A comprehensive X-ray view of the active nucleus in NGC 4258

TL;DR

This study provides a detailed analysis of NGC 4258's X-ray spectrum over 15 years, revealing variability in obscuration and luminosity, and indicating a low accretion rate consistent with a radiatively efficient disk rather than a hot RIAF.

Contribution

It offers the first comprehensive broadband X-ray spectral analysis of NGC 4258 across two decades, combining multiple observations and models to characterize its accretion flow and variability.

Findings

Obscuring column density shows possible 10-year periodic fluctuations.

Long-term intrinsic luminosity decreases by a factor of three over 15 years.

NGC 4258 is a low-luminosity Seyfert II with a radiatively efficient accretion disk.

Abstract

(Abridged) We present a detailed broadband X-ray spectrum of NGC 4258, with the goal of precisely measuring the coronal luminosity and accretion flow properties of the AGN, and track any possible variation across two decades of observations. We collect archival XMM-Newton, Chandra, Swift/BAT and NuSTAR spectroscopic observations spanning 15 years, and fit them with a suite of state of the art models, including a warped disk model which is suspected to provide the well known obscuration observed in the X-rays. We complement this information with archival results from the literature. Clear spectral variability is observed among the different epochs. The obscuring column density shows possibly periodic fluctuations on a timescale of 10 years, while the intrinsic luminosity displays a long term decrease of a factor of three in a time span of 15 years (from $L_{2-10~\text{keV}} \sim 10^{41}$ erg s$^{-1}$ in the early 2000s, to $L_{2-10~\text{keV}} \sim 3 \times 10^{40}$ erg s$^{-1}$ in 2016). The average absorption-corrected X-ray luminosity $L_{2-10~\text{keV}}$, combined with archival determinations of the bolometric luminosity, implies a bolometric correction $k_{\rm bol} \sim 20$, intriguingly typical for Seyferts powered by accretion through geometrically thin, radiatively efficient disks. Moreover, the X-ray photon index $\Gamma$ is consistent with the typical value of the broader AGN population. However, the accretion rate in Eddington units is very low, well within the expected RIAF regime. Our results suggest that NGC 4258 is a genuinely low-luminosity Seyfert II, with no strong indications in its X-ray emission for a hot, RIAF-like accretion flow.