572 research outputs found
Understanding the Fanaroff-Riley radio galaxy classification
The simple, yet profoundly far-reaching classification scheme based on
extended radio morphologies of radio galaxies, the Fanaroff-Riley
classification has been a cornerstone in our understanding of radio galaxies.
Over the decades since the recognition that there are two basic types of radio
galaxy morphologies there have been several findings in different wavebands
that have reported properties on different scales. Although it is realized that
there may be intrinsic as well external causes an overarching view of how we
may understand the two morphological types is missing. With the radio
power-absolute magnitude relation (the Owen-Ledlow diagram) as backdrop we
review and develop an understanding of the two radio galaxy types in the light
of what is known about them. We have for the first time included the dust
properties of the two FR classes together with the relative orientations of
dust, host major axis and the radio axis to present a qualitative framework
within which to understand the conditions under which they form. (Abridged).Comment: 15 pages, To appear in Astronomical Journa
Laser Doppler Velocimeter measurements in a 3-D impinging twin-jet fountain flow
Mean velocity and turbulence measurements were conducted on the three dimensional fountain flow field generated by the impingement of two axisymmetric jets on a ground plane with application to vertical takeoff and landing (VTOL) aircraft. The basic instantaneous velocity data were obtained using a two component laser Doppler velocimeter in a plane connecting the nozzle centerlines at different heights above the ground emphasizing the jet impingement region and the fountain upwash region formed by the collision of the wall jets. The distribution of mean velocity components and turbulence quantities, including the turbulence intensity and the Reynolds shear stress, were derived from the basic velocity data. Detailed studies of the characteristics of the fountain revealed self-similarity in the mean velocity and turbulence profiles across the fountain. The spread and mean velocity decay characteristics of the fountain were established. Turbulence intensities of the order of 50% were observed in the fountain
Simultaneous X-ray and optical observations of thermonuclear bursts in the LMXB EXO 0748-676
We report the detection of a large number of optical bursts in the Low Mass
X-ray Binary (LMXB) EXO 0748-676 simultaneous with the thermonuclear X-ray
bursts. The X-ray and the optical bursts are detected in a long observation of
this source with the XMM-Newton observatory. This has increased the number of
thermonuclear X-ray bursts in the LMXBs with simultaneous optical detection by
several factors. The optical bursts are found to have a linear rise followed by
a slow, somewhat exponential decay. Most of the optical bursts have longer rise
and decay timescale compared to the corresponding X-ray bursts. We have
determined the X-ray and optical excess photon counts in the bursts that allow
us to look at the optical to X-ray burst fluence ratio for each burst and the
ratio as a function of the X-ray burst intensity and as a function of the
orbital phase. The delay between the onset of the X-ray bursts and the onset of
the optical bursts have also been measured and is found to have an average
value of 3.25 seconds. We do not find any convincing evidence of orbital phase
dependence of the following parameters: X-ray to optical delay, rise time of
the optical bursts, and optical to X-ray burst intensity ratio as would be
expected if the optical bursts were produced by reprocessing from the surface
of the secondary star that is facing the compact star. On the other hand, if
the optical bursts are produced by reprocessing of the X-rays in the accretion
disk, the onset of the bursts is not expected to have a sharp, linear shape as
is observed in a few of the bursts in EXO 0748-676. We emphasise the fact that
simultaneous optical observations of the X-ray bursts in multiple wavelength
bands will enable further detailed investigations of the reprocessing
phenomena, including any non-linear effect of the X-ray irradiation.Comment: 12 pares, 5 figures (Submitted to the Bulletin of the Astronomical
Society of India
The Abundance of X-Shaped Radio Sources: Implications for the Gravitational Wave Background
Coalescence of super massive black holes (SMBH's) in galaxy mergers is
potentially the dominant contributor to the low frequency gravitational wave
background (GWB). IIt was proposed by Merritt and Ekers (2002) that X-shaped
radio galaxies are signposts of such coalescences, and that their abundance
might be used to predict the magnitude of the gravitational wave background. In
Roberts et al. (2015) we present radio images of all 52 X-shaped radio source
candidates out of the sample of 100 selected by Cheung (2007) for which
archival VLA data were available. These images indicate that at most 21% of the
candidates might be genuine X-shaped radio sources that were formed by a
restarting of beams in a new direction following a major merger. This suggests
that fewer than 1.3% of extended radio sources appear to be candidates for
genuine axis reorientations ("spin flips"), much smaller than the 7% suggested
by Leahy and Parma (1992). Thus the associated gravitational wave background
may be substantially smaller than previous estimates. These results can be used
to normalize detailed calculations of the SMBH coalescence rate and the GWB
Transport properties in nanocrystalline silicon and silicon germanium
Nanocrystalline(nc-) Si:H and (Si,Ge):H films are important electronic and optical materials. They consist of crystallite grains surrounded by amorphous tissues. To successfully apply these materials for solar cell applications we need to understand their fabrication procedures and develop ways to characterize the material. Transport properties determine whether the generated photons in the device effectively reach the terminal and contribute to the output current. While there are several reports in literature about growth and the carrier transport mechanism in nc-Si, nc-SiGe is a fairly new material. Motivation to study nc-SiGe comes from its lower bandgap which makes it an alternative material for lower cell in tandem devices. This thesis deals with measurement of fundamental transport properties in these materials like minority carrier lifetime and diffusion length. While there have been previous reports of diffusion length, lifetime was never measured before in nc-Si or nc-(Si,Ge). We adopt the reverse recovery method to calculate lifetime in device type structures. The typical values of lifetime in nc-Si and nc-SiGe were found to be in the range of 300-600ns and 150-250ns respectively. Systematic measurement of lifetimes and defect densities in the same solar cell devices enabled us to prove that Shockley Read-Hall (SRH) recombination holds true in these nanocrystalline materials. With respect to temperature we found that the lifetime follows a U shape, with a decreasing trend till a certain temperature and increasing thereafter. Both the effects could be explained by understanding the actual recombination process. We could get an estimate of the trap locations from lifetime versus temperature data. By examining the reverse recovery waveforms in different types of device structures, we deduced that this method is appropriate when the carrier transport is diffusion dominated.
Lifetime and diffusion length were measured in p(+)-n-n(+) devices where the base layer(n) was either nc-Si or nc-SiGe fabricated using VHF plasma deposition, at a frequency of 46MHz using mixtures of SiH4 and GeH4 with H2. From Raman spectroscopy it was observed that nc-SiGe requires significantly higher Hydrogen dilution ratios compared to nc-Si. Graded TMB doping was used to enhance carrier collection in the base layer. Defect densities were measured using capacitance spectroscopy. In SiGe devices it was observed that extreme care has to be taken at the p+/ n and n+/ n interfaces to ensure efficient carrier collection. Even though the actual process of grain growth is still unclear in nc-SiGe, from the lifetime versus defect density data, we do conclude that the carrier transport is primarily controlled by recombination at grain boundaries
Investigation of blown boundary layers with an improved wall jet system
The behavior of two dimensional incompressible turbulent wall jets submerged in a boundary layer when they are used to prevent boundary layer separation on plane surfaces is investigated. The experimental set-up and instrumentation are described. Experimental results of zero pressure gradient flow and adverse pressure gradient flow are presented. Conclusions are given and discussed
Investigation of blown boundary layers with an improved wall jet system
Measurements were made in a two dimensional incompressible wall jet submerged under a thick upstream boundary layer with a zero pressure gradient and an adverse pressure gradient. The measurements included mean velocity and Reynolds stresses profiles, skin friction, and turbulence spectra. The measurements were confined to practical ratios (less than 2) of the jet velocity to the free stream velocity. The wall jet used in the experiments had an asymmetric velocity profile with a relatively higher concentration of momentum away from the wall. An asymmetric jet velocity profile has distinct advantages over a uniform jet velocity profile, especially in the control of separation. Predictions were made using Irwin's (1974) method for blown boundary layers. The predictions clearly show the difference in flow development between an asymmetric jet velocity profile and a uniform jet velocity profile
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