1,376 research outputs found
Implications of a "Fast Radio Burst" from a Galactic Magnetar
A luminous radio burst was recently detected in temporal coincidence with a
hard X-ray flare from the Galactic magnetar SGR 1935+2154 with a time and
frequency structure consistent with cosmological fast radio bursts (FRB) and a
fluence within a factor of of the least energetic extragalactic
FRB previously detected. Although active magnetars are commonly invoked FRB
sources, several distinct mechanisms have been proposed for generating the
radio emission which make different predictions for the accompanying higher
frequency radiation. We show that the properties of the coincident radio and
X-ray flares from SGR 1935+2154, including their approximate simultaneity and
relative fluence , as well as the
duration and spectrum of the X-ray emission, are consistent with extant
predictions for the synchrotron maser shock model. Rather than arising from the
inner magnetosphere, the X-rays are generated by (incoherent) synchrotron
radiation from thermal electrons heated at the same shocks which produce the
coherent maser emission. Although the rate of SGR 1935+2154-like bursts in the
local universe is not sufficient to contribute appreciably to the extragalactic
FRB rate, the inclusion of an additional population of more active magnetars
with stronger magnetic fields than the Galactic population can explain both the
FRB rate as well as the repeating fraction, however only if the population of
active magnetars are born at a rate that is at least two-orders of magnitude
lower than that of SGR 1935+2154-like magnetars. This may imply that the more
active magnetar sources are not younger magnetars formed in a similar way to
the Milky Way population (e.g. via ordinary supernovae), but instead through
more exotic channels such as superluminous supernovae, accretion-induced
collapse or neutron star mergers.Comment: 21 pages, 9 figures; submitted to ApJL; comments welcome
Separation of Benzene and n-Hexane Using a Liquid Membrane Technique
The separation of a mixture of benzene and n-hexane was studied using a liquid membrane technique. Sparkleen^ detergent was used as the surfactant and heavy mineral oil was used as the solvent to receive the permeate.
The effects of the following variables on the separation factor were investigated:
1. The relative proportion of benzene and n-hexane in the original mixture.
2 . Surfactant concentration.
3 . Quantity of surfactant solution.
4. Amount of solvent.
5 . Contact time with solvent.
The effect of each variable was studied by varying the values of that variable while holding other variables constant.
It was found that both the separation factor and the permeate composition vary with the feed composition. Under the experimental conditions employed in the present study, the average value of the separation factor for 0.1 to 0.9 mole fraction of benzene in the feed was found to be approximately 6.
Under the experimental conditions employed in the present study, the separation factor was found to be low for the volumes of solvent less than 180 c.c. The separation factor was found to be about constant for the volumes of solvent between 180 and 250 c.c. Beyond 250 c.c. , the separation factor was again found to be low.
For low values of concentration of surfactant and contact times with solvent, the separation factor was found to be low. As the values of these variables were increased the separation factor increased and reached a maximum and for large values of these variables, the separation factor again decreased.
From the study of the variation of separation factor with the quantity of surfactant solution, it was found that the separation factor was independent of the quantity of surfactant solution beyond a certain volume of surfactant solution. For volumes less than this minimum, the separation factor was lower, but increased as the volume increased.
A rough estimate of the theoretical and experimental error involved in the present study has been made. The relative advantages and disadvantages of the separation by liquid membrane were also discussed
Switched Control of Electron Nuclear Spin Systems
In this article, we study control of electron-nuclear spin dynamics at
magnetic field strengths where the Larmor frequency of the nucleus is
comparable to the hyperfine coupling strength. The quantization axis for the
nuclear spin differs from the static B_0 field direction and depends on the
state of the electron spin. The quantization axis can be switched by flipping
the state of electron spin, allowing for universal control on nuclear spin
states. We show that by performing a sequence of flips (each followed by a
suitable delay), we can perform any desired rotation on the nuclear spins,
which can also be conditioned on the state of the electron spin. These
operations, combined with electron spin rotations can be used to synthesize any
unitary transformation on the coupled electron-nuclear spin system. We discuss
how these methods can be used for design of experiments for transfer of
polarization from the electron to the nuclear spins
Radio Nebul\ae\ from Hyper-Accreting X-ray Binaries as Common Envelope Precursors and Persistent Counterparts of Fast Radio Bursts
Roche lobe overflow from a donor star onto a black hole or neutron star
binary companion can evolve to a phase of unstable runaway mass-transfer,
lasting as short as hundreds of orbits ( yr for a giant
donor), and eventually culminating in a common envelope event. The highly
super-Eddington accretion rates achieved during this brief phase ( are accompanied by intense mass-loss in disk
winds, analogous but even more extreme than ultra-luminous X-ray (ULX) sources
in the nearby universe. Also in analogy with observed ULX, this expanding
outflow will inflate an energetic `bubble' of plasma into the circumbinary
medium. Embedded within this bubble is a nebula of relativistic electrons
heated at the termination shock of the faster wind/jet from
the inner accretion flow. We present a time-dependent, one-zone model for the
synchrotron radio emission and other observable properties of such ULX
`hyper-nebulae'. If ULX jets are sources of repeating fast radio bursts (FRB),
as recently proposed, such hyper-nebulae could generate persistent radio
emission and contribute large and time-variable rotation measure to the bursts,
consistent with those seen from FRB 20121102 and FRB 190520B. ULX hyper-nebulae
can be discovered independent of an FRB association in radio surveys such as
VLASS, as off-nuclear point-sources whose fluxes can evolve significantly on
timescales as short as years, possibly presaging energetic transients from
common envelope mergers.Comment: 27 pages, 9 figures, 1 table; accepted for publication in Ap
Quantitative analysis of pedestrian counterflow in a cellular automaton model
Pedestrian dynamics exhibits various collective phenomena. Here we study
bidirectional pedestrian flow in a floor field cellular automaton model. Under
certain conditions, lane formation is observed. Although it has often been
studied qualitatively, e.g., as a test for the realism of a model, there are
almost no quantitative results, neither empirically nor theoretically. As basis
for a quantitative analysis we introduce an order parameter which is adopted
from the analysis of colloidal suspensions. This allows to determine a phase
diagram for the system where four different states (free flow, disorder, lanes,
gridlock) can be distinguished. Although the number of lanes formed is
fluctuating, lanes are characterized by a typical density. It is found that the
basic floor field model overestimates the tendency towards a gridlock compared
to experimental bounds. Therefore an anticipation mechanism is introduced which
reduces the jamming probability.Comment: 11 pages, 12 figures, accepted for publication in Phys. Rev.
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