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BL Lac Objects and Blazars: Past, Present, and Future  [PDF]
C. Megan Urry
Physics , 1998,
Abstract: The past 20 years have seen phenomenal progress in our understanding of BL Lac objects. They form part of the blazar class, which are radio-loud AGN whose relativistic jets are aligned along our line of sight. Several critical milestones have helped establish this picture, first proposed at the Pittsburgh BL Lac meeting 20 years ago, most recently the EGRET and TeV detections of beamed gamma-ray emission. The spectral energy distributions are double peaked and follow a self-similar sequence in luminosity, which can be explained by electron cooling on ambient photons. This simple paradigm has yet to be tested, and further questions remain, notably about physical conditions in blazar jets --- the kinetic power, magnetic energy density, acceleration time scales, proton content, etc. --- and how this energy is transported in the innermost regions. Some clues are available from multiwavelength monitoring campaigns although better sampling over longer periods is clearly called for. Recent work on the host galaxies of BL Lac objects supports their unification with low-power, aligned FRI radio galaxies. Nature makes jets with a large range of kinetic powers, but the relative number densities of low-luminosity ("blue" BL Lacs) or high-luminosity ("red" BL Lacs and FSRQ) blazars --- is highly uncertain. Since according to unified schemes blazars are representative of all radio-loud AGN, their jet properties have broad implications. Future EUV/X-ray observations will illuminate the circumnuclear structure, especially the hot, highly ionized, high velocity gas on sub-parsec scales, which could play a role in jet dynamics and could possibly affect the formation of FRI vs. FRII type jets. The study of blazars may also help us eventually understand the difference between radio-quiet and radio-loud AGN. [slightly abridged]
The colours of BL Lac objects: a new approach to their classification  [PDF]
Enrico Massaro,Roberto Nesci,Silvia Piranomonte
Physics , 2012, DOI: 10.1111/j.1365-2966.2012.20782.x
Abstract: We selected a sample of 437 BL Lac objects, taken from the RomaBZCat catalogue, for which spectroscopic information and SDSS photometry is available. We propose a new classification of BL Lacs in which the sources' type is not defined only on the basis of the peak frequency of the synchrotron component in their Spectral Energy Distribution (types L and H), but also on the relevance of this component with respect to the brightness of the host galaxy (types N and G, for nuclear or galaxy dominated sources). We found that the SDSS colour index u-r=1.4 is a good separator between these two types. We used multiband colour-colour plots to study the properties of the BL Lac classes and found that in the X-ray to radio flux ratio vs u-r plot most of the N (blue) sources are located in a rather narrow strip, while the G-sources (red) are spread in a large area, and most of them are located in galaxy clusters or interacting systems, suggesting that their X-ray emission is not from a genuine BL Lac nucleus but it is related to their environment. Of the about 135 sources detected in the gamma-rays by Fermi-GST, nearly all belong to the N-type, indicating that only this type of sources should be considered as genuine BL Lac nuclei. The J-H, H-K plot of sources detected in the 2MASS catalogue is consistent with that of the "bona fide" BL Lac objects, independently of their N or G classification from the optical indices, indicating the existence in G-type sources of a K-band excess possibly due to a steep, low frequency peaked emission which deserves further investigations. We propose to use these colour plots as a further tool for searching candidate counterparts of newly discovered high-energy sources.
Synchrotron and inverse-Compton emission from blazar jets - IV. BL Lac type blazars and the physical basis for the blazar sequence  [PDF]
William J. Potter,Garret Cotter
Physics , 2013, DOI: 10.1093/mnras/stt1569
Abstract: In this paper we investigate the properties of a sample of six BL Lacs by fitting their spectra using our inhomogeneous jet model with an accelerating, magnetically dominated, parabolic base, which transitions to a slowly decelerating conical jet with a geometry based on observations of M87. Our model is able to fit very well to the simultaneous multiwavelength spectra of all the BL Lacs including radio observations. We find that the BL Lacs have lower jet powers and bulk Lorentz factors than the sample of Compton-dominant blazars investigated in Paper III, consistent with the blazar sequence. Excitingly, we find a correlation between the radius at which the jet first comes into equipartition and the jet power, in agreement with our prediction from Paper II. We interpret this result as one of two physical scenarios: a universal jet geometry which scales linearly with black hole mass or a dichotomy in Eddington accretion rates between flat spectrum radio quasars (FSRQs) and BL Lacs. If we assume that the jet geometry of all blazars scales linearly with black hole mass then we find a plausible range of masses (10^7-10^10 solar masses). We find that the quiescent gamma-ray spectrum of Markarian 421 is best fitted by scattering of external CMB photons. We are unable to fit the spectrum using synchrotron self-Compton emission due to strong gamma-ray absorption via pair production even using a compact, rapidly decelerating, jet with a very large bulk Lorentz factor (50), as has been suggested recently. Finally, we fit to the SEDs of the four high power HSP BL Lacs recently found by Padovani et al. 2012. We find that their high peak frequency emission is caused by high maximum electron energies whilst the rest of their jet properties are typical of relatively high power BL Lacs and consistent with our predictions.
The Optical Polarization Properties of X-ray Selected BL Lacertae Objects  [PDF]
B. T. Jannuzi,P. S. Smith,R. Elston
Physics , 1993, DOI: 10.1086/191765
Abstract: We discuss the optical polarization properties of X-ray selected BL Lacertae objects (XSBLs) as determined from three years of monitoring 37 BL Lac objects and candidates. The observed objects include a complete X-ray flux limited sample drawn from the EMS Survey. The majority of the XSBLs classi- fied solely on the appearance of their spectra are members of the class of BL~Lacs since they possess intrinsically polarized and variable continua. The duty cycle of polarized emission from XSBLs is 40\%. The majority of XSBLs ($\approx 85$\%) have preferred polarization position angles on time scales as long as three years. This reflects stability in the geometry of the region emitting the polarized optical emission. We discuss the observed spectral dependence of the degree of polarization and some of the possible mechanisms for producing the observed characteristics. While dilution of the polarized emission by the host galaxy starlight is present in some objects, we demon- strate that the average polarization properties derived from our observations are not drastically affected by this effect. The objects in our monitored sample did not display the larger than one magnitude variations generally used to characterize the optical variability of blazars in general.
Four New BL Lac Surveys: Sampling New Populations  [PDF]
S. A. Laurent-Muehleisen,R. H. Becker,W. Brinkmann,J. Siebert,E. D. Feigelson,R. I. Kollgaard,G. D. Schmidt,P. S. Smith
Physics , 1998,
Abstract: The advent of large area deep radio and X-ray surveys is leading to the creation of many new BL Lac samples. In particular, the ROSAT All-Sky, Green Bank and FIRST surveys are proving to be rich sources of new BL Lacs. We will discuss the methods used in four independent BL Lac searches based on these surveys. Comparison of the broadband spectral energy distributions of these BL Lacs with those of previously known objects clearly points to the existence of a large previously unrecognized population of objects with characteristics intermediate between those exhibited by Low and High energy peaked BL Lacs.
A Search for Low-Luminosity BL Lacertae Objects  [PDF]
Travis A. Rector,John T. Stocke,Eric S. Perlman
Physics , 1999, DOI: 10.1086/307076
Abstract: Many properties of BL Lacs have become explicable in terms of the "relativistic beaming" hypothesis whereby BL Lacs are FR-1 radio galaxies viewed nearly along the jet axis. However, a possible problem with this model is that a transition population between beamed BL Lacs and unbeamed FR-1s has not been detected. A transition population of "low-luminosity BL Lacs" was predicted to exist in abundance in X-ray-selected samples such as the Einstein Extended Medium Sensitivity Survey (EMSS) by Browne & Marcha. However, these BL Lacs may have been misidentified as clusters of galaxies. We have conducted a search for such objects in the EMSS with the ROSAT HRI; and here we present ROSAT HRI images, optical spectra and VLA radio maps for a small number of BL Lacs which were previously misidentified in the EMSS catalog as clusters of galaxies. While these objects are slightly lower in luminosity than other EMSS BL Lacs, their properties are too similar to the other BL Lacs in the EMSS sample to "bridge the gap" between BL Lacs and FR-1 radio galaxies. Also, the number of new BL Lacs found are too few to alter significantly the X-ray luminosity function or value for the X-ray-selected EMSS BL Lac sample. Thus, these observations do not explain fully the discrepancy between the X-ray and radio-selected BL Lac samples.
Does black hole spin play a key role in the FSRQ/BL Lac dichotomy?  [PDF]
Banibrata Mukhopadhyay
Physics , 2013,
Abstract: It is believed that jets emerging from blazars (Flat Spectrum Radio Quasars (FSRQs) and BL Lacs) are almost aligned to the line-of-sight. BL Lacs usually exhibit lower luminosity and harder power law spectra at gamma-ray energies than FSRQs. It was argued previously that the difference in accretion rates is mainly responsible for the large observed luminosity mismatch between them. However, when intrinsic luminosities are derived by correcting for beaming effects, this mismatch is significantly reduced. We show that spin plays an important role to reveal the dichotomy of luminosity distributions between BL Lacs and FSRQs, suggesting BL Lacs to be low luminous and slow rotators compared to FSRQs.
TeV BL Lac objects at the dawn of the Fermi era  [PDF]
F. Tavecchio,G. Ghisellini,G. Ghirlanda,L. Foschini,L. Maraschi
Physics , 2009, DOI: 10.1111/j.1365-2966.2009.15784.x
Abstract: We reconsider the emission properties of the BL Lac objects emitting in the high-energy gamma-ray band exploiting the new information in the MeV-GeV band obtained by the Large Area Telescope (LAT) onboard the Fermi Gamma-Ray Space Telescope in its first three months of operation. To this aim we construct the spectral energy distribution of all the BL Lacs revealed by LAT and of the known TeV BL Lacs not detected by LAT, also including data from the Swift satellite, and model them with a simple one-zone leptonic model. The analysis shows that the BL Lacs detected by LAT (being or not already detected in the TeV band) share similar physical parameters. While some of the TeV BL Lacs not revealed by LAT have spectral energy distributions and physical parameters very similar to the LAT BL Lacs, a group of objects displays peculiar properties (larger electron energies and smaller magnetic fields) suggesting different physical conditions in the emission region. Finally, we discuss possible criteria to effectively select good new candidates for the Cherenkov telescopes among the LAT sources, presenting a list of predicted fluxes in the very high-energy band calculated including the effect of the absorption by the extragalactic background light.
Whipple Observations of BL Lac Objects  [PDF]
M. D'Vali,I. H. Bond,S. M. Bradbury,J. H. Buckley,A. M. Burdett,D. A. Carter-Lewis,M. Catanese,M. F. Cawley,D. J. Fegan,S. J. Fegan,J. P. Finley,J. A. Gaidos,T. A. Hall,A. M. Hillas,J. Knapp,F. Krennrich,S. Le Bohec,R. W. Lessard,C. Masterson,S. D. Myles,J. Quinn,H. J. Rose,F. W. Samuelson,G. H. Sembroski,V. V. Vassiliev,T. C. Weekes
Physics , 1999,
Abstract: Only BL Lac objects have been detected as extragalactic sources of very high energy (E > 300 GeV) gamma rays. Using the Whipple Observatory Gamma-ray Telescope, we have attempted to detect more BL Lacs using three approaches. First, we have conducted surveys of nearby BL Lacs, which led to the detections of Mrk 501 and 1ES 2344+514. Second, we have observed X-ray bright BL Lacs when the RXTE All-Sky Monitor identifies high state X-ray emission in an object, in order to efficiently detect extended high emission states. Third, we have conducted rapid observations of several BL Lacs and QSOs located close together in the sky to search for very high flux, short time-scale flare states such as have been seen from Mrk 421. We will present the results of a survey using the third observational technique.
Hidden BL Lacertae Objects Near and Far  [PDF]
John T. Stocke
Physics , 2001,
Abstract: I describe two difficulties with current research into the cosmic evolution of BL Lacertae Objects: (1). Possible sample incompleteness due to unrecognized (i.e., ``hidden'') BL Lacs; and (2). The absence of a viable physical model of the evolution, to which current and future observations can be compared.
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