949 research outputs found
Magnetic Lensing near Ultramagnetized Neutron Stars
Extremely strong magnetic fields change the vacuum index of refraction. This
induces a lensing effect that is not unlike the lensing phenomenon in strong
gravitational fields. The main difference between the two is the polarization
dependency of the magnetic lensing, a behaviour that induces a handful of
interesting effects. The main prediction is that the thermal emission of
neutron stars with extremely strong magnetic fields is polarized - up to a few
percent for the largest fields known. This potentially allows a direct method
for measuring their magnetic fields.Comment: To appear in MNRAS, 12 pages, 9 figure
Dynamical Topological Quantum Phase Transitions for Mixed States
We introduce and study dynamical probes of band structure topology in the
post-quench time-evolution from mixed initial states of quantum many-body
systems. Our construction generalizes the notion of dynamical quantum phase
transitions (DQPTs), a real-time counterpart of conventional equilibrium phase
transitions in quantum dynamics, to finite temperatures and generalized Gibbs
ensembles. The non-analytical signatures hallmarking these mixed state DQPTs
are found to be characterized by observable phase singularities manifesting in
the dynamical formation of vortex-antivortex pairs in the interferometric phase
of the density matrix. Studying quenches in Chern insulators, we find that
changes in the topological properties of the Hamiltonian can be identified in
this scenario, without ever preparing a topologically non-trivial or
low-temperature initial state. Our observations are of immediate relevance for
current experiments aimed at realizing topological phases in ultracold atomic
gases.Comment: 4 pages, 3 figures, version close to publishe
The Long-Term Future of Space Travel
The fact that we apparently live in an accelerating universe places
limitations on where humans might visit. If the current energy density of the
universe is dominated by a cosmological constant, a rocket could reach a galaxy
observed today at a redshift of 1.7 on a one-way journey or merely 0.65 on a
round trip. Unfortunately these maximal trips are impractical as they require
an infinite proper time to traverse. However, calculating the rocket trajectory
in detail shows that a rocketeer could nearly reach such galaxies within a
lifetime (a long lifetime admittedly -- about 100 years). For less negative
values of the maximal redshift increases becoming infinite for .Comment: 5 pages, 3 figures, minor changes to reflect version accepted to PR
Polarization Evolution in Strong Magnetic Fields
Extremely strong magnetic fields change the vacuum index of refraction.
Although this polarization dependent effect is small for typical neutron stars,
it is large enough to decouple the polarization states of photons traveling
within the field. The photon states evolve adiabatically and follow the
changing magnetic field direction. The combination of a rotating magnetosphere
and a frequency dependent state decoupling predicts polarization phase lags
between different wave bands, if the emission process takes place well within
the light cylinder. This QED effect may allow observations to distinguish
between different pulsar emission mechanisms and to reconstruct the structure
of the magnetosphere.Comment: 22 pages, 10 figures, accepted for publication in MNRA
On the Lack of Type I X-ray Bursts in Black Hole X-ray Binaries: Evidence for the Event Horizon?
Type I X-ray bursts are very common in neutron star X-ray binaries, but no
Type I burst has been seen in the dozen or so binaries in which the accreting
compact star is too massive to be a neutron star and therefore is identified as
a black hole candidate. We have carried out a global linear stability analysis
of the accumulating fuel on the surface of a compact star to identify the
conditions under which thermonuclear bursts are triggered. Our analysis, which
improves on previous calculations, reproduces the gross observational trends of
bursts in neutron star systems. It further shows that, if black hole candidates
have surfaces, they would very likely exhibit instabilities similar to those
that lead to Type I bursts on neutron stars. The lack of bursts in black hole
candidates is thus significant, and indicates that these objects have event
horizons. We discuss possible caveats to this conclusion.Comment: 11 pages, 1 figure, to appear in 1 August 2002 edition of
Astrophysical Journal Letters, significant changes to the methods, results
unchange
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