1,141 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
Observational constraints on annihilation sites in 1E 1740.7-2942 and Nova Muscae
The region of the Galactic center contains several sources which demonstrate
their activity at various wavelengths and particularly above several hundred
keV. Escape of positrons from such a source or several sources into the
interstellar medium, where they slow down and annihilate, can account for the
511 keV narrow line observed from this direction. 1E 1740.7-2942 object has
been proposed as the most likely candidate to be responsible for this variable
source of positrons. Besides, Nova Muscae shows a spectrum which is consistent
with Comptonization by a thermal plasma kT<100 keV in its hard X-ray part,
while a relatively narrow annihilation line observed by SIGMA on January 20-21,
1991 implies that positrons annihilate in a much colder medium.
We estimate the electron number density and the size of the emitting regions
suggesting that annihilation features observed by SIGMA from Nova Muscae and 1E
1740.7-2942 are due to the positron slowing down and annihilation in thermal
plasma. We show that in the case of Nova Muscae the observed radiation is
coming from a pair plasma stream, N(e+)~N(e-), rather than from a gas cloud. We
argue that two models are probably relevant to the 1E source: annihilation in
(hydrogen) plasma N(e+)<~N(e-) at rest, and annihilation in the pair plasma
stream, which involves matter from the source environment.Comment: 5 pages including 2 figures, latex, aipproc.sty, aipproc.cls,
epsfig.sty. To be published in Proc. of 4th Compton Symp., 1997 (27-30 April,
Williamsburg, Virginia
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
Le Transsibérien, vecteur de la mise en tourisme des destinations enclavées. Le cas de la Russie
Le rôle du train dans le développement des nouvelles destinations constitue la question centrale de cette étude. Cet article analyse l’importance du Transsibérien dans le développement territorial des lieux qu’il traverse avec un intérêt particulier porté sur le tourisme à l’échelle d’un pays comme la Russie. L’étude apporte un certain nombre de réponses historiques et géographiques avec un accent singulier sur l’importance et les différentes formes d’interactions possibles entre le train et le territoire. L’utilisation de la méthode de récolte de données basée sur l’analyse des catalogues des voyagistes cherche à identifier l’importance des lieux du point de vue touristique sur les parcours des trains permettant de produire des données fiables concernant le Transsibérien
Efficiency of the Wang-Landau algorithm: a simple test case
We analyze the efficiency of the Wang-Landau algorithm to sample a multimodal
distribution on a prototypical simple test case. We show that the exit time
from a metastable state is much smaller for the Wang Landau dynamics than for
the original standard Metropolis-Hastings algorithm, in some asymptotic regime.
Our results are confirmed by numerical experiments on a more realistic test
case
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