Disc atmospheres and winds in X-ray binaries

We review the current status of studies of disc atmospheres and winds in low mass X-ray binaries. We discuss the possible wind launching mechanisms and compare the predictions of the models with the existent observations. We conclude that a combination of thermal and radiative pressure (the latter being relevant at high luminosities) can explain the current observations of atmospheres and winds in both neutron star and black hole binaries. Moreover, these winds and atmospheres could contribute significantly to the broad iron emission line observed in these systems.Comment: Accepted for publication in Acta Polytechnica. Invited review talk at the Vulcano Workshop 2012: "Frontier Objects in Astrophysics and Particle Physics

Intergovernmental grant rules, the "golden rule" of public finance and local expenditures

The Stability and Growth Pact and the process of fiscal consolidation in several European countries have enhanced the role of fiscal rules at sub-national level. This paper analyzes the combined effect of a rule to allocate capital and current block grants to local governments and the “golden rule” of public finance (surplus of current balance). We argue that the two fiscal rules introduce significant rigidities and distortions in local governments’ expenditures structure since these mimic the structure of revenues. This effect is particularly relevant in municipalities that are more dependent of intergovernmental grants, mainly rural. On the other hand, urban municipalities with greater tax revenues (current revenues) are constrained in their ability to make capital investments because they receive per capita capital grants below what economies of scale would suggest. An empirical analysis of Portuguese local governments shows that it is no longer the median voter, but fiscal rules, that command the broad pattern of expenditure (current versus capital) at a local level. This paper is a contribution to the literature on the perverse effects of fiscal rules.Intergovernmental block grants; Fiscal Rules; Local Government Expenditure; “Golden Rule”

The cooling, mass and radius of the neutron star in EXO 0748-676 in quiescence with XMM-Newton

We analyse four XMM-Newton observations of the neutron-star low-mass X-ray binary EXO 0748$-$676 in quiescence. We fit the spectra with an absorbed neutron-star atmosphere model, without the need for a high-energy (power-law) component; with a 95 per cent confidence the power-law contributes less than 1 per cent to the total flux of the source in $0.5-10.0$ keV. The fits show significant residuals at around 0.5 keV which can be explained by either a hot gas component around the neutron star or a moderately broad emission line from a residual accretion disc. The temperature of the neutron-star has decreased significantly compared to the previous observation, from 124 eV to 105 eV, with the cooling curve being consistent with either an exponential decay plus a constant or a (broken) power law. The best-fitting neutron-star mass and radius can be better constrained if we extend the fits down to the lowest possible energy available. For an assumed distance of 7.1 kpc, the best-fitting neutron-star mass and radius are $2.00_{-0.24}^{+0.07}~M_\odot$ and $11.3_{-1.0}^{+1.3}$ km if we fit the spectrum over the $0.3-10$ keV range, but $1.50_{-1.0}^{+0.4}~M_\odot$ and $12.2_{-3.6}^{+0.8}$ km if we restrict the fits to the $0.5-10$ keV range. We finally discuss the effect of the assumed distance to the source upon the best-fitting neutron-star mass and radius. As systematic uncertainties in the deduced mass and radius depending on the distance are much larger than the statistical errors, it would be disingenuous to take these results at face value.Comment: 12 pages, 6 figure

Citizens’ Freedom to Choose Representatives: Ballot Structure, Proportionality and “Fragmented” Parliaments

The analysis of the political consequences of electoral laws has emphasized how individual characteristics of the electoral system (electoral formulas, district magnitude, ballot structure) affect the degree of parliament “fragmentation” and proportionality. This paper argues that the personal attributes of representatives are also an important consequence of electoral laws, and that they are in part determined by citizens’ freedom to choose representatives. We clarify this concept and develop an index of citizens’ freedom to choose members of parliament as a function of the ballot structure, district size and electoral formulae. Using data from twenty nine countries, we find that neither proportionality nor the effective number of parties is significantly affected by voters’ freedom of choice. This result has important normative implications for electoral reform.Ballot structure; Electoral index; Freedom to choose; Personal vote.

Modelling the disk atmosphere of the low mass X-ray binary EXO 0748-676

Low mass X-ray binaries exhibit ionized emission from an extended disk atmosphere that surrounds the accretion disk. However, its nature and geometry is still unclear. In this work we present a spectral analysis of the extended atmosphere of EXO 0748-676 using high-resolution spectra from archival XMM-Newton observations. We model the RGS spectrum that is obtained during the eclipses. This enables us to model the emission lines that come only from the extended atmosphere of the source, and study its physical structure and properties. The RGS spectrum reveals a series of emission lines consistent with transitions of O VIII, O VII, Ne IX and N VII. We perform both Gaussian line fitting and photoionization modelling. Our results suggest that there are two photoionization gas components, out of pressure equilibrium with respect to each other. One with ionization parameter of 2.5 and a large opening angle, and one with 1.3. The second component is possibly covering a smaller fraction of the source. From the density diagnostics of the O vii triplet using photoionization modelling, we detect a rather high density plasma of > 10^13 cm^-3 for the lower ionization component. This latter component also displays an inflow velocity. We propose a scenario where the high ionization component constitutes an extended upper atmosphere of the accretion disk. The lower ionization component may instead be a clumpy gas created from the impact of the accretion stream with the disk.Comment: A&A accepted, 10 pages, 9 figure