Extreme spectral variations of the Seyfert galaxy Markarian 993

The nuclear spectrum of the broad-line Seyfert galaxy Mrk 993 was observed to display strong variations in the strengths of the broad Hα and Hβ emission lines, changing its classification from a Seyfert 1 to a Seyfert 1.9. Moderate to high resolution spectra were obtained with the IDS and CCD spectrograph on the 3 m Shane telescope at Lick Observatory in four different epochs spanning a period of approximately eight years. Using a spectral decomposition method which isolates the pure emission component of the active nucleus from the contaminating underlying host galaxy, our analysis indicates that the broad-line fluxes varied such that the Balmer decrement steepens whenever broad Hα and Hβ decrease in strength, and that the ionizing continuum correspondingly reddens in slope. These direct observational evidences suggest that variation in extinction along the line of sight to the broad-line region of Mrk 993 provides a simple explanation to the behavior of its spectrum, although a combination of possibilities may also be possible.