2,360 research outputs found
Radiation processes around accreting black holes
Accreting sources such as AGN, X-ray binaries or gamma-ray bursts are known
to be strong, high energy emitters. The hard emission is though to originate
from plasmas of thermal and/or non-thermal high energy particles. Not only does
this emission allow to probe the unique properties of the matter in an extreme
environment, but it also has a crucial backreaction on the energetics and the
dynamics of the emitting medium itself. Understanding interactions between
radiation and matter has become a key issue in the modelling of high energy
sources. Although most cross sections are well known, they are quite complex
and the way all processes couple non-linearly is still an open issue.
We present a new code that solves the local, kinetic evolution equations for
distributions of electrons, positrons and photons, interacting by radiation
processes such as self-absorbed synchrotron and brems-strahlung radiation,
Compton scattering, pair production/annihilation, and by Coulomb collisions.
The code is very general and aimed to modelled various high energy sources. As
an application, we study the spectral states of X-ray binaries, including
thermalization by Coulomb collisions and synchrotron self-absorption. It is
found that the low-hard and high-soft states can be modelled with different
illumination but the same non-thermal acceleration mechanism.Comment: 4 pages, 2 figures, proceedings of the SF2A conference 200
The existence of warm and optically thick dissipative coronae above accretion disks
In the past years, several observations of AGN and X-ray binaries have
suggested the existence of a warm T around 0.5-1 keV and optically thick, \tau
~ 10-20, corona covering the inner parts of the accretion disk. These
properties are directly derived from spectral fitting in UV to soft-X-rays
using Comptonization models. However, whether such a medium can be both in
radiative and hydrostatic equilibrium with an accretion disk is still
uncertain. We investigate the properties of such warm, optically thick coronae
and put constraints on their existence. We solve the radiative transfer
equation for grey atmosphere analytically in a pure scattering medium,
including local dissipation as an additional heating term in the warm corona.
The temperature profile of the warm corona is calculated assuming it is cooled
by Compton scattering, with the underlying dissipative disk providing photons
to the corona. Our analytic calculations show that a dissipative thick,
(\tau_{cor} ~ 10-12) corona on the top of a standard accretion disk can reach
temperatures of the order of 0.5-1 keV in its upper layers provided that the
disk is passive. But, in absence of strong magnetic fields, the requirement of
a Compton cooled corona in hydrostatic equilibrium in the vertical direction
sets an upper limit on the Thomson optical depth \tau_{cor} < 5 . We show this
value cannot be exceeded independently of the accretion disk parameters.
However, magnetic pressure can extend this result to larger optical depths.
Namely, a dissipative corona might have an optical depth up to ~ 20 when the
magnetic pressure is 100 times higher that the gas pressure. The observation of
warm coronae with Thomson depth larger than ~ 5 puts tights constraints on the
physics of the accretion disk/corona systems and requires either strong
magnetic fields or vertical outflows to stabilize the system.Comment: 9 pages 6 figure, submitted to A&A, comments are welcom
STUDY OF A SINGLE-CHARGED IONS ECR SOURCE MATCHING OF THE EXTRACTED BEAM TO AN ISOTOPE SEPARATOR
A new ECR ion-source has been designed and studied for single-charged ion beams. A very stable regime has been obtained with an ion-source made of two identical stages in cascade. The RF power supplies consist of two 2.45 GHz magnetrons. The discharge chamber is made of two coaxial Pyrex tubes. The external one ensures vacuum and HT insulation. The tubes are aligned inside the two multimode cavities axially limited by three magnetic coils. The ion beam is extracted at 20 kV and focused with electric lenses. For argon and xenon, 1 mA single-charged ion currents have been extracted. The influence of various parameters has been progressively achieved with a set-up including a 60° analyzing magnet and with the 120° on-line isotope separator at SARA. From emittances and images observed it appears difficult to compensate charge space effects. Suggestions and future developments are proposed to improve qualities of the isotopic separation
Simulating acceleration and radiation processes in X-ray binaries
The high energy emission of microquasars is thought to originate from high
energy particles. Depending on the spectral state, the distribution of these
particles can be thermal with a high temperature (typically 100 keV) or
non-thermal and extending to even higher energy. The properties of high energy
plasmas are governed by a rich microphysics involving particle-particle
collisions and particles-photons interactions.
We present a new code developed to address the evolution of relativistic
plasmas. This one-zone code focuses on the microphysics and solves the coupled
kinetic equations for particles and photons, including Compton scattering,
synchrotron emission and absorption, pair production and annihilation,
bremsstrahlung emission and absorption, Coulomb interactions, and prescriptions
for additional particle acceleration and heating. It can in particular describe
mechanisms such a thermalisation by synchrotron self-absorption and Coulomb
collisions.
Using the code, we investigate whether various acceleration processes, namely
thermal heating, non-thermal acceleration and stochastic acceleration, can
reproduce the different spectral states of microquasars. Premilinary results
are presented.Comment: 9 pages, 6 figures, proceedings of the VII Microquasar Workshop:
Microquasars and Beyond, September 1-5 2008, Foca, Izmir, Turkey; accepted
for publication in Po
Scaling of the electron dissipation range of solar wind turbulence
Electron scale solar wind turbulence has attracted great interest in recent
years. Clear evidences have been given from the Cluster data that turbulence is
not fully dissipated near the proton scale but continues cascading down to the
electron scales. However, the scaling of the energy spectra as well as the
nature of the plasma modes involved at those small scales are still not fully
determined. Here we survey 10 years of the Cluster search-coil magnetometer
(SCM) waveforms measured in the solar wind and perform a statistical study of
the magnetic energy spectra in the frequency range []Hz. We show that a
large fraction of the spectra exhibit clear breakpoints near the electon
gyroscale , followed by steeper power-law like spectra. We show that
the scaling below the electron breakpoint cannot be determined unambiguously
due to instrumental limitations that will be discussed in detail. We compare
our results to recent ones reported in other studies and discuss their
implication on the physical mechanisms and the theoretical modeling of energy
dissipation in the SW.Comment: 10 pages, submitte
On Random Sampling and Fourier Transform Estimation in Sea Waves Prediction
Improving the safety of a wide range of launch and recovery operations is of great international maritime interest. Deterministic sea wave prediction (DSWP) is a relatively new branch of science that can offer such opportunities by predicting the actual shape of the sea surface and its evolution for short time in the future. Fourier transform technique is the main building block in DSWP, which requires measurements of the sea surface. Nonetheless, uniformly sampled measurements of the sea surface cannot be practically achieved for various reasons. Conventional X-band radars are the most realistic candidate to provide a low-cost convenient source of two-dimensional wave profile information for DSWP purposes. Ship movement and mechanically rotating scanning antennas are among sources of irregularity in sea surface sampling. This in turn introduces errors when traditional Fourier transform based wave prediction methods are used. In this paper we show that by modelling the radar sampling instants as random variables and using the estimator of Tarczynski and Allay to process the samples, a reliable solution for DSWP can be constituted
Properties of AGN coronae in the NuSTAR era â II. Hybrid plasma
The corona, a hot cloud of electrons close to the centre of the accretion disc, produces the hard X-ray power-law continuum commonly seen in luminous active galactic nuclei. The continuum has a high-energy turnover, typically in the range of one to several 100 keV and is suggestive of Comptonization by thermal electrons. We are studying hard X-ray spectra of AGN obtained with NuSTAR after correction for X-ray reflection and under the assumption that coronae are compact, being only a few gravitational radii in size as indicated by reflection and reverberation modelling. Compact coronae raise the possibility that the temperature is limited and indeed controlled by electronâpositron pair production, as explored earlier (Paper I). Here, we examine hybrid plasmas in which a mixture of thermal and non-thermal particles is present. Pair production from the non-thermal component reduces the temperature leading to a wider temperature range more consistent with observations
Mild clinical course of covid-19 in 3 patients receiving therapeutic monoclonal antibodies targeting c5 complement for hematologic disorders
© Am J Case Rep, 2020. Objective: Rare co-existance of disease or pathology Background: Patients receiving immunosuppressive therapies might be more susceptible to COVID-19. Conversely, an exaggerated inflammatory response to the SARS-CoV-2 infection might be blunted by certain forms of immunosuppression, which could be protective. Indeed, there are data from animal models demonstrating that complement may be a part of the pathophysiology of coronavirus infections. There is also evidence from an autopsy series demonstrating complement deposition in the lungs of patients with COVID-19. This raises the question of whether patients on anti-complement therapy could be protected from COVID-19. Case Reports: Case 1 is a 39-year-old woman with an approximately 20-year history of paroxysmal nocturnal hemoglobinuria (PNH), who had recently been switched from treatment with eculizumab to ravulizumab prior to SARS-CoV-2 infection. Case 2 is a 54-year-old woman with a cadaveric renal transplant for lupus nephritis, complicated by thrombotic microangiopathy, who was maintained on eculizumab, which she started several months before she developed the SARS-CoV-2 infection. Case 3 is a 60-year-old woman with a 14-year history of PNH, who had been treated with eculizumab since 2012, and was diagnosed with COVID-19 at the time of her scheduled infusion. All 3 patients had a relatively mild course of COVID-19. Conclusions: We see no evidence of increased susceptibility to SARS-CoV-2 in these patients on anti-complement therapy, which might actually have accounted for the mild course of infection. The effect of anti-complement therapy on COVID-19 disease needs to be determined in clinical trials
Cross-species analysis of genetically engineered mouse models of MAPK-driven colorectal cancer identifies hallmarks of the human disease
Effective treatment options for advanced colorectal cancer (CRC) are limited, survival rates are poor and this disease continues to be a leading cause of cancer-related deaths worldwide. Despite being a highly heterogeneous disease, a large subset of individuals with sporadic CRC typically harbor relatively few established âdriverâ lesions. Here, we describe a collection of genetically engineered mouse models (GEMMs) of sporadic CRC that combine lesions frequently altered in human patients, including well-characterized tumor suppressors and activators of MAPK signaling. Primary tumors from these models were profiled, and individual GEMM tumors segregated into groups based on their genotypes. Unique allelic and genotypic expression signatures were generated from these GEMMs and applied to clinically annotated human CRC patient samples. We provide evidence that a Kras signature derived from these GEMMs is capable of distinguishing human tumors harboring KRAS mutation, and tracks with poor prognosis in two independent human patient cohorts. Furthermore, the analysis of a panel of human CRC cell lines suggests that high expression of the GEMM Kras signature correlates with sensitivity to targeted pathway inhibitors. Together, these findings implicate GEMMs as powerful preclinical tools with the capacity to recapitulate relevant human disease biology, and support the use of genetic signatures generated in these models to facilitate future drug discovery and validation efforts
A Hot Helium Plasma in the Galactic Center Region
Recent X-ray observations by the space mission Chandra confirmed the
astonishing evidence for a diffuse, hot, thermal plasma at a temperature of 9.
K (8 keV) found by previous surveys to extend over a few hundred parsecs
in the Galactic Centre region. This plasma coexists with the usual components
of the interstellar medium such as cold molecular clouds and a soft (~0.8 keV)
component produced by supernova remnants, and its origin remains uncertain.
First, simple calculations using a mean sound speed for a hydrogen-dominated
plasma have suggested that it should not be gravitationally bound, and thus
requires a huge energy source to heat it in less than the escape time. Second,
an astrophysical mechanism must be found to generate such a high temperature.
No known source has been identified to fulfill both requirements. Here we
address the energetics problem and show that the hot component could actually
be a gravitationally confined helium plasma. We illustrate the new prospects
this opens by discussing the origin of this gas, and by suggesting possible
heating mechanisms.Comment: 9 pages, accepted for publication in APJ
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