A Suzaku Observation of NGC 4593: Illuminating the Truncated Disk

TL;DR

This study analyzes Suzaku X-ray observations of NGC 4593, revealing a truncated accretion disk with no relativistic Fe K line, and discusses how the disk's structure and emission features change with flux variations.

Contribution

It provides new insights into the disk truncation and emission mechanisms in NGC 4593, especially regarding the Fe K alpha line profile and its relation to accretion rate changes.

Findings

Fe K alpha line width decreased from ~10000 to ~4000 km/s

The disk truncation radius increased from 1000-2000 to >5000 Rg

Soft excess decreased by a factor of ~20 from 2002 to 2007

Abstract

We report results from a 2007 Suzaku observation of the Seyfert 1 AGN NGC 4593. The narrow Fe K alpha emission line has a FWHM width ~4000 km/s, indicating emission from >~ 5000 Rg. There is no evidence for a relativistically broadened Fe K line, consistent with the presence of a radiatively efficient outer disk which is truncated or transitions to an interior radiatively inefficient flow. The Suzaku observation caught the source in a low-flux state; compared to a 2002 XMM observation, the hard X-ray flux decreased by 3.6, while the Fe K alpha line intensity and width each roughly halved. Two model-dependent explanations for the changes in Fe line profile are explored. In one, the Fe line width has decreased from ~10000 to ~4000 km/s from 2002 to 2007, suggesting that the thin disk truncation/transition radius has increased from 1000-2000 to >~5000 Rg. However, there are indications from other compact accreting systems that such truncation radii tend to be associated only with accretion rates relative to Eddington much lower than that of NGC 4593. In the second (preferred) model, the line profile in the XMM observation consists of a time-invariant narrow component plus a broad component originating from the inner part of the truncated disk (~300 Rg) which has responded to the drop in continuum flux. The Compton reflection component strength R is ~1.1, consistent with the measured Fe K alpha line total EQW with an Fe abundance 1.7 times solar. The modest soft excess has fallen by a factor of ~20 from 2002 to 2007, ruling out emission from a region 5 lt-yr in size.