2,555 research outputs found
Self-Consistent Thermal Accretion Disk Corona Models for Compact Objects: I. Properties of the Corona and the Spectrum of Escaping Radiation
We present the properties of accretion disk corona (ADC) models, where the
radiation field, the temperature, and the total opacity of the corona are
determined self-consistently. We use a non-linear Monte Carlo code to perform
the calculations. As an example, we discuss models where the corona is situated
above and below a cold accretion disk with a plane-parallel (slab) geometry,
similar to the model of Haardt and Maraschi. By Comptonizing the soft radiation
emitted by the accretion disk, the corona is responsible for producing the
high-energy component of the escaping radiation. Our models include the
reprocessing of radiation in the accretion disk. Here, the photons either are
Compton reflected or photo-absorbed, giving rise to fluorescent line emission
and thermal emission. The self-consistent coronal temperature is determined by
balancing heating (due to viscous energy dissipation) with Compton cooling,
determined using the fully relativistic, angle-dependent cross-sections. The
total opacity is found by balancing pair productions with annihilations. We
find that, for a disk temperature kT_bb \lta 200 eV, these coronae are unable
to have a self-consistent temperature higher than \sim 120 keV if the total
optical depth is \gta 0.2, regardless of the compactness parameter of the
corona and the seed opacity. This limitation corresponds to the angle-averaged
spectrum of escaping radiation having a photon index \gta 1.8 within the 5 keV
- 30 keV band. Finally, all models that have reprocessing features also predict
a large thermal excess at lower energies. These constraints make explaining the
X-ray spectra of persistent black hole candidates with ADC models very
problematic.Comment: 15 pages, Latex, 9 .eps figures, uses emulateapj.sty (included). To
be published in ApJ, October 1, 1997, Vol. 48
Determination of stress-strain characteristics of materials subjected to dynamic loading
In this thesis a method for predicating the stress-strain diagram for a material subjected to a dynamic load has been developed. This method involves the concept of unrelaxed and relaxed stresses within the material as well as the concept of a material property which determines how the relaxation of stress progresses with time. Theoretical background for these concepts may be found in the theory of anelasticity. Equations for evaluating the stress (sigma) associated with a given strain (epsilon) at a given time (t) have been developed for selected epsilon - t curves (Figures 1, 2, 3). The general differential equation used in obtaining the sigma - epsilon - t relationships for these selected epsilon - t curves is discussed in detail (equation 20). This general differential equation may be used to develop the sigma - epsilon - t relationship for any epsilon - t curve; numerical methods may have to be resorted to, however;An experimental technique for evaluating the material properties (f u(epsilon), fr(epsilon),gamma) involved in the sigma - epsilon - t relationships was developed and applied using rubber as the test material. Using the material properties determined by this experimental technique the theoretical sigma - epsilon - t relationships were used to predict the sigma - epsilon - t curves associated with various constant strain rates and with a constant rate strain cycle. These predicted sigma - epsilon curves were compared to experimentally determined curves. The velocity of strain wave propagation was studied experimentally to establish what, if any, relationship exists between the unrelaxed elastic modulus (Eu) and the velocity of wave propagation;The agreement between the theoretical sigma - epsilon curves, and the experimental curves (Figures 18 and 21) justifies the conclusion that, at laser for the material used in these tests and within the range of strains and strain rates investigated, the proposed method is suitable for predicting in advance how the rate of straining will affect sigma - epsilon characteristics. In the author\u27s opinion the results obtained in applying the proposed method to rubber are adequate to justify additional study in which the possibility of applying the proposed method to other materials would be investigated. A method similar to the one proposed in this thesis has been applied to the study of metals in Russia (44)
First and second order magnetic and structural transitions in BaFeCoAs
We present here high resolution magnetization measurements on high-quality
BaFeCoAs, 0x0.046 as-grown single crystals.
The results confirm the existence of a magnetic tricritical point in the
(,) plane at x0.022 and reveal the emergence of the
heat capacity anomaly associated with the onset of the structural transition at
x0.0064. We show that the samples with doping near x
do not show superconductivity, but rather superconductivity emerges at a
slightly higher cobalt doping, x0.0315Comment: 4 pages, 5 figure
Recommended from our members
Meta-analysis reveals ammonia-oxidizing bacteria respond more strongly to nitrogen addition than ammonia-oxidizing archaea
Shifts in microbial communities driven by anthropogenic nitrogen (N) addition have broad-scale ecological consequences. However, responses of microbial groups to exogenous N supply vary considerably across studies, hindering efforts to predict community changes. We used meta-analytical techniques to explore how amoA gene abundances of ammonia-oxidizing archaea (AOA) and bacteria (AOB) respond to N addition, and found that N addition increased AOA and AOB abundances by an average of 27% and 326%, respectively. Responses of AOB varied by study type, ecosystem, fertilizer type, and soil pH, and were strongest in unmanaged wildland soils and soils fertilized with inorganic N sources. Increases in nitrification potential with N addition significantly correlated with only AOB. Our analyses suggest that elevated N supply enhances soil nitrification potential by increasing AOB populations, and that this effect may be most pronounced in unmanaged wildland soils
Floodlight Quantum Key Distribution: A Practical Route to Gbps Secret-Key Rates
The channel loss incurred in long-distance transmission places a significant
burden on quantum key distribution (QKD) systems: they must defeat a passive
eavesdropper who detects all the light lost in the quantum channel and does so
without disturbing the light that reaches the intended destination. The current
QKD implementation with the highest long-distance secret-key rate meets this
challenge by transmitting no more than one photon per bit [Opt. Express 21,
24550-24565 (2013)]. As a result, it cannot achieve the Gbps secret-key rate
needed for one-time pad encryption of large data files unless an impractically
large amount of multiplexing is employed. We introduce floodlight QKD (FL-QKD),
which floods the quantum channel with a high number of photons per bit
distributed over a much greater number of optical modes. FL-QKD offers security
against the optimum frequency-domain collective attack by transmitting less
than one photon per mode and using photon-coincidence channel monitoring, and
it is completely immune to passive eavesdropping. More importantly, FL-QKD is
capable of a 2 Gbps secret-key rate over a 50 km fiber link, without any
multiplexing, using available equipment, i.e., no new technology need be
developed. FL-QKD achieves this extraordinary secret-key rate by virtue of its
unprecedented secret-key efficiency, in bits per channel use, which exceeds
those of state-of-the-art systems by two orders of magnitude.Comment: 18 pages, 5 figure
Solid-state diffusion in amorphous zirconolite
his research utilised Queen Mary's MidPlus computational facilities, supported by QMUL Research-IT and funded by EPSRC grant EP/K000128/1. We are grateful to E. Maddrell for discussions and to CSC for support
RXTE Observation of Cygnus X-1: Spectral Analysis
We present the results of the analysis of the broad-band spectrum of Cygnus
X-1 from 3.0 to 200 keV, using data from a 10 ksec observation by the Rossi
X-ray Timing Explorer. The spectrum can be well described phenomenologically by
an exponentially cut-off power law with a photon index Gamma = 1.45 +/- 0.02 (a
value considerably harder than typically found), e-folding energy E_fold = 162
+/- 9 keV, plus a deviation from a power law that formally can be modeled as a
thermal blackbody with temperature kT_bb = 1.2 +/1 0.2 keV. Although the 3 - 30
keV portion of the spectrum can be fit with a reflected power law with Gamma =
1.81 +/- 0.01 and covering fraction f = 0.35 +/- 0.02, the quality of the fit
is significantly reduced when the HEXTE data in the 30 - 100 keV range is
included, as there is no observed hardening in the power law within this energy
range. As a physical description of this system, we apply the accretion disc
corona models of Dove, Wilms & Begelman (1997) --- where the temperature of the
corona is determined self-consistently. A spherical corona with a total optical
depth tau = 1.6 +/- 0.1 and an average temperature kT_c = 87 +/- 5 keV,
surrounded by an exterior cold disc, does provide a good description of the
data (reduced chi-squared = 1.55). These models deviate from the data by up to
7% in the 5 - 10 keV range, and we discuss possible reasons for these
discrepancies. However, considering how successfully the spherical corona
reproduces the 10 - 200 keV data, such ``photon-starved'' coronal geometries
seem very promising for explaining the accretion processes of Cygnus X-1.Comment: Revised version (added content). 8 pages, 6 figures, 1 table.tex
file, latex, uses mn.sty. Accepted for publication in MNRA
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