X-ray variability time scales in active galactic nuclei

AbstractOne of the most important features in active galactic nuclei (AGN) is the variability of their emission. Variability has been discovered at X-ray, UV, and radio frequencies on time scales from hours to years. Among the AGN family and according to theoretical studies, Low-Ionization Nuclear Emission Line Region (LINER) nuclei would be variable objects on long time scales. Our purpose is to investigate spectral X-ray variability in LINERs and to understand the nature of these kinds of objects, as well as their accretion mechanism. Chandra and XMM–Newton public archives were used to compile X-ray spectra of LINER nuclei at different epochs with time scales of years. To search for variability we fit all the spectra from the same object with a set of models, in order to identify the parameters responsible for the variability pattern. We found that long term spectral variability is very common, with variations mostly related to hard energies (2-10 keV). These variations are due to changes in the soft excess, and/or changes in the absorber, and/or intrinsic variations of the source.

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X-Ray Variability in Active Galactic Nuclei: Two Things That Everybody Should Know

Highlights of Astronomy ◽

10.1017/s1539299600018839 ◽

1998 ◽

Vol 11 (2) ◽

pp. 804-807

Author(s):

Karen M. Leighly

Keyword(s):

Light Curve ◽

Active Galactic Nuclei ◽

Time Scales ◽

Time Scale ◽

Galactic Nuclei ◽

X Rays ◽

X Ray ◽

Central Engine ◽

Distinguishing Property

X-ray variability is a distinguishing property of Active Galactic Nuclei (AGN), and the energetics and time scales of the emission dictate that the X-rays must originate very close to the central engine. In this review I discuss two basic topics from AGN variability research. The first is the correlation of the variability time scale with the X-ray luminosity, and the second is the structure of the X-ray light curve. In each case, I first review the old results that have been known for approximately the last 10 years and then I discuss very new results which may force us to modify our ideas about the origin of AGN X-ray variability. Note that I am discussing the variability of non-blazar type AGN.

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X-Ray Variability of Active Galactic Nuclei

Highlights of Astronomy ◽

10.1017/s1539299600005505 ◽

1983 ◽

Vol 6 ◽

pp. 499-503 ◽

Cited By ~ 4

Author(s):

Daniel A. Schwartz ◽

Greg Madejski ◽

William H.-M. Ku

Keyword(s):

Active Galactic Nuclei ◽

Time Scales ◽

Galactic Nuclei ◽

Active Galaxies ◽

X Rays ◽

Active Nucleus ◽

Massive Black Hole ◽

Bl Lac Objects ◽

X Ray ◽

Structural Length

X-ray variability of active galactic nuclei is commonplace on scales from days to years. It also occurs, although rarely, on time scales as short as 200 seconds. Both these statements must be strongly qualified by the irregularity and insensitivity of the available observations. In the X-rays we expect that we are seeing deep within the active nucleus, near what is usually taken to be a massive black hole. The X-ray variability time scales may then give us the fundamental structural length scales. We would like to review data on active galaxies, present our new data on Einstein observations of BL Lac objects, and discuss all these in terms of a statistical quantification of the observations.

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Variability in Active Galactic Nuclei

International Astronomical Union Colloquium ◽

10.1017/s0252921100014299 ◽

1993 ◽

Vol 134 ◽

pp. 245-547

Author(s):

M. Matsuoka

Keyword(s):

Active Galactic Nuclei ◽

Time Scales ◽

Galactic Nuclei ◽

Wide Band ◽

Seyfert Galaxies ◽

Time Variability ◽

X Ray ◽

Central Engine ◽

Nearby Region ◽

Wide Range

AbstractAGN (Active Galactic Nuclei) have their profound time variability over a wide range of time scales. Although many results of AGN variability have been provided from wide band wavelength observations, I would like to concentrate the recent problems concerning a nearby region of their central engine based on the X-ray observations which are most efficient to investigate this region. In this paper we will investigate mainly the result of Seyfert galaxies which would be generalized to other AGN.

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Polarization Variability of Active Galactic Nuclei and X‐Ray Binaries

The Astrophysical Journal ◽

10.1086/304575 ◽

1997 ◽

Vol 487 (1) ◽

pp. 142-152 ◽

Cited By ~ 9

Author(s):

Gang Bao ◽

Petr Hadrava ◽

Paul J. Wiita ◽

Ying Xiong

Keyword(s):

Active Galactic Nuclei ◽

Galactic Nuclei ◽

X Ray ◽

X Ray Binaries

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Gamma-Ray and Neutrino Signals from Accretion Disk Coronae of Active Galactic Nuclei

Galaxies ◽

10.3390/galaxies9020036 ◽

2021 ◽

Vol 9 (2) ◽

pp. 36

Author(s):

Yoshiyuki Inoue ◽

Dmitry Khangulyan ◽

Akihiro Doi

Keyword(s):

Active Galactic Nuclei ◽

Gamma Ray ◽

Galactic Nuclei ◽

High Energy ◽

Thermal Activity ◽

X Ray ◽

Cascade Models ◽

Coronal Activity ◽

Ngc 1068 ◽

High Energy Particles

To explain the X-ray spectra of active galactic nuclei (AGN), non-thermal activity in AGN coronae such as pair cascade models has been extensively discussed in the past literature. Although X-ray and gamma-ray observations in the 1990s disfavored such pair cascade models, recent millimeter-wave observations of nearby Seyferts have established the existence of weak non-thermal coronal activity. In addition, the IceCube collaboration reported NGC 1068, a nearby Seyfert, as the hottest spot in their 10 yr survey. These pieces of evidence are enough to investigate the non-thermal perspective of AGN coronae in depth again. This article summarizes our current observational understanding of AGN coronae and describes how AGN coronae generate high-energy particles. We also provide ways to test the AGN corona model with radio, X-ray, MeV gamma ray, and high-energy neutrino observations.

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Hot gaseous atmospheres of rotating galaxies observed with XMM–Newton

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3182 ◽

2020 ◽

Vol 499 (4) ◽

pp. 5163-5174

Author(s):

A Juráňová ◽

N Werner ◽

P E J Nulsen ◽

M Gaspari ◽

K Lakhchaura ◽

...

Keyword(s):

Angular Momentum ◽

Active Galactic Nuclei ◽

Spiral Galaxy ◽

Galactic Nuclei ◽

X Ray ◽

Hot Gas ◽

Heating And Cooling ◽

Theoretical Predictions ◽

Central Regions ◽

Gaseous Atmospheres

ABSTRACT X-ray emitting atmospheres of non-rotating early-type galaxies and their connection to central active galactic nuclei have been thoroughly studied over the years. However, in systems with significant angular momentum, processes of heating and cooling are likely to proceed differently. We present an analysis of the hot atmospheres of six lenticulars and a spiral galaxy to study the effects of angular momentum on the hot gas properties. We find an alignment between the hot gas and the stellar distribution, with the ellipticity of the X-ray emission generally lower than that of the optical stellar emission, consistent with theoretical predictions for rotationally supported hot atmospheres. The entropy profiles of NGC 4382 and the massive spiral galaxy NGC 1961 are significantly shallower than the entropy distribution in other galaxies, suggesting the presence of strong heating (via outflows or compressional) in the central regions of these systems. Finally, we investigate the thermal (in)stability of the hot atmospheres via criteria such as the TI- and C-ratio, and discuss the possibility that the discs of cold gas present in these objects have condensed out of the hot atmospheres.

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