1,315 research outputs found
The High Energy Emission of the Crab Nebula from 20 keV to 6 MeV with INTEGRAL
The SPI spectrometer aboard the INTEGRAL mission observes regularly the Crab
Nebula since 2003. We report on observations distributed over 5.5 years and
investigate the variability of the intensity and spectral shape of this
remarkable source in the hard X-rays domain up to a few MeV. While single power
law models give a good description in the X-ray domain (mean photon index ~
2.05) and MeV domain (photon index ~ 2.23), crucial information are contained
in the evolution of the slope with energy between these two values. This study
has been carried out trough individual observations and long duration (~ 400
ks) averaged spectra. The stability of the emission is remarkable and excludes
a single power law model. The slopes measured below and above 100 keV agree
perfectly with the last values reported in the X-ray and MeV regions
respectively, but without indication of a localized break point. This suggests
a gradual softening in the emission around 100 keV and thus a continuous
evolution rather than an actual change in the mechanism parameters. In the MeV
region, no significant deviation from the proposed power law model is visible
up to 5-6 MeV. Finally, we take advantage of the spectroscopic capability of
the instrument to seek for previously reported spectral features in the covered
energy range with negative results for any significant cyclotron or
annihilation emission on 400 ks timescales. Beyond the scientific results, the
performance and reliability of the SPI instrument is explicitly demonstrated,
with some details about the most appropriate analysis method.Comment: accepted for publication in ApJ; 4 figures, 2 table
The emission of Cygnus X-1: observations with INTEGRAL SPI from 20 keV to 2 MeV
We report on Cyg X-1 observations performed by the SPI telescope onboard the
INTEGRAL mission and distributed over more than 6 years. We investigate the
variability of the intensity and spectral shape of this peculiar source in the
hard X-rays domain, and more particularly up to the MeV region. We first study
the total averaged spectrum which presents the best signal to noise ratio (4 Ms
of data). Then, we refine our results by building mean spectra by periods and
gathering those of similar hardness.
Several spectral shapes are observed with important changes in the curvature
between 20 and 200 keV, even at the same luminosity level. In all cases, the
emission decreases sharply above 700 keV, with flux values above 1 MeV (or
upper limits) well below the recently reported polarised flux (Laurent et al.
2011), while compatible with the MeV emission detected some years ago by
CGRO/COMPTEL (McConnell et al., 2002).
Finally, we take advantage of the spectroscopic capability of the instrument
to seek for spectral features in the 500 keV region with negative results for
any significant annihilation emission on 2 ks and days timescales, as well as
in the total dataset.Comment: 14 pages, 10 figures, accepted for publication in Ap
Absolute timing of the Crab pulsar with the INTEGRAL/SPI telescope
We have investigated the pulse shape evolution of the Crab pulsar emission in
the hard X-ray domain of the electromagnetic spectrum. In particular, we have
studied the alignment of the Crab pulsar phase profiles measured in the hard
X-rays and in other wavebands. To obtain the hard X-ray pulse profiles, we have
used six year (2003-2009, with a total exposure of about 4 Ms) of publicly
available data of the SPI telescope on-board of the INTEGRAL observatory,
folded with the pulsar time solution derived from the Jodrell Bank Crab Pulsar
Monthly Ephemeris. We found that the main pulse in the hard X-ray 20-100 keV
energy band is leading the radio one by milliperiods in phase, or
in time. Quoted errors represent only statistical
uncertainties.Our systematic error is estimated to be and is
mainly caused by the radio measurement uncertainties. In hard X-rays, the
average distance between the main pulse and interpulse on the phase plane is
. To compare our findings in hard X-rays with the soft 2-20
keV X-ray band, we have used data of quasi-simultaneous Crab observations with
the PCA monitor on-board the Rossi X-Ray Timing Explorer (RXTE) mission. The
time lag and the pulses separation values measured in the 3-20 keV band are
(corresponding to ) and
parts of the cycle, respectively. While the pulse separation values measured in
soft X-rays and hard X-rays agree, the time lags are statistically different.
Additional analysis show that the delay between the radio and X-ray signals
varies with energy in the 2 - 300 keV energy range. We explain such a behaviour
as due to the superposition of two independent components responsible for the
Crab pulsed emission in this energy band
Reactive Atom Plasma (RAP) figuring machine for meter class optical surfaces
A new surface figuring machine called Helios 1200 is presented in this paper. It is designed for the figuring of meter sized optical surfaces with form accuracy correction capability better than 20 nm rms within a reduced number of iterations. Unlike other large figuring facilities using energy beams, Helios 1200 operates a plasma torch at atmospheric pressure, offers a high material removal rate, and a relatively low running cost. This facility is ideal to process large optical components, lightweight optics, silicon based and difficult to machine materials, aspheric, and free form surfaces. Also, the surfaces processed by the reactive atom plasma (RAP) are easy to fine polish through hand conventional sub-aperture polishing techniques. These unique combined features lead to a new capability for the fabrication of optical components opening up novel design possibilities for optical engineers. The key technical features of this large RAP machine are fast figuring capabilities, non-contact material removal tool, the use of a near Gaussian footprint energy beam, and a proven tool path strategy for the management of the heat transfer. Helios 1200 complies with the European machine safety standard and can be used with different types of reactive gases using either fluorine or chlorine compounds. In this paper, first the need for large optical component is discussed. Then, the RAP facility is described: radio frequency R.F generator, plasma torch, and 3 axis computer numerically controlled motion system. Both the machine design and the performance of the RAP tool is assessed under specific production conditions and in the context of meter class mirror and lens fabrication
Propagation of chaos for rank-based interacting diffusions and long time behaviour of a scalar quasilinear parabolic equation
We study a quasilinear parabolic Cauchy problem with a cumulative
distribution function on the real line as an initial condition. We call
'probabilistic solution' a weak solution which remains a cumulative
distribution function at all times. We prove the uniqueness of such a solution
and we deduce the existence from a propagation of chaos result on a system of
scalar diffusion processes, the interactions of which only depend on their
ranking. We then investigate the long time behaviour of the solution. Using a
probabilistic argument and under weak assumptions, we show that the flow of the
Wasserstein distance between two solutions is contractive. Under more stringent
conditions ensuring the regularity of the probabilistic solutions, we finally
derive an explicit formula for the time derivative of the flow and we deduce
the convergence of solutions to equilibrium.Comment: Stochastic partial differential equations: analysis and computations
(2013) http://dx.doi.org/10.1007/s40072-013-0014-
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