scholarly journals Optical, Infrared and Ultraviolet Observations of Active Nuclei

1983 ◽  
Vol 6 ◽  
pp. 443-457
Author(s):  
M.S. Longair
Keyword(s):  
Radio Galaxies ◽  
High Energy ◽  
Broad Line ◽  
Narrow Line ◽  
Bl Lac Objects ◽  
Ngc 4151 ◽  
Bl Lac ◽  
Central Energy ◽  
Ultraviolet Observations ◽  
Blue Galaxy

I have been invited to present a brief review of optical, infrared and ultraviolet observations of active nuclei. I will only describe a small sub-set of these observations, in particular those which I believe are most likely to be important in elucidating the properties of the central energy source to which we attribute the origin of all high energy phenomena. I will therefore restrict my remarks to certain aspects of BL Lac objects, optically violently variable quasars, quasars, Seyfert I galaxies and broad line radio galaxies. If we could understand these systems, we should be able to account for most other types of active nuclei, the Seyfert II galaxies, narrow line radio galaxies, all other types of Markarian galaxy, compact blue galaxy, etc. In my view, the recent observations of NGC 4151 provide us with a benchmark against which we should measure all other observations of active nuclei.

scholarly journals The Environment of OJ 287: Nearby Galaxies and a Long Optical Jet?

1996 ◽  
Vol 175 ◽  
pp. 47-48
Author(s):  
A. Sillanpää ◽  
L. Takalo ◽  
K. Nilsson ◽  
T. Pursimo ◽  
P. Teerikorpi ◽  
...  
Keyword(s):  
Large Scale ◽  
Radio Galaxies ◽  
Trigger Mechanism ◽  
Galaxy Clustering ◽  
Bl Lac Objects ◽  
Close Interaction ◽  
Relativistic Jet ◽  
Bl Lac ◽  
Nearby Galaxies ◽  
Bl Lacs

A widely accepted model for BL Lac objects is that they are radio galaxies with a relativistic jet pointing almost directly towards us. But we need a clear trigger mechanism for these jets. One possibility is the close interaction between the BL Lac host and the closeby galaxies (e.g. Heckman et al. 1986). This interaction has been seen many times in the case of quasars (Hutchings et al. 1989) but not so much is known about the close surroundings of the BL Lac objects although there has been some pioneer work like Stickel et al. (1993). The problem has usually been that the images are not deep enough and that the seeing has not been so good. To clarify the situation we have started an observing program to get very deep images in the sub-arcsecond seeing conditions from the whole 1 Jy sample (Stickel et al. 1991) of BL Lac objects. The aims of this study are: 1. to search for very close companions to the BL Lacs, 2. to study the large scale galaxy clustering around the BL Lacs and 3. to study the BL Lac hosts themselves.

scholarly journals Characterizing the γ-ray long-term variability of PKS 2155−304 with H.E.S.S. and Fermi-LAT

2017 ◽  
Vol 598 ◽  
pp. A39 ◽  
Author(s):  
◽  
H. Abdalla ◽  
A. Abramowski ◽  
F. Aharonian ◽  
F. Ait Benkhali ◽  
...  
Keyword(s):  
Flicker Noise ◽  
High Energy ◽  
Long Term Results ◽  
Noise Power ◽  
Quiescent State ◽  
Physical Processes ◽  
Bl Lac Objects ◽  
Power Spectral ◽  
Bl Lac

Studying the temporal variability of BL Lac objects at the highest energies provides unique insights into the extreme physical processes occurring in relativistic jets and in the vicinity of super-massive black holes. To this end, the long-term variability of the BL Lac object PKS 2155−304 is analyzed in the high (HE, 100 MeV < E < 300 GeV) and very high energy (VHE, E > 200 GeV) γ-ray domain. Over the course of ~9 yr of H.E.S.S. observations the VHE light curve in the quiescent state is consistent with a log-normal behavior. The VHE variability in this state is well described by flicker noise (power-spectral-density index βVHE = 1.10+0.10-0.13) on timescales larger than one day. An analysis of ~5.5 yr of HE Fermi-LAT data gives consistent results (βHE = 1.20+0.21-0.23, on timescales larger than 10 days) compatible with the VHE findings. The HE and VHE power spectral densities show a scale invariance across the probed time ranges. A direct linear correlation between the VHE and HE fluxes could neither be excluded nor firmly established. These long-term-variability properties are discussed and compared to the red noise behavior (β ~ 2) seen on shorter timescales during VHE-flaring states. The difference in power spectral noise behavior at VHE energies during quiescent and flaring states provides evidence that these states are influenced by different physical processes, while the compatibility of the HE and VHE long-term results is suggestive of a common physical link as it might be introduced by an underlying jet-disk connection.

scholarly journals Gamma-ray counterparts of 2WHSP high-synchrotron-peaked BL Lac objects as possible signatures of ultra-high-energy cosmic ray emission

2020 ◽  
Vol 497 (2) ◽  
pp. 2455-2468
Author(s):  
Michael W Toomey ◽  
Foteini Oikonomou ◽  
Kohta Murase
Keyword(s):  
Energy Range ◽  
Gamma Ray ◽  
Cosmic Ray ◽  
High Energy ◽  
Potential Candidate ◽  
Ultra High Energy ◽  
Bl Lac Objects ◽  
Cherenkov Telescopes ◽  
Bl Lac ◽  
Γ Ray

ABSTRACT We present a search for high-energy γ-ray emission from 566 Active Galactic Nuclei at redshift z &gt; 0.2, from the 2WHSP catalogue of high-synchrotron peaked BL Lac objects with 8 yr of Fermi-LAT data. We focus on a redshift range where electromagnetic cascade emission induced by ultra-high-energy cosmic rays can be distinguished from leptonic emission based on the spectral properties of the sources. Our analysis leads to the detection of 160 sources above ≈5σ (TS ≥25) in the 1–300 GeV energy range. By discriminating significant sources based on their γ-ray fluxes, variability properties, and photon index in the Fermi-LAT energy range, and modelling the expected hadronic signal in the TeV regime, we select a list of promising sources as potential candidate ultra-high-energy cosmic ray emitters for follow-up observations by Imaging Atmospheric Cherenkov Telescopes.

scholarly journals Low-luminosity BL Lac Objects and Flat-spectrum Radio Galaxies

1998 ◽  
Vol 164 ◽  
pp. 171-172
Author(s):  
M. Bondi ◽  
D. Dallacasa ◽  
M. J. M. Marchã ◽  
C. Stanghellini
Keyword(s):  
Radio Galaxies ◽  
Radio Luminosity ◽  
Bl Lac Objects ◽  
Spectrum Radio ◽  
Bl Lac ◽  
First Results

AbstractWe present first results from a new sample of low radio luminosity flat spectrum radio galaxies.

scholarly journals THE H.E.S.S. EXTRAGALACTIC SKY

2014 ◽  
Vol 28 ◽  
pp. 1460163
Author(s):  
◽  
JEAN-PHILIPPE LENAIN
Keyword(s):  
Radio Galaxies ◽  
High Energy ◽  
Bl Lac Objects ◽  
Dominant Class ◽  
Cherenkov Telescopes ◽  
Spectrum Radio ◽  
System Experiment ◽  
Very High Energy ◽  
Stereoscopic System ◽  
Very High

More than fifty extragalactic very high energy (VHE; E > 100 GeV) sources have been found using ground-based imaging atmospheric Cherenkov telescopes, about twenty of which have been discovered using the H.E.S.S. (High Energy Stereoscopic System) experiment based in Namibia. Even though BL Lac objects are the dominant class of VHE detected extragalactic objects, other types of sources (starburst galaxies, radio galaxies or flat spectrum radio quasars) begin to emerge. A review of the extragalactic sources studied with H.E.S.S. is given, with an emphasis on new results.

scholarly journals Very high energy observations of the BL Lac objects 3C 66A and OJ 287

2007 ◽  
Vol 28 (3) ◽  
pp. 338-347 ◽  
Author(s):  
T. Lindner ◽  
D.S. Hanna ◽  
J. Kildea ◽  
J. Ball ◽  
D.A. Bramel ◽  
...  
Keyword(s):  
High Energy ◽  
Bl Lac Objects ◽  
Bl Lac ◽  
Very High Energy ◽  
Very High

scholarly journals High Energy Phenomena in AGN Jets

1998 ◽  
Vol 188 ◽  
pp. 153-156
Author(s):  
F. Takahara
Keyword(s):  
Gamma Ray ◽  
High Energy ◽  
Line Of Sight ◽  
Time Variability ◽  
Optical Polarization ◽  
Bl Lac Objects ◽  
Relativistic Jet ◽  
Bl Lac ◽  
Gamma Ray Emission ◽  
Superluminal Expansion

BL-Lac objects and optically violent variable quasars (OVVs), called together blazars, are characterized by rapid time variability, strong optical polarization, superluminal expansion and strong gamma-ray emission. Such properties are understood in the framework of a relativistic jet emanated from the central powerhouse. Blazars are considered to be objects for which the direction of the jet is very close to the line of sight.

scholarly journals Unification of Radio Galaxies

1998 ◽  
Vol 164 ◽  
pp. 81-82
Author(s):  
M.H. Cohen ◽  
P.M. Ogle ◽  
H.D. Iran ◽  
R.W. Goodrich
Keyword(s):  
Radio Galaxies ◽  
Broad Line ◽  
Narrow Line ◽  
Spectral Classification ◽  
Broad Line Region ◽  
Line Region ◽  
Direct View

AbstractMany FR 2 narrow-line radio galaxies also display polarized broad lines. The broad-line region is hidden from direct view and is seen by reflection (ie scattering). In these objects the spectral classification is controlled by the aspect at which they are viewed.

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