High‐energy astrophysics

Abstract

The european X-ray observatory (EXOSAT), which was launched in 1983 and which finished operations in April 1986, has brought a rich harvest of results in the period 1984–1987, surveyed here. The EXOSAT payload consisted of three sets of instruments : two low energy imaging telescopes (LE:E<2 KeV), a medium-energy experiment (ME:E=l-50KeV) and a gas scintillation proportional counter (GSPC:E=2-20KeV). Over most of the energy range covered, EXOSAT was not more sensitive than its predecessor, the american EINSTEIN satellite. But the EINSTEIN satellite is far from having exhausted the treasures of the X-ray sky. And EXOSAT, thanks to its elliptical 90-hour orbit, had the extra advantage of being able to make long, continuous observations of interesting objects, lasting up to 72 hours. Thus, EXOSAT was very well suited for variability studies, and many of its most important findings are in this area. EXOSAT observations sample a wide range of astrophysical sources : X-ray binaries, cataclysmic variables and active stars ; supernova remnants and the interstellar medium ; active galactic nuclei, and clusters of galaxies. Among the highlights, let us mention : 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.