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Under the unified model for active galactic nuclei (AGNs), narrow-line (Type2) AGNs are, in fact, broad-line (Type 1) AGNs but each with a heavily obscuredaccretion disk. We would therefore expect the optical continuum emission fromType 2 AGN to be composed mainly of stellar light and non-variable on thetime-scales of months to years. In this work we probe the spectroscopicvariability of galaxies and narrow-line AGNs using the multi-epoch data in theSloan Digital Sky Survey (SDSS) Data Release 6. The sample contains 18,435sources for which there exist pairs of spectroscopic observations (with amaximum separation in time of ~700 days) covering a wavelength range of3900-8900 angstrom. To obtain a reliable repeatability measurement between eachspectral pair, we consider a number of techniques for spectrophotometriccalibration resulting in an improved spectrophotometric calibration of a factorof two. From these data we find no obvious continuum and emission-linevariability in the narrow-line AGNs on average -- the spectroscopic variabilityof the continuum is 0.07+/-0.26 mag in the g band and, for the emission-lineratios log10([NII]/Halpha) and log10([OIII]/Hbeta), the variability is0.02+/-0.03 dex and 0.06+/-0.08 dex, respectively. From the continuumvariability measurement we set an upper limit on the ratio between the flux ofvarying spectral component, presumably related to AGN activities, and that ofhost galaxy to be ~30%. We provide the corresponding upper limits for otherspectral classes, including those from the BPT diagram, eClass galaxyclassification, stars and quasars.

Probing Spectroscopic Variability of Galaxies & Narrow-Line Active Galactic Nuclei in the Sloan Digital Sky Survey