Continued cooling of the crust in the neutron star low-mass X-ray binary KS 1731-260

Some neutron star low-mass X-ray binaries have very long outbursts (lasting several years) which can generate a significant amount of heat in the neutron star crust. After the system has returned to quiescence, the crust then thermally relaxes. This provides a rare opportunity to study the thermal properties of neutron star crusts, putting constraints on the thermal conductivity and hence the structure and composition of the crust. KS 1731-260 is one of only four systems where this crustal cooling has been observed. Here, we present a new Chandra observation of this source approximately 8 years after the end of the last outburst, and 4 years since the last observation. We find that the source has continued to cool, with the cooling curve displaying a simple power-law decay. This suggests that the crust has not fully thermally relaxed yet, and may continue to cool further. A simple power law decay is in contrast to theoretical cooling models of the crust, which predict that the crust should now have cooled to the same temperature as the neutron star core.Comment: Accepted to ApJ Letter

Magnetic resonance imaging: a tool for pork pie development

The traditional British pork pie consists of roughly chopped pork cooked in a hot water pastry crust. Due to shrinkage of the meat during cooking, the gap formed around the meat is usually sealed using a gelatin based jelly to exclude air and thus help to preserve the pie. The properties of the jelly are such that it will ingress into the pastry crust causing undesirable softening. The jelly is traditionally produced by simmering pig trotters with seasoning for several hours. In this work we demonstrate the potential of magnetic resonance imaging (MRI) as a tool for investigating the conditions required for producing jellies with different properties and present two examples of this use. Firstly we demonstrate that MRI can determine the ability of water to diffuse through the jelly which is critical in minimizing the amount of moisture moving from the jelly to the crust. Secondly, the impact of jelly temperature on the penetration length into the crust is investigated. These examples highlight the power of MRI as a tool for food assessment

Antineutrino Geophysics with Liquid Scintillator Detectors

Detecting the antineutrinos emitted by the decay of radioactive elements in the mantle and crust could provide a direct measurement of the total abundance of uranium and thorium in the Earth. In calculating the antineutrino flux at specific sites, the local geology of the crust and the background from the world's nuclear power reactors are important considerations. Employing a global crustal map, with type and thickness data, and using recent estimates of the uranium and thorium distribution in the Earth, we calculate the antineutrino event rate for two new neutrino detectors. We show that spectral features allow terrestrial antineutrino events to be identified above reactor antineutrino backgrounds and that the uranium and thorium contributions can be separately determined.Comment: Published paper differs from original submitted preprint because reviewers suggested updated continental crust U/Th abundances. Kamioka geographical location error was in preprint, partially corrected in published version. This version is the same as the published paper, with Kamioka fully corrected. Because of recent interest in this topic, this version is being made available, despite this work being 8 years ol

The Torque and X-Ray Flux Changes of OAO 1657-415

Combining previously published pulse frequencies and BATSE measurements, we estimate the noise strengths (or power density estimates) of angular accelerations by using the root mean square residuals of angular velocity time series of OAO 1657-415 and present the power spectra. The statistical interpretation of the angular velocity fluctuations are consistent with a random walk model. In order to investigate the short term angular velocity fluctuations in detail, a structure function analysis is applied for a two component neutron star model with a solid crust and a superfluid neutron core which is subjected to external white torque noise. No evidence for core-crust coupling on timescales longer than one day is found. The correlations between X-ray flux and angular acceleration ($\dot \Omega$) fluctuations are investigated. These are compared with disk accretion theory (Ghosh & Lamb 1979 a,b) and wind accretion theory (Blondin et al., 1990). It is found that the most natural explanation of X ray flux and angular acceleration fluctuations is the formation of episodic accretion disks in the case of stellar wind accretion.Comment: 39 pages, to appear in "Astronomy and Astrophysics

Global Seismic Oscillations in Soft Gamma Repeaters

There is evidence that soft gamma repeaters (SGRs) are neutron stars which experience frequent starquakes, possibly driven by an evolving, ultra-strong magnetic field. The empirical power-law distribution of SGR burst energies, analogous to the Gutenberg-Richter law for earthquakes, exhibits a turn-over at high energies consistent with a global limit on the crust fracture size. With such large starquakes occurring, the significant excitation of global seismic oscillations (GSOs) seems likely. Moreover, GSOs may be self-exciting in a stellar crust that is strained by many, randomly-oriented stresses. We explain why low-order toroidal modes, which preserve the shape of the star and have observable frequencies as low as ~ 30 Hz, may be especially susceptible to excitation. We estimate the eigenfrequencies as a function of stellar mass and radius, and their magnetic and rotational shiftings/splittings. We also describes ways in which these modes might be detected and damped. There is marginal evidence for 23 ms oscillations in the hard initial pulse of the 1979 March 5th event. This could be due to the $_3t_0$ mode in a neutron star with B ~ 10^{14} G or less; or it could be the fundamental toroidal mode if the field in the deep crust of SGR 0526-66 is ~ 4 X 10^{15} G, in agreement with other evidence. If confirmed, GSOs would give corroborating evidence for crust-fracturing magnetic fields in SGRs: B >~ 10^{14} G.Comment: 12 pages, AASTeX, no figures. Accepted for Astrophysical Journal Letter

Crustal structure of the Borderland-Continent Transition Zone of southern California adjacent to Los Angeles

We use data from the onshore-offshore component of Los Angeles Region Seismic Experiment (LARSE) to model the broad-scale features of the midcrust to upper mantle beneath a north-south transect that spans the continental borderland in the Los Angeles, California, region. We have developed an analysis method for wide-angle seismic data that consists primarily of refractions, lacks near-offset recordings, and contains wide gaps in coverage. Although the data restrict the analysis to the modeling of broad-scale structure, the technique allows one to explore the limits of the data and determine the resolving power of the data set. The resulting composite velocity model constrains the crustal thickness and location and width of the continent-Borderland transition zone. We find that the mid to lower crust layer velocities of the Inner Borderland are slightly lower than the corresponding layers in the average southern California crust model, while the upper mantle velocity is significantly higher. The data require the Moho to deepen significantly to the north. We constrain the transition zone to initiate between the offshore slope and the southwest Los Angeles Basin. If the Inner Borderland crust is 22 km thick, then the transition zone is constrained to initiate within a 2 km wide region beneath the southwest Los Angeles Basin, and have a width of 20–25 km. The strong, coherent, and continuous Pn phase suggests the Moho is coherent and laterally continuous beneath the Inner Borderland and transition zone. The Inner California Borderland seems to be modified and thickened oceanic crust, with the oceanic upper mantle intact beneath it