Comprehensive X-ray view of the active nucleus in NGC 4258

¹ SISSA – International School for Advanced Studies, Via Bonomea 265, 34151 Trieste, Italy
² INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
e-mail: alberto311290@gmail.com
³ Department of Physics and Astronomy, University of Padova, Via Francesco Marzolo 8, 35121 Padova, Italy
⁴ INAF – Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, Italy
⁵ IFPU – Institute for Fundamental Physics of the Universe, Via Beirut 2, 34151 Trieste, Italy
⁶ INFN – National Institute for Nuclear Physics, Via Valerio 2, 34127 Trieste, Italy
⁷ Astronomical Institute, Academy of Sciences, Boční II 1401, 14100 Prague, Czech Republic
⁸ Department of Physics & Astronomy, University of Southampton, Southampton SO17 1BJ, UK

Received: 31 January 2022
Accepted: 13 May 2022

Abstract

Context. The presence of water masers orbiting around the active galactic nucleus (AGN) in NGC 4258, one of the most studied extragalactic objects, has been crucial in developing a detailed picture of its nuclear environment. Nonetheless, its accretion rate and bolometric luminosity are still matter of debate, as there are indications that NGC 4258 may host a genuine radiatively inefficient accretion flow (RIAF).

Aims. In this context, 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, in addition to tracking any possible variation across two decades of observations.

Methods. We collected 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 that is suspected to provide the well known obscuration observed in the X-rays. We complemented this information with archival results from the literature.

Results. A 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 by a factor of three across a time span of 15 years (from L_{2−10 keV} ∼ 10⁴¹ erg s⁻¹ in the early 2000s to L_{2−10 keV} ∼ 3 × 10⁴⁰ erg s⁻¹ in 2016). The average absorption-corrected X-ray luminosity L_{2−10 keV}, combined with archival determinations of the bolometric luminosity, implies a bolometric correction k_bol ∼ 20; this result is intriguingly typical for Seyferts powered by accretion through geometrically thin, radiatively efficient disks. Moreover, the X-ray photon index Γ 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.

Conclusions. 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.