Download or read book On the Variable Nature of Low Luminosity Active Galactic Nuclei written by Lorena Hernández García and published by . This book was released on 2015 with total page 255 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book The Nature of Compact Objects in Active Galactic Nuclei written by Andrew Robinson and published by Cambridge University Press. This book was released on 1994-03-03 with total page 466 pages. Available in PDF, EPUB and Kindle. Book excerpt: What evidence is there for and against unified schemes for active galactic nuclei (AGN)? How do the AGN populations evolve over cosmological timescales? And what can the variability of their UV and X-ray emission tell us? These are just some of the exciting issues addressed in this volume of papers collected from the 33rd Herstmonceux conference in Cambridge. AGN are among the most spectacular objects known to astronomy. Yet, despite years of intense and wide-ranging research, the debate continues - what is their fundamental source of power? Rapid progress has been made towards answering this question by a variety of large-scale, multi-wavelength monitoring campaigns and the latest generation of satellite-borne observations. This volume provides a valuable overview and timely update of the exciting and rapidly developing field of AGN research - essential reading for graduate students and researchers.

Download or read book On the Study of Low Luminosity Active Galactic Nuclei written by 羅文斌 and published by . This book was released on 2023 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book The Astrophysics of Active Galactic Nuclei Variability in Large Scale Spectroscopic Surveys written by John J. Ruan and published by . This book was released on 2017 with total page 167 pages. Available in PDF, EPUB and Kindle. Book excerpt: More than 50 years after the initial discovery of the extragalactic nature of quasi-stellar objects (quasars) by Schmidt (1963), studies of luminous active galactic nuclei (AGN) have revolutionized our understanding of black hole growth across cosmic time, accretion and jet physics, as well as galaxy evolution and cosmology. In the coming decade, these studies will be further fueled by large (a few x10^6) samples of quasars from massive optical spectroscopic surveys (e.g., from eBOSS and DESI). These spectra will be accompanied by well-sampled photometric light curves from time-domain imaging surveys (e.g., from Pan-STARRS and LSST), enabling discovery of rare objects and new time-domain phenomena. Current spectroscopic and imaging surveys have well-established that nearly all Type 1 quasars are optically variable, although the origin of this variability is still unknown. The primary goal of this thesis is to investigate various AGN variability phenomena in the UV/optical, to understanding their origin. In particular, I investigate the origin of 10-20% flux variability ubiquitously observed in quasars, the apparent change in accretion states observed in ‘transition blazars’, as well as the rapid fading observed in the recently-discovered ‘changing-look quasars’ phenomenon. I also prepare for the science enabled by the large samples of AGN that will be discovered in future time-domain imaging surveys, by characterizing the unique properties of variability-selected AGN. The primary technique I use in this dissertation to probe AGN variability is repeat optical spectroscopy. AGN optical spectra contain a wealth of information about the central engine, encoded in the properties of the emission lines, absorption lines, and continuum emission. Repeat optical spectroscopy can further probe the time-variable nature of these emission components, but this has previously been little explored in comparison to single-epoch spectroscopy. One notable exception in repeat AGN spectroscopy is the well-established reverberation mapping technique of mapping the size of AGN broad line regions; this has lead to the development of black hole mass estimates based on broad Balmer emission line widths in single-epoch spectroscopy. However, these and other studies based on repeat AGN spectroscopy are only available for small samples of a few dozen AGN at low redshifts, due to the expensive nature of repeat spectroscopy for large samples of faint quasars at higher redshifts. The development of multi-object spectrographs now have the ability to do repeat spectroscopy for large numbers of quasars, opening new windows into AGN astrophysics in the time-domain. Surveys dedicated to repeat quasar spectroscopy, including currently in SDSS-IV and in the future in SDSS-V, will fuel the early science results from this dissertation. In this dissertation, I first use SDSS repeat spectroscopy of quasars to quantify the bluer-when-brighter trend of wavelength-dependent quasar spectral variability, and use it to con- strain simple models of quasar variability. In particular, I test whether the observed spectral variability is consistent with recent toy models of inhomogeneous accretion disks with large temperature fluctuations. These models provide a natural explanation for quasar UV/optical variability, and the first to be consistent with measurements of quasar accretion disk sizes and characteristic timescales of variability. I show that the observed spectral variability can be reproduced by strongly inhomogeneous disks with large temperature fluctuations. I then use SDSS repeat spectroscopy to investigate the origin of the ‘transition blazars’ phenomenon, which is observed in a handful of AGN with relativistic jets aligned with the line of sight. In transition blazars, the blazars appear to switch between BL Lac objects and Flat-Spectrum Radio Quasars (FSRQs) classifications, which correspond to low- and high- accretion rate states, respectively. I show that transition blazars are FSRQs with especially strong beaming, such that the strongly-beamed continuum swamps the broad emission lines. This occasionally causes the broad emission lines to disappear and reappear, producing the transitional behavior. Furthermore, I mine SDSS repeat spectroscopy to uncover the origin of the recently-discovered ‘changing-look quasars’ phenomenon. Repeat optical spectroscopy of this new class of objects show dramatic transitions from luminous broad line quasars into quiescent galaxies or low-luminosity AGN. Surprisingly, these changes occur over timescales of just a few years, a factor of >10^4 faster than both theoretical expectations and scaling spectral state transition timescales in X-ray binaries to 10^8 M_sun supermassive black holes (SMBHs). To understand this phenomenon, I perform the first large systematic search for CL quasars and I show that the fading of the continuum and broad emission lines in CL quasars is consistent with rapidly decreasing accretion rates, while disfavoring alternative interpretations including changes in intrinsic dust extinction and transient tidal disruption events or supernovae. Finally, future time-domain imaging surveys such as the ZTF and LSST will discover a few x10^7 variable objects, and AGN will constitute the majority of variable objects discovered. To understand the science enabled by these massive variability-selected samples of AGN, I utilized spectra from the Time-Domain Spectroscopic Survey (TDSS) to understand the unique properties of variability-selected quasars. TDSS is the first systematic spectroscopic survey of variable objects, and I show that variability-selected quasars complement color-based selection by selecting additional redder quasars, resulting in a smooth redshift distribution. Furthermore, I show that variability selection yields higher fractions of peculiar AGN such as broad absorption line quasars and blazars.

Download or read book Physical Conditions in Low luminosity Active Galactic Nuclei written by Alexei V. Filippenko and published by . This book was released on 1984 with total page 596 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Activity and Feeding of Nearby Low Luminosity Active Galactic Nuclei written by and published by . This book was released on 2014 with total page 131 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book The Physics and Evolution of Active Galactic Nuclei written by Hagai Netzer and published by Cambridge University Press. This book was released on 2013-09-16 with total page 369 pages. Available in PDF, EPUB and Kindle. Book excerpt: A comprehensive introduction to the theory underpinning our study of active galactic nuclei and the ways we observe them.

Download or read book Physical Conditions in Low luminosity Active Galactic Nuclei written by Alexei Vladim Filippenko and published by . This book was released on 1987 with total page 298 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book The Nearest Active Galaxies written by J. E. Beckman and published by Editorial CSIC - CSIC Press. This book was released on 1993 with total page 320 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Active Galactic Nuclei written by Volker Beckmann and published by John Wiley & Sons. This book was released on 2013-08-29 with total page 390 pages. Available in PDF, EPUB and Kindle. Book excerpt: Active Galactic Nuclei This AGN textbook gives an overview on the current knowledge of the Active Galacitc Nuclei phenomenon. The spectral energy distribution will be discussed, pointing out what can be observed in different wavebands. The different physical models are presented together with formula important for the understanding of AGN physics. Furthermore, the authors discuss the AGN with respect to its environment, host galaxy, feedback in galaxies and in clusters of galaxies, variability, etc. and finally the cosmological evolution of the AGN phenomenon. This book includes phenomena based on new results in the X-Ray and gamma-ray domain from new telescopes such as Chandra, XMM-Newton, the Fermi Gamma-Ray Space Telescope, and the VHE regime not mentioned so far in AGN books. Those and other new developments as well as simulations of AGN merging events and formations, enabled through latest super-computing capabilities. From the contents: The observational picture of AGN Radiative processes The central engine AGN types and unification AGN through the electromagnetic spectrum AGN variability Environment Quasars and cosmology Formation, evolution and the ultimate fate of AGN What we do not know (yet)

Download or read book Emission Mechanisms in Low luminosity Active Galactic Nuclei written by James Michael Anderson and published by . This book was released on 2007 with total page 1094 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Self consistent Dynamical and Radiative Models of Low luminosity Active Galactic Nuclei written by and published by . This book was released on 2012 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Investigating the Nature of High Energy Emission in Active Galactic Nuclei written by Michael Shea Dutka and published by . This book was released on 2014 with total page 119 pages. Available in PDF, EPUB and Kindle. Book excerpt: Active Galactic Nuclei (AGN) are extremely luminous objects that reside at the center of some galaxies. Radio loud AGN possess jets of plasma emanating from the center of their host galaxy. These jets are very bright at radio frequencies. Blazars are the most extreme subset of radio loud AGN, they are violently variable across the electromagnetic spectrum, highly polarized and their radio jets often show superluminal motion. The most likely explanation for the unusual properties of blazars is that their jets are pointed close to our line of sight. We study the high-energy emission mechanisms of blazars because gamma-rays account for a large fraction of the total energy emitted by blazars. Therefore, understanding the mechanisms of gamma-ray production in blazars is crucial to an overall understanding of how these objects work. In this dissertation we attempt to identify the basic mechanisms of gamma-ray emission in individual blazars by simultaneously monitoring their flux densities across the electromagnetic spectrum (radio though gamma-rays). We staged multi-wavelength observing campaigns using ground based and space based observatories for four blazars during active and quiescent gamma-ray states. Data from these campaigns are compiled into broadband spectral energy distributions (SEDs) ranging from radio to gamma-ray energies. The broadband SEDs are then fit with a one zone leptonic model of blazar emission in order to establish the most likely gamma-ray emission mechanism. One non-blazar radio loud AGN, 3C84, at the core of the galaxy NGC 1275 is studied in depth using the VLBA. Blazars and other radio loud AGN only differ by the viewing angle thus the same physical processes are common in both types of objects. The AGN 3C84 is interesting because it is nearby (z=0.017559), it is the brightest member of the Perseus cluster, and it is one of the few non-aligned gamma-ray emitting AGN. We obtained six epochs of data, at 22 and 43 gHz, with the VLBA and we constructed a map of the spectral index for every epoch. We find a pattern in the flares that we have modeled. Some flares can be fit by adjusting only the electron energy distribution while others require changes in other parameters in the model, such as the magnetic field or the size of the emitting region. We find two flat spectrum components in the parsec scale structure of 3C84. We do not see any obvious correlation between the parsec scale radio activity in the jet of 3C84 and its gamma-ray emission levels.

Download or read book Cosmic Rays at Earth written by P.K.F. Grieder and published by Elsevier. This book was released on 2001-07-27 with total page 1117 pages. Available in PDF, EPUB and Kindle. Book excerpt: In 1912 Victor Franz Hess made the revolutionary discovery that ionizing radiation is incident upon the Earth from outer space. He showed with ground-based and balloon-borne detectors that the intensity of the radiation did not change significantly between day and night. Consequently, the sun could not be regarded as the sources of this radiation and the question of its origin remained unanswered. Today, almost one hundred years later the question of the origin of the cosmic radiation still remains a mystery.Hess' discovery has given an enormous impetus to large areas of science, in particular to physics, and has played a major role in the formation of our current understanding of universal evolution. For example, the development of new fields of research such as elementary particle physics, modern astrophysics and cosmology are direct consequences of this discovery. Over the years the field of cosmic ray research has evolved in various directions: Firstly, the field of particle physics that was initiated by the discovery of many so-called elementary particles in the cosmic radiation. There is a strong trend from the accelerator physics community to reenter the field of cosmic ray physics, now under the name of astroparticle physics. Secondly, an important branch of cosmic ray physics that has rapidly evolved in conjunction with space exploration concerns the low energy portion of the cosmic ray spectrum. Thirdly, the branch of research that is concerned with the origin, acceleration and propagation of the cosmic radiation represents a great challenge for astrophysics, astronomy and cosmology. Presently very popular fields of research have rapidly evolved, such as high-energy gamma ray and neutrino astronomy. In addition, high-energy neutrino astronomy may soon initiate as a likely spin-off neutrino tomography of the Earth and thus open a unique new branch of geophysical research of the interior of the Earth. Finally, of considerable interest are the biological and medical aspects of the cosmic radiation because of it ionizing character and the inevitable irradiation to which we are exposed. This book is a reference manual for researchers and students of cosmic ray physics and associated fields and phenomena. It is not intended to be a tutorial. However, the book contains an adequate amount of background materials that its content should be useful to a broad community of scientists and professionals. The present book contains chiefly a data collection in compact form that covers the cosmic radiation in the vicinity of the Earth, in the Earth's atmosphere, at sea level and underground. Included are predominantly experimental but also theoretical data. In addition the book contains related data, definitions and important relations. The aim of this book is to offer the reader in a single volume a readily available comprehensive set of data that will save him the need of frequent time consuming literature searches.

Download or read book Active Galactic Nuclei written by Professor R. D. Blandford and published by Springer. This book was released on 2013-12-14 with total page 289 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Nature written by Sir Norman Lockyer and published by . This book was released on 1869 with total page 1544 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Low Ionization Nuclear Emission Line Regions written by Rachel Dudik (George Mason University graduate) and published by . This book was released on 2007 with total page 217 pages. Available in PDF, EPUB and Kindle. Book excerpt: "The horizon of the universe, once thought to extend only to the disk of the Milky Way, is now known to embrace a host of diverse galaxies, from active galaxies such as quasars and Seyfert galaxies to normal galaxies such as our own. The recent discovery that virtually all local galaxies harbor massive nuclear black holes, has provided convincing evidence that active galactic nuclei (AGNs) and normal galaxies are indeed connected. The nature of this connection and the evolutionary history connecting them, however, continues to be elusive. Low Ionization Nuclear Emission-line Regions (LINERs) are the dominant population of 'active' galaxies in our local universe and may indeed be the missing piece to the evolutionary puzzle. LINERs are dened by optical line ratios uncharacteristic of photoionization by normal main sequence stars. While classical AGNs represent at most a few percent of the galaxy population, LINERs constitute as much as 50% of the total local extragalactic population. However, despite several decades of intense research, the ionization mechanism responsible for the unusual LINER spectrum remains a mystery. What is the ionization mechanism responsible for the empirical line ratios characteristic of LINER galaxies? How do LINERs t into the overall evolution of galaxies as we know it? Are LINERs a subclass of AGN? What is the evolutionary connection, if any, between galaxies with heavy starburst activity and AGNs? The majority of LINERs are dust enshrouded and therefore very luminous in the far-infrared. The far-infrared (far-IR) luminosity to the luminosity in the optical Bband (center = 4400A), the so-called IR-brightness ratio, can be used as a gauge of the amount of dust in host galaxy. LINERs span a wide range of LFIR/LB ratios, tending predominantly toward the IR-bright end. However, the majority of research to-date has been based on optically selected samples which are partial toward IR-faint LINERs. This bias toward IR-faint galaxies could have important consequences on statistical analyses which examine the fraction of LINERs hosting AGNs. In order for an accurate picture of LINERs to emerge, IR-bright as well as IR-faint galaxies must be studied. What fraction of IR-bright LINERs are AGNs? In light of the open questions regarding these remarkable objects, the central goal of this dissertation is to carry out a systematic multi-wavelength X-ray imaging and Infrared spectroscopic survey of nearby LINERs spanning a wide range of IRbrightness ratios in order to 1) characterize the dominant energy source responsible for their optical line ratios, 2) compare the AGN detection rate in our infrared selected sample with the optically selected samples, 3) determine the luminosities, spectral characteristics and accretion properties of the AGN-LINERs and compare them with the standard active galaxies, and nally, 4) relate the host galaxies properties to the properties of the central source in an attempt to constrain the role of LINERs in galaxy evolution and formation models"--Abstract.