Dissertations / Theses on the topic 'BL Lac objects'

The investigation of the host galaxies of BL Lac objects (BL Lacs) is a promising new field made practical in recent years by advances in detector technology and improved telescope siting. By better understanding the nature of these host galaxies we can test the standard beaming and lensing models for BL Lacs. This thesis describes the techniques that we have developed for studying the host galaxies of BL Lac objects, and presents the results of a survey of BL Lac host galaxies that we have undertaken with the 4.2m William Herschel Telescope. This survey successfully resolved many new host galaxies, determined the morphology of three BL Lac host galaxies for the first time, and confirmed the morphology of an additional three objects. One BL Lac object, PKS1413+135, displayed a number of surprising properties, and was consequently studied in greater detail at multiple wavelengths. These observations are also presented in this thesis. We conclude with a description of Monte-Carlo simulations that we have undertaken in order to better determine the uncertainty in the results from our survey, and to assess the promise of future telescope/instrumentation combinations for host galaxy imaging.

STACEE is an atmospheric Cherenkov telescope that was built to detect gamma-rays of cosmic origin. By using a large primary mirror area we have reduced the detector's energy threshold to below 200 GeV. The present work continues STACEE's investigation of BL Lac objects, focusing on Markarian 421, 3C 66A and OJ 287. An important part of this work has been the development of software techniques for the correction of systematic biases and the improvement of background rejection. As a result of these improvements, STACEE made a convincing detection of Markarian 421 during the 2002-2004 seasons. The statistical excess was 3.9sigma for the 2002-2003 season and 10.9sigma for the 2003-2004 season; the latter represents the strongest single season detection ever achieved by STACEE. We compare our measured Mrk 421 rates to those of other instruments and present spectral measurements during various flaring states in 2003-2004. The LBLs 3C 66A and OJ 287 were not detected. We discuss the significance of our flux upper limits in the context of emission models for these two sources.

This thesis is an analysis of the X-ray, optical, infrared and radio properties of a sample of candidate BL Lac objects, identified from two catalogues of colour-selected point sources, the 2QZ and 6QZ. The importance of the sample lies in the fact that it is the first BL Lac sample in which initial selection has been made from optical spectra, independent of the objects' flux levels at X-ray and radio frequencies. These optically selected candidate BL Lac objects thus provide an unbiased sample (in terms of radio flux density) with which to study the proportion of radio- dominant and X-ray-dominant BL Lac objects in the global population. The observed number counts and redshift distribution of the 2BL are consistent with theoretical predictions based on the QSO evolutionary model. Given the small number statistics we are not, however, able to show that this distribution is significantly different to that of the 2QZ/6QZ white dwarfs. A median redshift of z=1.25 was computed for these candidate BL Lacs, a value supported by redshift information obtained for ~25% of the sample. This median redshift is much higher than redshifts found for X-ray selected BL Lac objects and suggests that in the past high redshift, low luminosity, radio-weak BL Lacs may have been missed in radio- and X-ray-selected samples. From results of the above studies it appears that this sample of optically selected candidate BL Lac objects is different to that of hitherto known radio-loud BL Lacs. We explore mechanisms that might produce such radio-weak/radio-quiet continuum objects, and also demonstrate that such radio-quiet BL Lac objects could exist at low redshift. Failure to include these radio-quiet BL Lacs in X-ray selected samples could explain the negative evolution found for this class of object.

This thesis is an analysis of the X-ray, optical, infrared and radio properties of a sample of candidate BL Lac objects, identified from two catalogues of colour-selected point sources, the 2QZ and 6QZ. The importance of the sample lies in the fact that it is the first BL Lac sample in which initial selection has been made from optical spectra, independent of the objects' flux levels at X-ray and radio frequencies. These optically selected candidate BL Lac objects thus provide an unbiased sample (in terms of radio flux density) with which to study the proportion of radio- dominant and X-ray-dominant BL Lac objects in the global population. The observed number counts and redshift distribution of the 2BL are consistent with theoretical predictions based on the QSO evolutionary model. Given the small number statistics we are not, however, able to show that this distribution is significantly different to that of the 2QZ/6QZ white dwarfs. A median redshift of z=1.25 was computed for these candidate BL Lacs, a value supported by redshift information obtained for ~25% of the sample. This median redshift is much higher than redshifts found for X-ray selected BL Lac objects and suggests that in the past high redshift, low luminosity, radio-weak BL Lacs may have been missed in radio- and X-ray-selected samples. From results of the above studies it appears that this sample of optically selected candidate BL Lac objects is different to that of hitherto known radio-loud BL Lacs. We explore mechanisms that might produce such radio-weak/radio-quiet continuum objects, and also demonstrate that such radio-quiet BL Lac objects could exist at low redshift. Failure to include these radio-quiet BL Lacs in X-ray selected samples could explain the negative evolution found for this class of object.

We report on long-term multiwavelength monitoring of blazar Mrk 421 by the GLAST-AGILE Support Program of the Whole Earth Blazar Telescope (GASP-WEBT) collaboration and Steward Observatory, and by the Swift and Fermi satellites. We study the source behaviour in the period 2007-2015, characterized by several extreme flares. The ratio between the optical, X-ray and gamma-ray fluxes is very variable. The gamma-ray flux variations show a fair correlation with the optical ones starting from 2012. We analyse spectropolarimetric data and find wavelength-dependence of the polarization degree (P), which is compatible with the presence of the host galaxy, and no wavelength dependence of the electric vector polarization angle (EVPA). Optical polarimetry shows a lack of simple correlation between P and flux and wide rotations of the EVPA. We build broad-band spectral energy distributions with simultaneous near-infrared and optical data from the GASP-WEBT and ultraviolet and X-ray data from the Swift satellite. They show strong variability in both flux and X-ray spectral shape and suggest a shift of the synchrotron peak up to a factor of similar to 50 in frequency. The interpretation of the flux and spectral variability is compatible with jet models including at least two emitting regions that can change their orientation with respect to the line of sight.

This thesis is concerned with multi-wavelength observations of the prominent southern hemisphere high energy BL Lac object, (HBL), PKS 2155-304. After a general introduction covering the particulars of Active Galactic Nuclei, the pysical processes and models that are believed to play an important role in the observations and ground based γ-ray astronomy, the results and interpretation of four individual multi-wavelength campaigns on PKS 2155-304 are presented. Lead by observations of the High Energy Stereoscopic System (H.E.S.S.), these campaigns include observations with RXTE, NTT at ESO, ROTSE, Spitzer, JCMT and the NRT. During all these campaigns, the source appeared to be in a quiescent state, allowing us to probe the jet paramaters and emission models for a quiescent HBL for the first time. The models for VHE emission from HBLs have evolved over the last 14 years since Mkn 421 was detected as a VHE emitter. However, the sensitivity of the past generations of VHE telescopes has only been sufficient to observe VHE emission from HBLs during apparent active states. These evolved models are therefore essentially high active state emission models. Since the sensitivity of H.E.S.S. now allows us to detect these objects in a quiescent state , the obvious question is, do the 'active state' models still accurately predict the SED and in particular the VHE emission?

The Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) was an atmospheric Cherenkov telescope operational until June 2007, which detected cosmic gamma rays by means of the wavefront-sampling technique. Using 64 of the large heliostats available at the National Solar Thermal Test Facility (NSTTF) near Albuquerque, New Mexico, its total mirror area of 2378 m^2 allowed it to achieve energy thresholds between 150 and 200 GeV. Following a review of the field of gamma-ray astrophysics and Active Galactic Nuclei, this work provides a detailed description of the detector, along with an introduction to simulation and data analysis techniques. During the springs of 2003 and 2004, STACEE observed the High-Energy-Peaked BL Lac object H1426+428, an established emitter of gamma rays in the TeV energy range. A full analysis of these data reveals that STACEE did not detect a statistically significant gamma-ray excess from H1426+428 in either data set. With the help of detailed detector simulations, upper limits on the integrated gamma-ray fluxes, F, at the 95% confidence level are obtained for both observing seasons: F(E > 163 GeV) < 1.83e-6 m^-2 s^-1 and F(E > 165 GeV) < 0.78e-6 m^-2 s^-1 for 2003 and 2004, respectively. The derived upper limits are discussed in the context of the results obtained by other gamma-ray observatories.
Le telescope a effet Cherenkov atmospherique STACEE (Solar Tower Atmospheric Cherenkov Effect Experiment), en operation jusqu'en juin 2007, servait a detecter des rayons gamma d'origine cosmique en utilisant la technique d'echantillonnage du front d'onde. L'utilisation de 64 heliostats de la centrale solaire NSTTF (National Solar Thermal Test Facility) situee pres d'Albuquerque au Nouveau Mexique permettait au telescope d'atteindre des seuils d'energie entre 150 et 200 GeV. Cette these commence par une revue dans le domaine de l'astrophysique des rayons gamma et des noyaux actifs galactiques, puis presente une description detaillee du detecteur STACEE, de meme qu'une introduction aux techniques de simulation et d'analyse de donnees. Entre mars et juin 2003 ainsi qu'entre mars et juillet 2004, STACEE a observe l'objet HBL H1426+428, un emetteur avere de rayons gamma TeV. L'analyse complete des donnees recueillies revele que STACEE n'a permis de detecter un exces de rayons gamma statistiquement significatif provenant de H1426+428 dans aucun des deux ensembles de donnees. Des simulations detaillees de l'effet du detecteur ont permis d'obtenir des limites superieures du flux integre de rayons gamma, F, a un niveau de confiance de 95% pour les deux saisons d'observation: F(E > 163 GeV) < 1.83e-6 m^-2 s^-1 pour les observations effectuees en 2003, et F(E > 165 GeV) < 0.78e-6 m^-2 s^-1 pour celles de 2004. Les limites superieures ainsi obtenues sont comparees aux mesures de flux enregistrees par d'autres detecteurs, et des implications possibles sont discutees.

Les blazars sont connus pour leur variabilité sur une large gamme d'échelles de temps à toutes les longueurs d'onde; et leur classification (en quasars radio à spectre plat, BL Lac à basse fréquence crête, intermédiaire ou haute fréquence; FSRQ, LBL, IBL, HBL, pour ses sigles en anglais) est basée sur des caractéristiques spectrales à large bande qui ne considèrent pas la source comme étant, éventuellement, dans différentes états d'activité. Récemment, il a été proposé de classer les blazars en fonction de la cinématique de leurs caractéristiques radio. La plupart des études sur les blazars à rayons gamma TeV se concentrent sur des échelles de temps courtes, en particulier pendant les éruptions, en raison de la rareté des campagnes d'observation ou de l'existence relativement récente de détecteurs spécialisés suffisamment sensibles.Avec une décennie d'observations du Fermi-LAT, VERITAS, Je présente une étude approfondie de la variabilité à long terme multi longueurs d'onde du blazar 1ES 1215+303, des rayons gamma à la radio. Cet ensemble de données sans précédent révèle de multiples éruptions de rayons gamma fortes et une augmentation à long terme de la ligne de base des rayons gamma et du flux optique de la source sur une période de dix ans, ce qui se traduit par une corrélation linéaire entre ces deux bandes d'énergie sur une décennie. Des comportements HBL typiques sont identifiés dans la morphologie radio de la source. Cependant, des analyses de la distribution d'énergie spectrale à large bande à différents états de flux de la source, révèlent un déplacement extrême de l'énergie de la fréquence de crête du synchrotron de l'IR aux rayons X mous; indiquant que la source présente les caractéristiques IBL pendant les états de repos et le comportement HBL pendant les états éruptifs. Un modèle synchrotron self-Compton à deux composantes est utilisé pour décrire ce changement spectaculaire.Un cadre détaillé de l'analyse des données de l'instrument Fermi-LAT est fourni et pourrait servir de guide aux chercheurs intéressés par ce domaine. Je présente les efforts approfondis de validation des méthodes utilisées et les contrôles d'intégrité des résultats effectués. Une description des analyses de niveau supérieur est fournie, comme la sélection des éruptions et la recherche d'un comportement plus dur quand plus lumineux dans les données de Fermi-LAT, l'analyse de corrélation croisée et de variabilité à plusieurs longueurs d'onde; la recherche de tendances, log-normalité et variabilité, la caractérisation des éruptions et des distributions spectrales d'énergie, et la recherche d'observations Fermi-LAT - VERITAS simultanées. Ce sont le cœur de ce travail de doctorat.Les différentes méthodes appliquées et présentées dans ce travail fournissent un panorama complet et détaillé de la nature complexe de ce blazar et peuvent même remettre en question notre système de classification actuel. De plus, ce travail fournit une illustration du type d'analyses à long terme que les futurs instruments d'imagerie atmosphérique, tels que le Cherenkov Telescope Array, non seulement permettront mais pourrait même d'améliorer
Blazars are known for their variability on a wide range of timescales at all wavelengths; and their classification (into flat spectrum radio quasars, low-, intermediate- or high-frequency-peaked BL Lac; FSRQ, LBL, IBL, HBL) is based on broadband spectral characteristics that do not consider the source being at, possibly, different states of activity. Recently, it was proposed that blazars could be classified according to the kinematics of their radio features. Most studies of TeV gamma-ray blazars focus on short timescales, especially during flares, due to the scarcity of observational campaigns or due to the relatively young existence of specialized, sensitive enough detectors.With a decade of observations from the Fermi-LAT and VERITAS, I present an extensive study of the long-term multi-wavelength variability of the blazar 1ES 1215+303 from gamma-rays to radio. This unprecedented data set reveals multiple strong gamma-ray flares and a long-term increase in the gamma-ray and optical flux baseline of the source over the ten-year period, which results in a linear correlation between these two energy bands over a decade. Typical HBL behaviors are identified in the radio morphology of the source. However, analyses of the broadband spectral energy distribution at different flux states of the source, unveil an extreme shift in energy of the synchrotron peak frequency from IR to soft X-rays; indicating that the source exhibits IBL characteristics during quiescent states and HBL behavior during high states. A two-component synchrotron self-Compton model is used to describe this dramatic change.A detailed framework of the analysis of the data from the Fermi-LAT instrument is provided, and could serve as a guideline for researchers interested in this field. I present the thorough efforts that were employed in validating the methods used and the sanity checks that were performed on the results obtained. A description of the higher-level analyses are provided, including the flare-selection algorithms, the search for harder-when-brighter behavior in the Fermi-LAT data, the multi-wavelength cross-correlation and variability analysis, the search for trends, log-normality and variability, the characterization of flares and of the spectral energy distributions, and the search for simultaneousFermi-LAT - VERITAS observations. These are the heart of this PhD work.The different methods applied and presented in this work provide a complete and detailed panorama of the intricate nature of this blazar, and possibly even challenge our current classification scheme. Moreover, this work provides an illustration of the type of long-term analyses that future imaging atmospheric instruments, such as the Cherenkov Telescope Array, will not only allow but potentially improve

: Ce manuscrit traite d'astronomie y de haute énergie (entre ~30 GeV et ~300 GeV) par l'intermédiaire du satellite Fermi-LAT et d'astronomie γ de très haute énergie (au-delà de GeV) via l'expérience H. E. S. S. La seconde phase de l'expérience H. E. S. S. A débuté en juillet 2012 avec l'ajout d'un cinquième télescope de 28 m de diamètre au réseau initialement constitué de quatre télescopes de 12m de diamètre d'imagerie atmosphérique Tcherenkov. Dans la première partie de cette thèse, nous présentons le-développement d'une analyse en mode hybride basée sur une méthode multivariée dédiée à la détection et à l'étude de sources ayant des caractéristiques spectrales différentes ainsi que les premiers résultats des analyses de données réelles. La seconde partie est dédiée à la recherche des candidats blazars parmi les sources non identifiées du Fermi-LAT du catalogue 2FGL. Un premier travail est consacré à l'identification des candidats blazars à l'aide de méthodes multivariées utilisant des variables discriminantes construites à partir des paramètres du catalogue 2FGL. Dans un second temps, à l'aide du catalogue de sources infrarouges obtenues par le satellite WISE et d'une méthode non-paramétrique nous recherchons les contreparties de type blazars des sources non-identifiées
This manuscript is about high energy -γ-ray astronomy (between GeV and GeV) with the Fermi-LAT satellite and very high energy astronomy above ~GeV) via the H. E. S. S. Experiment. The second phase of the H. E. S. S. Experiment began in July 2012 with the inauguration of a fifth 28 m diameter telescope added to the intial array composed of four 12m diameter imaging atmospheric Cherenkov telescopes. In the first part of this thesis, we present the development of an analysis in hybrid mode based on a multivariate method dedicated to detect and study sources with different spectral shapes and the first analysis results on real data. The second part is dedicated to the research of blazar candidates among the Fermi-LAT unidentified sources of the 2FGL catalog. A first development is based on a multivariate approach using discriminant parameters built with the 2FGL catalog parameters. A second development is done with the use of the WISE satellite catalog and a non-parametric technic in order to find the blazar-like infrared counterparts of the unidentified sources of the 2FGL catalog

碩士
國立中央大學
天文研究所
85
We observed BL Lac Objects PKS 0521-365 and PKS 2155-304 in Australia and tried to find out the characteristics of luminosity variability. However, it is hard to tell whether there were prominent changes of light curves during the observations because of the low signal-to-noise ratio. So it is still debatable that our data shows the changes in the longer wavelengths lead the shorter ones. The only thing we believed is that the amplitudes of variability of PKS 0521-365 in u band were greater than the ones in v band. We used the model of the synchrotron emission jet to simulate the light curve and the spectrum of BL Lac Objects, and successfully explained most of the characteristics what the other astronomers observed. So the jet model is supposed to be the best interpretation of the radiation mechanism of BL Lac Objects until now.