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Search Results: 1 - 10 of 540389 matches for " M. L. Lister "
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Gigahertz-Peaked Spectrum Radio Sources
M. L. Lister
Physics , 2003,
Abstract: The study of compact active galactic nuclei (AGN) that possess convex radio spectra (the gigahertz-peaked spectrum radio sources) offers a unique opportunity to probe both the early evolutionary stages of relativistic AGN jets and their immediate nuclear environments. In this article I trace Ken Kellermann's early investigations of these sources, which played a major role in justifying the development of modern-day VLBI techniques. I describe how our understanding of these AGN has progressed since Kellermann's early discoveries, and discuss several ways in which the current classification scheme can be simplified to reflect intrinsic source characteristics, rather than observer-biased quantities. Finally, I discuss recent results from the VLBA 2 cm survey concerning the relativistic jet kinematics of the two-sided peaked-spectrum sources 4C +12.50 (PKS 1345+125) and OQ 208 (1404+286).
Extended radio emission in MOJAVE Blazars: Challenges to Unification
P. Kharb,M. L. Lister,N. J. Cooper
Physics , 2010, DOI: 10.1088/0004-637X/710/1/764
Abstract: We present the results of a study on the 1.4 GHz kpc-scale radio emission in the complete flux density limited MOJAVE sample, comprising 135 radio-loud AGNs. While extended emission is detected in the majority of the sources, about 7% of the sources exhibit only radio core emission. Many BL Lacs exhibit extended radio power and kpc-scale morphology typical of powerful FRII jets, while a substantial number of quasars possess radio powers intermediate between FRIs and FRIIs. This poses challenges to the simple radio-loud unified scheme, which links BL Lacs to FRIs and quasars to FRIIs. We find a significant correlation between extended radio emission and pc-scale jet speeds: the more radio powerful sources possess faster jets. This indicates that the 1.4 GHz (or low frequency) radio emission is indeed related to jet kinetic power. Various properties such as extended radio power and apparent pc-scale jet speeds vary smoothly between different blazar subclasses, suggesting that, at least in terms of radio jet properties, the distinction between quasars and BL Lac objects, at an emission-line equivalent width of 5 Angstrom is essentially an arbitrary one. Based on the assumption that the extended radio luminosity is affected by the kpc-scale environment, we define the ratio of extended radio power to absolute optical magnitude as a proxy for environmental effects. Trends with this parameter suggest that the pc-scale jet speeds and the pc-to-kpc jet misalignments are not affected by the large-scale environment, but are more likely to depend upon factors intrinsic to the AGN, or its local pc-scale environment. We suggest that some of the extremely misaligned MOJAVE blazar jets could be "hybrid" morphology sources, with an FRI jet on one side and an FRII jet on the other. (Abridged)
Radio/gamma-ray time delay in the parsec-scale cores of active galactic nuclei
A. B. Pushkarev,Y. Y. Kovalev,M. L. Lister
Physics , 2010, DOI: 10.1088/2041-8205/722/1/L7
Abstract: We report the detection of a non-zero time delay between radio emission measured by the VLBA at 15.4 GHz and gamma-ray radiation (gamma-ray leads radio) registered by the Large Area Telescope (LAT) on board the Fermi Gamma-Ray Space Telescope for a sample of 183 radio and gamma-ray bright active galactic nuclei (AGNs). For the correlation analysis we used 100 MeV - 100 GeV gamma-ray photon fluxes, taken from monthly binned measurements from the first Fermi LAT catalog, and 15.4 GHz radio flux densities from the MOJAVE VLBA program. The correlation is most pronounced if the core flux density is used, strongly indicating that the gamma-ray emission is generated within the compact region of the 15 GHz VLBA core. Determining the Pearson's r and Kendall's tau correlation coefficients for different time lags, we find that for the majority of sources the radio/gamma-ray delay ranges from 1 to 8 months in the observer's frame and peaks at about 1.2 months in the source's frame. We interpret the primary source of the time delay to be synchrotron opacity in the nuclear region.
The Pearson-Readhead Survey of Compact Extragalactic Radio Sources From Space. II. Analysis of Source Properties
M. L. Lister,S. J. Tingay,R. A. Preston
Physics , 2001, DOI: 10.1086/321430
Abstract: We have performed a multi-dimensional correlation analysis on the observed properties of a statistically complete core-selected sample of compact radio-loud active galactic nuclei, based on data from the VLBI Space Observing Programme (Paper I) and previously published studies. Our sample is drawn from the well-studied Pearson-Readhead (PR) survey, and is ideally suited for investigating the general effects of relativistic beaming in compact radio sources. In addition to confirming many previously known correlations, we have discovered several new trends that lend additional support to the beaming model. These trends suggest that the most highly beamed sources in core-selected samples tend to have a) high optical polarizations; b) large pc/kpc-scale jet misalignments; c) prominent VLBI core components; d) one-sided, core, or halo radio morphology on kiloparsec scales; e) narrow emission line equivalent widths; and f) a strong tendency for intraday variability at radio wavelengths. We have used higher resolution space and ground-based VLBI maps to confirm the bi-modality of the jet misalignment distribution for the PR survey, and find that the sources with aligned parsec- and kiloparsec-scale jets generally have arcsecond-scale radio emission on both sides of the core. The aligned sources also have broader emission line widths. We find evidence that the BL Lacertae objects in the PR survey are all highly beamed, and have very similar properties to the high-optically polarized quasars, with the exception of smaller redshifts. A cluster analysis on our data shows that after partialing out the effects of redshift, the luminosities of our sample objects in various wave bands are generally well-correlated with each other, but not with other source properties.
A Multiwavelength Study of Three Hybrid Blazars
E. C. Stanley,P. Kharb,M. L. Lister,H. L. Marshall,C. O'Dea,S. Baum
Physics , 2015,
Abstract: We present multiwavelength imaging observations of PKS 1045-188, 8C 1849+670, and PKS 2216-038, three radio-loud active galactic nuclei from the MOJAVE-Chandra Sample that straddle the Fanaroff-Riley (FR) boundary between low- and high-power jets. These hybrid sources provide an excellent opportunity to study jet emission mechanisms and the influence of the external environment. We used archival VLA observations, and new Hubble and Chandra observations to identify and study the spectral properties of five knots in PKS 1045-188, two knots in 8C 1849+670, and three knots in PKS 2216-038. For the seven X-ray visible knots, we constructed and fit the broadband spectra using synchrotron and inverse Compton/cosmic microwave background (IC/CMB) emission models. In all cases, we found that the lack of detected optical emission ruled out the X-ray emission from the same electron population that produces radio emission. All three sources have high total extended radio power, similar to that of FR II sources. We find this is in good agreement with previously studied hybrid sources, where high-power hybrid sources emit X-rays via IC/CMB and the low-power hybrid sources emit X-rays via synchrotron emission. This supports the idea that it is total radio power rather than FR morphology that determines the X-ray emission mechanism. We found no significant asymmetries in the diffuse X-ray emission surrounding the host galaxies. Sources PKS 1045-188 and 8C 1849+670 show significant differences in their radio and X-ray termination points, which may result from the deceleration of highly relativistic bulk motion.
Full Polarization Spectra of 3C 279
D. C. Homan,M. L. Lister,H. D. Aller,M. F. Aller,J. F. C. Wardle
Physics , 2009, DOI: 10.1088/0004-637X/696/1/328
Abstract: We report the results of parsec-scale, multi-frequency VLBA observations of the core region of 3C 279 in Stokes I, linear polarization, and circular polarization. These full polarization spectra are modeled by radiative transfer simulations to constrain the magnetic field and particle properties of the parsec-scale jet in 3C 279. The polarization properties of the core region, including the amount of linear polarization, the amount and sign of Faraday rotation, and the amount and sign of circular polarization can be explained by a consistent physical picture. The base of the jet is modeled as an inhomogeneous Blandford-Konigl style conical jet dominated by a vector-ordered poloidal magnetic field along the jet axis, and we estimate its net magnetic flux. This poloidal field is responsible for the linear and circular polarization from this inhomogeneous component. Farther down the jet the magnetic field in two homogeneous features is dominated by local shocks and a smaller fraction of vector-ordered poloidal field remains along the jet axis. In this picture, we find the jet to be kinetically dominated by protons with the radiating particles being dominated by electrons at an approximate fraction of >~ 75%. Based on the amounts of Faraday conversion deduced for the homogeneous components, we find a plausible range for the lower cutoff in the relativistic particle energy spectrum to be 5 <~ gamma_l <~ 35. The physical picture described here is not unique if the observed Faraday rotation and depolarization occur in screens external to the jet; however, we find the joint explanation of linear and circular polarization observations from a single set of magnetic fields and particle properties internal to the jet to be compelling evidence for this picture. (Abridged)
Interval cancer review in the Leicestershire symptomatic breast service
SL Tennant, H Daintith, M Al-Attar, E Denton, L Grosvenor, D Lister, H Khan
Breast Cancer Research , 2009, DOI: 10.1186/bcr2370
Abstract: All patients referred to our centre with a breast symptom are seen in 'one-stop' triple assessment clinics. We identified, and reviewed the imaging of, patients diagnosed with in situ or invasive carcinoma at our unit during 2007 who had undergone breast imaging via the symptomatic service within 3 years prior to diagnosis.Between January 2005 and December 2007, 11,796 ultrasounds and 17,530 mammograms were performed within the symptomatic service. Of the total 681 cancers diagnosed in 2007, 34 of these were 'interval cancers' to the symptomatic service. The interval since discharge was ≤12 months in 9 patients, 13 to 24 months in 18 and 25 to 36 months in 7.Fourteen cancers were felt to be true intervals, six mammographically occult, and five suspicious. Nine were unclassifiable. Of the unclassifiable cases, one had missing imaging and eight did not have mammo-graphy at prior attendance. Only four of these were symptomatic in the same area as their cancer was later diagnosed.'Interval cancers' within the symptomatic population are a small but heterogeneous group. We discuss the details and present a pictorial review of the interesting cases.
VLBI detection of the HST-1 feature in the M87 jet at 2 cm
C. S. Chang,E. Ros,Y. Y. Kovalev,M. L. Lister
Physics , 2010, DOI: 10.1051/0004-6361/200913915
Abstract: A bright feature 80 pc away from the core in the powerful jet of M87 shows highly unusual properties. Earlier radio, optical and X-ray observations have shown that this feature, labeled HST-1, is superluminal, and is possibly connected with the TeV flare detected by HESS in 2005. It has been claimed that this feature might have a blazar nature, due to these properties. To examine the possible blazar-like nature of HST-1, we analyzed lambda 2 cm VLBA archival data from dedicated full-track observations and the 2 cm survey/MOJAVE VLBI monitoring programs obtained between 2000 and 2009. Applying VLBI wide-field imaging techniques, the HST-1 region was imaged at milliarcsecond resolution. Here we present the first 2 cm VLBI detection of this feature in observations from early 2003 to early 2007, and analyze its evolution over this time. Using the detections of HST-1, we find that the projected apparent speed is 0.61 +/- 0.31 c. A comparison of the VLA and VLBA flux densities of this feature indicate that is mostly resolved on molliarcsecond scales. This feature is optically thin between lambda 2 cm and lambda 20 cm. We do not find evidence of a blazar nature for HST-1.
The Relation between Radio Polarization and Gamma-ray Emission in AGN Jets
T. Hovatta,M. L. Lister,Y. Y. Kovalev,A. B. Pushkarev,T. Savolainen
Physics , 2010, DOI: 10.1142/S0218271810016683
Abstract: We have compared the parsec-scale jet linear polarization properties of the Fermi LAT-detected and non-detected sources in the complete flux-density-limited (MOJAVE-1) sample of highly beamed AGN. Of the 123 MOJAVE sources, 30 were detected by the LAT during its first three months of operation. We find that during the era since the launch of Fermi, the unresolved core components of the LAT-detected jets have significantly higher median fractional polarization at 15 GHz. This complements our previous findings that these LAT sources have higher apparent jet speeds, brightness temperatures and Doppler factors, and are preferentially found in higher activity states.
Apparent parsec-scale jet opening angles and γ-ray brightness of active galactic nuclei
A. B. Pushkarev,M. L. Lister,Y. Y. Kovalev,T. Savolainen
Physics , 2012,
Abstract: We have investigated the differences in apparent opening angles between the parsec-scale jets of the active galactic nuclei (AGN) detected by the Fermi Large Area Telescope (LAT) during its first 24 months of operations and those of non-LAT-detected AGN. We used 15.4 GHz VLBA observations of 215 sources from the 2 cm VLBA MOJAVE program. The apparent opening angles were determined by analyzing transverse jet profiles from the data in the image plane by using stacked images constructed from all available MOJAVE epochs for a given source. We confirm our earlier result based on the first three months of scientific operations of the LAT. The apparent opening angles of \gamma-ray bright AGN are preferentially larger than those of \gamma-ray weak sources, suggesting smaller viewing angles to the \gamma-ray bright AGN. Intrinsic opening angles for BL Lacs are wider than those in quasars.
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