4,899 research outputs found
Quintessence Model and Observational Constraints
The recent observations of type Ia supernovae strongly support that the
universe is accelerating now and decelerated in the recent past. By assuming a
general relation between the quintessence potential and the quintessence
kinetic energy, a general relation is found between the quintessence energy
density and the scale factor. The potential includes both the hyperbolic and
the double exponential potentials. A detailed analysis of the transition from
the deceleration phase to the acceleration phase is then performed. We show
that the current constraints on the transition time, the equation of state and
the energy density of the quintessence field are satisfied in the model.Comment: update references,add acknowledgements and correct some errors,
accepted for publication in class. and quant. gra
The X-ray Position and Optical Counterpart of the Accretion-Powered Millisecond Pulsar XTE J1814-338
We report the precise optical and X-ray localization of the 3.2 ms
accretion-powered X-ray pulsar XTE J1814-338 with data from the Chandra X-Ray
Observatory as well as optical observations conducted during the 2003 June
discovery outburst. Optical imaging of the field during the outburst of this
soft X-ray transient reveals an R = 18 star at the X-ray position. This star is
absent (R > 20) from an archival 1989 image of the field and brightened during
the 2003 outburst, and we therefore identify it as the optical counterpart of
XTE J1814-338. The best source position derived from optical astrometry is R.A.
= 18h13m39.s04, Dec.= -33d46m22.3s (J2000). The featureless X-ray spectrum of
the pulsar in outburst is best fit by an absorbed power-law (with photon index
= 1.41 +- 0.06) plus blackbody (with kT = 0.95 +- 0.13 keV) model, where the
blackbody component contributes approximately 10% of the source flux. The
optical broad-band spectrum shows evidence for an excess of infrared emission
with respect to an X-ray heated accretion disk model, suggesting a significant
contribution from the secondary or from a synchrotron-emitting region. A
follow-up observation performed when XTE J1814-338 was in quiescence reveals no
counterpart to a limiting magnitude of R = 23.3. This suggests that the
secondary is an M3 V or later-type star, and therefore very unlikely to be
responsible for the soft excess, making synchroton emission a more reasonable
candidate.Comment: Accepted for publication in ApJ. 6 pages; 3 figure
The Cosmological Constant is Back
A diverse set of observations now compellingly suggest that Universe
possesses a nonzero cosmological constant. In the context of quantum-field
theory a cosmological constant corresponds to the energy density of the vacuum,
and the wanted value for the cosmological constant corresponds to a very tiny
vacuum energy density. We discuss future observational tests for a cosmological
constant as well as the fundamental theoretical challenges---and
opportunities---that this poses for particle physics and for extending our
understanding of the evolution of the Universe back to the earliest moments.Comment: latex, 8 pages plus one ps figure available as separate compressed
uuencoded fil
Simulations of Electron Acceleration at Collisionless Shocks: The Effects of Surface Fluctuations
Energetic electrons are a common feature of interplanetary shocks and
planetary bow shocks, and they are invoked as a key component of models of
nonthermal radio emission, such as solar radio bursts. A simulation study is
carried out of electron acceleration for high Mach number, quasi-perpendicular
shocks, typical of the shocks in the solar wind. Two dimensional
self-consistent hybrid shock simulations provide the electric and magnetic
fields in which test particle electrons are followed. A range of different
shock types, shock normal angles, and injection energies are studied. When the
Mach number is low, or the simulation configuration suppresses fluctuations
along the magnetic field direction, the results agree with theory assuming
magnetic moment conserving reflection (or Fast Fermi acceleration), with
electron energy gains of a factor only 2 - 3. For high Mach number, with a
realistic simulation configuration, the shock front has a dynamic rippled
character. The corresponding electron energization is radically different:
Energy spectra display: (1) considerably higher maximum energies than Fast
Fermi acceleration; (2) a plateau, or shallow sloped region, at intermediate
energies 2 - 5 times the injection energy; (3) power law fall off with
increasing energy, for both upstream and downstream particles, with a slope
decreasing as the shock normal angle approaches perpendicular; (4) sustained
flux levels over a broader region of shock normal angle than for adiabatic
reflection. All these features are in good qualitative agreement with
observations, and show that dynamic structure in the shock surface at ion
scales produces effective scattering and can be responsible for making high
Mach number shocks effective sites for electron acceleration.Comment: 26 pages, 12 figure
Time-Resolved Intraband Relaxation of Strongly-Confined Electrons and Holes in Colloidal PbSe Nanocrystals
The relaxation of strongly-confined electrons and holes between 1P and 1S
levels in colloidal PbSe nanocrystals has been time-resolved using femtosecond
transient absorption spectroscopy. In contrast to II-VI and III-V semiconductor
nanocrystals, both electrons and holes are strongly confined in PbSe
nanocrystals. Despite the large electron and hole energy level spacings (at
least 12 times the optical phonon energy), we consistently observe picosecond
time-scale relaxation. Existing theories of carrier relaxation cannot account
for these experimental results. Mechanisms that could possibly circumvent the
phonon bottleneck in IV-VI quantum dots are discussed
Axion detection in the milli-eV mass range
We propose an experimental scheme to search for galactic halo axions with
mass eV, which is above the range accessible with cavity
techniques. The detector consists of a large number of parallel superconducting
wires embedded in a material transparent to microwave radiation. The wires
carry a current configuration which produces a static, inhomogeneous magnetic
field within the detector volume. Axions which enter this
volume may convert to photons. We discuss the feasibility of the detector and
its sensitivity.Comment: LaTex, 9 pages, 4 figures (sent upon request), UFIFT-HEP-93--
PHYCOBILISOMES AND ISOLATED PHYCOBILIPROTEINS. EFFECT OF GLUTARDIALDEHYDE AND BENZOQUINONE ON FLUORESCENCE
The fluorescence of the biliproteins C-phycocyanin from Spirulina platensis, B-phycoerythrin
from Porphyridium cruentum and of isolated whole P. cruentum phycobilisomes is quenched in the
presence of glutardialdehyde (GA) or benzoquinone (BQ). The kinetics of fluorescence decrease thus
induced is biphasic. If GA is used as a quencher, the fluorescence can be recovered at 77 K. Contrary to
the GA-effect, only a minor recovery takes place with BQ at 77K, thus demonstrating a different
mechanism of action of GA and BQ on biliprotein
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