51,637 research outputs found
Spectral evolution of the microquasar XTE J1550-564 over its entire 2000 outburst
We report on RXTE observations of the microquasar XTE J1550-564 during a ~70
day outburst in April-June 2000. We study the evolution of the PCA+HEXTE
spectra over the outburst. The source transited from an initial Low Hard State
(LS), to an Intermediate State (IS), and then back to the LS. The source shows
an hysteresis effect similar to what is observed in other sources, favoring a
common origin for the state transitions in soft X-ray transients. The first
transition occurs at a ~ constant 2-200 keV flux, which probably indicates a
change in the relative importance of the emitting media. The second transition
is more likely driven by a drop in the mass accretion rate.
In both LS, the spectra are characterized by the presence of a strong
power-law tail (Compton corona) with a variable high energy cut-off. During the
IS, the spectra show the presence of a ~0.8 keV thermal component (accretion
disk). We discuss the apparently independent evolution of the two media, and
show that right after the X-ray maximum on MJD 51662, the decrease of the
source luminosity is due to a decrease of the power-law luminosity, at a
constant disk luminosity. This, together with the detection of radio emission
(with a spectrum typical of optically thin synchrotron emission), may suggest
that the corona is ejected and further detected as a discrete radio ejection.Comment: Accepted for publication in ApJ. 9 pages, 4 figures, abstract
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Properties of unique hard X-ray dips observed from GRS 1915+105 and IGR J17091-3624 and their implications
We report a comprehensive study on spectral and timing properties of hard
X-ray dips uniquely observed in some so-called variability classes of the
micro-quasars GRS 1915+105 and IGR J17091-3624. These dips are characterized by
a sudden decline in the 2.0-60.0 keV X-ray intensity by a factor of 4-12
simultaneous with the increase in hardness ratio by a factor of 2-4. Using 31
observations of GRS 1915+105 with RXTE/PCA, we show that different behavior are
observed in different types of variability classes, and we find that a
dichotomy is observed between classes with abrupt transitions vs those with
smoother evolution. For example, both energy-lag spectra and frequency-lag
spectra of hard X-ray dips in classes with abrupt transitions and shorter dip
intervals show hard-lag (hard photons lag soft photons), while both lag spectra
during hard dips in classes with smoother evolution and longer dip intervals
show soft-lag. Both lag time-scales are of the order of 100-600 msec. We also
show that timing and spectral properties of hard X-ray dips observed in light
curves of IGR J17091-3624 during its 2011 outburst are consistent with the
properties of the abrupt transitions in GRS 1915+105 rather than smooth
evolution. A global correlation between the X-ray intensity cycle time and hard
dip time is observed for both abrupt and smooth transition which may be due to
two distinct physical processes whose time-scales are eventually correlated. We
discuss implications of our results in the light of some generic models.Comment: 17 pages, 5 figures, accepted for publication in the Astrophysical
Journa
Collective resonances in plasmonic crystals: Size matters
Periodic arrays of metallic nanoparticles may sustain Surface Lattice
Resonances (SLRs), which are collective resonances associated with the
diffractive coupling of Localized Surface Plasmon Resonances (LSPRs). By
investigating a series of arrays with varying number of particles, we traced
the evolution of SLRs to its origins. Polarization resolved extinction spectra
of arrays formed by a few nanoparticles were measured, and found to be in very
good agreement with calculations based on a coupled dipole model. Finite size
effects on the optical properties of the arrays are observed, and our results
provide insight into the characteristic length scales for collective plasmonic
effects: for arrays smaller than 5 x 5 particles, the Q-factors of SLRs are
lower than those of LSPRs; for arrays larger than 20 x 20 particles, the
Q-factors of SLRs saturate at a much larger value than those of LSPRs; in
between, the Q-factors of SLRs are an increasing function of the number of
particles in the array.Comment: 4 figure
Direct Measurement of Periodic Electric Forces in Liquids
The electric forces acting on an atomic force microscope tip in solution have
been measured using a microelectrochemical cell formed by two periodically
biased electrodes. The forces were measured as a function of lift height and
bias amplitude and frequency, providing insight into electrostatic interactions
in liquids. Real-space mapping of the vertical and lateral components of
electrostatic forces acting on the tip from the deflection and torsion of the
cantilever is demonstrated. This method enables direct probing of electrostatic
and convective forces involved in electrophoretic and dielectroforetic
self-assembly and electrical tweezer operation in liquid environments
Fermion Analogy for Layered Superconducting Films in Parallel Magnetic Field
The equivalence between the Lawrence-Doniach model for films of extreme
type-II layered superconductors and a generalization of the back-scattering
model for spin-1/2 electrons in one dimension is demonstrated. This fermion
analogy is then exploited to obtain an anomalous tail for
the parallel equilibrium magnetization of the minimal double layer case in the
limit of high parallel magnetic fields for temperatures in the
critical regime.Comment: 11 pages of plain TeX, 1 postscript figur
Direct and Heterodyne Detection of Microwaves in a Metallic Single Wall Carbon Nanotube
This letter reports measurements of microwave (up to 4.5 GHz) detection in
metallic single-walled carbon nanotubes. The measured voltage responsivity was
found to be 114 V/W at 77K. We also demonstrated heterodyne detection at 1 GHz.
The detection mechanism can be explained based on standard microwave detector
theory and the nonlinearity of the DC IV-curve. We discuss the possible causes
of this nonlinearity. While the frequency response is limited by circuit
parasitics in this measurement, we discuss evidence that indicates that the
effect is much faster and that applications of carbon nanotubes as terahertz
detectors are feasible
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