Showing papers by "Luigi Stella published in 1992"

Variability of the optical–UV continuum in NGC 5548: implications for accretion-disc models

Abstract: The recent campaign of systematic UV and optical observations of NGC 5548 has shown the continuum to vary virtually simultaneously at ultraviolet and optical frequencies, with an upper limit to the delay of less than 4 d. We examine this result in the context of accretion-disc models.

A new kind of oscillation in the persistent emission of the Rapid Burster.

Abstract: We present a study of a new kind of oscillation found in the persistent emission after some type II bursts from the Rapid Burster observed during 1985 August with EXOSAT. All 95 bursts observed were longer than 30 s. Ten of them were followed by these oscillations. The frequency of the oscillations ranged from 39 to 56 mHz. A period decrease (by 30-50 percent) over a time-scale of about 100 s was observed in seven cases. The fractional rms variation of the oscillations ranged from 5 to 15 percent. In eight cases, about 4-Hz quasi-periodic oscillations with fractional rms values from 6 to 19 percent were observed simultaneously with the new oscillations. There is a high probability that the two forms of oscillation are related to one another.

Iron K-alpha line from X-ray illuminated relativistic disks.

Abstract: The intensity and profile of the iron Kα fluorescence line from a flat, optically thick accretion disk rotating around a Schwarzschild black hole and illuminated by a central X-ray source are computed using a fully relativistic treatment of the photon intensity and shifts. The X-ray source is modelled as an isotropic point source located on the symmetry axis at a height h in units of the gravitational radius. These calculations represent a refinement and an extension of those presented in Paper I for h=20, carried out using a weak field approximation and therefore of validity limited to inclination angles <70°. Here it is shown that at high inclination angles purely relativistic effects lead to the growth of features between the two Doppler horns and that, as a consequence, the line equivalent width maintains a sizeable value, while the centroid energy and the line width go through a broad maximum at about ∼80°

Xronos: a Timing Analysis Software Package

Abstract: XRONOS is a general purpose timing analysis package Although it was designed primarily for X-ray astronomy, XRONOS is basically detector and wavelength-independent It has been used to analyse data from the Einstein Observatory, EXOSAT and Ginga, as well as optical photometry and helioseismology data In the currently available version (No 20), XRONOS includes programs for: light curve(s), hardness ratio and colour-colour plotting, epoch folding, power spectrum, autocorrelation, crosscorrelation, time skewness and statistical analysis

XRONOS: A timing analysis software package. User's guide, version 3.00

Abstract: XRONOS is a general purpose timing analysis package. Although it was designed mainly for x-ray astronomy, XRONOS is basically detector and wavelength independent. It has been used to analyze data from the Einstein Observatory, Exosat and Ginga, as well as optical photometry and helioseismology data. It includes programs for: light curve(s), hardness ratio and color-color plotting, epoch folding, power spectrum, autocorrelation, cross correlation, time skewness, and statistical analysis. An overview of XRONOS and walkthrough of a XRONOS session are presented. The QDP/PLT plotting and function fitting program used by XRONOS is outlined and the algorithm and features of XRONOS are detailed.