Showing papers by "Luigi Stella published in 1986"

Intermittent stellar wind accretion and the long-term activity of Population I binary systems containing an X-ray pulsar

Abstract: On compare les proprietes observees des binaires de population I contenant des pulsars RX avec differents regimes d'accretion par vent a l'interieur d'une etoile a neutrons magnetisee

Bimodal quasi-oscillatory and spectral behavior in Scorpius X-1

Abstract: X-ray observations were made to follow up the discovery of quasi-periodic oscillations (QPO) in Sco X-1. The QPO is observed in the quiescent state, and between flares in the active state. When the source is quiescent, the QPO frequency is 6 Hz and anticorrelated with intensity; in the active state, the frequency ranges from 10 to 20 Hz and is strongly correlated with intensity. At high intensities the QPO dissolves into continuum noise. The QPO rms amplitude decreases with count rate, and increases with photon energy. There are two modes of spectral behavior, with a one-to-one correspondence to the two modes of QPO: the spectral hardness ratio varies more steeply with intensity when the source shows 6 Hz QPO, than for 10 to 20 Hz QPO. Transitions between QPO modes, which occur when the flux is at its lowest levels, correspond to transitions between hardness ratio modes. The QPO frequency changes continuously through the transitions, which last a few hundred seconds.

The Evolution of the 1984 Outburst of the Transient X-Ray Source 4U 1630-47

Abstract: The results of a series of EXOSAT observations of the X-ray transient 4U 1630 - 47 during an outburst in 1984 are presented. The source decayed over an interval of about 100 days from a maximum 1-50 keV luminosity of about 2.8 x 10 to the 38th to about 4 x 10 to the 36th erg/s (for an assumed distance of 10 kpc). The 1-50 keV spectrum was best represented by a soft Wien-like spectrum with a temperature about 1 keV and a high-energy power-law tail. The relative contribution of the soft component to that of the high-energy power law decreased by atleast a factor of about 2 as the overall luminosity decreased. When the source was at its maximum observed luminosity, short time scale 7 percent intensity variations (1-7 keV) were seen with a characteristic time scale of about 20 s. The shortest time scale variability detected was about 50 ms. A 10-arcsec position obtained using an imaging telescope excludes a previously suggested optical counterpart. The spectral properties of 4U 1630 - 47 are reminiscent of those of the black hole candidate Cyg X-1 when it evolves from a high to a low state. 28 references.

A search for X-ray periodicities from 4U 1700–37

Abstract: On rapporte les resultats de la recherche des pulsations RX de 4U 1700-37, faite sur plus d'une periode orbitale complete avec l'observatoire EXOSAT

Uncorrelated soft and hard X-ray short-term variations from AM Herculis

Abstract: An investigation of the short-term X-ray variability of AM Her in its high state, based on data obtained with the high resolution imager and the monitor proportional counter on board the Einstein Observatory, is reported Analysis performed using correlation techniques reveals that the time variations in the soft and the hard X-ray spectral components of the source were uncorrelated at short time delays, although the light curves of the two components showed a very similar behavior, mostly consisting of about 400 s quasi-periodic fluctuations These findings are difficult to reconcile with the simplest current models of one pole accretion onto magnetized white dwarfs, since these predict the soft X-ray component will mimic the variations of the hard X-ray component 23 references

On the long term activity of pop. I binary systems containing an X-ray pulsar

Abstract: We review the observed properties of the population I binaries containing X-ray pulsars. We point out that many of the pulsing X-ray transients in OB star systems with pulse periods of 1–10 s, when in outburst, lie close to the critical equilibrium point where the corotation radius equals the magnetospheric radius. This suggests that many of these systems usually are dormant because of the centrifugal drag exerted by the pulsar magnetosphere which inhibits accretion and that a small increase in the mass accretion rate can turn them into very luminous transient X-ray sources. If a system lies close to the equilibrium point and it is in an eccentric orbit, luminous outbursts may occur around the time of periastron passage. A correlation between the X-ray luminosity and the pulsar frequency is found and interpreted as the result of centrifugal inhibition of accretion preventing low luminosity, fast spin period magnetised neutron stars accreting.