718 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
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
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
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
Understanding molecular abundances in star-forming regions using interpretable machine learning
Astrochemical modelling of the interstellar medium typically makes use of complex computational codes with parameters whose values can be varied. It is not always clear what the exact nature of the relationship is between these input parameters and the output molecular abundances. In this work, a feature importance analysis is conducted using SHapley Additive exPlanations (SHAP), an interpretable machine learning technique, to identify the most important physical parameters as well as their relationship with each output. The outputs are the abundances of species and ratios of abundances. In order to reduce the time taken for this process, a neural network emulator is trained to model each species’ output abundance and this emulator is used to perform the interpretable machine learning. SHAP is then used to further explore the relationship between the physical features and the abundances for the various species and ratios we considered. H2O and CO’s gas phase abundances are found to strongly depend on the metallicity. NH3 has a strong temperature dependence, with there being two temperature regimes (100 K). By analysing the chemical network, we relate this to the chemical reactions in our network and find the increased temperature results in increased efficiency of destruction pathways. We investigate the HCN/HNC ratio and show that it can be used as a cosmic thermometer, agreeing with the literature. This ratio is also found to be correlated with the metallicity. The HCN/CS ratio serves as a density tracer, but also has three separate temperature-dependence regimes, which are linked to the chemistry of the two molecules
Dynamical Quantum Phase Transitions in the Transverse Field Ising Model
A phase transition indicates a sudden change in the properties of a large
system. For temperature-driven phase transitions this is related to
non-analytic behavior of the free energy density at the critical temperature:
The knowledge of the free energy density in one phase is insufficient to
predict the properties of the other phase. In this paper we show that a close
analogue of this behavior can occur in the real time evolution of quantum
systems, namely non-analytic behavior at a critical time. We denote such
behavior a dynamical phase transition and explore its properties in the
transverse field Ising model. Specifically, we show that the equilibrium
quantum phase transition and the dynamical phase transition in this model are
intimately related.Comment: 4+4 pages, 4 figures, Appendix adde
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