(2003), since that method allows some freedom in disentangling the power-law continuum from any curvature in the spectrum (from dust extinction, for example). The method is explained in detail by (2003), but we provide a brief overview here.

Quasar continua are often described by a power law relation $f_{\lambda} \propto \lambda^{\alpha}$ with a spectral index $\alpha
\; (= \alpha_{\lambda} = -2 - \alpha_{\nu})$ and roughly Gaussian emission-line profiles. In this paper we make the assumption that deviations from a power law times the template composite spectrum over a wide wavelength range can be modeled as extinction by dust. The continuum is represented by $$f_{\lambda} \propto g_{\lambda} \lambda^{\alpha} 10^{-aE(B-V) \xi(\lambda)},$$ where $\lambda$ is in units of microns, $g_{\lambda}$ is the template composite spectrum, $a = 0.4(1+R_V)$, and $\xi(\lambda)$ is the extinction curve. We have adopted the SMC extinction curve fit given by (1992), for which the ratio of total to selective absorption is $R_V = A_V/E(B-V) = 2.93$. We choose the SMC curve because — unlike those of the LMC and the Milky Way — it lacks the “2200Å bump”, which has never been conclusively seen from quasar host galaxy dust, possibly due to the destruction of small graphite grains by 