Advances in Calibration and Imaging Techniques in Radio Interferometry

This paper summarizes some of the major calibration and image reconstruction techniques used in radio interferometry and describes them in a common mathematical framework. The use of this framework has a number of benefits, ranging from clarification of the fundamentals, use of standard numerical optimization techniques, and generalization or specialization to new algorithms

The long gamma-ray burst rate and the correlation with host galaxy properties

To answer questions on the start and duration of the epoch of reionisation, periods of galaxy mergers and properties of other cosmological encounters, the cosmic star formation history (CSFH), is of fundamental importance. Using the association of long gamma-ray bursts (LGRBs) with the death of massive stars and their ultra-luminous nature, the CSFH can be probed to higher redshifts than current conventional methods. Unfortunately, no consensus has been reached on the manner in which the LGRB rate (LGRBR) traces the CSFH, leaving many of the questions mentioned mostly unexplored by this method. Observations by the GRB NIR detector (GROND) over the past 4 years have, for the first time, acquired highly complete LGRB samples. Driven by these completeness levels and new evidence of LGRBs also occurring in more massive and metal rich galaxies than previously thought, the possible biases of the LGRBR-CSFH connection are investigated over a large range of galaxy properties. The CSFH is modelled using empirical fits to the galaxy mass function and galaxy star formation rates. Biasing the CSFH by metallicity cuts, mass range boundaries, and other unknown redshift dependencies, a LGRBR is generated and compared to the highly complete GROND sample. It is found that there is no strong preference for a metallicity cut or fixed galaxy mass boundaries and that there are no unknown redshift effects, in contrast to previous work which suggest values of Z/Z_sun~0.1-0.3. From the best-fit models, we predict that ~1.2% of the LGRB burst sample exists above z=6. The linear relationship between the LGRBR and the CSFH suggested by our results implies that redshift biases present in previous LGRB samples significantly affect the inferred dependencies of LGRBs on their host galaxy properties. Such biases can lead to, e.g., an interpretation of metallicity limitations and evolving LGRB luminosity functions.Comment: 15 pages, 14 figures, 7 tables, accepted for publication in Astronomy & Astrophysic

Comptonization and reflection of X-ray radiation and the X-ray-radio correlation in the chi-states of GRS 1915+105

We present a comprehensive X-ray study of four years of pointed RXTE observations of GRS 1915+105 in the chi-state. We interpret the behavior of the hard power law tail spectrum as coming from inverse Compton scattering of soft disk photons on a thermally dominated hybrid corona above the accretion disk. GRS 1915+105 shows a strong, variable reflection amplitude. As in other BHC and in Seyfert galaxies, a correlation between the power law slope and the reflection was found. Also, the radio fluxes at 2.25 GHz and 15 GHz correlate with the power law slope, thus revealing a connection between the outflowing matter and the comptonizing region in the chi-states

Analysis of a turbine rotor containing a transverse crack at Oak Creek Unit 17

Transient increases in one, two and three per revolution vibration characteristics of a low pressure steam turbine were observed during steam temperature reduction operations. Vibration and fracture mechanics analyses suggested the presence of a transverse shaft crack which was eventually identified by ultrasonic inspection and confirmed by destructive sectioning. Signature analyses of vibration data recorded over a two-year period prior to crack identification are correlated with fatigue crack growth, which occurred intermittently during transient temperature decreases. The apparent increased response of the rotor to vibration is due to asymmetric stiffness changes introduced by the growing transverse crack. The vibration response is predicted to increase with increasing crack depths in excess of 10% of the shaft diameter. Fracture mechanics analyses predict that fatigue crack growth occurred during periods of steam temperature decrease, when high surface tensile stresses are present. These same transient thermal stresses are shown to have retarded and prevented subsequent fatigue crack growth during steady operation

The X-ray spectrum of LSI+61o303

It had been proposed earlier that the hard X-ray and gamma-ray radiation of the Be/X-ray system LSI+61o303 could be due to inverse Compton scattering of optical photons from the Be star by the same electron population which also produces the radio emission. Recently, Apparao (2001) has calculated this inverse Compton emission in more detail, and predicted that the X-ray spectrum should show a break at around 20 keV. We investigated archival RXTE data, but do not find such a break in the 2--25 keV range. The implications of this finding are shortly discussed.Comment: 4 pages, 3 figures, to appear in A&

Three-level spin system under decoherence-minimizing driving fields: Application to nitrogen-vacancy spin dynamics

Within the framework of a general three-level problem, the dynamics of the nitrogen-vacancy (NV) spin is studied for the case of a special type of external driving consisting of a set of continuous fields with decreasing intensities. Such a set has been proposed for minimizing coherence losses. Each new driving field with smaller intensity is designed to protect against the fluctuations induced by the driving field at the preceding step with larger intensity. We show that indeed this particular type of external driving minimizes the loss of coherence, using purity and entropy as quantifiers for this purpose. As an illustration, we study the coherence loss of an NV spin due to a surrounding spin bath of $^{13}$C nuclei.Comment: 10 pages, 8 figures, to be published in Phys. Rev.

A survey for GRB orphan afterglows

Gamma-ray bursts are believed to be produced in highly-relativistic collimated outflows. Support for this comes among others from the association of the times of detected breaks in the decay of afterglow light curves with the collimation angle of the jets. An alternative approach to estimate a limit on the collimation angle uses GRB afterglows without detected prompt-emission counterparts. Here we report on the analysis of a dedicated survey for the search of these orphan afterglows using the Wide Field Imager at the 2.2m telescope at La Silla, Chile

Confined One Dimensional Harmonic Oscillator as a Two-Mode System

The one-dimensional harmonic oscillator in a box problem is possibly the simplest example of a two-mode system. This system has two exactly solvable limits, the harmonic oscillator and a particle in a (one-dimensional) box. Each of the two limits has a characteristic spectral structure describing the two different excitation modes of the system. Near each of these limits, one can use perturbation theory to achieve an accurate description of the eigenstates. Away from the exact limits, however, one has to carry out a matrix diagonalization because the basis-state mixing that occurs is typically too large to be reproduced in any other way. An alternative to casting the problem in terms of one or the other basis set consists of using an "oblique" basis that uses both sets. Through a study of this alternative in this one-dimensional problem, we are able to illustrate practical solutions and infer the applicability of the concept for more complex systems, such as in the study of complex nuclei where oblique-basis calculations have been successful. Keywords: one-dimensional harmonic oscillator, particle in a box, exactly solvable models, two-mode system, oblique basis states, perturbation theory, coherent states, adiabatic mixing.Comment: 11 pages and 9 figures; Submitted to American Journal of Physic