Under the unified model for active galactic nuclei ( AGNs ) , narrow-line ( Type 2 ) AGNs are , in fact , broad-line ( Type 1 ) AGNs but each with a heavily obscured accretion disk .
We would therefore expect the optical continuum emission from Type 2 AGN to be composed mainly of stellar light and non-variable on the time-scales of months to years .
In this work we probe the spectroscopic variability of galaxies and narrow-line AGNs using the multi-epoch data in the Sloan Digital Sky Survey ( SDSS ) Data Release 6 .
The sample contains 18,435 sources for which there exist pairs of spectroscopic observations ( with a maximum separation in time of \sim { 700 } days ) covering a wavelength range of 3900 - 8900 Å .
To obtain a reliable repeatability measurement between each spectral pair , we consider a number of techniques for spectrophotometric calibration resulting in an improved spectrophotometric calibration of a factor of two .
From these data we find no obvious continuum and emission-line variability in the narrow-line AGNs on average – the spectroscopic variability of the continuum is 0.07 \pm 0.26 mag in the g band and , for the emission-line ratios \log _ { 10 } ( [ N ii ] /H \alpha ) and \log _ { 10 } ( [ O iii ] /H \beta ) , the variability is 0.02 \pm 0.03 dex and 0.06 \pm 0.08 dex , respectively .
From the continuum variability measurement we set an upper limit on the ratio between the flux of varying spectral component , presumably related to AGN activities , and that of host galaxy to be \sim { 30 } % .
We provide the corresponding upper limits for other spectral classes , including those from the BPT diagram , eClass galaxy classification , stars and quasars .