A comparative study of optical/ultraviolet variability of narrow-line Seyfert 1 and broad-line Seyfert 1 active galactic nuclei

摘要

The ensemble optical/ultraviolet variability of narrow-line Seyfert 1 (NLS1)type active galactic nuclei (AGNs) is investigated, based on a sample selectedfrom the Sloan Digital Sky Survey (SDSS) Stripe-82 region with multi-epochphotometric scanning data. As a comparison a control sample of broad-lineSeyfert 1 (BLS1) type AGNs is also incorporated. To quantify properly theintrinsic variation amplitudes and their uncertainties, a novel method ofparametric maximum-likelihood is introduced, that has, as we argued, certainvirtues over previously used methods. The majority of NLS1-type AGNs exhibitsignificant variability on timescales from about ten days to a few years with,however, on average smaller amplitudes compared to BLS1-type AGNs. About 20NLS1- type AGNs showing relatively large variations are presented, that maydeserve future monitoring observations, for instance, reverberation mapping.The averaged structure functions of variability, constructed using the samemaximumlikelihood method, show remarkable similarity in shape for the two typesof AGNs on timescales longer than about 10 days, which can be approximated by apower-law or an exponential function. This, along with other similarproperties, such as the wavelength-dependent variability, are indicative of acommon dominant mechanism responsible for the long-term optical/UV variabilityof both NLS1- and BLS1-type AGNs. Towards the short timescales, however, thereis tentative evidence that the structure function of NLS1-type AGNs continuesdeclining, whereas that of BLS1-type AGNs flattens with some residualvariability on timescales of days. If this can be confirmed, it may suggestthat an alternative mechanism, such as X-ray reprocessing, starts to becomedominating in BLS1-type AGNs, but not in NLS1-, on such timescales.