapping. @lia03 find a black hole mass of $2\times 10^7$ M$_\sun$ by generalizing the Elliot-Shapiro relation to the Klein-Nishina regime for 3C 273’s gamma-ray flux obtained from EGRET. Another method is to use the @mclure correlation between host galaxy luminosity and black hole mass which obtains a mass of $1.6 \times 10^9$ M$_\sun$ with an uncertainty of 0.6 dex [@wang].

Underlying Physical Process for the QPO in 3C 273
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If the 