W. Penninx

Searches for millisecond pulsations on low-mass x-ray binaries

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Radio and X-ray states in the X-ray binary Scorpius X-1

Radio observations of Sco X-1 were made on March 10-11, 1989 with the VLA, the WSRT, and the Australian Telescope as part of a multiwavelength campaign with the Ginga X-ray satellite, the IUE satellite, and other ground-based instruments. Two source states were detected in the radio and X-ray data, with Sco X-1 being radio-quiet when it was in the X-ray flaring branch and radio-loud when it was in the X-ray normal branch, the same type of radio-X-ray correlation seen in the Z-sources GX 17 + 2 and Cyg X-2. Both radio-quiet and radio-loud states showed radio spectrum changes indicating a multicomponent radio source, where the high-frequency component is variable on time scales of 2-3 hr. 14 refs.

Correlation of X-ray burst properties with source state in the 'atoll' source 4U/MXB 1636-53

A series of Exosat observations of the atoll source 4U/MXB 1636 - 53 shows that duration and temperature of the X-ray bursts strongly correlate with the X-ray spectral and fast variability characteristics of the persistent emission of the source. This implies that spectral shape, fast variability, and burst duration and temperature all correlate well with accretion rate M. This provides a strong argument that in the atoll sources, source-state is determined by M, just as in the Z sources. These observations also show that the persistent X-ray intensity can vary independently from the other mentioned characteristics. Therefore, intensity is probably not a good measure for the accretion rate. 16 refs.

EXOSAT observations of 4U/MXB 1636-53 - On the relation between the amount of accreted fuel and the strength of an X-ray burst

During Exosat observations of 4U/MXB 1636-53 in August 1985, three bursts were observed in 6 h and 24 bursts in 79 h of uninterrupted observation. The burst times, burst fluences, peak burst fluxes, burst rise times, decay times, burst intervals, mean persistent fluxes, and the product of the latter two are listed, analyzed, and discussed. The implications of the results for thermonuclear flash models are examined. A model which fits the data well involves bursts which suffer from nuclear energy losses due to stable hydrogen burning between bursts. Thus, the percentage of lost energy increases with increasing burst intervals. For individual bursts, however, there are significant deviations from the approximate linear relationship. 43 references.

Multipeaked X-ray bursts from 4U/MXB 1636-53 - Evidence against burst-induced accretion disk coronae

The burst-induced accretion-disk corona (BIADC) model proposed by Melia (1987) to explain the multiple-peak burst profiles of 4U/MXB 1636-53 in terms of direct and scattered components is examined critically. Published observational data (Sztajno et al., 1986, and Lewin et al., 1987) are presented in tables and graphs and analyzed. A number of possible BIADC scenarios are discussed, and it is argued that the observed characteristics of the 1636-53 bursts are not well accounted for by the BIADC model. 11 references.