Creation of magnetic spots at the neutron star surface

According to the partially screened gap scenario, an efficient electron-positron pair creation, a general precondition of radio-pulsar activity, relies on the existence of magnetic spots, i.e., local concentrations of strong and small scale magnetic field structures at the surface of neutron stars. They have a strong impact on the surface temperature, which is potentially observable. Here we reinforce the idea that such magnetic spots can be formed by extracting magnetic energy from the toroidal field that resides in deep crustal layers, via Hall drift. We study and discuss the magneto-thermal evolution of qualitatively different neutron star models and initial magnetic field configurations that lead to the creation of magnetic spots. We find that magnetic spots can be created on a timescale of $10^4$ years with magnetic field strengths $\gtrsim 5\times 10^{13}$ G, provided almost the whole magnetic energy is stored in its toroidal component, and that the conductivity in the inner crust is not too large. The lifetime of the magnetic spots is at least $\sim$one million of years, being longer if the initial field permeates both core and crust.Comment: Accepted by M.N.R.A.

Population Synthesis of Isolated Neutron Stars with magneto-rotational evolution II: from radio-pulsars to magnetars

Population synthesis studies constitute a powerful method to reconstruct the birth distribution of periods and magnetic fields of the pulsar population. When this method is applied to populations in different wavelengths, it can break the degeneracy in the inferred properties of initial distributions that arises from single-band studies. In this context, we extend previous works to include $X$-ray thermal emitting pulsars within the same evolutionary model as radio-pulsars. We find that the cumulative distribution of the number of X-ray pulsars can be well reproduced by several models that, simultaneously, reproduce the characteristics of the radio-pulsar distribution. However, even considering the most favourable magneto-thermal evolution models with fast field decay, log-normal distributions of the initial magnetic field over-predict the number of visible sources with periods longer than 12 s. We then show that the problem can be solved with different distributions of magnetic field, such as a truncated log-normal distribution, or a binormal distribution with two distinct populations. We use the observational lack of isolated NSs with spin periods P>12 s to establish an upper limit to the fraction of magnetars born with B > 10^{15} G (less than 1\%). As future detections keep increasing the magnetar and high-B pulsar statistics, our approach can be used to establish a severe constraint on the maximum magnetic field at birth of NSs.Comment: 12 pages, 11 figures, 5 table

Pulsar timing irregularities and the imprint of magnetic field evolution

(Abridged) The rotational evolution of isolated neutron stars is dominated by the magnetic field anchored to the solid crust of the star. Assuming that the core field evolves on much longer timescales, the crustal field evolves mainly though Ohmic dissipation and the Hall drift, and it may be subject to relatively rapid changes with remarkable effects on the observed timing properties. We investigate whether changes of the magnetic field structure and strength during the star evolution may have observable consequences in the braking index, which is the most sensitive quantity to reflect small variations of the timing properties that are caused by magnetic field rearrangements. By performing axisymmetric, long-term simulations of the magneto-thermal evolution of neutron stars with state-of-the-art microphysical inputs, we find that the effect of the magnetic field evolution on the braking index can be divided into three qualitatively different stages depending on the age and the internal temperature: a first stage that may be different for standard pulsars (with n~3) or low field neutron stars that accreted fallback matter during the supernova explosion (systematically n<3); in a second stage, the evolution is governed by almost pure Ohmic field decay, and a braking index n>3 is expected; in the third stage, at late times, when the interior temperature has dropped to very low values, Hall oscillatory modes in the neutron star crust result in braking indices of high absolute value and both positive and negative signs. Models with strong (1e14 G) multipolar or toroidal components, even with a weak (~1e12 G) dipolar field are consistent with the observed trend of the timing properties.Comment: 7 pages, 5 figures, accepted for publication in Astronomy & Astrophysics (submitted July 24, 2012

Pushing context-awareness down to the core: moreflexibility for the PerLa language

Information technology is increasingly pervading our envi- ronment, making real Mark Weiser’s vision of a “disappear- ing technology”. The work described in this paper focuses on using context to enable pervasive system personaliza- tion, allowing context-aware sensor-data tailoring. Since sensor networks, besides data collection, are also able to pro- duce active behaviours, the tailoring capabilities are also ex- tended to these, thus applying context-awareness to generic system operations. Moreover, because the number of pos- sible context can grow rapidly with the complexity of the application, the design phase is also supported by the possi- bility to speed-up and modularize the definition of the data and operations associated with each specific context, pro- ducing a support tool that eases the job of the designers of modern context-aware pervasive systems

Towards autonomic pervasive systems: the PerLa context language

The property of context-awareness, inherent to a Pervasive System, requires a clear definition of context and of how the context parameter values must be extracted from the real world. Since often the same variables are common to the operational system and to the context it operates into, the usage of the same language to manage both the application and the context can lead to substantial savings in application development time and costs. In this paper we propose a context-management extension to the PerLa language and middleware that allows for declarative gathering of context data from the environment, feeding this data to the internal context model and, once a context is active, acting on the relevant resources of the pervasive system, according to the chosen contextual policy

Non-collaborative Attackers and How and Where to Defend Flawed Security Protocols (Extended Version)

Security protocols are often found to be flawed after their deployment. We present an approach that aims at the neutralization or mitigation of the attacks to flawed protocols: it avoids the complete dismissal of the interested protocol and allows honest agents to continue to use it until a corrected version is released. Our approach is based on the knowledge of the network topology, which we model as a graph, and on the consequent possibility of creating an interference to an ongoing attack of a Dolev-Yao attacker, by means of non-collaboration actuated by ad-hoc benign attackers that play the role of network guardians. Such guardians, positioned in strategical points of the network, have the task of monitoring the messages in transit and discovering at runtime, through particular types of inference, whether an attack is ongoing, interrupting the run of the protocol in the positive case. We study not only how but also where we can attempt to defend flawed security protocols: we investigate the different network topologies that make security protocol defense feasible and illustrate our approach by means of concrete examples.Comment: 29 page

Assessing the potential of molten carbonate fuel cell-based schemes for carbon capture in natural gas-fired combined cycle power plants

Abstract This work explores two configurations of natural gas-fired combined cycles (NGCC) with molten carbonate fuel cells (MCFC) for CO2 capture. Special attention is devoted to the selection of MCFC operating conditions (trade-off between CO2 capture and voltage losses), heat integration scheme, fuel use and CO2 purification. Two schemes are considered: (i) in the first "integrated" scheme, MCFC modules are installed between the gas turbine and the heat recovery steam generator (HRSG) to maximize the efficiency of the integrated power plant; (ii) in the second "non-integrated" layout, the MCFC is located downstream of the HRSG and a regenerative heat exchanger is designed to preheat cathode reactants up to the MCFC working temperature. This study includes a full techno-economic analysis of the two layouts based on a preliminary sizing of the key-components, and a sensitivity analysis on the CO2 utilization factor. Compared to a benchmark amine scrubbing process, the "integrated" configuration shows considerably better performance (Specific Primary Energy Consumption for CO2 Avoided - SPECCA = 0.31 MJ kgCO2-1; Cost of CO2 avoided - CCA = 50 $tCO2−1), whereas the "non-integrated" solution shows higher energy penalties but similar CO2 avoidance cost (SPECCA = 2.4  MJ  kgCO2−1; CCA = 76$ tCO2−1)

The influence of magnetic field geometry on magnetars X-ray spectra

Nowadays, the analysis of the X-ray spectra of magnetically powered neutron stars or magnetars is one of the most valuable tools to gain insight into the physical processes occurring in their interiors and magnetospheres. In particular, the magnetospheric plasma leaves a strong imprint on the observed X-ray spectrum by means of Compton up-scattering of the thermal radiation coming from the star surface. Motivated by the increased quality of the observational data, much theoretical work has been devoted to develop Monte Carlo (MC) codes that incorporate the effects of resonant Compton scattering in the modeling of radiative transfer of photons through the magnetosphere. The two key ingredients in this simulations are the kinetic plasma properties and the magnetic field (MF) configuration. The MF geometry is expected to be complex, but up to now only mathematically simple solutions (self-similar solutions) have been employed. In this work, we discuss the effects of new, more realistic, MF geometries on synthetic spectra. We use new force-free solutions in a previously developed MC code to assess the influence of MF geometry on the emerging spectra. Our main result is that the shape of the final spectrum is mostly sensitive to uncertain parameters of the magnetospheric plasma, but the MF geometry plays an important role on the angle-dependence of the spectra.Comment: 6 pages, 4 figures To appear in Proceedings of II Iberian Nuclear Astrophysics Meeting held in Salamanca, September 22-23, 201