On isomorphisms between Siegel modular threefolds

The Satake compactification of the moduli space of principally polarized abelian surfaces with a level two structure has a degree 8 endomorphism. The aim of this paper is to show that this result can be extended to other modular threefolds. The main tools are Siegel modular forms and Satake compactifications of arithmetic quotients of the Siegel upper-half space. Indeed, the construction of the degree 8 endomorphism on suitable modular threefolds is done via an isomorphism of graded rings of modular forms. By studying the action of the Fricke involution one gets a further extension of the previous result to other modular threefolds. The possibility of a similar situation in higher dimensions is also discussed.Comment: 13 pages, revised versio

Book review: happiness and social policy in Europe edited by Bent Greve

Happiness has become a central research issue in recent years, and this book brings together researchers from around Europe to present, analyse and discuss the relationship between happiness and social policy. Pierpaolo Perna finds a very informative and insightful collection of essays, and recommends it for policymakers, researchers and academics interested in combining economics, psychology and sociology

From Smart Cities To Playable Cities. Towards Playful Intelligence In The Urban Environment

In the last decade, we have seen the rise of urban play as a tool for community building, and city-making and Western society is actively focusing on play/playfulness and intelligent systems as a way to approach complex challenges and emergent situations. In this paper, we aim to initiate a dialogue between game scholars and architects. Like many creative professions, we believe that the architectural practice may benefit significantly from having more design methodologies at hand, thus improving lateral thinking. We aim at providing new conceptual and operative tools to discuss and reflect on how games and smart systems facilitate long-term the shift from the Smart Cities to the Playable one, where citizens/players have the opportunity to hack the city and use the smart city’s data and digital technology for their purposes to reactivate the urban environment

Accretion flows in early-type galaxies and CMB experiments

We investigate the possible contribution from the emission of accretion flows around supermassive black holes in early type galaxies to current measurements of the Cosmic Microwave Background (CMB) at radio frequencies. We consider a range of luminosities suggested by targeted radio observations and accretion models and compute the residual contribution of these sources to the spectrum and bispectrum of the observed CMB maps. As for high-resolution CMB experiments, we find that the unresolved component of these sources could make up to ~40-50% of the observed CBI and BIMA power spectrum at l > 2000. As a consequence, the inferred sigma_8^{SZ} value could be biased high by up to 6-7%. As for all sky experiments, we find that the contribution of accretion-flow sources to the WMAP bispectrum is at the 2-3 per cent level at most. At the flux limit that Planck will achieve, however, these sources may contribute up to 15 per cent of the bispectrum in the 60-100 GHz frequency range. Moreover, Planck should detect hundreds of these sources in the 30-300 GHz frequency window. These detections, possibly coupled with galaxy type confirmation from optical surveys, will allow number counts to put tighter constraints on early-type galaxies radio luminosity and accretion flows properties. These sources may also contribute up to the 30 per cent level to the residual radio sources power spectrum in future high-resolution SZ surveys (like ACT or APEX) reaching mJy flux limits.Comment: 7 pages, 3 figures, accepted to MNRA

Broadband Modeling of GRB 021004

We present a broadband modeling of the afterglow of GRB 021004. The optical transient of this burst has been detected very early and densely sampled in several bands. Its light curve shows significant deviations from a simple power law. We use the data from the X-ray to the $I-$band gathered in the first month of observations, and examine three models. Two involve variations in the energy of the shock. The first (energy injection) allows only increases to the shock energy, while the second (patchy shell) allows the energy to increase or decrease. In the final model (clumpy medium) the energy of the shock is constant while the density varies. While all three models reproduce well the optical bands, the variable density model can best account for the X-ray data, and the energy-injection model has the poorest fit. None of the models can account for the modest color variations observed during the first few days of the burst.Comment: 13 pages, 5 figures, accepted to Astrophys. J. Letters, added analysis of X-ray lightcurv

Constraints on the Emission and Viewing Geometry of the Transient Anomalous X-ray Pulsar XTE J1810-197

The temporal decay of the flux components of Transient Anomalous X-ray Pulsar XTE J1810-197 following its 2002 outburst presents a unique opportunity to probe the emission geometry of a magnetar. Toward this goal, we model the magnitude of the pulsar's modulation in narrow spectral bands over time. Following previous work, we assume that the post-outburst flux is produced in two distinct thermal components arising from a hot spot and a warm concentric ring. We include general relativistic effects on the blackbody spectra due to gravitational redshift and light bending near the stellar surface, which strongly depend on radius. This affects the model fits for the temperature and size of the emission regions. For the hot spot, the observed temporal and energy-dependent pulse modulation is found to require an anisotropic, pencil-beamed radiation pattern. We are able to constrain an allowed range for the angles that the line-of-sight (psi) and the hot spot pole (xi) make with respect to the spin-axis. Within errors, this is defined by the locus of points in the xi-psi plane that lie along the line (xi+beta(R))(psi+beta(R)) ~ constant, where beta(R) is a function of the radius R of the star. For a canonical value of R=12 km, the viewing parameters range from psi=xi=37 deg to (psi,xi)=(85 deg,15 deg). We discuss our results in the context of magnetar emission models.Comment: 8 pages, accepted to Ap

Strategies for EU survival: bind states to cooperate, improve democratic legitimacy, and make the decision making process more efficient

Public unease with the European Union, Euro problems, and dysfunctional institutions give rise to the real danger that the EU will become increasing irrelevant, just as its member states face more and more challenges in a globalised world. Jean-Claude Piris, a leading figure in the conception and drafting of the EU’s legal structures, works through the options available in light of the economic and political climate, assessing their effectiveness. Reviewed by Pierpaolo Perna. The Future of Europe: Towards a Two-Speed EU? Jean-Claude Piris. Cambridge University Press. December 2011

Atmospheres of Hot Super-Earths

Hot super-Earths likely possess minimal atmospheres established through vapor saturation equilibrium with the ground. We solve the hydrodynamics of these tenuous atmospheres at the surface of Corot-7b, Kepler 10b and 55 Cnc-e, including idealized treatments of magnetic drag and ohmic dissipation. We find that atmospheric pressures remain close to their local saturation values in all cases. Despite the emergence of strongly supersonic winds which carry sublimating mass away from the substellar point, the atmospheres do not extend much beyond the day-night terminators. Ground temperatures, which determine the planetary thermal (infrared) signature, are largely unaffected by exchanges with the atmosphere and thus follow the effective irradiation pattern. Atmospheric temperatures, however, which control cloud condensation and thus albedo properties, can deviate substantially from the irradiation pattern. Magnetic drag and ohmic dissipation can also strongly impact the atmospheric behavior, depending on atmospheric composition and the planetary magnetic field strength. We conclude that hot super-Earths could exhibit interesting signatures in reflection (and possibly in emission) which would trace a combination of their ground, atmospheric and magnetic properties.Comment: 14 pages, 4 figures, 1 table, accepted for publication in ApJ