Using the quasars with z em < 0.9 from the Baryon Oscillation Spectroscopic Survey, we measure the spectral characteristics, including continuum and emission lines, around the Hβ and Hα spectral regions, which are lacking in Quasar Data Release 12 (DR12Q). We estimate the virial black hole mass from broad Hα and/or Hβ, and infer quasar redshifts from [O III] λ5007 emission lines. All the measurements and derived quantities are publicly available. The comparison between [O III] λ5007 based redshifts and the visual inspection redshifts included in DR12Q indicates that the visual inspection redshifts are robust. We find that the FWHMs of the broad Hα are consistent with those of the broad Hβ, while both the equivalent widths and line luminosities of the broad Hα are obviously larger than the corresponding quantities of the broad Hβ. We also find that there is an obviously systematic offset between the Hβ and Hα based mass if they are inferred from the empirical relationships in the literature. Using our large quasar sample, we have improved the Hβ and Hα based mass estimators by minimizing the difference between the Hβ and Hαbased mass. For the black hole mass estimator (Equation (1)), we find that the coefficients (a, b) = (7.00, 0.50) for the Hα and (a, b) = (6.96, 0.50) for the Hβ are the best choices.

Using the SDSS spectra of quasars included in the DR7Q or DR12Q catalogs, we search for Mg II λλ2796,2803 narrow absorption doublets in the spectra data around Mg II λ2798 emission lines. We obtain 17 316 Mg II doublets, within the redshift range of 0.3299 ≤ z abs ≤ 2.5663. We find that a velocity offset of υ r < 6000 km s −1 is a safe boundary to constrain the vast majority of associated Mg II systems, although we find some doublets at υ r > 6000 km s −1 . If associated Mg II absorbers are defined by υ r < 6000 km s −1 , ∼33.3% of the absorbers supposed to be contaminants of intervening systems. Removing the 33.3% contaminants, ∼4.5% of the quasars present at least one associated Mg II system with W λ2796 r ≥ 0.2 Å. The fraction of associated Mg II systems with high velocity outflows correlates with the average luminosities of their central quasars, indicating a relationship between outflows and the quasar feedback power. The υ r distribution of the outflow Mg II absorbers is peaked at 1023 km s −1 , which is smaller than the corresponding value of the outflow C IV absorbers. The redshift number density evolution of absorbers (dn/dz) limited by υ r > −3000 km s −1 differs from that of absorbers constrained by υ r > 2000 km s −1 . While, absorbers limited by υ r > 2000 km s −1 and higher values exhibit similar profile of dn/dz. In addition, the dn/dz is smaller when absorbers are constrained with larger υ r . The distributions of equivalent widths, and the ratio of W λ2796 r /W λ2803 r is the same for associated and intervening systems, and independent on quasar luminosity.

This paper statistically investigates the properties of C IV and Mg II narrow absorption lines (NALs) to look for velocity cuts that can well constrain quasar-associated NALs. The coverage fraction ( f c ) is defined as the ratio between the number of quasars exhibiting at least one detected absorber and the total number of quasars that can be used to detect absorptions with given criteria. We find that, for both C IV and Mg II absorbers, both the number density of absorbers in given velocity intervals (dn/dβ) and the f c show very significant excess at the low-velocity offset from the quasars, relative to the random occurrence that is expected for cosmologically intervening absorbers. These relative excess extensions for Mg II absorptions are not only evidently related to absorption strength but also to quasar luminosity, while they are mainly constrained within 2000 km s . Turning to C IV absorptions, the relative excess extensions of both dn/dβ and f c are mainly limited within v abs < 4000 km s, and depend neither on absorption strength nor on quasar luminosity. And also, the absorbers with v abs < 4000 km sshow obviously different redshift number density evolution from those with v abs > 4000 km s −1. We suggest velocity cuts of 4000 km s to define quasar C IV and Mg II associated NALs, respectively.

In this Letter, we report the discovery of a strong correlation between the variability of narrow absorption lines (NALs) and the ionizing continuum from a two-epoch spectra sample of 40 quasars containing 52 variable C iv λλ1548, 1551 absorption doublets. According to the concordance index, this sample is classified into two subsamples. Subsample I shows an anti-correlation between the variations of absorption lines and the continuum, while Subsample II exhibits a positive correlation. These results imply that these variable C iv λλ1548, 1551 absorption doublets are intrinsic to the corresponding quasars and that their variations are caused primarily by the fluctuations of the ionizing continuum. Based on our analysis, we propose that there might be two kinds of absorption gas: one that is very sensitive to the continuum variations, the another that is not. In addition, we suggest that in many cases the emergence or disappearance of NALs is caused by fluctuations of the ionizing continuum.

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