The Chandra/HETG view of NGC 1365 in a Compton-thick state

TL;DR

This study analyzes Chandra HETG observations of NGC 1365 in a Compton-thick state, revealing detailed properties of the photoionized gas, outflows, and the origin of iron fluorescence lines, enhancing understanding of obscured active galaxies.

Contribution

First high-resolution X-ray spectral analysis of NGC 1365 in a Compton-thick state isolating nuclear emission, revealing detailed line profiles and outflow velocities.

Findings

Detection of broad soft X-ray emission lines indicating outflows up to a few thousand km/s.

Resolution of the neutral iron Kα line with a width suggesting origin in broad-line region clouds.

Identification of photoionized gas features distinct from larger aperture observations.

Abstract

We present the analysis of a Chandra High-Energy Transmission Grating (HETG) observation of the local Seyfert galaxy NGC 1365. The source, well known for its dramatic X-ray spectral variability, was caught in a reflection-dominated, Compton-thick state. The high spatial resolution afforded by Chandra allowed us to isolate the soft X-ray emission from the active nucleus, neglecting most of the contribution from the kpc-scale starburst ring. The HETG spectra thus revealed a wealth of He- and H-like lines from photoionized gas, whereas in larger aperture observations these are almost exclusively produced through collisional ionization in the circumnuclear environment. Once the residual thermal component is accounted for, the emission-line properties of the photoionized region close to the hard X-ray continuum source indicate that NGC 1365 has some similarities to the local population of obscured active galaxies. In spite of the limited overall data quality, several soft X-ray lines seem to have fairly broad profiles (~800-1300 km/s full-width at half maximum), and a range of outflow velocities (up to ~1600 km/s, but possibly reaching a few thousands km/s) appears to be involved. At higher energies, the K$\alpha$ fluorescence line from neutral iron is resolved with > 99 per cent confidence, and its width of ~3000 km/s points to an origin from the same broad-line region clouds responsible for eclipsing the X-ray source and likely shielding the narrow-line region.