Disc winds spectral energy distribution and the variability time-scale of active galactic nuclei

ABSTRACT Accretion disc winds are commonly observed in active galactic nuclei (AGN). The winds may be associated with the changing spectral properties of such sources, yet such connections have not been studied in detail so far. This paper presents detailed calculations of accretion disc winds and their impact on the observed spectrum of AGN, emphasizing recent observations of Mrk 817. The model consists of a radial and time-dependent mass outflow rate with a half ejection radius of 50 $R_{\rm g}$ and a variability time-scale of 100 d. The resulting spectral energy distribution (SED) is characterized by a large drop in the ionizing luminosity and less significant changes in the optical luminosity. The time-dependent intensities of the broad emission lines and the spectrum emitted by the wind material reflect these variations. For Mrk 817, the variability time-scale of the 1500–5500 Å continuum, the thermal time of the disc at the locations where most of this radiation is emitted, and the range of radii over which most of the mass outflow is taking place are all in agreement with the black hole mass and the radial-dependent accretion rate through the disc. This suggests a general connection between time-dependent disc winds, SED shape, thermal time-scale, and optical-UV variability in AGN.