Long-term Variability of the Composite Galaxy SDSS J103911-000057: A True Type-2 AGN Candidate

In this manuscript, the composite galaxy SDSS J103911-000057 (=SDSS J1039) is reported as a true Type-2 AGN candidate without hidden BLRs. Only narrow emission lines, not broad, detected in SDSS J1039 can be confirmed both by the F-test technique and by the expected broad emission lines with an EW smaller than 13.5 Å with a 99% confidence level. Meanwhile, a reliable AGN power-law component is preferred with a confidence level higher than 7 σ in SDSS J1039. Furthermore, the long-term variability of SDSS J1039 from Catalina Sky Survey can be described by the DRW process with an intrinsic variability timescale τ ∼ 100 days, similar to normal quasars. And, based on BH mass in SDSS J1039 through the M BH – σ relation and on the correlation between AGN continuum luminosity and total H α luminosity, the expected broad H α , if there was one, could be reconstructed with a line width of about 300–1000 km s −1 and with a line flux of about 666 × 10 −17 erg s −1 cm −2 under the virialization assumption to BLRs, providing robust evidence to reject the probability that the intrinsic probable broad H α was overwhelmed by noises of the SDSS spectrum in SDSS J1039. Moreover, SDSS J1039 follows the same correlation between continuum luminosity and [O iii ] line luminosity as the one for normal broad-line AGN, indicating SDSS J1039 classified as a changing-look AGN in the dim state can be ruled out. Therefore, under the current knowledge, SDSS J1039 is a better candidate for a true Type-2 AGN.