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We present light curves and spectra of a ∼35ksec RXTE observation of Vela X-1 and a ∼300 ksec observation of Cen X-3. Both light curves show strong absorption which increases from no absorption at low orbital phases to large values at high orbital phases; in the case of Vela X-1 by a factor of almost 100. Superimposed are absorption dips which can be observed at any orbital phase, though mostly at high orbital phases. We study an unusually strong dip in Vela X-1 during which the flux is reduced by ∼90% and almost no pulses are detectable. The increase of N[sub H] and the scattered component show the in- and egress of material in the line of sight. A similar behaviour is observed in a pre-eclipse dip in Cen X-3. [ABSTRACT FROM AUTHOR]

We present the spectral analysis of our 35 ksec continuous RXTE observation of the High Mass X-ray Binary Vela X-1. The spectral models used normally to describe pulsar spectra like a power law with an exponential cutoff or the NPEX model (Negative Positive EXponential), do not work for our data. Our successful model consists of two NPEX components: one is heavily absorbed, while the other is due to reflection. But we still find large deviations around ∼21 keV and ∼50 keV. We interpret the feature at ∼55 keV as a cyclotron absorption line while the lower deviation might be due to instrument calibration problems around 25 keV in the PCA. [ABSTRACT FROM AUTHOR]

We present first results from the spectral and temporal analysis of an RXTE monitoring campaign of the black hole candidate Cygnus X-1 in 1999. The timing properties of this hard state black hole show considerable variability, even though the state does not change. This has previously been noted for the power spectral density, but is probably even more pronounced for the time lags. From an analysis of four monitoring observations of Cyg X-1, separated by 2 weeks from each other, we find that a shortening of the time lags is associated with a hardening of the X-ray spectrum, as well as with a longer characteristic “shot time scale.” We briefly discuss possible physical/geometrical reasons for this variability of the hard state properties. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]