A study of fault prediction and reliability assessment in the SEL environment

An empirical study on estimation and prediction of faults, prediction of fault detection and correction effort, and reliability assessment in the Software Engineering Laboratory environment (SEL) is presented. Fault estimation using empirical relationships and fault prediction using curve fitting method are investigated. Relationships between debugging efforts (fault detection and correction effort) in different test phases are provided, in order to make an early estimate of future debugging effort. This study concludes with the fault analysis, application of a reliability model, and analysis of a normalized metric for reliability assessment and reliability monitoring during development of software

Sub-milliarcsec-scale structure of the gravitational lens B1600+434

In the gravitational lens system B1600+434 the brighter image, A, is known to show rapid variability which is not detected in the weaker image, B (Koopmans & de Bruyn 2000). Since correlated variability is one of the fundamental properties of gravitational lensing, it has been proposed that image A is microlensed by stars in the halo of the lensing galaxy (Koopmans & de Bruyn 2000). We present VLBA observations of B1600+434 at 15 GHz with a resolution of 0.5 milliarcsec to determine the source structure at high spatial resolution. The surface brightness of the images are significantly different, with image A being more compact. This is in apparent contradiction with the required property of gravitational lensing that surface brightness be preserved. Our results suggest that both the lensed images may show two-sided elongation at this resolution, a morphology which does not necessarily favour superluminal motion. Instead these data may suggest that image B is scatter-broadened at the lens so that its size is larger than that of A, and hence scintillates less than image A.Comment: 4 pages, 2 figures, accepted in AA Letter

Constraints on the Inner Mass Profiles of Lensing Galaxies from Missing Odd Images

Most gravitational lens systems consist of two or four observable images. The absence of detectable odd images allows us to place a lower limit on the power-law slope of the inner mass profile of lensing galaxies. Using a sample of six two-image radio-loud lens systems and assuming a singular power-law surface density (Sigma proportional to r^{-beta}) for the inner several kpc of the mass distribution, we find that there is less than a 10% probability that the data are consistent with profile slopes beta < 0.80. Furthermore, individual mass modeling yields beta > 0.85 for B0739+366 and beta > 0.91 for B1030+074. Modeling central black holes as additional point masses changes the constraints in these systems to beta > 0.84 and beta > 0.83, respectively. The inner mass profiles of lensing galaxies are therefore not much shallower than isothermal.Comment: Final published version, minor typos corrected, 13 page

Cell-free protein expression systems in microdroplets: stabilization of interdroplet bilayers

Cell-free protein expression with bacterial lysates has been demonstrated to produce soluble proteins in microdroplets. However, droplet assays with expressed membrane proteins require the presence of a lipid bilayer. A bilayer can be formed in between lipid-coated aqueous droplets by bringing these into contact by electrokinetic manipulation in a continuous oil phase, but it is not known whether such interdroplet bilayers are compatible with high concentrations of biomolecules. In this study we have characterized the lifetime and the structural integrity of interdroplet bilayers by measuring the bilayer current in the presence of three different commercial cell-free expression mixtures and their individual components. Samples of pure proteins and of a polymer were included for comparison. It is shown that complete expression mixtures reduce the bilayer lifetime to several minutes or less, and that this is mainly due to the lysate fraction itself. The fraction that contains the molecules for metabolic energy generation does not reduce the bilayer lifetime but does give rise to current steps that are indicative of lipid packing defects. Gel electrophoresis confirmed that proteins are only present at significant amounts in the lysate fractions and, when supplied separately, in the T7 enzyme mixture. Interestingly, it was also found that pure-protein and pure-polymer solutions perturb the interdroplet bilayer at higher concentrations; 10% (w/v) PEG 8000 and 3 mM lysozyme induce large bilayer currents without a reduction in bilayer lifetime, whereas 3 mM albumin causes rapid bilayer failure. It can therefore be concluded that the high protein content of the lysates and the presence of PEG polymer, a typical lysate supplement, compromise the structural integrity of interdroplet bilayers. However, we established that the addition of lipid vesicles to the cell-free expression mixture stabilizes the interdroplet bilayer, allowing the exposure of interdroplet bilayers to cell-free expression solutions. Given that cell-free expressed membrane proteins can insert in lipid bilayers, we envisage that microdroplet technology may be extended to the study of in situ expressed membrane receptors and ion channel

Strong lensing constraints on the velocity dispersion and density profile of elliptical galaxies

We use the statistics of strong gravitational lensing from the CLASS survey to impose constraints on the velocity dispersion and density profile of elliptical galaxies. This approach differs from much recent work, where the luminosity function, velocity dispersion and density profile were typically {\it assumed} in order to constrain cosmological parameters. It is indeed remarkable that observational cosmology has reached the point where we can consider using cosmology to constrain astrophysics, rather than vice versa. We use two different observables to obtain our constraints (total optical depth and angular distributions of lensing events). In spite of the relatively poor statistics and the uncertain identification of lenses in the survey, we obtain interesting constraints on the velocity dispersion and density profiles of elliptical galaxies. For example, assuming the SIS density profile and marginalizing over other relevant parameters, we find 168 km/s < sigma_* < 200 km/s (68% CL), and 158 km/s < sigma_* < 220 km/s (95% CL). Furthermore, if we instead assume a generalized NFW density profile and marginalize over other parameters, the slope of the profile is constrained to be 1.50 < beta < 2.00 (95% CL). We also constrain the concentration parameter as a function of the density profile slope in these models. These results are essentially independent of the exact knowledge of cosmology. We briefly discuss the possible impact on these constraints of allowing the galaxy luminosity function to evolve with redshift, and also possible useful future directions for exploration.Comment: Uses the final JVAS/CLASS sample, more careful choice of ellipticals, added discussion of possible biases. Final results essentially unchanged. Matches the MNRAS versio

Interferometric Phase Calibration Sources in the Declination Range 0deg to -30deg

We present a catalog of 321 compact radio sources in the declination range 0deg > delta > -30deg. The positions of these sources have been measured with a two-dimensional rms accuracy of 35 milliarcseconds using the NRAO Very Large Array. Each source has a peak flux density >50 mJy at 8.4 GHz. We intend for this catalog to be used mainly for selection of phase calibration sources for radio interferometers, although compact radio sources have other scientific uses.Comment: 9 pages. To appear in ApJS. Catalog (Table 3) is abbreviated in printed version. Complete catalog available at ftp://ftp.aoc.nrao.edu/pub/staff/jwrobel/WPW2003_ApJS.tx

A determination of H_0 with the CLASS gravitational lens B1608+656: II. Mass models and the Hubble constant from lensing

EDITED FROM PAPER: We present mass models of the four-image gravitational lens system B1608+656. A mass model for the lens galaxies has been determined that reproduces the image positions, two out of three flux-density ratios and the model time delays. Using the time delays determined by Fassnacht et al. (1999a), we find that the best isothermal mass model gives H_0=59^{+7}_{-6} km/s/Mpc for Omega_m=1 and Omega_l=0.0, or H_0=(65-63)^{+7}_{-6} km/s/Mpc for Omega_m=0.3 and Omega_l = 0.0-0.7 (95.4% statistical confidence). A systematic error of +/-15 km/s/Mpc is estimated. This cosmological determination of H_0 agrees well with determinations from three other gravitational lens systems (i.e. B0218+357, Q0957+561 and PKS1830-211), SNe Ia, the S-Z effect and local determinations. The current agreement on H_0 from four out of five gravitational lens systems (i) emphasizes the reliability of its determination from isolated gravitational lens systems and (ii) suggests that a close-to-isothermal mass profile can describe disk galaxies, ellipticals and central cluster ellipticals. The average of H_0 from B0218+357, Q0957+561, B1608+656 and PKS1830-211, gives H_0(GL)=69 +/-7 km/s/Mpc for a flat universe with Omega_m=1 or H_0(GL)=74 +/-8 km/s/Mpc for Omega_m=0.3 and Omega_l=0.0-0.7. When including PG1115+080, these values decrease to 64 +/-11 km/s/Mpc and 68 +/-13 km/s/Mpc (2-sigma errors), respectively.Comment: Accepted for publication in ApJ. 34 pages, 4 figure