scholarly journals A systematic survey for z< 0.04 CLAGNs

2021 ◽  
Vol 503 (2) ◽  
pp. 2583-2597
Author(s):  
Madhooshi R Senarath ◽  
Michael J I Brown ◽  
Michelle E Cluver ◽  
Thomas H Jarrett ◽  
Christian Wolf ◽  
...  
Keyword(s):  
Active Galactic Nuclei ◽  
Galactic Nuclei ◽  
Selection Method ◽  
Selection Methods ◽  
Small Aperture ◽  
Systematic Survey ◽  
The Past ◽  
Spectral Class ◽  
Dust Component ◽  
Over Time

ABSTRACT We have conducted a systematic survey for z &lt; 0.04 active Galactic nuclei (AGNs) that may have changed spectral class over the past decade. We use SkyMapper, Pan-STARRS and the Véron-Cetty & Véron catalogue to search the entire sky for these ‘changing-look’ AGNs (CLAGNs) using a variety of selection methods, where Pan-STARRS has a coverage of 3$\pi$ steradians (sky north of Declination −30○) and SkyMapper has coverage of ∼21000 $~\rm {deg^2}$ (sky south of Declination 0○). We use small aperture photometry to measure how colour and flux have changed over time, where a change may indicate a change in spectral type. Optical colour and flux are used as a proxy for changing Hα equivalent width, while WISE 3.4 μm flux is used to look for changes in the hot dust component. We have identified four AGNs with varying spectra selected using our optical colour selection method. Three AGNs were confirmed from recent observations with WiFeS on the 2.3 m telescope at Siding Spring and the other was identified from archival spectra alone. From this, we identify two new CLAGNs; NGC 1346 and 2MASX J20075129–1108346. We also recover Mrk 915 and Mrk 609, which are known to have varying spectra in the literature, but they do not meet our specific criteria for CLAGNs.

scholarly journals Environments of dwarf galaxies with optical AGN characteristics

2020 ◽  
Vol 496 (3) ◽  
pp. 2577-2590 ◽  
Author(s):  
Mikkel T Kristensen ◽  
Kevin Pimbblet ◽  
Samantha Penny
Keyword(s):  
Emission Line ◽  
Active Galactic Nuclei ◽  
Dwarf Galaxies ◽  
Galactic Nuclei ◽  
Local Environment ◽  
Selection Method ◽  
Estimation Methods ◽  
Selection Methods ◽  
Low Mass ◽  
Future Work

ABSTRACT This study aims to explore the relation between dwarf galaxies (M* ≤ 5 × 109 M⊙) with active galactic nuclei (AGNs) and their environment by comparing neighbourhood parameters of AGN and non-AGN samples. Using the NASA-Sloan Atlas, both the local environment and the immediate environment of dwarf galaxies with z ≤ 0.055 are analysed. Of the 145 155 galaxies in the catalogue, 62 258 of them are classified as dwarf galaxies, and by employing two AGN selection methods based on emission-line fluxes (BPT and WHAN), 4476 are found to have AGN characteristics in their optical spectra. Regardless of selection method, this study finds no discernible differences in environment between AGN and non-AGN host dwarf galaxies and these results indicate that environment is not an important factor in triggering AGN activity in dwarf galaxies. This is in line with existing literature on environments of regular galaxies with AGNs and suggests universality in terms of reaction to environment across the mass regime. The biases of AGN selection in low-mass galaxies, and the biases of different measures of environment are also considered. It is found that there are several mass trends in emission-line ratios and that the SDSS fibre covers galaxies non-uniformly with redshift. These biases should be accounted for in future work by possibly including other wavelength regimes or mass weighting of emission-line ratios. Lastly, a discussion of the environment estimation methods is included since they may not gauge the desired properties due to factors such as time delay or using loosely constrained proxy parameters.

scholarly journals Collimated Bipolar Outflow and the Formation of Nuclear Spirals; Possible Role of Magnetic Fields

1990 ◽  
Vol 140 ◽  
pp. 447-448
Author(s):  
P. Pismis ◽  
E. Moreno ◽  
A. Garcia-Barreto
Keyword(s):  
Magnetic Fields ◽  
Active Galactic Nuclei ◽  
Galactic Nuclei ◽  
Radio Galaxies ◽  
The Past ◽  
Bipolar Outflow ◽  
The Difference ◽  
Observational Techniques ◽  
Quasars And Radio Galaxies

The existence of non–steady phenomena, namely activity in the form of radial motions (outflow) of matter from the nuclei of galaxies is well established at present. Active Galactic Nuclei (AGN) constitute a topic of great interest and are intensively studied by all existing observational techniques. Conventionally objects classified as AGN span a range from quasars, radio galaxies to Seyferts 1 and 2. It appears, however, that there exist galaxies which exhibit somewhat milder activity which does not qualify their inclusion in the AGN group. The designation of MAGN (M for mildly) was suggested in the past (Pismis, 1986) to cover the less energetic nuclei. It may be reasonable to consider that active nuclei form a sequence, the difference along it being due to the energetics of the nuclei, from the most active quasars and radio galaxies down to the mildest ones like M31 or our Galaxy. The phenomenon underlying the activity may thus be universal, subject to the intrinsic energetics of the nuclei (Pismis, 1987).

High energy neutrino sources

1994 ◽  
Vol 346 (1678) ◽  
pp. 93-98 ◽  
Keyword(s):  
Active Galactic Nuclei ◽  
Galaxy Evolution ◽  
Cosmic Strings ◽  
Galactic Nuclei ◽  
High Energy ◽  
Big Bang ◽  
The Past ◽  
The Earth ◽  
The Big Bang ◽  
Bright Phase

High energy cosmic neutrinos can be produced by protons and nuclei accelerated in cosmic sources (‘acceleration neutrinos) as well as by relic Big Bang particles, cosmic strings, etc. (neutrinos of non-acceleration origin). The most promising ‘acceleration’ sources of neutrinos are supernovae in our Galaxy and active galactic nuclei (AGN). Detectable diffuse fluxes of ‘ acceleration ’ neutrinos can be produced by AGN and during the ‘bright phase’ of galaxy evolution. During the past few years it has been realized that the detectable flux of high energy neutrinos can be also produced by the relic Big Bang particles. The possible sources are annihilation of the neutralinos accumulated inside the Earth and the Sun, decay of neutralinos (due to the weak breaking of R-parity), and the decay of exotic long-lived particles from the Big Bang.

scholarly journals Observational View of Magnetic Fields in Active Galactic Nuclei Jets

2016 ◽  
Vol 12 (S324) ◽  
pp. 149-156
Author(s):  
Talvikki Hovatta
Keyword(s):  
Black Hole ◽  
Magnetic Fields ◽  
Active Galactic Nuclei ◽  
Galactic Nuclei ◽  
Short Review ◽  
Relativistic Jets ◽  
The Past ◽  
Emission Models ◽  
Active Galactic Nuclei Jets ◽  
The Magnetic Field

AbstractAccording to the currently favored picture, relativistic jets in active galactic nuclei (AGN) are launched in the vicinity of the black hole by magnetic fields extracting energy from the spinning black hole or the accretion disk. In the past decades, various models from shocks to magnetic reconnection have been proposed as the energy dissipation mechanism in the jets. This paper presents a short review on how linear polarization observations can be used to constrain the magnetic field structure in the jets of AGN, and how the observations can be used to constrain the various emission models.

scholarly journals The X-ray view of merger-induced active galactic nuclei activity at low redshift

2020 ◽  
Vol 499 (2) ◽  
pp. 2380-2389 ◽  
Author(s):  
Nathan J Secrest ◽  
Sara L Ellison ◽  
Shobita Satyapal ◽  
Laura Blecha
Keyword(s):  
Black Holes ◽  
Active Galactic Nuclei ◽  
Galactic Nuclei ◽  
Sloan Digital Sky Survey ◽  
Galaxy Mergers ◽  
Significant Evidence ◽  
X Ray ◽  
The Past ◽  
Sky Survey ◽  
Theoretical Predictions

ABSTRACT Galaxy mergers are predicted to trigger accretion on to the central supermassive black holes, with the highest rates occurring during final coalescence. Previously, we have shown elevated rates of both optical and mid-IR selected active galactic nuclei (AGNs) in post-mergers, but to date the prevalence of X-ray AGNs has not been examined in the same systematic way. We present XMM–Newton data of 43 post-merger galaxies selected from the Sloan Digital Sky Survey along with 430 non-interacting control galaxies matched in stellar mass, redshift, and environment in order to test for an excess of hard X-ray (2–10 keV) emission in post-mergers attributable to triggered AGNs. We find two X-ray detections in the post-mergers ($4.7^{+9.3}_{-3.8}{{\ \rm per\ cent}}$) and nine in the controls ($2.1^{+1.5}_{-1.0}{{\ \rm per\ cent}}$), an excess of $2.22^{+4.44}_{-2.22}$, where the confidence intervals are 90 per cent. While, we therefore do not find statistically significant evidence for an X-ray AGN excess in post-mergers (p = 0.26), we find a factor of ∼17 excess of mid-IR AGNs in our sample, consistent with the past work and inconsistent with the observed X-ray excess (p = 2.7 × 10−4). Dominant, luminous AGNs are therefore more frequent in post-mergers, and the lack of a comparable excess of 2–10 keV X-ray AGNs suggests that AGNs in post-mergers are more likely to be heavily obscured. Our results are consistent with the post-merger stage being characterized by enhanced AGN fueling, heavy AGN obscuration, and more intrinsically luminous AGN, in line with theoretical predictions.

scholarly journals Variable radio AGN at high redshift identified in the VLA Sky Survey

2020 ◽  
Vol 15 (S359) ◽  
pp. 27-32
Author(s):  
Kristina Nyland ◽  
Dillon Dong ◽  
Pallavi Patil ◽  
Mark Lacy ◽  
Amy Kimball ◽  
...  
Keyword(s):  
Active Galactic Nuclei ◽  
High Redshift ◽  
Galactic Nuclei ◽  
Very Large Array ◽  
The Past ◽  
Sky Survey ◽  
Upper Limits ◽  
Radio Transients ◽  
Variable Radio

AbstractAs part of an on-going study of radio transients in Epoch 1 (2017–2019) of the Very Large Array Sky Survey (VLASS), we have discovered a sample of 0.2 < z < 3.2 active galactic nuclei (AGN) selected in the optical/infrared that have recently brightened dramatically in the radio. These sources would have previously been classified as radio-quiet based on upper limits from the Faint Images of the Radio Sky at Twenty-centimeters (FIRST; 1993-2011) survey; however, they are now consistent with radio-loud quasars. We present a quasi-simultaneous, multi-band (1–18 GHz) VLA follow-up campaign of our sample of AGN with extreme radio variability. We conclude that the radio properties are most consistent with AGN that have recently launched jets within the past few decades, potentially making them among the youngest radio AGN known.

scholarly journals Analyzing temporal variations of AGN emission line profiles in the context of (dusty) cloud structure formation in the broad line region

2019 ◽  
Vol 621 ◽  
pp. A46 ◽  
Author(s):  
J. Esser ◽  
J.-U. Pott ◽  
H. Landt ◽  
W. D. Vacca
Keyword(s):  
Active Galactic Nuclei ◽  
Accretion Disk ◽  
Galactic Nuclei ◽  
Temporal Variations ◽  
Cloud Formation ◽  
Broad Line ◽  
Ngc 4151 ◽  
Broad Line Region ◽  
The Past ◽  
Line Region

The formation processes and the exact appearance of the dust torus and broad line region (BLR) of active galactic nuclei (AGN) are under debate. Theoretical studies show a possible connection between the dust torus and BLR through a common origin in the accretion disk. However observationally the dust torus and BLR are typically studied separately. NGC 4151 is possibly one of the best suited Seyfert 1 galaxies for simultaneous examinations because of its high number of both photometric and spectroscopic observations in the past. Here we compare changes of the dust radius to shape variations of broad emission lines (BEL). While the radius of the dust torus decreased by almost a factor of two from 2004 to 2006 shape variations can be seen in the red wing of BELs of NGC 4151. These simultaneous changes are discussed in a dust and BEL formation scheme. We also use the BEL shape variations to assess possible cloud distributions, especially in azimuthal direction, which could be responsible for the observed variations. Our findings can best be explained in the framework of a dust inflated accretion disk. The changes in the BELs suggest that this dusty cloud formation does not happen continuously, and over the whole accretion disk, but on the contrary in spatially confined areas over rather short amount of times. We derive limits to the azimuthal extension of the observed localized BEL flux enhancement event.

scholarly journals Diffuse gamma-ray emission from the Galactic center and implications of its past activities

2016 ◽  
Vol 11 (S322) ◽  
pp. 214-217
Author(s):  
Yutaka Fujita ◽  
Shigeo S. Kimura ◽  
Kohta Murase
Keyword(s):  
Active Galactic Nuclei ◽  
Gamma Ray ◽  
Galactic Center ◽  
Galactic Nuclei ◽  
Cosmic Ray ◽  
High Energy ◽  
Accretion Flows ◽  
The Past ◽  
Sagittarius A ◽  
Sgr A

AbstractIt has been indicated that low-luminosity active galactic nuclei (LLAGNs) are accelerating high-energy cosmic-ray (CR) protons in their radiatively inefficient accretion flows (RIAFs). If this is the case, Sagittarius A* (Sgr A*) should also be generating CR protons, because Sgr A* is a LLAGN. Based on this scenario, we calculate a production rate of CR protons in Sgr A* and their diffusion in the central molecular zone (CMZ) around Sgr A*. The CR protons diffusing in the CMZ create gamma-rays through pp interaction. We show that the gamma-ray luminosity and spectrum are consistent with observations if Sgr A* was active in the past.

scholarly journals On the acceleration of ultra-high-energy cosmic rays

2008 ◽  
Vol 366 (1884) ◽  
pp. 4417-4428 ◽  
Author(s):  
Federico Fraschetti
Keyword(s):  
Cosmic Rays ◽  
Active Galactic Nuclei ◽  
Galactic Nuclei ◽  
High Energy ◽  
New Physics ◽  
Acceleration Process ◽  
Tennis Ball ◽  
Ultra High Energy ◽  
High Energy Cosmic Rays ◽  
The Past

Ultra-high-energy cosmic rays (UHECRs) hit the Earth's atmosphere with energies exceeding 10 18  eV. This is the same energy as carried by a tennis ball moving at 100 km h −1 , but concentrated on a subatomic particle. UHECRs are so rare (the flux of particles with E >10 20  eV is 0.5 km −2 per century) that only a few such particles have been detected over the past 50 years. Recently, the HiRes and Auger experiments have reported the discovery of a high-energy cut-off in the UHECR spectrum, and Auger has found an apparent clustering of the highest energy events towards nearby active galactic nuclei. Consensus is building that the highest energy particles are accelerated within the radio-bright lobes of these objects, but it remains unclear how this actually happens, and whether the cut-off is due to propagation effects or reflects an intrinsically physical limitation of the acceleration process. The low event statistics presently allows for many different plausible models; nevertheless observations are beginning to impose strong constraints on them. These observations have also motivated suggestions that new physics may be implicated. We present a review of the key theoretical and observational issues related to the processes of propagation and acceleration of UHECRs and proposed solutions.

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