Long-term variability of the composite galaxy SDSS J103911-000057: A candidate of true Type-2 AGN

Abstract

In the manuscript, the composite galaxy SDSS J103911-000057 (=SDSS J1039) is reported as a better candidate of true Type-2 AGN without hidden BLRs. None broad but only narrow emission lines detected in SDSS J1039 can be well confirmed both by the F-test technique and by the expected broad emission lines with EW smaller than 13.5\AA~ with 99\% confidence level. Meanwhile, a reliable AGN power law component is preferred with confidence level higher than 7sigma in SDSS J1039. Furthermore, the long-term variability of SDSS J1039 from CSS can be well described by the DRW process with intrinsic variability timescale $\tau\sim100{\rm days}$, similar as normal quasars. And, based on BH mass in SDSS J1039 through the msig relation and on the correlation between AGN continuum luminosity and total H$\alpha$ luminosity, the expected broad H$\alpha$, if there was, could be re-constructed with line width about $300-1000{\rm km~s^{-1}}$ and with line flux about $666\times10^{-17}{\rm erg~s^{-1}~cm^{-2}}$ under the Virialization assumption to BLRs, providing robust evidence to reject the probability that the intrinsic probable broad H$\alpha$ were overwhelmed by noises of the SDSS spectrum in SDSS J1039. Moreover, the SDSS J1039 does follow the same correlation between continuum luminosity and [O~{\sc iii}] line luminosity as the one for normal broad line AGN, indicating SDSS J1039 classified as a changing-look AGN at dim state can be well ruled out. Therefore, under the current knowledge, SDSS J1039 is a better candidate of true Type-2 AGN.