6,108 research outputs found
Goals, Strategies and First Discoveries of AO327, the Arecibo All-Sky 327 MHz Drift Pulsar Survey
We report initial results from AO327, a drift survey for pulsars with the
Arecibo telescope at 327 MHz. The first phase of AO327 will cover the sky at
declinations of -1 to 28 degrees, excluding the region within 5 degrees of the
Galactic plane, where high scattering and dispersion make low-frequency surveys
sub-optimal. We record data from a 57 MHz bandwidth with 1024 channels and 125
us sampling time. The 60 s transit time through the AO327 beam means that the
survey is sensitive to very tight relativistic binaries even with no
acceleration searches. To date we have detected 44 known pulsars with periods
ranging from 3 ms to 2.21 s and discovered 24 new pulsars. The new discoveries
include three millisecond pulsars, three objects with periods of a few tens of
milliseconds typical of young as well as mildly recycled pulsars, a nuller, and
a rotating radio transient. Five of the new discoveries are in binary systems.
The second phase of AO327 will cover the sky at declinations of 28 to 38
degrees. We compare the sensitivity and search volume of AO327 to the Green
Bank North Celestial Cap survey and the GBT350 drift survey, both of which
operate at 350 MHz.Comment: Accepted for publication in Ap
The Specific Heat of a Ferromagnetic Film.
We analyze the specific heat for the vector model on a -dimensional
film geometry of thickness using ``environmentally friendly''
renormalization. We consider periodic, Dirichlet and antiperiodic boundary
conditions, deriving expressions for the specific heat and an effective
specific heat exponent, \alpha\ef. In the case of , for , by
matching to the exact exponent of the two dimensional Ising model we capture
the crossover for \xi_L\ra\infty between power law behaviour in the limit
{L\over\xi_L}\ra\infty and logarithmic behaviour in the limit
{L\over\xi_L}\ra0 for fixed , where is the correlation length in
the transverse dimensions.Comment: 21 pages of Plain TeX. Postscript figures available upon request from
[email protected]
Towards a new theory of practice for community health psychology
The article sets out the value of theorizing collective action from a social science perspective that engages with the messy actuality of practice. It argues that community health psychology relies on an abstract version of Paulo Freireâs earlier writing, the Pedagogy of the Oppressed, which provides scholar-activists with a âmapâ approach to collective action. The article revisits Freireâs later work, the Pedagogy of Hope, and argues for the importance of developing a âjourneyâ approach to collective action. Theories of practice are discussed for their value in theorizing such journeys, and in bringing maps (intentions) and journeys (actuality) closer together
Chuva sob dossel em vegetação secundåria em åreas anteriormente ocupadas por capoeiras enriquecidas.
Completeness and consistency of renormalisation group flows
We study different renormalisation group flows for scale dependent effective
actions, including exact and proper-time renormalisation group flows. These
flows have a simple one loop structure. They differ in their dependence on the
full field-dependent propagator, which is linear for exact flows. We
investigate the inherent approximations of flows with a non-linear dependence
on the propagator. We check explicitly that standard perturbation theory is not
reproduced. We explain the origin of the discrepancy by providing links to
exact flows both in closed expressions and in given approximations. We show
that proper-time flows are approximations to Callan-Symanzik flows. Within a
background field formalism, we provide a generalised proper-time flow, which is
exact. Implications of these findings are discussed.Comment: 33 pages, 15 figures, revtex, typos corrected, to be published in
Phys.Rev.
Deppining of a Superfluid Vortex Inside a Circular Defect
In this work we study the process of depinning of a quantum of circulation
trapped inside a disk by an applied two dimensional superflow. We use the
Gross-Pitaevskii model to describe the neutral superfluid. The collective
coordinate dynamics is derived directly from the condensate equation of motion,
the nonlinear Schroedinger equation, and it is used to obtain an expression for
the critical velocity as a function of the defect radius. This expression is
compared with a numerical result obtained from the time independent nonlinear
Schroedinger equation. Below the critical velocity, we obtain the dependence of
the semiclassical nucleation rate with the flow velocity at infinity. Above the
critical velocity, the classical vortex depinning is illustrated with a
numerical simulation of the time dependent nonlinear Schroedinger equation.Comment: 8 pages, 5 figures, uses revtex and epsf.st
Pulsar J0453+1559: A Double Neutron Star System with a Large Mass Asymmetry
To understand the nature of supernovae and neutron star (NS) formation, as
well as binary stellar evolution and their interactions, it is important to
probe the distribution of NS masses. Until now, all double NS (DNS) systems
have been measured to have a mass ratio close to unity (q 0.91). Here we
report the measurement of the individual masses of the 4.07-day binary pulsar
J0453+1559 from measurements of the rate of advance of periastron and Shapiro
delay: The mass of the pulsar is 1.559(5) and that of its companion
is 1.174(4) ; q = 0.75. If this companion is also a neutron star
(NS), as indicated by the orbital eccentricity of the system (e=0.11), then its
mass is the smallest precisely measured for any such object. The pulsar has a
spin period of 45.7 ms and a spin derivative of 1.8616(7) x; from these
we derive a characteristic age of ~ 4.1 x years and a magnetic field of
~ 2.9 x G,i.e, this pulsar was mildly recycled by accretion of matter
from the progenitor of the companion star. This suggests that it was formed
with (very approximately) its current mass. Thus NSs form with a wide range of
masses, which is important for understanding their formation in supernovae. It
is also important for the search for gravitational waves released during a
NS-NS merger: it is now evident that we should not assume all DNS systems are
symmetric
- âŠ