Showing papers by "L. Maraschi published in 1986"

Spectral properties of blazars. I. Objects observed in the far-ultraviolet. II. An X-ray observed sample

Abstract: All blazars observed with the IUE are studied and shown to form a well-defined subgroup according to their spectral properties. These properties are discussed with respect to theoretical models and are compared with those of quasars. Radio, ultraviolet, and X-ray fluxes are used to construct composite spectral indices, and systematic differences between X-ray selected and otherwise selected objects are discussed. It is confirmed that X-ray selected objects have flatter overall spectra, and are therefore weaker radio emitters relative to their X-ray emission than objects selected otherwise. It is found that X-ray selected blazars have the same average X-ray luminosity as blazars selected otherwise and are underluminous at UV and radio frequencies. This finding is used to argue that the radio-weak, X-ray selected BL Lac objects are, in terms of space density, the dominant members of the blazar population. The results are interpreted in the framework of synchrotron emission models involving relativistic plasma jets. 134 references.

X-ray variability of the BL Lacertae object PKS 2155 - 304 in the 0. 1-6 keV range

Abstract: Observations of the bright BL Lac object PKS 2155 - 304 obtained at 1-6 keV using the ME argon counters and channel-multiplier array at the focus of the Exosat LE telescope, in conjunction with the 0.05-2-keV-bandpass 3000-A Lexan filter, during a total of 30 h in October-November 1983 and November 1984 are reported. The data are presented in tables and graphs and characterized. Findings discussed include an overall variation of a factor of 10, one factor-of-four increase over 4 h, and maximum luminosity variation dL/dt = 2 x 10 to the 42nd erg/s sq for H = 100 km/s Mpc (corresponding to a lower limit of mass of 10 to the 8th solar mass and a gravitational radius of 3 x 10 to the 13th cm). The implications of these results for theoretical models of the X-ray emission source are considered. 17 references.

Multifrequency observations of the Blazar PKS 0537-441 in a moderately active state

Abstract: PKS 0537-441 was observed during an active state in February 1985 at infrared, optical, UV, and X-ray frequencies. Comparison with earlier measurements indicates that the source brightened by a factor of approximately 2 in all bands. This suggests that the same spatial region may be responsible for the emission in the whole spectral range observed. 19 references.

Gamma-ray emission from accretion onto a rotating black hole

Abstract: Gamma-ray production from the decay of pi(0) mesons deriving from proton-proton interactions in a thermal plasma around a rotating black hole is considered. The temperature and density profile of the accreting plasma are calculated for an adiabatic flow in a Kerr metric with detached electron and proton populations and using the geodesic approximation. The computed gamma-ray luminosity turns out to be lower than that obtained by other authors. The relevance of the process for observable gamma-ray sources is discussed. 16 references.

Variability of the BL Lacertae Objects PKS 2155-304 and OJ 287 in the Far-Ultraviolet

Abstract: All the ultraviolet spectra of the two bright BL Lacertae objects PKS 2155 - 304 and OJ 287 taken with the International Ultraviolet Explorer in the period 1978-1984 are examined. For each spectrum the best-fitting power law is determined and a correlation between spectral slope and intensity is searched for. The correlation, if present, is weak. This is discussed in terms of models of the continuum emission of active galactic nuclei. 31 references.