AGN/starburst connection in action: the half million second RGS spectrum of NGC1365

TL;DR

This study presents a high-resolution X-ray spectrum of NGC1365 revealing the coexistence of collisional and photoionised gas, providing insights into the AGN/starburst connection and the obscuration effects in this galaxy.

Contribution

First high-resolution spectrum of an obscured AGN showing both collisional and photoionised plasma components, advancing understanding of AGN environments.

Findings

NGC1365's spectrum requires both collisional and photoionisation models.

Photoionisation features are weaker, possibly due to line-of-sight obscuration.

The galaxy's X-ray luminosity is ~10^{42} erg/s, indicating a luminous AGN or starburst.

Abstract

We present a deep (~5.8 days) 0.3-2 keV high-resolution spectrum of NGC1365, collected with the reflection grating spectrometer (RGS) on board XMM-Newton. The spectrum is dominated by strong recombination lines of He- and H-like transitions from carbon to silicon, as well as by L transitions from FeXVII. The continuum is strong, especially in the 10 to 20 Angstrom, range. Formal fits require two optically thin, collisionally ionised plasma components, with temperatures ~300 and ~640 eV. However, they leave the bulk of the forbidden components of the He-alpha OVII and NVI triplets unaccounted for. These features can be explained as being produced by photoionised gas. NGC1365 is therefore the first obscured AGN, whose high-resolution X-ray spectrum requires both collisional ionisation and photoionisation. The relative weakness of photoionisation does not stem from the intrinsic weakness of its AGN, whose X-ray luminosity is ~10^{42} erg/s. We suggest that it may instead come from the line-of-sight from the active nucleus to the NLR being blocked by optically thick matter in the broad line region, at the same time responsible for the large observed variation of the column density obscuring the X-ray active nucleus. Alternatively, NGC1365 could host a remarkably luminous nuclear starburst when compared to the AGN accretion power [abriged].