MOJAVE: Monitoring of Jets in AGN with VLBA Experiments. VII. Blazar Jet Acceleration

We discuss acceleration measurements for a large sample of extragalactic radio jets from the MOJAVE program which studies the parsec-scale jet structure and kinematics of a complete, flux-density-limited sample of Active Galactic Nuclei (AGN). Accelerations are measured from the apparent motion of individual jet features or "components" which may represent patterns in the jet flow. We find that significant accelerations are common both parallel and perpendicular to the observed component velocities. Parallel accelerations, representing changes in apparent speed, are generally larger than perpendicular acceleration that represent changes in apparent direction. The trend for larger parallel accelerations indicates that a significant fraction of these changes in apparent speed are due to changes in intrinsic speed of the component rather than changes in direction to the line of sight. We find an overall tendency for components with increasing apparent speed to be closer to the base of their jets than components with decreasing apparent speed. This suggests a link between the observed pattern motions and the underlying flow which, in some cases, may increase in speed close to the base and decrease in speed further out; however, common hydro-dynamical processes for propagating shocks may also play a role. About half of the components show "non-radial" motion, or a misalignment between the component's structural position angle and its velocity direction, and these misalignments generally better align the component motion with the downstream emission. Perpendicular accelerations are closely linked with non-radial motion. When observed together, perpendicular accelerations are usually in the correct direction to have caused the observed misalignment.Comment: 17 pages, 11 figures, 1 table, accepted by the Astrophysical Journa

The Relation between Radio Polarization and Gamma-ray Emission in AGN Jets

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.Comment: 6 pages, 3 figures, to appear in the proceedings of "High Energy Phenomena In Relativistic Outflows II" (Buenos Aires, Argentina, October 26-30, 2009) International Journal of Modern Physics

MOJAVE: Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments. VI. Kinematics Analysis of a Complete Sample of Blazar Jets

We discuss the jet kinematics of a complete flux-density-limited sample of 135 radio-loud active galactic nuclei (AGN) resulting from a 13 year program to investigate the structure and evolution of parsec-scale jet phenomena. Our analysis is based on new 2 cm Very Long Baseline Array (VLBA) images obtained between 2002 and 2007, but includes our previously published observations made at the same wavelength, and is supplemented by VLBA archive data. In all, we have used 2424 images spanning the years 1994-2007 to study and determine the motions of 526 separate jet features in 127 jets. The data quality and temporal coverage (a median of 15 epochs per source) of this complete AGN jet sample represents a significant advance over previous kinematics surveys. In all but five AGNs, the jets appear one-sided, most likely the result of differential Doppler boosting. In general the observed motions are directed along the jet ridge line, outward from the optically thick core feature. We directly observe changes in speed and/or direction in one third of the well-sampled jet components in our survey. While there is some spread in the apparent speeds of separate features within an individual jet, the dispersion is about three times smaller than the overall dispersion of speeds among all jets. This supports the idea that there is a characteristic flow that describes each jet, which we have characterized by the fastest observed component speed. The observed maximum speed distribution is peaked at ~10c, with a tail that extends out to ~50c. This requires a distribution of intrinsic Lorentz factors in the parent population that range up to ~50. We also note the presence of some rare low-pattern speeds or even stationary features in otherwise rapidly flowing jets... (abridged)Comment: 19 pages, 10 figures, 2 tables, accepted by the Astronomical Journal; online only material is available from http://www.cv.nrao.edu/2cmVLBA/pub/MOJAVE_VI_suppl.zi

Low NO(x) heavy fuel combustor program

The 'low nitrogen oxides heavy fuel combustor' program is described. Main program objectives are to generate and demonstrate the technology required to develop durable gas turbine combustors for utility and industrial applications, which are capable of sustained, environmentally acceptable operation with minimally processed petroleum residual fuels. The program will focus on 'dry' reductions of oxides of nitrogen, improved combustor durability, and satisfactory combustion of minimally processed petroleum residual fuels. Other technology advancements sought include: fuel flexibility for operation with petroleum distillates, blends of petroleum distillates and residual fuels, and synfuels (fuel oils derived from coal or shale); acceptable exhaust emissions of carbon monoxide, unburned hydrocarbons, sulfur oxides and smoke; and retrofit capability to existing engines

The use of comprehension strategies by good and poor learners : a longitudinal study.

SIGLEAvailable from British Library Document Supply Centre-DSC:DXN026381 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Synthesis, structures and properties of inorganic framework materials

Chapter 1 reviews the literature on the structures and properties of inorganic framework materials that are of relevance to this thesis. In particular the phenomenon of negative thermal expansion and the AM(_2)O(_8)/AM(_2)O(_7) families of materials are discussed. Chapter 2 describes the methods of synthesis and characterisation of the materials investigated in this thesis. Chapter 3 discusses the dehydration reaction of M(_o)O(_2).H(_2)O.PO(_3)OH. The study involved the introduction of a new methodology for whole pattern powder fitting; this method was later verified by full Rietveld analysis. This investigation led to the discovery and structure solution of two new molybdenum phosphates using powder XRD. These materials have been named β-(MoO(_2))(_2)P(_2)0(_7) and β -(Mo0(_2))(_2)P(_2)0(_7). A structural pathway for the dehydration reactions has been proposed which is consistent with all of these structures and other analytical data obtained. Chapter 4 describes investigations into the structures of a-(Mo02)2P207 by powder diffraction and NMR methods. The high temperature structure was confirmed to be related to a literature model. The low temperature structure was further studied by electron diffraction, second harmonic generation and solid state NMR. The use of these complementary techniques with powder X-ray and neutron diffraction data, led to the solution of the complex superstructure. Chapter 5 describes a study into the structures of (MoO)(_2)P(_4)0(_13). The material undergoes a phase transition at 523 K. The low temperature structure contains 441 unique atoms and as such is the most crystallographically complex oxide solved to date. The high temperature structure contains 253 unique atoms and is the second most complex oxide in the ICSD. Chapter 6 describes the in-situ X-ray studies on the synthesis of M0P(_2)O(_7) from precursors Mo0(_2)(P0(_3))(_2) and (MoO)(_2)P(_4)0(_13) in an H(_2) environment. (Mo0(_2))(_2)P(_2)0(_7) was studied under similar conditions and found to decompose to an unidentifiable poorly crystalline phase. Chapter 7 describes the discovery of a new high temperature synthetic route to cubic ZrMo(_2)0(_8) using extremely rapid time-resolved XRD data recorded at the ESRF. The cubic material forms from its constituent oxides at 1350 K and can be isolated back at room temperature using a quench cooling method. A pure phase sample can be prepared using a Zr0(_2):Mo0(_3) ratio of 1:3. The entire synthesis occurs within seconds and precise control of temperature and time is crucial for this synthesis

Public assistance area supervision in Rhode Island -

Thesis (M.S.)--Boston University, 1944. This item was digitized by the Internet Archive

Relativistic Beaming and Flux Variability in Active Galactic Nuclei

We discuss the impact of special relativistic effects on the observed light curves and variability duty cycles of AGNs. We model the properties of AGN light curves at radio wavelengths using a simulated shot noise process in which the occurrence of major flaring events in a relativistic jet is governed by Poisson statistics. We show that flaring sources whose radiation is highly beamed toward us are able to reach very high flux levels, but will in fact spend most of their time in relatively low flaring states due to relativistic contraction of flare time scales in the observer frame. The fact that highly beamed AGNs do not return to a steady-state quiescent level between flares implies that their weakly beamed counterparts should have highly stable flux densities that result from a superposition of many long-term, low-amplitude flares. The ``apparent'' quiescent flux levels of these weakly beamed AGNs (identified in many unified models as radio galaxies) will be significantly higher than their ''true'' quiescent (i.e., non-flaring) levels. We use Monte Carlo simulations to investigate flux variability bias in the selection statistics of flat-spectrum AGN samples. In the case of the Caltech-Jodrell Flat-spectrum survey, the predicted orientation bias towards jets seen end-on is weakened if the parent population is variable, since the highly beamed sources have a stronger tendency to be found in low flaring states. This effect is small, however, since highly beamed sources are relatively rare, and their fluxes tend to be boosted sufficiently above the survey limit such that they are selected regardless of their flaring level. We find that for larger flat-spectrum AGN surveys with fainter flux cutoffs, variability should not be an appreciable source of selection bias.Comment: Accepted for publication in the Astrophysical Journa