6,156 research outputs found
Examining the perceptions and attitudes of staff working in community based children's homes: are their needs being met?
There remains considerable ambiguity and negativity around the purpose and effectiveness of children’s homes. High levels of unqualified staff, low status and poor pay and conditions have continued to be the norm within residential child care. In light of this situation it is appropriate to ask why staff are viewed as a key ingredient in the service provision. It is particularly significant given the views of staff and the tasks they undertake have not been widely researched. This article provides a summary of findings from a doctoral study that attempted to address this deficit by examining the context of children’s homes, especially those social processes and interactions that shape key tasks
Correlation among QPO frequencies and Quiescence-state Duration in Black Hole Candidate GRS 1915+105
We discover a definite correlation between the frequency of the
quasi-periodic oscillations (QPO) in quiescence states and the duration of the
quiescence state of the transient X-ray source GRS 1915+105. We find that while
the QPO frequency can be explained with the oscillation of shocks in accretion
flows, the switching of burst to quiescence states (and vice versa) and their
duration can be explained by assuming an outflow from the post-shock region.
The duration of the quiescence state is inversely related to the QPO-frequency.
We derive this relation. We also find the correlation between the observed low
(Hz) and the intermediate (1-10Hz) QPO frequencies. Our
analytical solutions are verified by analyzing several days of public-domain
data from RXTE.Comment: Latex, 13 pages with 3 figures; Accepted for Publication in
Astrophysical Journal Letter
Accretion flow behaviour during the evolution of the Quasi Periodic Oscillation Frequency of XTE J1550-564 in 1998 outburst
Low and intermediate frequency quasi-periodic oscillations (QPOs) are thought
to be due to oscillations of Comptonizing regions or hot regions embedded in
Keplerian discs. Observational evidence of evolutions of QPOs would therefore
be very important as they throw lights on the dynamics of the hotter region.
Our aim is to find systems in which there is a well-defined correlation among
the frequencies of the QPOs over a range of time so as to understand the
physical picture. In this paper, we concentrate on the archival data of XTE
J1550-564 obtained during 1998 outburst, and study the systematic drifts during
the rising phase from the 1998 September 7 to the 1998 September 19, when the
QPO frequency increased monotonically from 81mHz to 13.1Hz. Immediately after
that, QPO frequency started to decrease and on the 1998 September 26, the QPO
frequency became 2.62Hz. After that, its value remained almost constant. This
frequency drift can be modelled satisfactorily with a propagatory oscillating
shock solution where the post-shock region behaves as the Comptonized region.
Comparing with the nature of a more recent 2005 outburst of another black hole
candidate GRO 1655-40, where QPOs disappeared at the end of the rising phase,
we conjecture that this so-called `outburst' may not be a full-fledged
outburst.Comment: 15 pages, 6 figure
Fluctuation Cumulant Behavior for the Field-Pulse Induced Magnetisation-Reversal Transition in Ising Models
The universality class of the dynamic magnetisation-reversal transition,
induced by a competing field pulse, in an Ising model on a square lattice,
below its static ordering temperature, is studied here using Monte Carlo
simulations. Fourth order cumulant of the order parameter distribution is
studied for different system sizes around the phase boundary region. The
crossing point of the cumulant (for different system sizes) gives the
transition point and the value of the cumulant at the transition point
indicates the universality class of the transition. The cumulant value at the
crossing point for low temperature and pulse width range is observed to be
significantly less than that for the static transition in the same
two-dimensional Ising model. The finite size scaling behaviour in this range
also indicates a higher correlation length exponent value. For higher
temperature and pulse width range, the transition seems to fall in a mean-field
like universality class.Comment: 5 pages, 8 eps figures, thoroughly revised manuscript with new
figures, accepted in Phys. Rev. E (2003
Simulation of Thick Accretion Disks with Standing Shocks by Smoothed Particle Hydrodynamics
We present results of numerical simulation of inviscid thick accretion disks
and wind flows around black holes. We use Smoothed Particle Hydrodynamics (SPH)
technique for this purpose. Formation of thick disks are found to be preceded
by shock waves travelling away from the centrifugal barrier. For a large range
of the parameter space, the travelling shock settles at a distance close to the
location obtained by a one-and-a-half dimensional model of inviscid accretion
disks. Occasionally, it is observed that accretion processes are aided by the
formation of oblique shock waves, particularly in the initial transient phase.
The post-shock region (where infall velocity suddenly becomes very small)
resembles that of the usual model of thick accretion disk discussed in the
literature, though they have considerable turbulence. The flow subsequently
becomes supersonic before falling into the black hole. In a large number of
cases which we simulate, we find the formation of strong winds which are hot
and subsonic when originated from the disk surface very close to the black hole
but become supersonic within a few tens of the Schwarzschild radius of the
blackhole. In the case of accretion of high angular momentum flow, very little
amount of matter is accreted directly onto the black hole. Most of the matter
is, however, first squeezed to a small volume close to the black hole, and
subsequently expands and is expelled as a strong wind. It is quite possible
that this expulsion of matter and the formation of cosmic radio jets is aided
by the shock heating in the inner parts of the accretion disks.Comment: LaTeX, 16 pages, Astrophysical Journal (in press
Maps and twists relating and the nonstandard : unified construction
A general construction is given for a class of invertible maps between the
classical and the Jordanian algebras. Different maps
are directly useful in different contexts. Similarity trasformations connecting
them, in so far as they can be explicitly constructed, enable us to translate
results obtained in terms of one to the other cases. Here the role of the maps
is studied in the context of construction of twist operators between the
cocommutative and noncocommutative coproducts of the and
algebras respectively. It is shown that a particular map called
the `minimal twist map' implements the simplest twist given directly by the
factorized form of the -matrix of Ballesteros-Herranz. For other
maps the twist has an additional factor obtainable in terms of the similarity
transformation relating the map in question to the minimal one. The series in
powers of for the operator performing this transformation may be obtained
up to some desired order, relatively easily. An explicit example is given for
one particularly interesting case. Similarly the classical and the Jordanian
antipode maps may be interrelated by a similarity transformation. For the
`minimal twist map' the transforming operator is determined in a closed form.Comment: LaTeX, 13 page
Steady shocks around black holes produced by sub-keplerian flows with negative energy
We discuss a special case of formation of axisymmetric shocks in the
accretion flow of ideal gas onto a Schwarzschild black hole: when the total
energy of the flow is negative. The result of our analysis enlarges the
parameter space for which these steady shocks are exhibited in the accretion of
gas rotating around relativistic stellar objects. Since keplerian disks have
negative total energy, we guess that, in this energy range, the production of
the shock phenomenon might be easier than in the case of positive energy. So
our outcome reinforces the view that sub-keplerian flows of matter may
significantly affect the physics of the high energy radiation emission from
black hole candidates. We give a simple procedure to obtain analytically the
position of the shocks. The comparison of the analytical results with the data
of 1D and 2D axisymmetric numerical simulations confirms that the shocks form
and are stable.Comment: 5 pages, 5 figures, accepted by MNRAS on 10 November 200
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