Comprehensive X-ray view of the active nucleus in NGC4258

Context. The presence of water masers orbiting around the active galactic nucleus (AGN) in NGC4258, 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 NGC4258 may host a genuine radiatively inefficient accretion flow (RIAF). Aims. In this context, we present a detailed broadband X-ray spectrum of NGC4258, 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 L2-10 (keV) similar to 10(41) erg s(-1) in the early 2000s to L2-10 keV similar to 3 x 10(40) 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) similar to 20; this result is intriguingly typical for Seyferts powered by accretion through geometrically thin, radiatively efficient disks. Moreover, the X-ray photon index 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 NGC4258 is a genuinely low-luminosity Seyfert II, with no strong indications in its X-ray emission for a hot, RIAF-like accretion flow.

Automated summary

By analyzing the X-ray spectrum of NGC4258, researchers found that its luminosity varies, the obscuring column density possibly fluctuates periodically, and the bolometric luminosity shows a long-term decrease. However, the X-ray photon index is consistent with other AGNs. The results suggest that NGC4258 is a genuinely low-luminosity Seyfert II galaxy.