Large-Amplitude X-Ray Outbursts from Galactic Nuclei: A Systematic Survey using [ITAL]ROSAT[/ITAL] Archival Data

Abstract SDSS J2232−0806 (the ‘Big Dipper’) has been identified as a ‘slow-blue nuclear hypervariable’: a galaxy with no previously known active nucleus, blue colours, and large-amplitude brightness evolution occurring on a time-scale of years. Subsequent observations have shown that this source does indeed contain an active galactic nucleus (AGN). Our optical photometric and spectroscopic monitoring campaign has recorded one major dimming event (and subsequent rise) over a period of around 4 yr; there is also evidence of previous events consistent with this in archival data recorded over the last 20 yr. Here we report an analysis of the 11 optical spectra obtained to date and we assemble a multiwavelength data set including infrared, ultraviolet, and X-ray observations. We find that an intrinsic change in the luminosity is the most favoured explanation of the observations, based on a comparison of continuum and line variability and the apparent lagged response of the hot dust. This source, along with several other recently discovered ‘changing-look’ objects, demonstrate that AGNs can exhibit large-amplitude luminosity changes on time-scales much shorter than those predicted by standard thin accretion disc models.

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Testing warm Comptonization models for the origin of the soft X-ray excess in AGNs

Astronomy and Astrophysics ◽

10.1051/0004-6361/201731580 ◽

2018 ◽

Vol 611 ◽

pp. A59 ◽

Cited By ~ 43

Author(s):

P.-O. Petrucci ◽

F. Ursini ◽

A. De Rosa ◽

S. Bianchi ◽

M. Cappi ◽

...

Keyword(s):

Active Galactic Nuclei ◽

Large Fraction ◽

Galactic Nuclei ◽

High Energy ◽

Archival Data ◽

X Rays ◽

X Ray ◽

Accretion Flow ◽

High Energy Tail ◽

Intrinsic Emission

The X-ray spectra of many active galactic nuclei (AGNs) show a soft X-ray excess below 1–2 keV on top of the extrapolated high-energy power law. The origin of this component is uncertain. It could be a signature of relativistically blurred, ionized reflection or the high-energy tail of thermal Comptonization in a warm (kT ~ 1 keV), optically thick (τ ≃ 10–20) corona producing the optical/UV to soft X-ray emission. The purpose of the present paper is to test the warm corona model on a statistically significant sample of unabsorbed, radio-quiet AGNs with XMM-Newton archival data, providing simultaneous optical/UV and X-ray coverage. The sample has 22 objects and 100 observations. We use two thermal Comptonization components to fit the broadband spectra, one for the warm corona emission and one for the high-energy continuum. In the optical/UV, we also include the reddening, the small blue bump, and the Galactic extinction. In the X-rays, we include a warm absorber and a neutral reflection. The model gives a good fit (reduced χ2 < 1.5) to more than 90% of the sample. We find the temperature of the warm corona to be uniformly distributed in the 0.1–1 keV range, while the optical depth is in the range ~10–40. These values are consistent with a warm corona covering a large fraction of a quasi-passive accretion disk, i.e., that mostly reprocesses the warm corona emission. The disk intrinsic emission represents no more than 20% of the disk total emission. According to this interpretation, most of the accretion power would be released in the upper layers of the accretion flow.

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BAT AGN Spectroscopic Survey – XVII. The parsec-scale jet properties of the ultrahard X-ray-selected local AGNs

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz1995 ◽

2019 ◽

Vol 488 (3) ◽

pp. 4317-4328 ◽

Cited By ~ 5

Author(s):

Junhyun Baek ◽

Aeree Chung ◽

Kevin Schawinski ◽

Kyuseok Oh ◽

O Ivy Wong ◽

...

Keyword(s):

Galactic Nuclei ◽

Archival Data ◽

Imaging Data ◽

X Ray ◽

Tidal Interactions ◽

Long Baseline ◽

Hydrogen Column Density ◽

Wide Range ◽

Burst Alert Telescope ◽

Gas Supply

ABSTRACT We have performed a very long baseline interferometry (VLBI) survey of local (z < 0.05) ultrahard X-ray (14–195 keV) selected active galactic nuclei (AGNs) from the Swift Burst Alert Telescope (BAT) using KVN, KaVA, and VLBA. We first executed fringe surveys of 142 BAT-detected AGNs at 15 or 22 GHz. Based on the result from the fringe surveys and archival data, we find 10/279 nearby AGN (∼4 per cent) VLBI have 22 GHz flux above 30 mJy. This implies that the X-ray AGNs with a bright nuclear jet are not common. Among these 10 radio-bright AGNs, we obtained 22 GHz VLBI imaging data of our own for four targets and reprocessed archival data for six targets. We find that, although our 10 AGNs observed with VLBI span a wide range of pc-scale morphological types, they lie on a tight linear relation between accretion luminosity and nuclear jet luminosity. Our result suggests that a powerful nuclear radio jet correlates with the accretion disc luminosity. We also probed the Fundamental Plane of black hole activity at VLBI scales (e.g. few milliarcsecond). The jet luminosity and size distribution among our sample roughly fit into the proposed AGN evolutionary scenario, finding powerful jets after the blow-out phase based on the Eddington ratio (λEdd)–hydrogen column density (NH) relation. In addition, we find some hints of gas inflow or galaxy–galaxy merger in the majority of our sample. This implies that gas supply via tidal interactions in galactic scale may help the central AGN to launch a powerful parsec-scale jet.

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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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Active Nuclei and Redshift Problems

Highlights of Astronomy ◽

10.1017/s1539299600005542 ◽

1983 ◽

Vol 6 ◽

pp. 531-533

Author(s):

Geoffrey Burbidge

Keyword(s):

Galactic Nuclei ◽

Seyfert Galaxies ◽

Radio Sources ◽

Powerful Radio ◽

Radiation Mechanism ◽

X Ray ◽

Major Interest ◽

Relativistic Particles

More than 20 years ago V. A. Ambartsumian proposed that much of the activity in galaxies was dominated and even generated by their nuclei. Subsequent observational work in radio, optical and x-ray frequencies has borne out his prophecy, and major interest has centered about the nature of the machine in the galactic nucleus. The major characteristic of this machine is that it releases energy rapidly and often spasmodically by processes which are not thermonuclear in origin.The original studies which led to the conclusion that nuclei were all important were observations of the powerful radio sources and Seyfert galaxies, and evidence for the ejection of gas from galaxies of many types. The realization that the synchrotron mechanism was the dominant radiation mechanism and the later studies of Compton radiation were fundamental in leading to the conclusion that large fluxes of relativistic particles must be generated in galactic nuclei.

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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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Reconciling galaxy cluster shapes, measured by theorists versus observers

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3193 ◽

2020 ◽

Vol 500 (2) ◽

pp. 2627-2644

Author(s):

David Harvey ◽

Andrew Robertson ◽

Sut-Ieng Tam ◽

Mathilde Jauzac ◽

Richard Massey ◽

...

Keyword(s):

Hubble Space Telescope ◽

Orientation Angle ◽

Galaxy Cluster ◽

Galactic Nuclei ◽

Weak Lensing ◽

Strongly Correlated ◽

The Bahamas ◽

Cluster Galaxy ◽

Moments Of Inertia ◽

X Ray

ABSTRACT If properly calibrated, the shapes of galaxy clusters can be used to investigate many physical processes: from feedback and quenching of star formation, to the nature of dark matter. Theorists frequently measure shapes using moments of inertia of simulated particles’. We instead create mock (optical, X-ray, strong-, and weak-lensing) observations of the 22 most massive ($\sim 10^{14.7}\, \mathrm{ M}_\odot$) relaxed clusters in the BAHAMAS simulations. We find that observable measures of shape are rounder. Even when moments of inertia are projected into 2D and evaluated at matched radius, they overestimate ellipticity by 56 per cent (compared to observable strong lensing) and 430 per cent (compared to observable weak lensing). Therefore, we propose matchable quantities and test them using observations of eight relaxed clusters from the Hubble Space Telescope (HST) and Chandra X-Ray Observatory. We also release our HST data reduction and lensing analysis software to the community. In real clusters, the ellipticity and orientation angle at all radii are strongly correlated. In simulated clusters, the ellipticity of inner (<rvir/20) regions becomes decoupled: for example, with greater misalignment of the central cluster galaxy. This may indicate overly efficient implementation of feedback from active galactic nuclei. Future exploitation of cluster shapes as a function of radii will require better understanding of core baryonic processes. Exploitation of shapes on any scale will require calibration on simulations extended all the way to mock 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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X-ray properties of He ii λ 1640 emitting galaxies in VANDELS

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa1805 ◽

2020 ◽

Vol 496 (3) ◽

pp. 3796-3807

Author(s):

A Saxena ◽

L Pentericci ◽

D Schaerer ◽

R Schneider ◽

R Amorin ◽

...

Keyword(s):

Formation Rate ◽

Galactic Nuclei ◽

X Ray ◽

Star Forming ◽

Set Constraints ◽

Significant Difference ◽

Low Metallicity ◽

Matched Samples ◽

X Ray Binaries

ABSTRACT We explore X-ray emission from a sample of 18 He ii λ1640 emitting star-forming galaxies at z ∼ 2.3–3.6 from the VANDELS survey in the Chandra Deep Field South, to set constraints on the role of X-ray sources in powering the He ii emission. We find that 4 He ii emitters have tentative detections with S/N ∼ 2 and have X-ray luminosities, LX = 1.5−4.9 × 1041 erg s−1. The stacked luminosity of all 18 He ii emitters is 2.6 × 1041 erg s−1, and that of a subset of 13 narrow He ii emitters (FHWM(He ii) < 1000 km s−1) is 3.1 × 1041 erg s−1. We also measure stacked LX for non-He ii emitters through bootstrapping of matched samples, and find LX = 2.5 × 1041 erg s−1, which is not significantly different from LX measured for He ii emitters. The LX per star formation rate for He ii emitters (log (LX/SFR) ∼ 40.0) and non-emitters (log (LX/SFR) ∼ 39.9) are also comparable and in line with the redshift evolution and metallicity dependence predicted by models. Due to the non-significant difference between the X-ray emission from galaxies with and without He ii, we conclude that X-ray binaries or weak or obscured active galactic nuclei are unlikely to be the dominant producers of He ii ionizing photons in VANDELS star-forming galaxies at z ∼ 3. Given the comparable physical properties of both He ii emitters and non-emitters reported previously, alternative He ii ionizing mechanisms such as localized low-metallicity stellar populations, Pop-III stars, etc. may need to be explored.

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