679 research outputs found
Approximate Solutions of the Boltzmann Equation for Secondary Electron Emission: Results and Comparisons to Experiments
The aim of the present paper is to survey the theoretical work performed in Brussels about secondary electron emission (SEE). Two new approximate solutions of the Boltzmann equation for internal secondary electrons are applied to both electron and ion induced SEE. Using a realistic set of interaction cross sections, most calculated characteristics of electron emission compare fairly well to experiments.
The improved age-diffusion model can be used to calculate the electron yield, the energy and angular spectrum and also the depth and radial distributions of outgoing electrons for incident electrons and ions.
The transport-albedo model assumes an uniform internal electron source in a semiinfinite medium and gives the electron yield and the energy spectrum of secondary electrons for incident light ions. Taking into account the anisotropy of the internal electron source, the ratio of the forward and backward yields and the influence of the angle of incidence have been calculated for thin targets
Extreme Blazars Studied with Fermi-LAT and Suzaku: 1ES 0347-121 and Blazar Candidate HESS J1943+213
We report on our study of high-energy properties of two peculiar TeV
emitters: the "extreme blazar" 1ES 0347-121 and the "extreme blazar candidate"
HESS J1943+213 located near the Galactic Plane. Both objects are characterized
by quiescent synchrotron emission with flat spectra extending up to the hard
X-ray range, and both were reported to be missing GeV counterparts in the
Fermi-LAT 2-year Source Catalog. We analyze a 4.5 year accumulation of the
Fermi-LAT data, resulting in the detection of 1ES 0347-121 in the GeV band, as
well as in improved upper limits for HESS J1943+213. We also present the
analysis results of newly acquired Suzaku data for HESS J1943+213. The X-ray
spectrum is well represented by a single power law extending up to 25 keV with
photon index 2.00+/-0.02 and a moderate absorption in excess of the Galactic
value, in agreement with previous X-ray observations. No short-term X-ray
variability was found over the 80 ks duration of the Suzaku exposure. Under the
blazar hypothesis, we modeled the spectral energy distributions of 1ES 0347-121
and HESS J1943+213, and derived constraints on the intergalactic magnetic field
strength and source energetics. We conclude that although the classification of
HESS J1943+213 has not yet been determined, the blazar hypothesis remains the
most plausible option, since in particular the broad-band spectra of the two
analyzed sources along with the source model parameters closely resemble each
other, and the newly available WISE and UKIDSS data for HESS J1943+213 are
consistent with the presence of an elliptical host at the distance of
approximately ~600 Mpc.Comment: 10 pages, 7 figures, 3 tables. Accepted by Ap
Hypoxia promotes chemoresistance in acute lymphoblastic leukemia cell lines by modulating death signaling pathways
International audienceBackground: Several studies show that bone marrow (BM) microenvironment and hypoxia condition can promote the survival of leukemic cells and induce resistance to anti-leukemic drugs. However, the molecular mechanism for chemoresistance by hypoxia is not fully understood.Methods: In the present study, we investigated the effect of hypoxia on resistance to two therapies, methotrexate (MTX) and prednisolone (PRD), in two cell models for acute lymphoblastic leukemia (ALL). To look for an implication of hypoxia in chemoresistance, cell viability, total cell density and cell proliferation were analyzed. Survival and death signaling pathways were also screened by "reverse phase protein array" (RPPA) and western blotting experiments conducted on selected proteins to confirm the results.Results: We found that hypoxia promotes chemoresistance in both ALL cell lines. The induction of drug-resistance by hypoxia was not associated with an increase in total cell density nor an increase in cell proliferation. Using RPPA, we show that chemoresistance induced by hypoxia was mediated through an alteration of cell death signaling pathways. This protective effect of hypoxia seems to occur via a decrease in pro-apoptotic proteins and an increase in anti-apoptotic proteins. The results were confirmed by immunoblotting. Indeed, hypoxia is able to modulate the expression of anti-apoptotic proteins independently of chemotherapy while a pro-apoptotic signal induced by a chemotherapy is not modulated by hypoxia.Conclusions: Hypoxia is a factor in leukemia cell resistance and for two conventional chemotherapies modulates cell death signaling pathways without affecting total cell density or cell proliferation
Spitzer Space Telescope Observations of the Magnetic Cataclysmic Variable AE Aqr
The magnetic cataclysmic variable AE Aquarii hosts a rapidly rotating white
dwarf which is thought to expel most of the material streaming onto it.
Observations of AE Aqr have been obtained in the wavelength range of 5 - 70
microns with the IRS, IRAC, and MIPS instruments on board the Spitzer Space
Telescope. The spectral energy distribution reveals a significant excess above
the K4V spectrum of the donor star with the flux increasing with wavelength
above 12.5 microns. Superposed on the energy distribution are several hydrogen
emission lines, identified as Pf alpha and Hu alpha, beta, gamma. The infrared
spectrum above 12.5 microns can be interpreted as synchrotron emission from
electrons accelerated to a power-law distribution dN=E^{-2.4}dE in expanding
clouds with an initial evolution timescale in seconds. However, too many
components must then be superposed to explain satisfactorily both the
mid-infrared continuum and the observed radio variability. Thermal emission
from cold circumbinary material can contribute, but it requires a disk
temperature profile intermediate between that produced by local viscous
dissipation in the disk and that characteristic of a passively irradiated disk.
Future high-time resolution observations spanning the optical to radio regime
could shed light on the acceleration process and the subsequent particle
evolution.Comment: 15 pages, 3 figures, accepted for publication in Ap
VLT spectroscopy of XTE J2123-058 during quiescence
We present VLT low resolution spectroscopy of the neutron star X-ray
transient XTE J2123-058 during its quiescent state. Our data reveal the
presence of a K7V companion which contributes 77 % to the total flux at 6300 A
and orbits the neutron star at K_2 = 287 +/- 12 km/s. Contrary to other soft
X-ray transients (SXTs), the Halpha emission is almost exactly in antiphase
with the velocity curve of the optical companion. Using the light-center
technique we obtain K_1 = 140 +/- 27 km/s and hence q=K_1/K_2=M_2/M_1= 0.49 +/-
0.10. This, combined with a previous determination of the inclination angle
(i=73 +/- 4) yields M_1 = 1.55 +/- 0.31 Msun and M_2 = 0.76 +/- 0.22 Msun. M_2
agrees well with the observed spectral type. Doppler tomography of the Halpha
emission shows a non-symmetric accretion disc distribution mimicking that seen
in SW Sex stars. Although we find a large systemic velocity of -110 +/- 8 km/s
this value is consistent with the galactic rotation velocity at the position of
J2123-058, and hence a halo origin. The formation scenario of J2123-058 is
still unresolved.Comment: 10 pages, 3 figures, accepted for publication in MNRAS with very
minor change
An alpha theory of time-dependent warped accretion discs
The non-linear fluid dynamics of a warped accretion disc was investigated in
an earlier paper by developing a theory of fully non-linear bending waves in a
thin, viscous disc. That analysis is here extended to take proper account of
thermal and radiative effects by solving an energy equation that includes
viscous dissipation and radiative transport. The problem is reduced to simple
one-dimensional evolutionary equations for mass and angular momentum, expressed
in physical units and suitable for direct application. This result constitutes
a logical generalization of the alpha theory of Shakura & Sunyaev to the case
of a time-dependent warped accretion disc. The local thermal-viscous stability
of such a disc is also investigated.Comment: 16 pages, 3 figures, to be published in MNRA
Search for radio pulsations in LS I +61 303
Context. LS I +61 303 is a member of the select group of gamma-ray binaries:
galactic binary systems that contain a massive star and a compact object, show
a changing milliarcsecond morphology and a similar broad spectral energy
distribution (SED) that peaks at MeV-TeV energies and is modulated by the
orbital motion. The nature of the compact object is unclear in LS I +61 303, LS
5039 and HESS J0632+057, whereas PSR B1259-63 harbours a 47.74 ms radio pulsar.
Aims. A scenario in which a young pulsar wind interacts with the stellar wind
has been proposed to explain the very high energy (VHE, E > 100 GeV) gamma-ray
emission detected from LS I +61 303, although no pulses have been reported from
this system at any wavelength. We aim to find evidence of the pulsar nature of
the compact object. Methods. We performed phased array observations with the
Giant Metrewave Radio Telescope (GMRT) at 1280 MHz centred at phase 0.54.
Simultaneous data from the multi-bit phased array (PA) back-end with a sampling
time of tsamp = 128 microsec and from the polarimeter (PMT) back-end with tsamp
= 256 microsec where taken. Results. No pulses have been found in the data set,
with a minimum detectable mean flux density of \sim 0.38 mJy at 8-sigma level
for the pulsed emission from a putative pulsar with period P >2 ms and duty
cycle D = 10% in the direction of LS I +61 303. Conclusions. The detection of
posible radio pulsations will require deep and sensitive observations at
frequencies \sim0.5-5 GHz and orbital phases 0.6-0.7. However, it may be
unfeasible to detect pulses if the putative pulsar is not beamed at the Earth
or if there is a strong absorption within the binary system.Comment: 8 pages. 5 figures. Accepted for publication in Astronomy and
Astrophysic
Characterising anomalous transport in accretion disks from X-ray observations
Whilst direct observations of internal transport in accretion disks are not yet possible, measurement of the energy emitted from accreting astrophysical systems can provide useful information on the physical mechanisms at work. Here we examine the unbroken multi-year time variation of the total X-ray flux from three sources: Cygnus X-1 , the microquasar GRS 1915+105 , and for comparison the nonaccreting Crab nebula. To complement previous analyses, we demonstrate that the application of advanced statistical methods to these observational time-series reveals important contrasts in the nature and scaling properties of the transport processes operating within these sources. We find the Crab signal resembles Gaussian noise; the Cygnus X-1 signal is a leptokurtic random walk whose self-similar properties persist on timescales up to three years; and the GRS 1915+105 signal is similar to that from Cygnus X-1, but with self-similarity extending possibly to only a few days. This evidence of self-similarity provides a robust quantitative characterisation of anomalous transport occuring within the systems
XMM-Newton Observations of the Ultraluminous Nuclear X-ray Source in M33
We present observations with XMM-Newton of M33 X-8, the ultraluminous X-ray
source (L_{0.5-10 keV} ~ 2x10^39 erg/s) closest to the centre of the galaxy.
The best-fit model is similar to the typical model of Galactic black holes in
very high state. Comparison with previous observations indicates that the
source is still in a very high state after about 20 years of observations. No
state transition has been observed even during the present set of XMM-Newton
observations. We estimate the lower limit of the mass of the black hole >6
M_sun, but with proper parameters taking into account different effects, the
best estimate becomes 12 M_sun. Our analysis favours the hypothesis that M33
X-8 is a stellar mass black hole candidate, in agreement with the findings of
other authors. In addition, we propose a different model where the high
luminosity of the source is likely to be due to orientation effects of the
accretion disc and anisotropies in the Comptonized emission.Comment: 9 pages, 5 figures. Accepted for publication on A&A Main Journal. A
bug in the introduction has been corrected (a citation
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