2,794 research outputs found
Cloud tracking in cloud-resolving models
In recent years Cloud Resolving Models (CRMs) have become an increasingly important tool for the study of convective phenomena. CRMs should not be regarded as simply providing surrogates for observations; rather
The truncated and evolving inner accretion disc of the black hole GX 339-4
The nature of accretion onto stellar mass black holes in the low/hard state
remains unresolved, with some evidence suggesting that the inner accretion disc
is truncated and replaced by a hot flow. However, the detection of relativistic
broadened Fe emission lines, even at relatively low luminosities, seems to
require an accretion disc extending fully to its innermost stable circular
orbit. Modelling such features is however highly susceptible to degeneracies,
which could easily bias any interpretation. We present the first systematic
study of the Fe line region to track how the inner accretion disc evolves in
the low/hard state of the black hole GX 3394. Our four observations display
increased broadening of the Fe line over two magnitudes in luminosity, which we
use to track any variation of the disc inner radius. We find that the disc
extends closer to the black hole at higher luminosities, but is consistent with
being truncated throughout the entire low/hard state, a result which renders
black hole spin estimates inaccurate at these stages of the outburst.
Furthermore, we show that the evolution of our spectral inner disc radius
estimates corresponds very closely to the trend of the break frequency in
Fourier power spectra, supporting the interpretation of a truncated and
evolving disc in the hard state.Comment: Accepted for publication in A&A. Some typos corrected from version
Revealing accretion onto black holes: X-ray reflection throughout three outbursts of GX 339-4
Understanding the dynamics behind black hole state transitions and the
changes they reflect in outbursts has become long-standing problem. The X-ray
reflection spectrum describes the interaction between the hard X-ray source
(the power-law continuum) and the cool accretion disc it illuminates, and thus
permits an indirect view of how the two evolve. We present a systematic
analysis of the reflection spectrum throughout three outbursts (500+
observations) of the black hole binary GX 339-4, representing the largest study
applying a self-consistent treatment of reflection to date. Particular
attention is payed to the coincident evolution of the power-law and reflection,
which can be used to determine the accretion geometry. The hard state is found
to be distinctly reflection weak, however the ratio of reflection to power-law
gradually increases as the source luminosity rises. In contrast the reflection
is found dominate the power-law throughout most of the soft state, with
increasing supremacy as the source decays. We discuss potential dynamics
driving this, favouring inner disc truncation and decreasing coronal height for
the hard and soft states respectively. Evolution of the ionisation parameter,
power-law slope and high-energy cut-off also agree with this interpretation.Comment: Accepted for publication in MNRA
Stochastic parameterization: uncertainties from convection
In 2005, the ECMWF held a workshop on stochastic parameterisation, at which the convection was seen as being
a key issue. That much is clear from the working group reports and particularly the statement from working group
1 that âit is clear that a stochastic convection scheme is desirableâ. The present note aims to consider our current
status in comparison with some of the issues raised and hopes expressed in that working group report
A PC parallel port button box provides millisecond response time accuracy under Linux
For psychologists, it is sometimes necessary to measure people's reaction times to the nearest millisecond. This article describes how to use the PC parallel port to receive signals from a button box to achieve millisecond response time accuracy. The workings of the parallel port, the corresponding port addresses, and a simple Linux program for controlling the port are described. A test of the speed and reliability of button box signal detection is reported. If the reader is moderately familiar with Linux, this article should provide sufficient instruction for him or her to build and test his or her own parallel port button box. This article also describes how the parallel port could be used to control an external apparatus
Studying the Solar system with the International Pulsar Timing Array
Pulsar-timing analyses are sensitive to errors in the Solar-system ephemerides (SSEs) that timing models utilize to estimate the location of the Solar-system barycentre, the quasi-inertial reference frame to which all recorded pulse times-of-arrival are referred. Any error in the SSE will affect all pulsars, therefore pulsar timing arrays (PTAs) are a suitable tool to search for such errors and impose independent constraints on relevant physical parameters. We employ the first data release of the International Pulsar Timing Array to constrain the masses of the planetâmoons systems and to search for possible unmodelled objects (UMOs) in the Solar system. We employ 10 SSEs from two independent research groups, derive and compare mass constraints of planetary systems, and derive the first PTA mass constraints on asteroid-belt objects. Constraints on planetary-system masses have been improved by factors of up to 20 from the previous relevant study using the same assumptions, with the mass of the Jovian system measured at 9.5479189(3) Ă 10^(â4)âMâ. The mass of the dwarf planet Ceres is measured at 4.7(4) Ă 10^(â10)âMâ. We also present the first sensitivity curves using real data that place generic limits on the masses of UMOs, which can also be used as upper limits on the mass of putative exotic objects. For example, upper limits on dark-matter clumps are comparable to published limits using independent methods. While the constraints on planetary masses derived with all employed SSEs are consistent, we note and discuss differences in the associated timing residuals and UMO sensitivity curves
Results of an aqueous source term model for a radiological risk assessment of the Drigg LLW Site, U.K.
A radionuclide source term model has been developed which simulates the biogeochemical evolution of the Drigg low level waste (LLW) disposal site. The DRINK (DRIgg Near field Kinetic) model provides data regarding radionuclide concentrations in groundwater over a period of 100,000 years, which are used as input to assessment calculations for a groundwater pathway. The DRINK model also provides input to human intrusion and gaseous assessment calculations through simulation of the solid radionuclide inventory. These calculations are being used to support the Drigg post closure safety case. The DRINK model considers the coupled interaction of the effects of fluid flow, microbiology, corrosion, chemical reaction, sorption and radioactive decay. It represents the first direct use of a mechanistic reaction-transport model in risk assessment calculations
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