THE ASTROPHYSICAL JOURNAL, 454:77-94, 1995 November 20

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

         ANETA SIEMIGINOWSKA, OLGA KUHN, MARTIN ELVIS, FABRIZIO FIORE,
                  JONATHAN MCDOWELL, AND BELINDA J. WILKES
        Harvard-Smithsonian Center for Astrophysics, 60 Garden Street,
                              Cambridge, MA 02138


                                   ABSTRACT

        We disuss several models of quasar "big blue bump" emission in
     color-color and color-luminosity diagrams.  We define several broad
     passbands: IR (0.8-1.6 um), VIS (4000-8000 A), UV (1000-2000 A), UV1
     (1400-2000 A) and UV2 (1000-1400 A), and SX (0.2-0.4 keV).  The colors
     have been chosen to investigate characteristics of the big blue bump;
     (1) IR/VIS color represents the importance of the IR component and
     shows the contribution around ~1 um; (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 combination 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 indicator 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 investigate the range of
     model parameters.
        We describe the colors for several 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 the disk surface.  We test models
     against the sample of 47 low-redshift quasars from Elvis et al.  We
     find that (1) modified blackbody emission from an accretion disk in a
     Kerr geometry can successfully reproduce both the luminosities and
     colors of the quasars except for the soft X-ray emission; (2) no
     additional components (hot dust or power-law) are needed to fit the
     optical-UV colors when the irradiation of the surface of the disk is
     included in the model; (3) even modest (10%) irradiation of the
     surface of the disk modifies significantly the optical colors; (4) the
     simplest, single-temperature, free-free models need either an
     additional component or a range of temperatures to explain the
     observations.
        Tables of predicted colors for each model family are provided on
     the AAS CD-ROM.  A part of the tables is listed in the Appendix.

     Subject headings: cosmology: theory -- galaxies: photometry --
                       quasars: general -- radiation mechanisms: thermal