TESTING MODELS FOR THE QUASAR BIG BLUE BUMP VIA COLOR-COLOR DIAGRAMS

We discuss several models of quasar big blue bump emission in color-co lor and color-luminosity diagrams. We define several broad passbands: IR (0.8-1.6 mu m), VIS (4000-8000 Angstrom), UV (1000-2000 Angstrom), UV1 (1400-2000 Angstrom) and UV2 (1000-1400 Angstrom), and SX (0.2-0.4 keV). The colors have been chosen to investigate characteristics of t he big blue bump: (1) IR/VIS color represents the importance of the IR component and shows the contribution around similar to 1 mu m; (2) UV /VIS color shows the slope of the big blue bump (in a region where it dominates, a higher value means the bump gets steeper); (3) the combin ation of IR/VIS/UV colors shows the relative strength of the big blue bump and the IR component; (4) UV1/UV2 color is important as an indica tor of a flattening of the spectrum in this region and the presence of the far-UV turnover, (5) UV/SX tests the relationship between the big blue bump and the soft X-ray component. All colors are needed to inve stigate the range of model parameters. We describe the colors for seve ral models: accretion disk models in Schwarzschild and Kerr geometries , single-temperature optically thin emission, combination of the main emission model and nonthermal power law or dust, and irradiation of th e disk surface. We test models against the sample of 47 low-redshift q uasars from Elvis et al. We and that (1) modified blackbody emission f rom an accretion disk in a Kerr geometry can successfully reproduce bo th the luminosities and colors of the quasars except for the soft X-ra y emission; (2) no additional components (hot dust or power-law) are n eeded to fit the optical-UV colors when the irradiation of the surface of the disk is included in the model; (3) even modest (10%) irradiati on of the surface of the disk modifies significantly the optical color s; (4) the simplest, single-temperature, free-free models need either an additional component or a range of temperatures to explain the obse rvations. Tables of predicted colors for each model family are provide d on the AAS CD-ROM. A part of the tables is listed in the Appendix.