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

Towards operational measures of computer security

Ideally, a measure of the security of a system should capture quantitatively the intuitive notion of ‘the ability of the system to resist attack’. That is, it should be operational, reflecting the degree to which the system can be expected to remain free of security breaches under particular conditions of operation (including attack). Instead, current security levels at best merely reflect the extensiveness of safeguards introduced during the design and development of a system. Whilst we might expect a system developed to a higher level than another to exhibit ‘more secure behaviour’ in operation, this cannot be guaranteed; more particularly, we cannot infer what the actual security behaviour will be from knowledge of such a level. In the paper we discuss similarities between reliability and security with the intention of working towards measures of ‘operational security’ similar to those that we have for reliability of systems. Very informally, these measures could involve expressions such as the rate of occurrence of security breaches (cf rate of occurrence of failures in reliability), or the probability that a specified ‘mission’ can be accomplished without a security breach (cf reliability function). This new approach is based on the analogy between system failure and security breach. A number of other analogies to support this view are introduced. We examine this duality critically, and have identified a number of important open questions that need to be answered before this quantitative approach can be taken further. The work described here is therefore somewhat tentative, and one of our major intentions is to invite discussion about the plausibility and feasibility of this new approach

Compton Heating of the Intergalactic Medium by the Hard X-ray Background

High-resolution hydrodynamics simulations of the Ly-alpha forest in cold dark matter dominated cosmologies appear to predict line widths that are substantially narrower than those observed. Here we point out that Compton heating of the intergalactic gas by the hard X-ray background (XRB), an effect neglected in all previous investigations, may help to resolve this discrepancy. The rate of gain in thermal energy by Compton scattering will dominate over the energy input from hydrogen photoionization if the XRB energy density is 0.2x/ times higher than the energy density of the UV background at a given epoch, where x is the hydrogen neutral fraction in units of 1e-6 and is the mean X-ray photon energy in units of m_ec^2. The numerical integration of the time-dependent rate equations shows that the intergalactic medium approaches a temperature of about 1.5e4 K at z>3 in popular models for the redshift evolution of the extragalactic background radiation. The importance of Compton heating can be tested experimentally by measuring the Ly-alpha line-width distribution as a function of redshift, thus the Lyman-alpha forest may provide a useful probe of the evolution of the XRB at high redshifts.Comment: LaTeX, 10 pages, 2 figures, final version to be published in the Ap

Recommended approach to sofware development

A set of guideline for an organized, disciplined approach to software development, based on data collected and studied for 46 flight dynamics software development projects. Methods and practices for each phase of a software development life cycle that starts with requirements analysis and ends with acceptance testing are described; maintenance and operation is not addressed. For each defined life cycle phase, guidelines for the development process and its management, and the products produced and their reviews are presented

Transient Observers and Variable Constants, or Repelling the Invasion of the Boltzmann's Brains

If the universe expands exponentially without end, ``ordinary observers'' like ourselves may be vastly outnumbered by ``Boltzmann's brains,'' transient observers who briefly flicker into existence as a result of quantum or thermal fluctuations. One might then wonder why we are so atypical. I show that tiny changes in physics--for instance, extremely slow variations of fundamental constants--can drastically change this result, and argue that one should be wary of conclusions that rely on exact knowledge of the laws of physics in the very distant future.Comment: 4 pages, LaTeX; v2: added references; v3: more discussion of setting, alternative approaches, now 5 pages; v4: added discussion of the effect of quantum fluctuations on varying constants, appendix added, now 7 pages; v5: new reference, minor correctio

Proof of the Generalized Second Law for Quasistationary Semiclassical Black Holes

A simple direct explicit proof of the generalized second law of black hole thermodynamics is given for a quasistationary semiclassical black hole.Comment: 12 pages, LaTeX, report Alberta-Thy-10-93 (revision of paper in response to Phys. Rev. Lett. referees' comments, which suffered a series of long delays

No Time Asymmetry from Quantum Mechanics

With CPT-invariant initial conditions that commute with CPT-invariant final conditions, the respective probabilities (when defined) of a set of histories and its CPT reverse are equal, giving a CPT-symmetric universe. This leads me to question whether the asymmetry of the Gell-Mann--Hartle decoherence functional for ordinary quantum mechanics should be interpreted as an asymmetry of {\it time} .Comment: 14 pages, Alberta-Thy-11-9