On the accuracy of the S/N estimates obtained with the exposure time calculator of the Wide Field Planetary Camera 2 on board the Hubble Space Telescope

We have studied the accuracy and reliability of the exposure time calculator (ETC) of the Wide Field Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope (HST) with the objective of determining how well it represents actual observations and, therefore, how much confidence can be invested in it and in similar software tools. We have found, for example, that the ETC gives, in certain circumstances, very optimistic values for the signal-to-noise ratio (SNR) of point sources. These values overestimate by up to a factor of 2 the HST performance when simulations are needed to plan deep imaging observations, thus bearing serious implications on observing time allocation. For this particular case, we calculate the corrective factors to compute the appropriate SNR and detection limits and we show how these corrections vary with field crowding and sky background. We also compare the ETC of the WFPC2 with a more general ETC tool, which takes into account the real effects of pixel size and charge diffusion. Our analysis indicates that similar problems may afflict other ETCs in general showing the limits to which they are bound and the caution with which their results must be taken.Comment: 14 pages, 13 figures, to be published in PASP on July 200

Optimization of commercial net spacers in spiral wound membrane modules

CFD simulations have been used to determine mass transfer coefficients and power consumption of commercial net spacers. The simulations show transversal and longitudinal vortices, vortex shedding and instationary flow behavior leading to the enhanced mass transfer in spacer filled-channels compared to empty channels. The results of the simulations were validated with experiments and compared with data reported in literature, showing satisfactory agreement. Furthermore, CFD simulations were used to optimize the geometry of commercial net spacers in terms of mass transfer and power consumption. The performance of these optimized spacer geometries will be used as reference for future work on the development of new high-performance spacer shapes

Gamma-ray upper limits on magnetars with 6 years of Fermi-LAT observations

We report on the search for gamma-ray emission from 20 magnetars using 6 years of Fermi, Large Area Telescope (LAT) observations. No significant evidence for gamma-ray emission from any of the currently-known magnetars is found. We derived the most stringent upper limits to date on the 0.1--10 GeV emission of Galactic magnetars, which are estimated between $\sim10^{-12}-10^{-11}$ erg s$^{-1}$ cm$^{-2}$. Gamma-ray pulsations were searched for the four magnetars having reliable ephemerides over the observing period, but none were detected. On the other hand, we also studied the gamma-ray morphology and spectra of seven Supernova Remnants associated or adjacent to the magnetars.Comment: 22 pages, 4 figures, 2 tables, submitted to Ap

Critical point of Nf=3N_f = 3 QCD from lattice simulations in the canonical ensemble

A canonical ensemble algorithm is employed to study the phase diagram of $N_f = 3$ QCD using lattice simulations. We lock in the desired quark number sector using an exact Fourier transform of the fermion determinant. We scan the phase space below $T_c$ and look for an S-shape structure in the chemical potential, which signals the coexistence phase of a first order phase transition in finite volume. Applying Maxwell construction, we determine the boundaries of the coexistence phase at three temperatures and extrapolate them to locate the critical point. Using an improved gauge action and improved Wilson fermions on lattices with a spatial extent of 1.8 \fm and quark masses close to that of the strange, we find the critical point at $T_E = 0.925(5) T_c$ and baryon chemical potential $\mu_B^E = 2.60(8) T_c$.Comment: 5 pages, 7 figures, references added, published versio

Gamma-ray emission from PSR J0007+7303 using 7 years of Fermi Large Area Telescope observations

Based on more than seven years of Fermi Large Area Telescope (LAT) Pass 8 data, we report on a detailed analysis of the bright gamma-ray pulsar (PSR) J0007+7303. We confirm that PSR J0007+7303 is significantly detected as a point source also during the off-peak phases with a TS value of 262 ($\sim$ 16 $\sigma$). In the description of PSR J0007+7303 off-peak spectrum, a power law with an exponential cutoff at 2.7$\pm$1.2$\pm$1.3 GeV (the first/second uncertainties correspond to statistical/systematic errors) is preferred over a single power law at a level of 3.5 $\sigma$. The possible existence of a cutoff hints at a magnetospheric origin of the emission. In addition, no extended gamma-ray emission is detected compatible with either the supernova remnant (CTA 1) or the very high energy (> 100 GeV) pulsar wind nebula. A flux upper limit of 6.5$\times$10$^{-12}$ erg cm$^{-2}$ s$^{-1}$ in the 10-300 GeV energy range is reported, for an extended source assuming the morphology of the VERITAS detection. During on-peak phases, a sub-exponential cutoff is significantly preferred ($\sim$11 $\sigma$) for representing the spectral energy distribution, both in the phase-averaged and in the phase-resolved spectra. Three glitches are detected during the observation period and we found no flux variability at the time of the glitches or in the long-term behavior. We also report the discovery of a previously unknown gamma-ray source in the vicinity of PSR J0007+7303, Fermi J0020+7328, which we associate with the z = 1.781 quasar S5 0016+73. A concurrent analysis of this source is needed to correctly characterize the behavior of CTA 1 and it is also presented in the paper.Comment: 26 pages, 10 figures, 4 tables; Accepted for publication in Ap

LLAGN and jet-scaling probed with the EVN

Accreting black holes on all mass scales (from stellar to supermassive) appear to follow a nonlinear relation between X-ray luminosity, radio luminosity and BH mass, indicating that similar physical processes drive the central engines in X-ray binaries and active galactic nuclei (AGN). However, in recent years an increasing number of BH systems have been identified that do not fit into this scheme. These outliers may be the key to understand how BH systems are powered by accretion. Here we present results from EVN observations of a sample of low-luminosity AGN (LLAGN) with known mass that have unusually high radio powers when compared with their X-ray luminosity.Comment: Presented at the 11th EVN Symposium, Bordeaux, France, 2012 October 9-12. Six pages, including a figure and a table. Final, accepted versio

Dynamic critical exponents of the Ising model with multispin interactions

We revisit the short-time dynamics of 2D Ising model with three spin interactions in one direction and estimate the critical exponents $z,$ $\theta,$ $\beta$ and $\nu$. Taking properly into account the symmetry of the Hamiltonian we obtain results completely different from those obtained by Wang et al.. For the dynamic exponent $z$ our result coincides with that of the 4-state Potts model in two dimensions. In addition, results for the static exponents $\nu$ and $\beta$ agree with previous estimates obtained from finite size scaling combined with conformal invariance. Finally, for the new dynamic exponent $\theta$ we find a negative and close to zero value, a result also expected for the 4-state Potts model according to Okano et al.Comment: 12 pages, 9 figures, corrected Abstract mistypes, corrected equation on page 4 (Parameter Q

Possible complex annihilation and B -> K pi direct CP asymmetry

We point out that a sizable strong phase could be generated from the penguin annihilation in the soft-collinear effective theory for B meson decays. Keeping a small scale suppressed by O(Lambda/m_b), Lambda being a hadronic scale and m_b the b quark mass, in the denominators of internal particle propagators without expansion, the resultant strong phase can accommodate the data of the B^0 -> K^-+ pi^+- direct CP asymmetry. Our study reconciles the opposite conclusions on the real or complex penguin annihilation amplitude drawn in the soft-collinear effective theory and in the perturbative QCD approach based on k_T factorization theorem.Comment: 8 pages, 1 figure, added reference

Properties of canonical determinants and a test of fugacity expansion for finite density lattice QCD with Wilson fermions

We analyze canonical determinants, i.e., grand canonical determinants projected to a fixed net quark number. The canonical determinants are the coefficients in a fugacity expansion of the grand canonical determinant and we evaluate them as the Fourier moments of the grand canonical determinant with respect to imaginary chemical potential, using a dimensional reduction technique. The analysis is done for two mass-degenerate flavors of Wilson fermions at several temperatures below and above the confinement/deconfinement crossover. We discuss various properties of the canonical determinants and analyse the convergence of the fugacity series for different temperatures.Comment: Typo removed, paragraph added in the discussion. Version to appear in Phys. Rev.

Effects of Flavor-dependent qqˉq\bar{q} Annihilation on the Mixing Angle of the Isoscalar Octet-Singlet and Schwinger's Nonet Mass Formula

By incorporating the flavor-dependent quark-antiquark annihilation amplitude into the mass-squared matrix describing the mixing of the isoscalar states of a meson nonet, the new version of Schwinger's nonet mass formula which holds with a high accuracy for the $0^{-+}$, $1^{--}$, $2^{++}$, $2^{-+}$ and $3^{--}$ nonets is derived and the mixing angle of isoscalar octet-singlet for these nonets is obtained. In particular, the mixing angle of isoscalar octet-singlet for pseudoscalar nonet is determined to take the value of $-12.92^\circ$, which is in agreement with the value of $-13^\circ\sim-17^\circ$ deduced from a rather exhaustive and up-to-date analysis of data. It is also pointed out that the omission of the flavor-dependent $q\bar{q}$ annihilation effect might be a factor resulting in the invalidity of Schwinger's original nonet mass formula for pseudoscalar nonet.Comment: Latex, 7 page