Variability of the optical UV continuum in NGC 5548 : implications for accretion-disc models.
Abstract
The recent campaign of systematic UV and optical observations of NGC 5548 has shown the continuum to vary virtually simultaneously at ultraviolet and optical frequencies, with an upper limit to the delay of less than 4 d. We examine this result in the context of accretion-disc models. Within the framework of the standard accretion-disc model, it is found that variations in the accretion rate and propagation of sound waves cannot be reconciled with the time-delay constraint. Reprocessing of the hard radiation, produced in the inner region, by the outer part of the disc emitting in the optical band, which would be consistent with the time- delay limit, is unimportant in the standard model, due to the small solid angle subtended by the relevant region of the disc. Therefore we consider a modified self-irradiating disc model, in which the central region is blown up into a bulge-like shape. We find that this model is consistent with the current observational information on the optical and UV variability of NGC 5548, provided that the bulge has a radius of 6 X 10^14^ cm and a luminosity ranging between ~5 and 10X10^44^ erg s^-1^.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- March 1992
- DOI:
- 10.1093/mnras/255.1.27
- Bibcode:
- 1992MNRAS.255...27M
- Keywords:
-
- Accretion Disks;
- Astronomical Models;
- Continuous Radiation;
- Seyfert Galaxies;
- Ultraviolet Radiation;
- Variability;
- Active Galactic Nuclei;
- Light Curve;
- Visible Spectrum;
- X Ray Astronomy;
- Astrophysics