First order phase transitions in classical lattice gas spin models

The present paper considers some classical ferromagnetic lattice--gas models, consisting of particles that carry $n$--component spins ($n=2,3$) and associated with a $D$--dimensional lattice ($D=2,3$); each site can host one particle at most, thus implicitly allowing for hard--core repulsion; the pair interaction, restricted to nearest neighbors, is ferromagnetic, and site occupation is also controlled by the chemical potential $\mu$. The models had previously been investigated by Mean Field and Two--Site Cluster treatments (when D=3), as well as Grand--Canonical Monte Carlo simulation in the case $\mu=0$, for both D=2 and D=3; the obtained results showed the same kind of critical behaviour as the one known for their saturated lattice counterparts, corresponding to one particle per site. Here we addressed by Grand--Canonical Monte Carlo simulation the case where the chemical potential is negative and sufficiently large in magnitude; the value $\mu=-D/2$ was chosen for each of the four previously investigated counterparts, together with $\mu=-3D/4$ in an additional instance. We mostly found evidence of first order transitions, both for D=2 and D=3, and quantitatively characterized their behaviour. Comparisons are also made with recent experimental results.Comment: 9 pages, 12 figure

Nematic order by thermal disorder in a three-dimensional lattice-spin model with dipolar-like interactions

At low temperatures, some lattice spin models with simple ferromagnetic or antiferromagnetic interactions (for example nearest-neighbour interaction being isotropic in spin space on a bipartite three-dimensional lattice) produce orientationally ordered phases exhibiting nematic (second--rank) order, in addition to the primary first-rank one; on the other hand, in the Literature, they have been rather seldom investigated in this respect. Here we study the thermodynamic properties of a three-dimensional model with dipolar-like interaction. Its ground state is found to exhibit full orientational order with respect to a suitably defined staggered magnetization (polarization), but no nematic second-rank order. Extensive Monte Carlo simulations, in conjunction with Finite-Size Scaling analysis have been used for characterizing its critical behaviour; on the other hand, it has been found that nematic order does indeed set in at low temperatures, via a mechanism of order by disorder.Comment: 24 pages, 9 figure

Realistic Sensitivity Curves For Pulsar Timing Arrays

We construct realistic sensitivity curves for pulsar timing array searches for gravitational waves, incorporating both red and white noise contributions to individual pulsar noise spectra, and the effect of fitting to a pulsar timing model. We demonstrate the method on both simulated pulsars and a realistic array consisting of a subset of NANOGrav pulsars used in recent analyses. A comparison between the results presented here and measured upper limit curves from actual analyses shows agreement to tens of percent. The resulting sensitivity curves can be used to assess the detectability of predicted gravitational-wave signals in the nanohertz frequency band in a coherent, flexible, and computationally efficient manner

Reforming the U. S. health care system: where there's a will, there could be a way

The essay in the 2005 annual report summarizes the themes and consensus-based prescriptions for action that emerged from the Boston Fed's 50th economic conference, Wanting It All: The Challenge of Reforming the U.S. Health Care System, held in June 2005.Health care reform ; Medical care, Cost of

Quantum and Superquantum Nonlocal Correlations

We present a simple hidden variable model for the singlet state of a pair of qubits, characterized by two kinds, hierarchically ordered, of hidden variables. We prove that, averaging over both types of variables, one reproduces all the quantum mechanical correlations of the singlet state. On the other hand, averaging only over the hidden variables of the lower level, one obtains a general formal theoretical scheme exhibiting correlations stronger than the quantum ones, but with faster-than-light communication forbidden. This result is interesting by itself since it shows that a violation of the quantum bound for nonlocal correlations can be implemented in a precise physical manner and not only mathematically, and it suggests that resorting to two levels of nonlocal hidden variables might led to a deeper understanding of the physical principles at the basis of quantum nonlocality.Comment: 5 pages, 1 figure. Submitted for publicatio

Two years of monitoring Supergiant Fast X-ray Transients with Swift

We present two years of intense Swift monitoring of three SFXTs, IGR J16479-4514, XTE J1739-302, and IGR J17544-2619 (since October 2007). Out-of-outburst intensity-based X-ray (0.3-10keV) spectroscopy yields absorbed power laws with by hard photon indices (G~1-2). Their outburst broad-band (0.3-150 keV) spectra can be fit well with models typically used to describe the X-ray emission from accreting NSs in HMXBs. We assess how long each source spends in each state using a systematic monitoring with a sensitive instrument. These sources spend 3-5% of the total in bright outbursts. The most probable flux is 1-2E-11 erg cm^{-2} s^{-1} (2-10 keV, unabsorbed), corresponding to luminosities in the order of a few 10^{33} to 10^{34} erg s^{-1} (two orders of magnitude lower than the bright outbursts). The duty-cycle of inactivity is 19, 39, 55%, for IGR J16479-4514, XTE J1739-302, and IGR J17544-2619, respectively. We present a complete list of BAT on-board detections further confirming the continued activity of these sources. This demonstrates that true quiescence is a rare state, and that these transients accrete matter throughout their life at different rates. X-ray variability is observed at all timescales and intensities we can probe. Superimposed on the day-to-day variability is intra-day flaring which involves variations up to one order of magnitude that can occur down to timescales as short as ~1ks, and whichcan be explained by the accretion of single clumps composing the donor wind with masses M_cl~0.3-2x10^{19} g. (Abridged)Comment: Accepted for publication in MNRAS. 17 pages, 11 figures, 8 table

Swift monitoring of IGR J16418-4532

We report on the Swift observations of the candidate supergiant fast X-ray transient (SFXT) IGR J16418-4532, which has an orbital period of ~3.7 d. Our monitoring, for a total of ~43 ks, spans over three orbits and represents the most intense and complete sampling along the orbital period of the light curve of this source. If one assumes a circular orbit, the X-ray emission from this source can be explained by accretion from a spherically symmetric clumpy wind from a blue supergiant, composed of clumps with different masses, ranging from ~5x10^16 g to 10^21g.Comment: 4 pages; Proceedings, 5th International Symposium on High-Energy Gamma-Ray Astronomy, (Gamma2012) Heidelberg, Germany, July 9-13th, 201

Exploring the role of X-ray reprocessing and irradiation in the anomalous bright optical outbursts of A0538-66

In 1981, the Be/X-ray binary (Be/XRB) A0538-66 showed outbursts characterized by high peak luminosities in the X-ray and optical bands. The optical outbursts were qualitatively explained as X-ray reprocessing in a gas cloud surrounding the binary system. Since then, further important information about A0538-66 have been obtained, and sophisticated photoionization codes have been developed to calculate the radiation emerging from a gas nebula illuminated by a central X-ray source. In the light of the new information and tools available, we studied again the enhanced optical emission displayed by A0538-66 to understand the mechanisms responsible for these unique events among the class of Be/XRBs. We performed about 10^5 simulations of a gas envelope photoionized by an X-ray source. We assumed for the shape of the gas cloud either a sphere or a circumstellar disc observed edge-on. We studied the effects of varying the main properties of the envelope and the influence of different input X-ray spectra on the optical/UV emission emerging from the photoionized cloud. We compared the computed spectra with the IUE spectrum and photometric UBV measurements obtained during the outburst of 29 April 1981. We also explored the role played by the X-ray heating of the surface of the donor star irradiated by the X-ray emission of the neutron star (NS). We found that reprocessing in a spherical cloud with a shallow radial density distribution can reproduce the optical/UV emission. To our knowledge, this configuration has never been observed either in A0538-66 during other epochs or in other Be/XRBs. We found, contrary to the case of most other Be/XRBs, that the optical/UV radiation produced by the X-ray heating of the surface of the donor star irradiated by the NS is non-negligible, due to the particular orbital parameters of this system that bring the NS very close to its companion.Comment: Accepted for publication in Astronomy & Astrophysics. Abstract abridged to meet arXiv requirement