Line-driven winds and the UV turnover in AGN accretion discs

Ari Laor, Shane W. Davis

Research output: Contribution to journalArticlepeer-review


AGN SEDs generally show a turnover at λ ∼ 1000 Å, implying a maximal accretion disc (AD) temperature of Tmax ∼ 50 000 K. Massive O stars display a similar Tmax, associated with a sharp rise in a line-driven mass-loss Mwind with increasing surface temperature. AGN AD are also characterized by similar surface gravity to massive O stars. The Mwind of O stars reaches ∼10-5 M yr-1. Since the surface area of AGN AD can be 106 larger, the implied Mwind in AGN AD can reach the accretion rate M. A rise to Mwind ~ M towards the AD centre may therefore set a similar cap of Tmax ∼ 50 000 K. To explore this idea, we solve the radial structure of an AD with a mass-loss term, and calculate the implied AD emission using the mass-loss term derived from observations of O stars. We find that Mwind becomes comparable to M typically at a few tens of GM/c2. Thus, the standard thin AD solution is effectively truncated well outside the innermost stable orbit. The calculated AD SED shows the observed turnover at λ ∼ 1000 Å, which is weakly dependent on the AGN luminosity and black hole mass. The AD SED is generally independent of the black hole spin, due to the large truncation radius. However, a cold AD (low M, high black hole mass) is predicted to be windless, and thus its SED should be sensitive to the black hole spin. The accreted gas may form a hot thick disc with a low radiative efficiency inside the truncation radius, or a strong line-driven outflow, depending on its ionization state.

Original languageEnglish
Pages (from-to)3024-3038
Number of pages15
Issue number4
StatePublished - Mar 2014


  • Accretion, accretion discs
  • Black hole physics
  • Galaxies: active
  • Quasars: general

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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