830 research outputs found
A window into the neutron star: Modelling the cooling of accretion heated neutron star crusts
In accreting neutron star X-ray transients, the neutron star crust can be
substantially heated out of thermal equilibrium with the core during an
accretion outburst. The observed subsequent cooling in quiescence (when
accretion has halted) offers a unique opportunity to study the structure and
thermal properties of the crust. Initially crust cooling modelling studies
focussed on transient X-ray binaries with prolonged accretion outbursts (> 1
year) such that the crust would be significantly heated for the cooling to be
detectable. Here we present the results of applying a theoretical model to the
observed cooling curve after a short accretion outburst of only ~10 weeks. In
our study we use the 2010 outburst of the transiently accreting 11 Hz X-ray
pulsar in the globular cluster Terzan 5. Observationally it was found that the
crust in this source was still hot more than 4 years after the end of its short
accretion outburst. From our modelling we found that such a long-lived hot
crust implies some unusual crustal properties such as a very low thermal
conductivity (> 10 times lower than determined for the other crust cooling
sources). In addition, we present our preliminary results of the modelling of
the ongoing cooling of the neutron star in MXB 1659-298. This transient X-ray
source went back into quiescence in March 2017 after an accretion phase of ~1.8
years. We compare our predictions for the cooling curve after this outburst
with the cooling curve of the same source obtained after its previous outburst
which ended in 2001.Comment: 4 pages, 1 figure, to appear in the proceedings of "IAUS 337: Pulsar
Astrophysics - The Next 50 Years" eds: P. Weltevrede, B.B.P. Perera, L. Levin
Preston & S. Sanida
Hyperfine-interaction- and magnetic-field-induced Bose-Einstein-statistics suppressed two-photon transitions
Two-photon transitions between atomic states of total electronic angular
momentum and are forbidden when the photons are of the same
energy. This selection rule is analogous to the Landau-Yang theorem in particle
physics that forbids decays of vector particle into two photons. It arises
because it is impossible to construct a total angular momentum
quantum-mechanical state of two photons that is permutation symmetric, as
required by Bose-Einstein statistics. In atoms with non-zero nuclear spin, the
selection rule can be violated due to hyperfine interactions. Two distinct
mechanisms responsible for the hyperfine-induced two-photon transitions are
identified, and the hyperfine structure of the induced transitions is
evaluated. The selection rule is also relaxed, even for zero-nuclear-spin
atoms, by application of an external magnetic field. Once again, there are two
similar mechanisms at play: Zeeman splitting of the intermediate-state
sublevels, and off-diagonal mixing of states with different total electronic
angular momentum in the final state. The present theoretical treatment is
relevant to the ongoing experimental search for a possible
Bose-Einstein-statistics violation using two-photon transitions in barium,
where the hyperfine-induced transitions have been recently observed, and the
magnetic-field-induced transitions are being considered both as a possible
systematic effect, and as a way to calibrate the measurement
Magneto-optical imaging of magnetic deflagration in Mn12-Acetate
For the first time, the morphology and dynamics of spin avalanches in
Mn12-Acetate crystals using magneto-optical imaging has been explored. We
observe an inhomogeneous relaxation of the magnetization, the spins reversing
first at one edge of the crystal and a few milliseconds later at the other end.
Our data fit well with the theory of magnetic deflagration, demonstrating that
very slow deflagration rates can be obtained, which makes new types of
experiments possible.Comment: 5 two-column pages, 3 figures, EPL styl
The S(0) structure in highly compressed hydrogen and the orientational transition
A calculation of the rotational S(0) frequencies in high pressure solid
para-hydrogen is performed. Convergence of the perturbative series at high
density is demonstrated by the calculation of second and third order terms. The
results of the theory are compared with the available experimental data to
derive the density behaviour of structural parameters. In particular, a strong
increase of the value of the lattice constant ratio and of the
internuclear distance is determined. Also a decrease of the anisotropic
intermolecular potential is observed which is attributed to charge transfer
effects. The structural parameters determined at the phase transition may be
used to calculate quantum properties of the rotationally ordered phase.Comment: accepted Europhysics Letter
Consistent accretion-induced heating of the neutron-star crust in MXB 1659-29 during two different outbursts
Monitoring the cooling of neutron-star crusts heated during accretion
outbursts allows us to infer the physics of the dense matter present in the
crust. We examine the crust cooling evolution of the low-mass X-ray binary MXB
1659-29 up to ~505 days after the end of its 2015 outburst (hereafter outburst
II) and compare it with what we observed after its previous 1999 outburst
(hereafter outburst I) using data obtained from the Swift, XMM-Newton, and
Chandra observatories. The observed effective surface temperature of the
neutron star in MXB 1659-29 dropped from ~92 eV to ~56 eV from ~12 days to ~505
days after the end of outburst II. The most recently performed observation
after outburst II suggests that the crust is close to returning to thermal
equilibrium with the core. We model the crust heating and cooling for both its
outbursts collectively to understand the effect of parameters that may change
for every outburst (e.g., the average accretion rate, the length of outburst,
the envelope composition of the neutron star at the end of the outburst) and
those which can be assumed to remain the same during these two outbursts (e.g.,
the neutron star mass, its radius). Our modelling indicates that all parameters
were consistent between the two outbursts with no need for any significant
changes. In particular, the strength and the depth of the shallow heating
mechanism at work (in the crust) were inferred to be the same during both
outbursts, contrary to what has been found when modelling the cooling curves
after multiple outburst of another source, MAXI J0556-332. This difference in
source behaviour is not understood. We discuss our results in the context of
our current understanding of cooling of accretion-heated neutron-star crusts,
and in particular with respect to the unexplained shallow heating mechanism.Comment: Submitted to A&A. The supplementary video can be found at
https://www.youtube.com/watch?v=OpJ053zq9-
High pressure effects in fluorinated HgBa2Ca2Cu3O(8+d)
We have measured the pressure sensitivity of Tc in fluorinated
HgBa2Ca2Cu3O(8+d) (Hg-1223) ceramic samples with different F contents, applying
pressures up to 30 GPa. We obtained that Tc increases with increasing pressure,
reaching different maximum values, depending on the F doping level, and
decreases for a further increase of pressure. A new high Tc record (166 K +/- 1
K) was achieved by applying pressure (23 GPa) in a fluorinated Hg-1223 sample
near the optimum doping level. Our results show that all our samples are at the
optimal doping, and that fluorine incorporation decreases the crystallographic
-parameter concomitantly increasing the maximum attainable Tc. This effect
reveals that the compression of the axes is one of the keys that controls
the Tc of high temperature superconductors.Comment: 4 pages, 4 figures, submitted to Phys. Rev.
The effect of diffusive nuclear burning in neutron star envelopes on cooling in accreting systems
Valuable information about the neutron star (NS) interior can be obtained by comparing observations of thermal radiation from a cooling NS crust with theoretical models. Nuclear burning of lighter elements that diffuse to deeper layers of the envelope can alter the relation between surface and interior temperatures and can change the chemical composition over time. We calculate new temperature relations and consider two effects of diffusive nuclear burning (DNB) for H–C envelopes. First, we consider the effect of a changing envelope composition and find that hydrogen is consumed on short time-scales and our temperature evolution simulations correspond to those of a hydrogen-poor envelope within ∼100 d. The transition from a hydrogen-rich to a hydrogen-poor envelope is potentially observable in accreting NS systems as an additional initial decline in surface temperature at early times after the outburst. Second, we find that DNB can produce a non-negligible heat flux, such that the total luminosity can be dominated by DNB in the envelope rather than heat from the deep interior. However, without continual accretion, heating by DNB in H–C envelopes is only relevant for <1–80 d after the end of an accretion outburst, as the amount of light elements is rapidly depleted. Comparison to crust cooling data shows that DNB does not remove the need for an additional shallow heating source. We conclude that solving the time-dependent equations of the burning region in the envelope self-consistently in thermal evolution models instead of using static temperature relations would be valuable in future cooling studies
Relation between self-organized criticality and grain aspect ratio in granular piles
We investigate experimentally whether self-organized criticality (SOC) occurs in granular piles composed of different grains, namely, rice, lentils, quinoa, and mung beans. These four grains were selected to have different aspect ratios, from oblong to oblate. As a function of aspect ratio, we determined the growth (β) and roughness (α) exponents, the avalanche fractal dimension (D), the avalanche size distribution exponent (τ), the critical angle (γ), and its fluctuation. At superficial inspection, three types of grains seem to have power-law-distributed avalanches with a well-defined τ. However, only rice is truly SOC if we take three criteria into account: a power-law-shaped avalanche size distribution, finite size scaling, and a universal scaling relation relating characteristic exponents. We study SOC as a spatiotemporal fractal; in particular, we study the spatial structure of criticality from local observation of the slope angle. From the fluctuation of the slope angle we conclude that greater fluctuation (and thus bigger avalanches) happen in piles consisting of grains with larger aspect ratio. © 2012 American Physical Society
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