High Energy Astrophysics
the detection of quasi-periodic oscillations (QPO) in seven well known X-ray sources, starting with the galactic bulge source GX 5-1, and including Sco X-l, the brightest X-ray source in the sky, Cyg X2 and the Rapid Burster. The 38 majority of the QPO sources are very luminous (>1038 erg/sec), and the oscillations are strong (5% rms variation in flux), persistent (> 10 cycles), and take place at high frequency (5-50 Hz). Other characteristics of the QPO, however, such as their relation to the source intensity or the spectral shape, vary widely from source to source. QPO sources are believed to be binary systems containing a neutron star, and the models proposed generally involve interactions between the accretion disk and the neutron star magnetosphere.
the mapping out of the X-ray galactic ridge in the 2–6 KeV band. The “ridge” is a disk of radius 10 to 12 Kpsec and a height of a few hundred parsecs ; its total luminosity is 1038 erg/sec.
the detailed observations of active galactic nuclei (AGN), revealing, in some cases, the presence of a soft X-ray component, in the .05-1 KeV range. Long terra monitoring of AGN allowed to study their variability on timescales from minutes to years. For NGC 4151, the flux in the 2-10 KeV band can vary by a factor 2 over periods of six months, while the soft component remains constant. A possible interpretation is that the soft component originates in a hot intercloud medium in the narrow-line region, while the hard X-ray flux is emitted by regions surrounding the nucleus. EXOSAT observations indicate that rapid variability, on time scales of the order of an hour, is frequent in AGNs.
Astrophysique Des Hautes Energies
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