2016ApJ...824...38X

We investigate the UV continuum slope α of a large quasar sample from SDSS DR7. By using specific continuum windows, we build two samples at lower ( 0.71\lt z\lt 1.19 ) and higher ( 1.90\lt z\lt 3.15 ) redshifts, which correspond to the continuum slopes at longer (near-UV) and shorter (far-UV) rest wavelength ranges, respectively. Overall, the average continuum slopes are -0.36 and -0.51 for α_{NUV} and α_{FUV} with similar dispersions σ_α∼0.5 . For both samples, we confirm the luminosity dependence of the continuum slope, i.e., fainter quasars have redder spectra. We further find that both α_{NUV} and α_{FUV} have a common upper limit ( ∼1/3 ), which is almost independent of the quasar luminosity L_{bol} . This finding implies that the intrinsic quasar continuum (or the bluest quasar), at any luminosity, obeys the standard thin-disk model. We propose that the other quasars with redder α are caused by the reddening from the dust locally. With this assumption, we employ the dust extinction scenario to model the observed L_{bol}–αrelation. We find that a typical value of E(B-V)∼0.1–0.3 mag (depending on the types of extinction curve) of the quasar local dust is enough to explain the discrepancy of α between the observation ( ∼-0.5 ) and the standard accretion disk model prediction ( ∼1/3 ).