Study gives X-Ray insight into the active core of Galaxy NGC 4258

(ORDO NEWS) — After analyzing data from four space observatories, European astronomers have unveiled a complete and thorough X-ray study of the active galactic nucleus in a nearby spiral galaxy known as NGC 4258. Published May 16 on the arXiv website, the results of the study shed more light on the nature of this galaxy.

Active galactic nuclei (AGNs) are accreting supermassive black holes (SMBHs) located at the centers of some galaxies and emit powerful high-energy radiation when they absorb gas and dust. These cores can form jets that are generally cylindrical, conical or parabolic in shape, which are observed even at megaparsec scales.

At a distance of about 24.7 million light-years from Earth, NGC 4258 hosts one of the closest AGNs, making NGC 4258 the clearest evidence for the existence of extragalactic supermassive black holes.

Further study of these objects revealed a huge molecular dust disk with Keplerian rotation around a central region with a mass of about 40 million solar masses.

The AGN in NGC 4258 has also attracted astronomers because of its particularly low bolometric luminosity. They suggested that this could be due to either a low accretion rate or a radiatively inefficient accretion flow. However, although several previous studies have examined the nature of the accretion flow in NGC 4258, it still remains a mystery.

So a team of astronomers led by Alberto Masini at the International School for Advanced Study in Trieste, Italy, conducted a comprehensive review of NGC 4258’s X-ray properties to shed new light on its accretion flow and evolution.

To do this, they analyzed archival observations of this galaxy made with the Chandra, Swift, NuSTAR and XMM-Newton space telescopes.

“The results obtained were supplemented with other data from the literature (for the period ∼ 1993-2000) to obtain a complete and thorough X-ray survey of the nearby low-luminosity active galactic nucleus over 23 years of observations,” the researchers explained.

The study showed that the X-ray properties of the AGN in NGC 4258 have changed over the years. The results suggest that these changes were caused by a twofold change in the density of the absorbing column, probably related to the dust disk and internal changes in the radiation of the central “engine”.

The short timescales of variability observed with NuSTAR indicate that the variations may be due to changes in the rate of accretion, which in turn is related to the rate of deposition of energy at the center. The astronomers noted that the variability in the accretion rate also explains the long-term decline in intrinsic luminosity observed for this AGN.

In addition, luminosity variations seem to follow two patterns of behavior. The first is observed when the source is brighter than a certain critical value of the Eddington x-ray ratio – then no clear trend is seen. The second was found at a lower accretion rate, since there is an anti-correlation between the two, suggesting a transition between hot and cold accretion states.

In general, the authors of the paper concluded that the average spectral properties of NGC 4258 are typical of a low-luminosity type II Seyfert galaxy, with a photon index in the range of 1.6-2.2.

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