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The most highly accreting quasars (xA) are of special interest in studies ofthe physics of AGNs and host galaxy evolution. Quasars accreting at high rates(L/LEdd $\sim$ 1) hold promise for use as 'standard candles': distanceindicators detectable at very high redshift. However, their observationalproperties are largely unknown. A large sample of xA can clarify the mainproperties of quasars radiating near L/LEdd $\sim$ 1 in the Hb spectral rangefor redshift $\lt$ 0.8. We use selection criteria derived from 4DE1 studies toidentify and analyze spectra for a sample of 334 candidate sources identifiedfrom the SDSS DR7 database. The source spectra were chosen to show a ratioRFeII between the FeII emission blend at 4570 \AA\ and Hb, RFeII $\gt$ 1.Composite spectra were analyzed for systematic trends as a function of FeIIstrength, line width, and [OIII] strength. We introduced tighter constraints onthe signal-to-noise ratio (S/N) and RFeII values that allowed us to isolatesources most likely to be extreme accretors. We provide a database of detailedmeasurements. Analysis of the data allows us to confirm that Hb shows aLorentzian function with a FWHM of Hb $\lt$ 4000 km s-1. Systematic [OIII] blueshifts, as well as a blueshifted component in Hb are revealed. We interpret theblueshifts as related to the signature of outflowing gas from the quasarcentral engine. The FWHM of Hb is still affected by the blueshifted emission,however, the effect is non-negligible if the FWHM Hb is used as a 'virialbroadening estimator'. A effect of the viewing angle on Hb broadening, derivinga correction for those sources that shows disagreement between virial andconcordance cosmology luminosity values. The scatter between concordancecosmology and virial luminosity estimates can be reduced if a correction fororientation effects is included in the FWHM Hb value.

Highly accreting quasars: The SDSS low-redshift catalog