Evidence for periodicity in 43 year-long monitoring of NGC 5548

Understand

We present an analysis of 43 years (1972 to 2015) of spectroscopic observations of the Seyfert 1 galaxy NGC 5548.

• This includes 12 years of new unpublished observations (2003 to 2015).
• We compiled about 1600 H$\beta$ spectra and analyzed the long-term spectral variations of the 5100 \AA\ continuum and the H$\beta$ line.
• Our analysis is based on standard procedures including the Lomb-Scargle method, which is known to be rather limited to such heterogeneous data sets, and new method developed specifically for this project that is more robust and reveals a $\sim$5700 day periodicity in the continuum light curve, the H$\beta$ light curve and the radial velocity curve of the red wing of the H$\beta$ line.