2003MNRAS.338L...1T

The X-ray spectrum of a transient dip of the black-hole binary GRO J1655-40 observed with ASCA in 1994, August, displays a deep K-absorption edge by neutral or weakly ionized iron, but the fluorescence K-line is absent. This dip of GRO J1655-40 is explained in terms of partial covering of the central X-ray emitting region, involving two layers of absorber (`double partial covering'). The results of the spectral analysis indicate that the dip is caused by an absorber of a column density as large as one Compton depth, crossing the line of sight. The physical conditions of such absorbers imply that they are located in the outer part of the accretion disc. An interesting property of the double partial covering is that, while the intrinsic luminosity is constant, the observed flux can vary by a large factor without changing the spectral shape. This might account for some cases of variabilities of active galactic nuclei showing a similarly deep iron K-absorption.