H. J. van der Woerd

Observations of the late superhump in VW Hydri

This paper presents the results of simultaneous optical and near-IR photometry, optical fast spectroscopy, and Exosat X-ray observations of the dwarf nova VW Hyi, obtained simultaneously during three consecutive orbital cycles, approximately two days after the 1983 November superoutburst terminated. The optical data show clear evidence for a late superhump, which is shifted + 0.7 in phase relative to the orbital modulation. An attempt is made to derive from the observed spectral distribution the contribution of the late superhump. The orbital hump and the late superhump apparently are not related to each other. This important effect excludes all models, in which the late superhump phenomenon is interpreted in terms of variations in the bright-spot brightness. 43 references.

X-ray spectroscopy of the ultrasoft transient 4U 1543 - 47

The X-ray transient 4U 1543 - 47 was observed in August 1983 by the Exosat observatory near the maximum of an outburst. The X-ray spectrum was measured using a gas scintillation proportional counter (GSPC) and a transmission grating spectrometer (TGS). A broad (FWHM about 2.7 keV) line at 5.9 keV is detected in the GSPC, which is interpreted as a redshifted and broadened iron K-alpha line. The line broadening and redshift may arise from either Compton scattering in a cool plasma with small optical depth and/or from Droppler and relativistic effects in the vicinity of compact object. The spectrum below 2 keV, obtained with the TGS, shows evidence for a broad emission feature at 0.74 keV, which may be an iron L-transition complex. However, such an emission feature could be an artifact caused by an anomalously low interstellar absorption by neutral oxygen. The contimuum emission is extremely soft and is well described by an unsaturated Comptonized spectrum from very cool plasma (kT = 0.84 keV) with large scattering depth. 30 refs.

EPISODIC DISTORTION AND DUST FORMATION IN THE WIND OF WR 140

In 1985 April, the WC7+abs star WR140 = HD 193793 was observed to have brightened by over two mag. in the infrared owing to the formation of a new dust shell. The growth and evolution of the shell was monitored by infrared observations using UKIRT during the remainder of 1985. Examination of infrared photometry of this star since 1979 and previously published data indicate that the dust formation occurs at intervals of 7.9 years. Phasing the published radial velocities of the absorption line component with this period confirms that it is a member of an eccentric (e = 0.7-0.8) binary system having periastron passage shortly before dust formation. Further evidence of the distortion of the stellar wind from the WC7 component is provided by changes in the P Cygni profile of the C IV lines observed with IUE while the dust was forming. The X-ray spectrum also changed between 1984 and 1985 in becoming significantly “harder” while the non-thermal radio source was extinguished. An interpretation is sought in terms of a model wherein the non-thermal radio and X-ray emission is excited by collision of the WC and O star winds and modulated by the orbital motion. A suitably compressed region of the wind must be advected far enough from the stars to form dust.

The complex 0. 4-12 keV X-ray spectrum of Cygnus X-1

Simultaneous observations of the 0.4-2 keV and 2-12 low-state spectrum of Cygnus X-1, made with the low-energy transmission grating spectrometer and gas scintillation proportional counter on board EXOSAT, are presented. At least four spectral components are required to describe the data: a hard (energy spectral index alpha about 0.4) power law dominating the emission above 2 keV, a broad (FWHM 170 eV) emission line near 6.3 keV, a steep (alpha = 2.2) continuum below 3 keV, and a broad emission feature between 600 and 800 eV. Conventional viscous accretion disk models (alpha disks) cannot simultaneously explain both the spectral shape and the luminosity of the steep soft continuum component. 25 refs.

Medium resolution imaging spectrometer data for monitoring tropical coastal waters: a case study of Berau estuary, East Kalimantan, Indonesia

This article investigates the performance of MERIS reduced resolution data to monitor water quality parameters in the Berau estuary waters, Indonesia. Total suspended matter (TSM), Chlorophyll-a (Chl-a) concentration and diffuse attenuation coefficient (Kd ) were derived from MERIS data using three different algorithms for coastal waters: standard global processor (MERIS L2), C2R and FUB. The outcomes were compared to in situ measurements collected in 2007. MERIS data processed with C2R gave the best retrieval of Chl-a, while MERIS L2 performed the best for TSM retrieval, but large deviations from in situ data were observed, pointing at inversion problems over these tropical waters for all standard processors. Nevertheless, MERIS can be of use for monitoring equatorial coastal waters like the Berau estuary and reef system. Applying a Kd (490) local algorithm to the MERIS RR data over the study area showed a sufficient good correlation to the in situ measurements (R 2 = 0.77).

A Comparison of H2252-035/AO PSC and 4U1849-31/V1223 SGR

From five-colour photometry of the triple-periodic systems H2252-035 and V1223 Sgr, which have very similar orbital and pulsation periods, we find a significant difference in the wavelength dependence of their short period pulsations. Our results indicate that in H2252-035 this pulse originates in a hot region, probably on the white dwarf (Motch and Pakull, 1981), whereas in V1223 Sgr both pulses are the result of reprocessing of X-rays (in the far side of the accretion disk, and in the companion star, respectively), as suggested by Warner (1985).