Radiation pressure acceleration by X-rays in active galactic nuclei

Doron Chelouche, Hagai Netzer

Research output: Contribution to journalArticlepeer-review


We present calculations of the dynamics of highly ionized gas (HIG) clouds that are confined by external pressure, and are photoionized by AGN continuum. We focus on the gas that is seen, in absorption, in the X-ray spectrum of many AGN and show that such gas can reach hydrostatic equilibrium under various conditions. The principal conclusion is that the clouds can be accelerated to high velocities by the central X-ray source. The dynamical problem can be reduced to the calculation of a single parameter, the average force multiplier, 〈M〉. The typical value of 〈M〉 is ∼10 suggesting that radiation pressure acceleration by X-rays is efficient for L/LEdd ≳ 0.1. The terminal velocity scales with the escape velocity at the base of the flow and can exceed it by a large factor. The typical velocity for a HIG flow that originates at R = 1017 cm in a source with Lx = 1044 erg s-1 is ∼1000 km s-1, i.e. similar to the velocities observed in several X-ray and UV absorption systems. Highly ionized AGN clouds are driven mainly by bound-free absorption, and bound-bound processes are less important unless the lines are significantly broadened or the column density is very small. Pressure laws that result in constant or outward decreasing ionization parameters are most effective in accelerating the flow.

Original languageEnglish
Pages (from-to)916-926
Number of pages11
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
StatePublished - 21 Sep 2001
Externally publishedYes


  • Galaxies: active
  • Galaxies: nuclei
  • ISM: jets and outflows
  • Quasars: absorption lines
  • X-rays: general

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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