Nuclear composition and heating in accreting neutron-star crusts

Nuclear reactions in accreting neutron-star crusts and the heat release accompanying them are studied, under different assumptions concerning the composition of the outermost layer formed of the ashes of X-ray bursts. Particular examples of ashes containing nuclides with A ~ 90-110 are considered and compared with a standard A=56 case. In all cases, evolution of a crust shell is followed from 10^8 g/cm^3 to a few times 10^{13} g/cm^3. The total crustal heating produced in the non-equilibrium processes in the accreting crust is 1.1-1.5 MeV per one accreted nucleon. The composition of the accreted crust at densities exceeding the threshold for the pycnonuclear fusion is essentially independent of the assumed initial composition of the X-ray burst ashes.Comment: 5 pages, 2 figures, accepted for publication in A&A Letter

Neutron star cooling after deep crustal heating in the X-ray transient KS 1731-260

We simulate the cooling of the neutron star in the X-ray transient KS 1731-260 after the source returned to quiescence in 2001 from a long (>~ 12.5 yr) outburst state. We show that the cooling can be explained assuming that the crust underwent deep heating during the outburst stage. In our best theoretical scenario the neutron star has no enhanced neutrino emission in the core, and its crust is thin, superfluid, and has the normal thermal conductivity. The thermal afterburst crust-core relaxation in the star may be not over.Comment: 5 pages, 2 figures, accepted by MNRAS. In v.2, two references added and typos correcte

Models of crustal heating in accreting neutron stars

Heating associated with non-equilibrium nuclear reactions in accreting neutron-star crusts is reconsidered, taking into account suppression of neutrino losses demonstrated recently by Gupta et al. Two initial compositions of the nuclear burning ashes, A=56 and A=106, are considered. Dependence of the integrated crustal heating on uncertainties plaguing pycnonuclear reaction models is studied. One-component plasma approximation is used, with compressible liquid-drop model of Mackie and Baym to describe nuclei. Evolution of a crust shell is followed from 10^8 g/cm^3 to 10^(13.6) g/cm^3 The integrated heating in the outer crust agrees nicely with results of self-considtent multicomponent plasma simulations of Gupta et al.; their results fall between our curves obtained for A=56 and A=106. Total crustal heat per one accreted nucleon ranges between 1.5 MeV to 1.9 MeV for A=106 and A=56, respectively. The value of total crustal heat per nucleon depends weakly on the presence of pycnonuclear reactions at densities 10^(12)-10^(13) g/cm^3. Remarkable insensitivity of the total crustal heat on the details of the distribution of nuclear processes in accreted crust is explained.Comment: 8 pages, 5 figures, Submitted to A&

Wireless Medical Sensor Networks: Design Requirements and Enabling Technologies

This article analyzes wireless communication protocols that could be used in healthcare environments (e.g., hospitals and small clinics) to transfer real-time medical information obtained from noninvasive sensors. For this purpose the features of the three currently most widely used protocols—namely, Bluetooth® (IEEE 802.15.1), ZigBee (IEEE 802.15.4), and Wi-Fi (IEEE 802.11)—are evaluated and compared. The important features under consideration include data bandwidth, frequency band, maximum transmission distance, encryption and authentication methods, power consumption, and current applications. In addition, an overview of network requirements with respect to medical sensor features, patient safety and patient data privacy, quality of service, and interoperability between other sensors is briefly presented. Sensor power consumption is also discussed because it is considered one of the main obstacles for wider adoption of wireless networks in medical applications. The outcome of this assessment will be a useful tool in the hands of biomedical engineering researchers. It will provide parameters to select the most effective combination of protocols to implement a specific wireless network of noninvasive medical sensors to monitor patients remotely in the hospital or at home

Small and beautiful? The programme of activities and the least developed countries

Most carbon abatement projects under the Kyoto Protocol's Clean Development Mechanism (CDM) have been implemented in rapidly industrializing countries, notably China and India. To support small carbon abatement projects and to promote decarbonization in the least developed countries, the Programme of Activities (PoA) modality was introduced. Are the determinants of project implementation different under the PoA from those of conventional CDM projects? To answer this question, we conduct a statistical analysis of the global distribution of CDM projects and PoAs during the years 2007–2012. In regard to country size, large countries clearly dominate both the CDM and PoA, suggesting that the PoA may do only little to facilitate project implementation in small countries. However, the number of PoAs has a strong negative association with a country's corruption level, while the importance of corruption for the CDM is much smaller. Moreover, per capita income has no effect on PoA implementation, while high wealth levels have a weak positive effect on CDM projects. Thus, the PoA modality seems to promote sustainable development in poor countries that have exceeded a certain threshold of good governance. In this regard, PoAs are directing carbon credits to new areas, as many had initially hoped

Exploring Outliers in Crowdsourced Ranking for QoE

Outlier detection is a crucial part of robust evaluation for crowdsourceable assessment of Quality of Experience (QoE) and has attracted much attention in recent years. In this paper, we propose some simple and fast algorithms for outlier detection and robust QoE evaluation based on the nonconvex optimization principle. Several iterative procedures are designed with or without knowing the number of outliers in samples. Theoretical analysis is given to show that such procedures can reach statistically good estimates under mild conditions. Finally, experimental results with simulated and real-world crowdsourcing datasets show that the proposed algorithms could produce similar performance to Huber-LASSO approach in robust ranking, yet with nearly 8 or 90 times speed-up, without or with a prior knowledge on the sparsity size of outliers, respectively. Therefore the proposed methodology provides us a set of helpful tools for robust QoE evaluation with crowdsourcing data.Comment: accepted by ACM Multimedia 2017 (Oral presentation). arXiv admin note: text overlap with arXiv:1407.763

Heating in the Accreted Neutron Star Ocean: Implications for Superburst Ignition

We perform a self-consistent calculation of the thermal structure in the crust of a superbursting neutron star. In particular, we follow the nucleosynthetic evolution of an accreted fluid element from its deposition into the atmosphere down to a depth where the electron Fermi energy is 20 MeV. We include temperature-dependent continuum electron capture rates and realistic sources of heat loss by thermal neutrino emission from the crust and core. We show that, in contrast to previous calculations, electron captures to excited states and subsequent gamma-emission significantly reduce the local heat loss due to weak-interaction neutrinos. Depending on the initial composition these reactions release up to a factor of 10 times more heat at densities < 10^{11} g/cc than obtained previously. This heating reduces the ignition depth of superbursts. In particular, it reduces the discrepancy noted by Cumming et al. between the temperatures needed for unstable 12C ignition on timescales consistent with observations and the reduction in crust temperature from Cooper pair neutrino emission.Comment: 10 pages, 11 figures, the Astrophysical Journal, in press (scheduled for v. 662). Revised from v1 in response to referee's comment

The rp Process Ashes from Stable Nuclear Burning on an Accreting Neutron Star

We calculate the nucleosynthesis during stable nuclear burning on an accreting neutron star. This is appropriate for weakly magnetic neutron stars accreting at near-Eddington rates in low mass X-ray binaries, and for most accreting X-ray pulsars. We show that the nuclear burning proceeds via the rapid proton capture process (rp process), and makes nuclei far beyond the iron group. The final mixture of nuclei consists of elements with a range of masses between approximately A=60 and A=100. The average nuclear mass of the ashes is set by the extent of helium burning via (alpha,p) reactions, and depends on the local accretion rate. Our results imply that the crust of these accreting neutron stars is made from a complex mixture of heavy nuclei, with important implications for its thermal, electrical and structural properties. A crustal lattice as impure as our results suggest will have a conductivity set mostly by impurity scattering, allowing more rapid Ohmic diffusion of magnetic fields than previously estimated.Comment: To appear in the Astrophysical Journal (33 pages, LaTeX, including 11 postscript figures