A Quasar Catalog with Simultaneous UV, Optical and X-ray Observations by Swift

TL;DR

This study presents a catalog of 843 optically-selected quasars with simultaneous UV, optical, and X-ray observations from Swift, analyzing their spectral energy distributions and correlations to improve understanding of quasar emission properties.

Contribution

The paper provides the first large catalog of quasars with simultaneous UV, optical, and X-ray data, and revises key spectral relations with reduced dispersion using this data.

Findings

60% X-ray detection rate overall, rising to 85% with longer exposure

Big blue bump contributes about 0.3 dex to quasar luminosity

Reduced dispersion in the alpha_ox-L_uv relation by 15% with simultaneous data

Abstract

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 UVOT and 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 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 alpha_ox-L_uv 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 to studies that use non-simultaneous UV/optical and X-ray data. We only found a weak correlation between L/L_Edd and alpha_uv. We do not find significant correlations between alpha_x and alpha_ox, alpha_ox and alpha_uv, and alpha_x and Log L(0.3-10 keV). The correlations between alpha_uv and alpha_x, alpha_ox and alpha_x, alpha_ox and alpha_uv, L/L_Edd and alpha_x, and L/L_Edd and alpha_ox are stronger amongst low-redshift quasars, indicating that these correlations are likely driven by the changes of SED shape with accretion state.