Simulation of adaptive optics for the Vacuum Tower Telescope

A simulation model of the adaptive optics of the German Vacuum Tower Telescope (VTT), Observatorio del Teide, Tenerife, is presented. The model uses modules from the integrated model of the Euro50 extremely large telescope, and includes submodels of a Shack-Hartmann wavefront sensor, a de-formable mirror, a tip-tilt mirror, high-voltage amplifier low-pass filters, a reconstructor and a controller. We investigate the impact on the closed loop bandwidth of changes in controller configuration and certain system parameters, such as low pass filter bandwidth and camera integration and readout time. Control strategies were tested on simple models before implementation on the full VTT model. Using the models, different control strategies are compared.     

Longitudinal dispersion compensation for the Very Large Telescope Interferometer

The Very Large Telescope Interferometer [1,13] will be operated in air which will introduce small optical path fluctuations due to internal turbulence [4,6] and dispersion effects. Both may contribute to fringe contrast decrease. Longitudinal dispersion effects can be corrected by inserting an appropriate glass of variable thickness in one arm of the interferometer [3,5,11,12]. This paper presents a new method applied to VLTI in order to select the optimum glass, according to both the observing wavelength and the spectral bandpath, and to calculate its thickness. Results are presented in terms of improvement on the fringe contrast.     

Wolf-Rayet stars

Summary Recent literature on Population I Wolf-Rayet star research extending from the Milky Way to blue compact dwarf galaxies is reviewed, broken down into inventory, basic parameters and galactic distribution, atmospheres, binaries, intrinsic variability, mass loss, enrichment and evolution. Also the incidence of Wolf-Rayet stars with variable non-thermal radio emission, excess X-ray fluxes, and episodic/periodic IR excesses is reviewed. These phenomena appear to be associated with wind-wind interaction in wide long-period WR+OB binaries and with wind-compact object interaction in WR+c binaries, with orbit sizes of the order of magnitude of the WR radio photosphere sizes or larger.     

Modeling X-Ray Photoionized Plasmas Produced at the Sandia Z-Facility

In experiments at the high-power Z-facility at Sandia National Laboratory in Albuquerque, New Mexico, we have been able to produce a low density photoionized laboratory plasma of Fe mixed with NaF. The conditions in the experiment allow a meaningful comparison with X-ray emission from astrophysical sources. The charge state distributions of Fe, Na and F are determined in this plasma using high resolution X-ray spectroscopy. Independent measurements of the density and radiation flux indicate unprecedented values for the ionization parameter ξ = 20–25 erg cm s⁻¹ under nearly steady-state conditions. First comparisons of the measured charge state distributions with X-ray photoionization models show reasonable agreement, although many questions remain.     

On least squares estimation of generalized covariance functions

Least squares estimation (LSE) is theoretically related to quadratic unbiased estimation of variance components. It is argued that these methods of estimation of variance components essentially generalize LSE though they are not formally equivalent.     

A search for the optical and near-infrared counterpart of the accreting millisecond X-ray pulsar XTE J1751−305

We have obtained optical and near-infrared images of the field of the accreting millisecond X-ray pulsar XTE J1751−305. There are no stars in the 0.7-arcsec error circle (0.7 arcsec is the overall uncertainty arising from tying the optical and X-ray images and from the intrinsic uncertainty in the Chandra X-ray astrometric solution). We derive limiting magnitudes for the counterpart of   R > 23.1, I > 21.6, Z > 20.6, J > 19.6  and   K > 19.2  . We compare these upper limits with the magnitudes one would expect for simple models for the possible donor stars and the accretion disc subject to the reddening observed in X-rays for XTE J1751−305 and when put at the distance of the Galactic Centre (8.5 kpc). We conclude that our non-detection does not constrain any of the models for the accretion disc or possible donor stars. Deep, near-infrared images obtained during quiescence will, however, constrain possible models for the donor stars in this ultracompact system.     

MS 1603.6+2600: an accretion disc corona source?

We have observed the eclipsing low-mass X-ray binary MS 1603.6+2600 with Chandra for 7 ks. The X-ray spectrum is well fit with a single absorbed power law with an index of ∼2. We find a clear sinusoidal modulation in the X-ray light curve with a period of  1.7 ± 0.2 h  , consistent with the period of 1.85 h found before. However, no (partial) eclipses were found. We argue that if the X-ray flare observed in earlier X-ray observations was a type I X-ray burst, then the source can only be an accretion disc corona source at a distance of ∼11–24 kpc (implying a height above the Galactic disc of ∼8–17 kpc). It has also been proposed in the literature that MS 1603.76+2600 is a dipper at ∼75 kpc. We argue that, in this dipper scenario, the observed optical properties of MS 1603.6+2600 are difficult to reconcile with the optical properties one would expect on the basis of comparisons with other high-inclination, low-mass X-ray binaries, unless the X-ray flare was not a type I X-ray burst. In that case, the source can be a nearby soft X-ray transient accreting at a quiescent rate, as was proposed by Hakala et al., or a high-inclination source at ∼15–20 kpc.