We present a detailed characterization of the 849 broad-line quasars from the Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project. Our quasar sample covers a redshift range of 0.1 < z < 4.5 and is flux-limited to i PSF < 21.7 without any other cuts on quasar properties. The main sample characterization includes: 1) spectral measurements of the continuum and broad emission lines for individual objects from the coadded first-season spectroscopy in 2014; 2) identification of broad and narrow absorption lines in the spectra; 3) optical variability properties for continuum and broad lines from multi-epoch spectroscopy. We provide improved systemic redshift estimates for all quasars, and demonstrate the effects of signal-to-noise ratio on the spectral measurements. We compile measured properties for all 849 quasars along with supplemental multi-wavelength data for subsets of our sample from other surveys. The SDSS-RM sample probes a diverse range in quasar properties, and shows well detected continuum and broad-line variability for many objects from first-season monitoring data. The compiled properties serve as the benchmark for follow-up work based on SDSS-RM data. The spectral fitting tools are made public along with this work.

We report the results of near-infrared spectroscopic observations of 37 quasars in the redshift range 6.3 < z ≤ 7.64, including 32 quasars at z > 6.5, forming the largest quasar near-infrared spectral sample at this redshift. The spectra, taken with Keck, Gemini, VLT, and Magellan, allow investigations of central black hole mass and quasar rest-frame ultraviolet spectral properties. The black hole masses derived from the Mg ii emission lines are in the range (0.3–3.6) × 109 M ⊙, which requires massive seed black holes with masses ≳103–104 M ⊙, assuming Eddington accretion since z = 30. The Eddington ratio distribution peaks at λ Edd ∼ 0.8 and has a mean of 1.08, suggesting high accretion rates for these quasars. The C iv–Mg ii emission-line velocity differences in our sample show an increase of C iv blueshift toward higher redshift, but the evolutionary trend observed from this sample is weaker than the previous results from smaller samples at similar redshift. The Fe ii/Mg ii flux ratios derived for these quasars up to z = 7.6, compared with previous measurements at different redshifts, do not show any evidence of strong redshift evolution, suggesting metal-enriched environments in these quasars. Using this quasar sample, we create a quasar composite spectrum for z > 6.5 quasars and find no significant redshift evolution of quasar broad emission lines and continuum slope, except for a blueshift of the C iv line. Our sample yields a strong broad absorption line quasar fraction of ∼24%, higher than the fractions in lower-redshift quasar samples, although this could be affected by small sample statistics and selection effects.

The broad Mg ii line in quasars has distinct variability properties compared with broad Balmer lines: it is less variable and usually does not display a “breathing” mode, the increase in the average cloud distance when luminosity increases. We demonstrate that these variability properties of Mg ii can be reasonably well explained by simple locally optimally emitting cloud (LOC) photoionization models, confirming earlier photoionization results. In the fiducial LOC model, the Mg ii-emitting gas is on average more distant from the ionizing source than the Hα/Hβ gas and responds with a lower amplitude to continuum variations. If the broad-line region (BLR) is truncated at a physical radius of ∼0.3 pc (for a 108.5 M ⊙ BH accreting at Eddington ratio of 0.1), most of the Mg ii flux will always be emitted near this outer boundary and hence will not display breathing. These results indicate that reverberation mapping results on broad Mg ii, while generally more difficult to obtain owing to the lower line responsivity, can still be used to infer the Mg ii BLR size and hence black hole mass. But it is possible that Mg ii does not have a well-defined intrinsic BLR size–luminosity relation for individual quasars, even though a global one for the general population may still exist. The dramatic changes in broad Hα/Hβ emission in the observationally rare changing-look quasars are fully consistent with photoionization responses to extreme continuum variability, and the LOC model provides natural explanations for the persistence of broad Mg ii in changing-look quasars defined on Hα/Hβ and the rare population of broad Mg ii emitters in the spectra of massive inactive galaxies.