Comprehensive X-ray view of the active nucleus in NGC 4258
Comprehensive X-ray view of the active nucleus in NGC 4258
Context: the presence of water masers orbiting around the activegalactic 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 along-term decrease by a factor of three across a time span of 15 years(from L2‒10 keV ∼ 1041 ergs‒1 in the early 2000s to L2‒10 keV∼ 3 × 1040 erg s‒1 in 2016). The average absorption-corrected X-ray luminosity L2‒10 keV, combined with archival determinations of the bolometric luminosity,implies a bolometric correction kbol ∼ 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-likeaccretion flow.
Accretion, accretion disks, Galaxies: Seyfert, Galaxies: active, Methods: observational, Techniques: spectroscopic, X-rays: galaxies
Masini, A.
ad4d2e5c-b780-4519-8a69-1eca38b8ec86
Wijesekera, J. V.
19be4ff1-abb2-4604-bdc8-94599923c8f1
Celotti, A.
6c62e9eb-dbb9-4beb-b786-e83aa57a6a63
Boorman, P.G.
e5240793-37c1-4b7b-9840-3b996cef8dd3
14 July 2022
Masini, A.
ad4d2e5c-b780-4519-8a69-1eca38b8ec86
Wijesekera, J. V.
19be4ff1-abb2-4604-bdc8-94599923c8f1
Celotti, A.
6c62e9eb-dbb9-4beb-b786-e83aa57a6a63
Boorman, P.G.
e5240793-37c1-4b7b-9840-3b996cef8dd3
Masini, A., Wijesekera, J. V., Celotti, A. and Boorman, P.G.
(2022)
Comprehensive X-ray view of the active nucleus in NGC 4258.
Astronomy & Astrophysics, 663, [A87].
(doi:10.1051/0004-6361/202243231).
Abstract
Context: the presence of water masers orbiting around the activegalactic 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 along-term decrease by a factor of three across a time span of 15 years(from L2‒10 keV ∼ 1041 ergs‒1 in the early 2000s to L2‒10 keV∼ 3 × 1040 erg s‒1 in 2016). The average absorption-corrected X-ray luminosity L2‒10 keV, combined with archival determinations of the bolometric luminosity,implies a bolometric correction kbol ∼ 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-likeaccretion flow.
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Accepted/In Press date: 13 May 2022
Published date: 14 July 2022
Keywords:
Accretion, accretion disks, Galaxies: Seyfert, Galaxies: active, Methods: observational, Techniques: spectroscopic, X-rays: galaxies
Identifiers
Local EPrints ID: 472742
URI: http://eprints.soton.ac.uk/id/eprint/472742
ISSN: 0004-6361
PURE UUID: cd6da9be-1e0d-47af-8220-30c9e0e5f0ef
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Date deposited: 16 Dec 2022 17:32
Last modified: 16 Mar 2024 22:39
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Contributors
Author:
A. Masini
Author:
J. V. Wijesekera
Author:
A. Celotti
Author:
P.G. Boorman
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