1,857 research outputs found
A neutron star progenitor for FRBs? Insights from polarisation measurements
Fast Radio Bursts (FRBs) are intense, millisecond-duration broadband radio
transients, the emission mechanisms of which are not understood. Masui et al.
recently presented Green Bank Telescope observations of FRB 110523, which
displayed temporal variation of the linear polarisation position angle (PA).
This effect is commonly seen in radio pulsars and is attributed to a changing
projected magnetic field orientation in the emission region as the star
rotates. If a neutron star is the progenitor of this FRB, and the emission
mechanism is pulsar-like, we show that the progenitor is either rapidly
rotating, or the emission originates from a region of complex magnetic field
geometry. The observed PA variation could also be caused by propagation effects
within a neutron-star magnetosphere, or by spatially varying magnetic fields if
the progenitor lies in a dense, highly magnetised environment. Although we urge
caution in generalising results from FRB 110523 to the broader FRB population,
our analysis serves as a guide to interpreting future polarisation measurements
of FRBs, and presents another means of elucidating the origins of these
enigmatic ephemera.Comment: 7 pages, 2 figures, submitted to MNRA
Pulsar timing noise and the minimum observation time to detect gravitational waves with pulsar timing arrays
The sensitivity of pulsar timing arrays to gravitational waves is, at some
level, limited by timing noise. Red timing noise - the stochastic wandering of
pulse arrival times with a red spectrum - is prevalent in slow-spinning pulsars
and has been identified in many millisecond pulsars. Phenomenological models of
timing noise, such as from superfluid turbulence, suggest that the timing noise
spectrum plateaus below some critical frequency, , potentially aiding the
hunt for gravitational waves. We examine this effect for individual pulsars by
calculating minimum observation times, , over which the
gravitational wave signal becomes larger than the timing noise plateau. We do
this in two ways: 1) in a model-independent manner, and 2) by using the
superfluid turbulence model for timing noise as an example to illustrate how
neutron star parameters can be constrained. We show that the superfluid
turbulence model can reproduce the data qualitatively from a number of pulsars
observed as part of the Parkes Pulsar Timing Array. We further show how a value
of , derived either through observations or theory, can be related to
. This provides a diagnostic whereby the usefulness of timing
array pulsars for gravitational-wave detection can be quantified.Comment: Accepted for publication in MNRA
Indirect Effects of Direct Election: A Structural Examination of the Seventh Amendment
Federalism is hot. Courts are trying to preserve it.\u27 Politicians are trying to reinvent it. And academics are trying just to understand it. Inspired by this renewed interest in the relationship between federal and state governments, I decided to undertake a fresh examination of the Seventeenth Amendment which requires direct election-by the People of each State-of members of the United States Senate. After all, although direct election has not received extensive academic attention, the amendment\u27s removal of state legislatures from the federal electoral process would seem to have significantly reworked the Constitution\u27s federal framework; state legislative election of Senators was seen in 1787 as a (if not the) central device for the protection of States\u27 rights and interests. And in fairly short order I identified some currently important federalism implications of the amendment. For example, much of the current flack over unfunded federal mandates and federal conscription of state instrumentalities is, I think, a result of state legislatures having been cut out of the electoral loop.
But as my structural inquiry into direct election became more systematic, I came to see that some of the Seventeenth Amendment\u27s most important and heretofore unobserved implications concern not federalism, but rather separation of powers-the relationships and processes of the three co-equal federal branches. Although it does so indirectly, the Seventeenth Amendment alters and casts important light on the dynamic between organs within the federal government. As James Madison keenly suggested in Federalist No. 51,11 the two great themes of the Constitution\u27s design-federalism and separation of powers-are intricately and interestingly related., And when we enact structural changes in one of these two areas, we simply cannot ignore the spillover effects in the other.
In this Article, I identify and begin to explore three ways in which direct election bears on important separation of powers questions. First, I argue that direct election systematically reduces rotation between the Senate and Executive Branch offices. This is so because involvement of the People of each State makes more difficult deals by which Senators leave the Senate voluntarily to perform other public service on the implicit understanding that they will be reelected to the Senate when openings present themselves. Put another way, because the Seventeenth Amendment introduces new transaction costs, certain kinds of rotation arrangements have be- come harder to fashion
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Effect of neurostimulation on cognition and mood in refractory epilepsy.
Epilepsy is a common, debilitating neurological disorder characterized by recurrent seizures. Mood disorders and cognitive deficits are common comorbidities in epilepsy that, like seizures, profoundly influence quality of life and can be difficult to treat. For patients with refractory epilepsy who are not candidates for resection, neurostimulation, the electrical modulation of epileptogenic brain tissue, is an emerging treatment alternative. Several forms of neurostimulation are currently available, and therapy selection hinges on relative efficacy for seizure control and amelioration of neuropsychiatric comorbidities. Here, we review the current evidence for how invasive and noninvasive neurostimulation therapies affect mood and cognition in persons with epilepsy. Invasive therapies include vagus nerve stimulation (VNS), deep brain stimulation (DBS), and responsive neurostimulation (RNS). Noninvasive therapies include trigeminal nerve stimulation (TNS), repetitive transcranial magnetic stimulation (rTMS), and transcranial direct current stimulation (tDCS). Overall, current evidence supports stable cognition and mood with all neurostimulation therapies, although there is some evidence that cognition and mood may improve with invasive forms of neurostimulation. More research is required to optimize the effects of neurostimulation for improvements in cognition and mood
Photonic Anomalous Quantum Hall Effect
We experimentally realize a photonic analogue of the anomalous quantum Hall
insulator using a two-dimensional (2D) array of coupled ring resonators.
Similar to the Haldane model, our 2D array is translation invariant, has zero
net gauge flux threading the lattice, and exploits next-nearest neighbor
couplings to achieve a topologically non-trivial bandgap. Using direct imaging
and on-chip transmission measurements, we show that the bandgap hosts
topologically robust edge states. We demonstrate a topological phase transition
to a conventional insulator by frequency detuning the ring resonators and
thereby breaking the inversion symmetry of the lattice. Furthermore, the
clockwise or the counter-clockwise circulation of photons in the ring
resonators constitutes a pseudospin degree of freedom. We show that the two
pseudospins acquire opposite hopping phases and their respective edge states
propagate in opposite directions. These results are promising for the
development of robust reconfigurable integrated nanophotonic devices for
applications in classical and quantum information processing
Enhancing electrochemical intermediate solvation through electrolyte anion selection to increase nonaqueous Li-O battery capacity
Among the 'beyond Li-ion' battery chemistries, nonaqueous Li-O batteries
have the highest theoretical specific energy and as a result have attracted
significant research attention over the past decade. A critical scientific
challenge facing nonaqueous Li-O batteries is the electronically insulating
nature of the primary discharge product, lithium peroxide, which passivates the
battery cathode as it is formed, leading to low ultimate cell capacities.
Recently, strategies to enhance solubility to circumvent this issue have been
reported, but rely upon electrolyte formulations that further decrease the
overall electrochemical stability of the system, thereby deleteriously
affecting battery rechargeability. In this study, we report that a significant
enhancement (greater than four-fold) in Li-O cell capacity is possible by
appropriately selecting the salt anion in the electrolyte solution. Using
Li nuclear magnetic resonance and modeling, we confirm that this
improvement is a result of enhanced Li stability in solution, which in turn
induces solubility of the intermediate to LiO formation. Using this
strategy, the challenging task of identifying an electrolyte solvent that
possesses the anti-correlated properties of high intermediate solubility and
solvent stability is alleviated, potentially providing a pathway to develop an
electrolyte that affords both high capacity and rechargeability. We believe the
model and strategy presented here will be generally useful to enhance Coulombic
efficiency in many electrochemical systems (e.g. Li-S batteries) where
improving intermediate stability in solution could induce desired mechanisms of
product formation.Comment: 22 pages, 5 figures and Supporting Informatio
Nuclear Equation of State from Observations of Short Gamma-Ray Burst Remnants
The favoured progenitor model for short -ray bursts (SGRBs) is the
merger of two neutron stars that triggers an explosion with a burst of
collimated -rays. Following the initial prompt emission, some SGRBs
exhibit a plateau phase in their -ray light curves that indicates additional
energy injection from a central engine, believed to be a rapidly rotating,
highly magnetised neutron star. The collapse of this `protomagnetar' to a black
hole is likely to be responsible for a steep decay in -ray flux observed at
the end of the plateau. In this letter, we show that these observations can be
used to effectively constrain the equation of state of dense matter. In
particular, we show that the known distribution of masses in binary neutron
star systems, together with fits to the -ray light curves, provide
constraints that exclude the softest and stiffest plausible equations of state.
We further illustrate how a future gravitational wave observation with Advanced
LIGO/Virgo can place tight constraints on the equation of state, by adding into
the picture a measurement of the chirp mass of the SGRB progenitor.Comment: accepted for publication in Phys. Rev.
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