459 research outputs found
USA Observation of Spectral and Timing Evolution During the 2000 Outburst of XTE J1550-564
We report on timing and spectral observations of the 2000 outburst of XTE
J1550-564 made by the Unconventional Stellar Aspect (USA) Experiment on board
the Advanced Research and Global Observation Satellite (ARGOS). We observe a
low-frequency quasi-periodic oscillation (LFQPO) with a centroid frequency that
tends to increase with increasing flux and a fractional rms amplitude which is
correlated with the hardness ratio. The evolution of the hardness ratio (4--16
keV/1--4 keV) with time and source flux is examined. The hardness-intensity
diagram (HID) shows a cyclical movement in the clockwise direction and possibly
indicates the presence of two independent accretion flows. We observe a
relationship between the USA 4--16 keV count rate and radio observations and
discuss this in the context of previously observed correlations between X-ray,
radio, optical and IR data. We examine our results in the context of models
invoking two accretion flows: a thin disk and a hot sub-Keplerian flow.Comment: 11 pages, 2 figure
Temporal Properties of Cygnus X-1 During the Spectral Transitions
We report the results from our timing analysis of 15 RXTE observations of
Cygnus X-1 throughout its 1996 spectral transitions. The entire period can be
divided into 3 distinct phases: (1) transition from the hard to soft state, (2)
soft state, and (3) transition from the soft state back to the hard state. The
observed X-ray properties in phases 1 and 3 are remarkably similar, suggesting
that the same physical processes are likely involved in triggering such
transitions. The power density spectrum (PDS) during the transition can be
characterized by a red noise component, followed by a white noise component
which extends to roughly 1-3 Hz where it is cut off, and a steeper power law at
higher frequencies. The X-ray flux also exhibits apparent quasi-periodic
oscillation (QPO) with the centroid frequency varying in the range of 4-12 Hz.
The QPO shows no correlation with the source flux, but becomes more prominent
at higher energies. This type of PDS bears resemblance to that of other black
hole candidates often observed in a so-called very high state, although the
origin of the observed QPO may be very different. The low-frequency red noise
has not been observed in the hard state, thus seems to be positively correlated
with the disk mass accretion rate which is presumably low in the hard state and
high in the soft state; in fact, it completely dominates the PDS in the soft
state. In the framework of thermalComptonization models, Cui et al. (see
astro-ph/9610071 and astro-ph/9610072) speculated that the difference in the
observed spectral and timing properties between the hard and soft states is due
to the presence of a ``fluctuating'' Comptonizing corona during the transition.
Here we present the measured hard X-ray time lags and coherence functions
between various energy bands, and show that the results strongly support such a
scenario.Comment: AASTex file. 29 pages including 11 figures. To appear in Ap
Defining Meyer's loop-temporal lobe resections, visual field deficits and diffusion tensor tractography
Anterior temporal lobe resection is often complicated by superior quadrantic visual field deficits (VFDs). In some cases this can be severe enough to prohibit driving, even if a patient is free of seizures. These deficits are caused by damage to Meyer's loop of the optic radiation, which shows considerable heterogeneity in its anterior extent. This structure cannot be distinguished using clinical magnetic resonance imaging sequences. Diffusion tensor tractography is an advanced magnetic resonance imaging technique that enables the parcellation of white matter. Using seed voxels antero-lateral to the lateral geniculate nucleus, we applied this technique to 20 control subjects, and 21 postoperative patients. All patients had visual fields assessed with Goldmann perimetry at least three months after surgery. We measured the distance from the tip of Meyer's loop to the temporal pole and horn in all subjects. In addition, we measured the size of temporal lobe resection using postoperative T1-weighted images, and quantified VFDs. Nine patients suffered VFDs ranging from 22% to 87% of the contralateral superior quadrant. In patients, the range of distance from the tip of Meyer's loop to the temporal pole was 24–43 mm (mean 34 mm), and the range of distance from the tip of Meyer's loop to the temporal horn was –15 to +9 mm (mean 0 mm). In controls the range of distance from the tip of Meyer's loop to the temporal pole was 24–47 mm (mean 35 mm), and the range of distance from the tip of Meyer's loop to the temporal horn was –11 to +9 mm (mean 0 mm). Both quantitative and qualitative results were in accord with recent dissections of cadaveric brains, and analysis of postoperative VFDs and resection volumes. By applying a linear regression analysis we showed that both distance from the tip of Meyer's loop to the temporal pole and the size of resection were significant predictors of the postoperative VFDs. We conclude that there is considerable variation in the anterior extent of Meyer's loop. In view of this, diffusion tensor tractography of the optic radiation is a potentially useful method to assess an individual patient's risk of postoperative VFDs following anterior temporal lobe resection
RXTE Observation of Cygnus X-1 In Its High State
We present the results from the RXTE observations of Cygnus X-1 in its high
state. In the energy range of 2-200 keV, the observed X-ray spectrum can be
described by a model consisting of a soft blackbody component and a broken
power-law with a high energy cutoff. The low energy spectrum (below about 11
keV) varies significantly from observation to observation while the high energy
portion changes little. The X-ray flux varies on all timescales down to
milliseconds. The power density spectrum (PDS) can be characterized by excess
red noise (``1/f'') at low frequencies and a white noise component that extends
to 1-3 Hz before being cut off. At higher frequencies, the PDS becomes
power-law again, with a slope of roughly -2 (i.e., ``1/f^2''). Broad peaks in
the range of 3-9 Hz are present, and might be due to quasi-periodic
oscillations. The PDS shows interesting spectral dependence: the 1/f component
becomes more prominent when the low-energy spectrum becomes softer. The
difference in the observed spectral and timing properties between the low and
high states is qualitatively consistent with a simple ``fluctuating corona''
model.Comment: 11 pages including 3 figures, LaTeX (aaspp4.sty). Accepted for
publication in ApJ Letter
Rms-flux relation of Cyg X-1 with RXTE: dipping and nondipping cases
The rms (root mean square) variability is the parameter for understanding the
emission temporal properties of X-ray binaries (XRBs) and active galactic
nuclei (AGN).
The rms-flux relation with Rossi X-ray Timing Explorer (RXTE) data for the
dips and nondip of black hole Cyg X-1 has been investigated in this paper. Our
results show that there exist the linear rms-flux relations in the frequency
range 0.1-10 Hz for the dipping light curve. Moreover, this linear relation
still remains during the nondip regime, but with the steeper slope than that of
the dipping case in the low energy band. For the high energy band, the slopes
of the dipping and nondipping cases are hardly constant within errors. The
explanations of the results have been made by means of the ``Propagating
Perturbation'' model of Lyubarskii (1997).Comment: 15 pages, 12 figures, Accepted for publication in Astrophysics &
Space Scienc
Constraints on the Cosmic-Ray Density Gradient beyond the Solar Circle from Fermi gamma-ray Observations of the Third Galactic Quadrant
We report an analysis of the interstellar -ray emission in the third
Galactic quadrant measured by the {Fermi} Large Area Telescope. The window
encompassing the Galactic plane from longitude 210\arcdeg to 250\arcdeg has
kinematically well-defined segments of the Local and the Perseus arms, suitable
to study the cosmic-ray densities across the outer Galaxy. We measure no large
gradient with Galactocentric distance of the -ray emissivities per
interstellar H atom over the regions sampled in this study. The gradient
depends, however, on the optical depth correction applied to derive the \HI\
column densities. No significant variations are found in the interstellar
spectra in the outer Galaxy, indicating similar shapes of the cosmic-ray
spectrum up to the Perseus arm for particles with GeV to tens of GeV energies.
The emissivity as a function of Galactocentric radius does not show a large
enhancement in the spiral arms with respect to the interarm region. The
measured emissivity gradient is flatter than expectations based on a cosmic-ray
propagation model using the radial distribution of supernova remnants and
uniform diffusion properties. In this context, observations require a larger
halo size and/or a flatter CR source distribution than usually assumed. The
molecular mass calibrating ratio, , is
found to be
in the Local-arm clouds and is not significantly sensitive to the choice of
\HI\ spin temperature. No significant variations are found for clouds in the
interarm region.Comment: Corresponding authors: I. A. Grenier ([email protected]); T.
Mizuno ([email protected]); L. Tibaldo
([email protected]) accepted for publication in Ap
Fermi LAT observations of the Geminga pulsar
We report on the \textit{Fermi}-LAT observations of the Geminga pulsar, the
second brightest non-variable GeV source in the -ray sky and the first
example of a radio-quiet -ray pulsar. The observations cover one year,
from the launch of the satellite through 2009 June 15. A data sample of
over 60,000 photons enabled us to build a timing solution based solely on
rays. Timing analysis shows two prominent peaks, separated by = 0.497 0.004 in phase, which narrow with increasing energy. Pulsed
rays are observed beyond 18 GeV, precluding emission below 2.7 stellar
radii because of magnetic absorption. The phase-averaged spectrum was fitted
with a power law with exponential cut-off of spectral index = (1.30
0.01 0.04), cut-off energy = (2.46 0.04 0.17)
GeV and an integral photon flux above 0.1 GeV of (4.14 0.02 0.32)
10 cm s. The first uncertainties are statistical
and the second are systematic. The phase-resolved spectroscopy shows a clear
evolution of the spectral parameters, with the spectral index reaching a
minimum value just before the leading peak and the cut-off energy having maxima
around the peaks. Phase-resolved spectroscopy reveals that pulsar emission is
present at all rotational phases. The spectral shape, broad pulse profile, and
maximum photon energy favor the outer magnetospheric emission scenarios.Comment: 32 pages, 12 figures, 3 tables. Accepted for publication in The
Astrophysical Journal. Corresponding authors: Denis Dumora
([email protected]), Fabio Gargano ([email protected]),
Massimiliano Razzano ([email protected]
Fermi LAT Search for Photon Lines from 30 to 200 GeV and Dark Matter Implications
Dark matter (DM) particle annihilation or decay can produce monochromatic
-rays readily distinguishable from astrophysical sources. -ray
line limits from 30 GeV to 200 GeV obtained from 11 months of Fermi Large Area
Space Telescope data from 20-300 GeV are presented using a selection based on
requirements for a -ray line analysis, and integrated over most of the
sky. We obtain -ray line flux upper limits in the range , and give corresponding DM annihilation
cross-section and decay lifetime limits. Theoretical implications are briefly
discussed.Comment: 6 pages, 1 figure. Accepted for publication by The Physical Review
Letter
Fermi Large Area Telescope Observations of the Cosmic-Ray Induced gamma-ray Emission of the Earth's Atmosphere
We report on measurements of the cosmic-ray induced gamma-ray emission of
Earth's atmosphere by the Large Area Telescope onboard the Fermi Gamma-ray
Space Telescope. The LAT has observed the Earth during its commissioning phase
and with a dedicated Earth-limb following observation in September 2008. These
measurements yielded 6.4 x 10^6 photons with energies >100MeV and ~250hours
total livetime for the highest quality data selection. This allows the study of
the spatial and spectral distributions of these photons with unprecedented
detail. The spectrum of the emission - often referred to as Earth albedo
gamma-ray emission - has a power-law shape up to 500 GeV with spectral index
Gamma = 2.79+-0.06.Comment: Accepted for publication in PR
Measurements of single top quark production cross sections and |Vtb| in ppbar collisions at sqrt{s}=1.96 TeV
We present measurements of production cross sections of single top quarks in
\ppbar collisions at in a data sample corresponding
to an integrated luminosity of collected by the D0 detector
at the Fermilab Tevatron Collider. We select events with an isolated electron
or muon, an imbalance in transverse energy, and two, three, or four jets, with
one or two of them containing a bottom hadron. We obtain an inclusive cross
section of \sigma({\ppbar}{\rargap}tb+X, tqb+X) = 3.43\pm^{0.73}_{0.74}\;\rm
pb and use it to extract the CKM matrix element at
the 95% C.L. We also measure \sigma({\ppbar}{\rargap}tb+X) =
0.68\pm^{0.38}_{0.35}\;\rm pb and \sigma({\ppbar}{\rargap}tqb+X) =
2.86\pm^{0.69}_{0.63}\;\rm pb when assuming, respectively, and
production rates as predicted by the standard model.Comment: 11 pages, 8 figures, submitted to Phys. Rev.
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