897 research outputs found
Photometric support for future astonomical research
The I.A.P.P.P. is described and how that organization can provide photometric support for future astronomical research projects such as the 1982-1984 eclipse of epsilon Aurigae discussed at this workshop. I.A.P.P.P., International Amateur-Professional Photoelectric Photometry, is an organization founded in Fairborn, Ohio by the authors in 1980. Its purpose is to encourage contact between amateur and professional astronomers interested in photoelectric photometry, for their mutual benefit and for the benefit of astronomical research. Aspects dealt with include instrumentation, electronics, computer hardware and software, observing techniques, data reduction, and observing programs. Starting with the June 1980 issue, I.A.P.P.P. has published the quarterly I.A.P.P.P. Communications. The Communications contain articles dealing with all the above aspects of photoelectric photometry, although it does not publish observational results as such. Photoelectric photometry obtained by amateurs is published in the same journals which publish photometry obtained by professionals
Giant slip lengths of a simple fluid at vibrating solid interfaces
It has been shown recently [PRL 102, 254503 (2009)] that in the plane-plane
configuration a mechanical resonator vibrating close to a rigid wall in a
simple fluid can be overdamped to a frozen regime. Here, by solving
analytically the Navier Stokes equations with partial slip boundary conditions
at the solid fluid interface, we develop a theoretical approach justifying and
extending these earlier findings. We show in particular that in the perfect
slip regime the above mentioned results are, in the plane-plane configuration,
very general and robust with respect to lever geometry considerations. We
compare the results with those obtained previously for the sphere moving
perpendicularly and close to a plane in a simple fluid and discuss in more
details the differences concerning the dependence of the friction forces with
the gap distance separating the moving object (i.e., plane or sphere) from the
fixed plane. Finally, we show that the submicron fluidic effect reported in the
reference above, and discussed further in the present work, can have dramatic
implications in the design of nano-electromechanical systems (NEMS).Comment: submitted to PRE (see also PRL 102, 254503 (2009)
Casimir energy and geometry : beyond the Proximity Force Approximation
We review the relation between Casimir effect and geometry, emphasizing
deviations from the commonly used Proximity Force Approximation (PFA). We use
to this aim the scattering formalism which is nowadays the best tool available
for accurate and reliable theory-experiment comparisons. We first recall the
main lines of this formalism when the mirrors can be considered to obey
specular reflection. We then discuss the more general case where non planar
mirrors give rise to non-specular reflection with wavevectors and field
polarisations mixed. The general formalism has already been fruitfully used for
evaluating the effect of roughness on the Casimir force as well as the lateral
Casimir force or Casimir torque appearing between corrugated surfaces. In this
short review, we focus our attention on the case of the lateral force which
should make possible in the future an experimental demonstration of the
nontrivial (i.e. beyond PFA) interplay of geometry and Casimir effect.Comment: corrected typos, added references, QFEXT'07 special issue in J. Phys.
GRB 081028 and its late-time afterglow re-brightening
‘The definitive version is available at www3.interscience.wiley.com '. Copyright Royal Astronomical SocietySwift captured for the first time a smoothly rising X-ray re-brightening of clear non-flaring origin after the steep decay in a long gamma-ray burst (GRB): GRB 081028. A rising phase is likely present in all GRBs but is usually hidden by the prompt tail emission and constitutes the first manifestation of what is later to give rise to the shallow decay phase. Contemporaneous optical observations reveal a rapid evolution of the injection frequency of a fast cooling synchrotron spectrum through the optical band, which disfavours the afterglow onset (start of the forward shock emission along our line of sight when the outflow is decelerated) as the origin of the observed re-brightening. We investigate alternative scenarios and find that the observations are consistent with the predictions for a narrow jet viewed off-axis. The high on-axis energy budget implied by this interpretation suggests different physical origins of the prompt and (late) afterglow emission. Strong spectral softening takes place from the prompt to the steep decay phase: we track the evolution of the spectral peak energy from the γ-rays to the X-rays and highlight the problems of the high latitude and adiabatic cooling interpretations. Notably, a softening of both the high and low spectral slopes with time is also observed. We discuss the low on-axis radiative efficiency of GRB 081028 comparing its properties against a sample of Swift long GRBs with secure Eγ,iso measurements.Peer reviewe
UBVJHKLM photometry and modeling of R Coronae Borealis
We present the results of UBVJHKLM photometry of R CrB spanning the period
from 1976 to 2001. Studies of the optical light curve have shown no evidence of
any stable harmonics in the variations of the stellar emission. In the L band
we found semi-regular oscillations with the two main periods of ~3.3 yr and
11.9 yr and the full amplitude of ~0.8 mag and ~0.6 mag, respectively. The
colors of the warm dust shell (resolved by Ohnaka et al. 2001) are found to be
remarkably stable in contrast to its brightness. This indicates that the inner
radius is a constant, time-independent characteristic of the dust shell. The
observed behavior of the IR light curve is mainly caused by the variation of
the optical thickness of the dust shell within the interval \tau(V)= 0.2-0.4.
Anticorrelated changes of the optical brightness (in particular with P ~ 3.3
yr) have not been found. Their absence suggests that the stellar wind of R CrB
deviates from spherical symmetry. The light curves suggest that the stellar
wind is variable. The variability of the stellar wind and the creation of dust
clouds may be caused by some kind of activity on the stellar surface. With some
time lag, periods of increased mass-loss cause an increase in the dust
formation rate at the inner boundary of the extended dust shell and an increase
in its IR brightness. We have derived the following parameters of the dust
shell (at mean brightness) by radiative transfer modeling: inner dust shell
radius r_in ~ 110 R_*, temperature T_dust(r_in) ~ 860 K, dust density
\rho_dust(r_in) ~ 1.1x10^{-20} g cm^-3, optical depth \tau(V) ~ 0.32 at 0.55
micron, mean dust formation rate [dM/dt]_dust ~ 3.1x10^-9 M_sun / yr, mass-loss
rate [dM/dt]_gas ~ 2.1x10^-7 M_sun / yr, size of the amorphous carbon grains
<(~) 0.01 micron, and B-V ~ -0.28.Comment: 9 pages, 6 figures, accepted for publication in A&
Ultrasonographic evaluation of three approaches for botulinum toxin injection into tibialis posterior muscle in chronic stroke patients with equinovarus foot: An observational study
Spastic equinovarus (SEV) foot deformity is commonly observed in patients with post-stroke spasticity. Tibialis posterior (TP) is a common target for botulinum toxin type-A (BoNT-A) injection, as a first-line treatment in non-fixed SEV deformity. For this deep muscle, ultrasonographic guidance is crucial to achieving maximum accuracy for the BoNT-A injection. In current clinical practice, there are three approaches to target the TP: an anterior, a posteromedial, and a posterior. To date, previous studies have failed to identify the best approach for needle insertion into TP. To explore the ultrasonographic characteristics of these approaches, we investigated affected and unaffected legs of 25 stroke patients with SEV treated with BoNT-A. We evaluated the qualitative (echo intensity) and quantitative (muscle depth, muscle thickness, overlying muscle, subcutaneous tissue, cross-sectional area) ultrasound characteristics of the three approaches for TP injection. In our sample, we observed significant differences among almost all the parameters of the three approaches, except for the safety window. Moreover, our analysis showed significant differences in cross-sectional area between treated and untreated. Advantages and disadvantages of each approach were investigated. Our findings can thus provide a suitable reference for clinical settings, especially for novice operators
Casimir energy and entropy between dissipative mirrors
We discuss the Casimir effect between two identical, parallel slabs,
emphasizing the role of dissipation and temperature. Starting from quite
general assumptions, we analyze the behavior of the Casimir entropy in the
limit T->0 and link it to the behavior of the slab's reflection coefficients at
low frequencies. We also derive a formula in terms of a sum over modes, valid
for dissipative slabs that can be interpreted in terms of a damped quantum
oscillator.Comment: 8 pages, 1 figur
A multiwavelength study of Swift GRB 060111B constraining the origin of its prompt optical emission
In this work, we present the results obtained from a multi-wavelength
campaign, as well as from the public Swift/BAT, XRT, and UVOT data of GRB
060111B for which a bright optical emission was measured with good temporal
resolution during the prompt phase. We identified the host galaxy at R~25 mag;
its featureless spectral continuum and brightness, as well as the non-detection
of any associated supernova 16 days after the trigger and other independent
redshift estimates, converge to z~1-2. From the analysis of the early afterglow
SED, we find that non-negligible host galaxy dust extinction, in addition to
the Galactic one, affects the observed flux in the optical regime. The
extinction-corrected optical-to-gamma-ray spectral energy distribution during
the prompt emission shows a flux density ratio =0.01-0.0001
with spectral index , strongly suggesting a
separate origin of the optical and gamma-ray components. This result is
supported by the lack of correlated behavior in the prompt emission light
curves observed in the two energy domains. The properties of the prompt optical
emission observed during GRB 060111B favor interpretation of this optical light
as radiation from the reverse shock in a thick shell limit and in the slow
cooling regime. The expected peak flux is consistent with the observed one
corrected for the host extinction, likely indicating that the starting time of
the TAROT observations is very near to or coincident with the peak time. The
estimated fireball initial Lorentz factor is >260-360 at z=1-2, similar to the
Lorentz factors obtained from other GRBs. GRB 060111B is a rare, good test case
of the reverse shock emission mechanism in both the X-ray and optical energy
ranges.Comment: Accepted for publication in Astronomy and Astrophysics, 15 pages,10
figures and 7 table
Afterglow Model for the Radio Emission from the Jetted Tidal Disruption Candidate Swift J1644+57
The recent transient event Swift J1644+57 has been interpreted as emission
from a collimated relativistic jet, powered by the sudden onset of accretion
onto a supermassive black hole following the tidal disruption of a star. Here
we model the radio-microwave emission as synchrotron radiation produced by the
shock interaction between the jet and the gaseous circumnuclear medium (CNM).
At early times after the onset of the jet (t < 5-10 days) a reverse shock
propagates through and decelerates the ejecta, while at later times the outflow
approaches the Blandford-McKee self-similar evolution (possibly modified by
additional late energy injection). The achromatic break in the radio light
curve of J1644+57 is naturally explained as the transition between these
phases. We show that the temporal indices of the pre- and post-break light
curve are consistent with those predicted if the CNM has a wind-type radial
density profile n ~ 1/r^2. The observed synchrotron frequencies and
self-absorbed flux constrain the fraction of the post-shock thermal energy in
relativistic electrons epsilon_e ~ 0.03-0.1; the CNM density at 1e18 cm ~ 1-10
1/cm^3; and the initial Lorentz factor Gamma_j ~ 10-20 and opening angle
theta_j ~ (0.1-1)Gamma_j^(-1) ~ 0.01-0.1 of the jet. Radio modeling thus
provides robust independent evidence for a narrowly collimated outflow.
Extending our model to the future evolution of J1644+57, we predict that the
radio flux at low frequencies (<~ few GHz) will begin to brighten more rapidly
once the characteristic frequency crosses below the self-absorption frequency
on a timescale of months (indeed, such a transition may already have begun).
Our results demonstrate that relativistic outflows from tidal disruption events
provide a unique probe of the conditions in distant, previously inactive
galactic nuclei, complementing studies of normal AGN.Comment: 10 pages, 9 figures, now accepted to MNRA
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