The Variable O VIII Warm Absorber in MCG–6–30–15

Abstract

We present the results of a 4-day ASCA observation of the Seyfert galaxy MCG–6–30–15, focusing on the nature of X-ray absorption by the warm absorber, characterized by the K-edges of highly ionized oxygen, O VII and O VIII. We have confirmed that the column density of O VIII changes on a timescale of ∼ 104 s when the X-ray continuum flux decreases. A significant anti-correlation of the column density with the continuum flux gives direct evidence that the warm absorber is photoionized by the X-ray continuum. From the timescale of the variation of the O VIII column density, we have estimated that it originates from gas within a radius of about 1017 cm of the central engine. In contrast, the depth of the O VII edge shows no response to the continuum flux, which indicates that it originates in gas at larger radii. Our results strongly suggest that there are two warm absorbing regions: one is located near or within the Broad Line Region; the other is associated with the outer molecular torus, scattering medium or Narrow Line Region.