SIMBAD references

We have compiled a catalog of optically selected quasars with simultaneous observations in UV/optical and X-ray bands by the Swift Gamma-ray Burst Explorer. Objects in this catalog are identified by matching the Swift pointings with the Sloan Digital Sky Survey Data Release 5 quasar catalog. The final catalog contains 843 objects, among which 637 have both Ultraviolet Optical Telescope (UVOT) and X-Ray Telescope (XRT) observations and 354 of which are detected by both instruments. The overall X-ray detection rate is ∼60% which rises to ∼85% among sources with at least 10 ks of XRT exposure time. We construct the time-averaged spectral energy distribution (SED) for each of the 354 quasars using UVOT photometric measurements and XRT spectra. From model fits to these SEDs, we find that the big blue bump contributes about ∼0.3 dex to the quasar luminosity. We re-visit the α_ox-L_2500 Å_relation by selecting a clean sample with only Type 1 radio-quiet quasars; the dispersion of this relation is reduced by at least 15% compared with studies that use non-simultaneous UV/optical and X-ray data. We only found a weak correlation between L_bol/L_Edd and α_UV. We do not find significant correlations between α_x and α_ox, α_oxand α_UV, and α_x and log L(0.3-10 keV). The correlations between α_UV and α_x, α_ox and α_x, α_oxand α_UV, L_bol/L_Edd and α_x, and L_bol/L_Edd and α_oxare stronger among low-redshift quasars, indicating that these correlations are likely driven by the changes of SED shape with accretion state.