48,338 research outputs found
A Comment on "A note on polarized light from Magnetars: QED effects and axion-like particles" by L.M. Capparelli, L. Maiani and A.D. Polosa
The recent detection of a large polarization degree in the optical emission
of an isolated neutron star led to the suggestion that this has been the first
evidence of vacuum polarization in a strong magnetic field, an effect predicted
by quantum electrodynamics but never observed before. This claim was challanged
in a paper by Capparelli, Maiani & Polosa (2017), according to whom a much
higher polarization degree would be necessary to positively identify vacuum
polarization. Here we show that their conclusions are biased by several
inadequate assumptions and have no impact on the original claim.Comment: 10 pages, 2 figure
Recommended from our members
Numerical investigation of high-speed droplet impact using a multiscale two-fluid approach
A single droplet impact onto solid surfaces remains a fundamental and challenging topic in both experimental and numerical studies with significant importance in a plethora of industrial applications, ranging from printing technologies to fuel injection in internal combustion engines. Under high-speed impact conditions, additional complexities arise as a result of the prompt droplet splashing and the subsequent violent fragmentation; thus, different flow regimes and a vast spectrum of sizes for the produced secondary flow structures coexist in the flow field. The present work introduces a numerical methodology to capture the multiscale processes involved with respect to local topological characteristics. The proposed methodology concerns a compressible Σ-Υ two-fluid model with dynamic interface sharpening based on an advanced flow topology detection algorithm. The model has been developed in OpenFOAM® and provides the flexibility of dealing with the multiscale character of droplet splashing, by switching between a sharp and a diffuse interface within the Eulerian-Eulerian framework in segregated and dispersed flow regions, respectively. An additional transport equation for the interface surface area density (Σ) introduces important information for the sub-grid scale phenomena, which is exploited in the dispersed flow regions to provide an insight into the extended cloud of secondary droplets after impact on the target. A high-speed water droplet impact case has been examined and evaluated against new experimental data; these refer to a millimetre size droplet impacting a solid dry smooth surface at velocity as high as 150m/s, which corresponds to a Weber number of ~7.6×10^5. At the investigated impact conditions compressibility effects dominate the early stages of droplet splashing. A strong shock wave forms and propagates inside the droplet, where transonic Mach numbers occur; local Mach numbers up to 2.5 are observed for the expelled surrounding gas outside the droplet. The proposed numerical approach is found to capture relatively accurately the phenomena and provide significant information regarding the produced flow structure dimensions, which is not available from the experiment
Megawatt solar power systems for lunar surface operations
The work presented here shows that a solar power system can provide power on the order of one megawatt to a lunar base with a fairly high specific power. The main drawback to using solar power is still the high mass, and therefore, cost of supplying energy storage through the solar night. The use of cryogenic reactant storage in a fuel cell system, however, greatly reduces the total system mass over conventional energy storage schemes
Non-Equilibrium Dynamics and Superfluid Ring Excitations in Binary Bose-Einstein Condensates
We revisit a classic study [D. S. Hall {\it et al.}, Phys. Rev. Lett. {\bf
81}, 1539 (1998)] of interpenetrating Bose-Einstein condensates in the
hyperfine states and of Rb and observe striking new non-equilibrium
component separation dynamics in the form of oscillating ring-like structures.
The process of component separation is not significantly damped, a finding that
also contrasts sharply with earlier experimental work, allowing a clean first
look at a collective excitation of a binary superfluid. We further demonstrate
extraordinary quantitative agreement between theoretical and experimental
results using a multi-component mean-field model with key additional features:
the inclusion of atomic losses and the careful characterization of trap
potentials (at the level of a fraction of a percent).Comment: 4 pages, 3 figures (low res.), to appear in PR
Evidence of vacuum birefringence from the polarisation of the optical emission from an Isolated Neutron Star
Isolated Neutron Stars are some of the most exciting stellar objects known to
astronomers: they have the most extreme magnetic fields, with values up to
G, and, with the exception of stellar-mass black holes, they are the
most dense stars, with densities of g cm. As such,
they are perfect laboratories to test theories of electromagnetism and nuclear
physics under conditions of magnetic field and density unattainable on Earth.
In particular, the interaction of radiation with strong magnetic fields is the
cause of the {\em vacuum birefringence}, an effect predicted by quantum
electrodynamics in 1936 but that lacked an observational evidence until now.
Here, we show how the study of the polarisation of the optical radiation from
the surface of an isolated neutron star yielded such an observational evidence,
opening exciting perspectives for similar studies at other wavelengths.Comment: 5 pages, 1 figure, Contributed to the 13th Patras Workshop on Axions,
WIMPs and WISPs, Thessaloniki, May 15 to 19, 201
Spectral Evolution of Two High-Energy Gamma-Ray Bursts
The prompt emission of the gamma-ray bursts is found to be very energetic,
releasing ~10^51 ergs in a flash. However, their emission mechanism remains
unclear and understanding their spectra is a key to determining the emission
mechanism. Many GRB spectra have been analyzed in the sub-MeV energy band, and
are usually well described with a smoothly broken power-law model. We present a
spectral analysis of two bright bursts (GRB910503 and GRB930506), using BATSE
and EGRET spectra that cover more than four decades of energy (30 keV - 200
MeV). Our results show time evolutions of spectral parameters (low-energy &
high-energy photon indices and break energy) that are difficult to reconcile
with a simple shock-acceleration model.Comment: 8 pages, 2 figures, to appear in the proceedings of "Astrophysical
Particle Acceleration in Geospace and Beyond", Chattanooga, 2002, AGU
monograp
The accretion disk in the post period-minimum cataclysmic variable SDSS J080434.20+510349.2
This study of SDSS0804 is primarily concerned with the double-hump shape in
the light curve and its connection with the accretion disk in this bounce-back
system. Time-resolved photometric and spectroscopic observations were obtained
to analyze the behavior of the system between superoutbursts. A geometric model
of a binary system containing a disk with two outer annuli spiral density waves
was applied to explain the light curve and the Doppler tomography. Observations
were carried out during 2008-2009, after the object's magnitude decreased to
V~17.7(0.1) from the March 2006 eruption. The light curve clearly shows a
sinusoid-like variability with a 0.07 mag amplitude and a 42.48 min
periodicity, which is half of the orbital period of the system. In Sept. 2010,
the system underwent yet another superoutburst and returned to its quiescent
level by the beginning of 2012. This light curve once again showed a
double-humps, but with a significantly smaller ~0.01mag amplitude. Other types
of variability like a "mini-outburst" or SDSS1238-like features were not
detected. Doppler tomograms, obtained from spectroscopic data during the same
period of time, show a large accretion disk with uneven brightness, implying
the presence of spiral waves. We constructed a geometric model of a bounce-back
system containing two spiral density waves in the outer annuli of the disk to
reproduce the observed light curves. The Doppler tomograms and the
double-hump-shape light curves in quiescence can be explained by a model system
containing a massive >0.7Msun white dwarf with a surface temperature of
~12000K, a late-type brown dwarf, and an accretion disk with two outer annuli
spirals. According to this model, the accretion disk should be large, extending
to the 2:1 resonance radius, and cool (~2500K). The inner parts of the disk
should be optically thin in the continuum or totally void.Comment: 12 pages, 15 figures, accepted for publication in A&
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