1,857 research outputs found
New observations with the gamma ray imager SIGMA
Results from the use of the gamma ray telescope SIGMA are given. An identification and an extensive study was done of sources contributing to the emission of the Galactic center region above 30 keV. A strong line was observed at 480 keV from Nova Muscae, which may be interpreted as an annihilation line with a redshift due to the presence of a compact object. The soft x-ray tails observed by SIGMA in some transient sources already identified as soft x-ray transients might be a common characteristic of these objects and has to be explained. The unusual spectrum of NGC4151 with a break around 50 keV can characterize a particular state of this kind of object. If it is the case, it has interesting implications for the origin of the Cosmic Diffuse Background
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
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Generation Adequacy and Investment Incentives in Britain: from the Pool to NETA
Three years after the controversial change of the British market design from compulsory Pool with capacity payments to decentralised energy-only New Electricity Trading Arrangements (NETA) market framework, we compare the two designs in terms of investment incentives. We review the biases of the Pool capacity payments design, the drought of investment following the introduction of NETA, and the reaction of the market during the first âstress-testâ of NETA during the winter 2003. In an energy-only market such as NETA, it is essential that price signals are right and the system operator has a crucial role in contracting ahead for reserve. We recommend that NETA adopt a single marginal imbalance price as dual imbalance pricing distorts price signals in times of scarcity. The lack of long-term contracting that causes hedging and financing difficulties for power projects can becompensated by vertical and horizontal reintegration at a cost of increased market power
Low-energy gamma rays from Cygnus X-1
The Cyg X-1 was observed by the balloonborne telescope OPALE, in June 1976. The high energy spectrum of the source, which was in its superlow state, was seen to extend well beyond 1 MeV. The observed low energy gamma ray component of Cyg X-1 is compared with the predictions of recent models involving accretion onto a stellar black hole, and including a possible contribution from the pair annihilation 511 keV gamma ray line
Scintillation Caustics in Planetary Occultation Light Curves
We revisit the GSC5249-01240 light curve obtained during its occultation by
Saturn's North polar region. In addition to refractive scintillations, the
power spectrum of intensity fluctuations shows an enhancement of power between
refractive and diffractive regimes. We identify this excess power as due to
high amplitude spikes in the light curve and suggest that these spikes are due
to caustics associated with ray crossing situations. The flux variation in
individual spikes follows the expected caustic behavior, including diffraction
fringes which we have observed for the first time in a planetary occultation
light curve. The presence of caustics in scintillation light curves require an
inner scale cut off to the power spectrum of underlying density fluctuations
associated with turbulence. Another possibility is the presence of gravity
waves in the atmosphere. While occultation light curves previously showed the
existence of refractive scintillations, a combination of small projected
stellar size and a low relative velocity during the event have allowed us to
identify caustics in this occultation. This has led us to re-examine previous
data sets, in which we have also found likely examples of caustics.Comment: 4 pages, 3 figures; ApJL submitte
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
Kuiper Belt Object Occultations: Expected Rates, False Positives, and Survey Design
A novel method of generating artificial scintillation noise is developed and
used to evaluate occultation rates and false positive rates for surveys probing
the Kuiper Belt with the method of serendipitous stellar occultations. A
thorough examination of survey design shows that: (1) diffraction-dominated
occultations are critically (Nyquist) sampled at a rate of 2 Fsu^{-1},
corresponding to 40 s^{-1} for objects at 40 AU, (2) occultation detection
rates are maximized when targets are observed at solar opposition, (3) Main
Belt Asteroids will produce occultations lightcurves identical to those of
Kuiper Belt Objects if target stars are observed at solar elongations of: 116
deg < epsilon < 125 deg, or 131 deg < epsilon < 141 deg, and (4) genuine KBO
occultations are likely to be so rare that a detection threshold of >7-8 sigma
should be adopted to ensure that viable candidate events can be disentangled
from false positives.Comment: Accepted AJ, 12 pages, 12 figure
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
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