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

Abstract 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 7σ 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 Hα luminosity, the expected broad Hα, 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 Hα 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.