The Most Precise Extra-Galactic Black-Hole Mass Measurement

I use archival data to measure the mass of the central black hole in NGC 4526, M = (4.70 ± 0.14) X 10^8 Msun. This 3% error bar is the most precise for an extra-galactic black hole and is close to the precision obtained for Sgr A* in the Milky Way. The factor 7 improvement over the previous measurement is entirely due to correction of a mathematical error, an error that I suggest may be common among astronomers.