Extreme X-ray spectral variability in the Seyfert 2 Galaxy NGC 1365

TL;DR

This study documents extreme and rapid X-ray spectral variability in the Seyfert galaxy NGC 1365, revealing changes in the line-of-sight absorber rather than intrinsic emission, with implications for the structure of the circumnuclear material.

Contribution

It provides detailed observations of rapid spectral state changes in an AGN, highlighting the role of variable absorption over intrinsic emission variability.

Findings

Spectral state switched from reflection to transmission dominance in 6 weeks.

Reflection component remains constant, indicating stable circumnuclear reflection.

Variability driven by line-of-sight absorber changes, not intrinsic emission.

Abstract

We present multiple Chandra and XMM-Newton observations of the type 1.8 Seyfert Galaxy NGC 1365, which shows the most dramatic X-ray spectral changes observed so far in an AGN: the source switched from reflection dominated to transmission dominated and back in just 6 weeks. During this time the soft thermal component, arising from a ~1 kpc region around the center, remained constant. The reflection component is constant at all timescales, and its high flux relative to the primary component implies the presence of thick gas covering a large fraction of the solid angle. The presence of this gas, and the fast variability time scale, suggest that the Compton-thick to Compton thin change is due to variation in the line-of-sight absorber, rather than to extreme intrinsic emission variability. We discuss a structure of the circumnuclear absorber/reflector which can explain the observed X-ray spectral and temporal properties.