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Bl-Lacertae Objects from the EXOSAT High Galactic Latitude Survey - Constraints on the Logn-Logs and on the Cosmological Evolution

Tagliaferri, G.; Beuermann, K.; Branduardi-Raymont, G. +8 more

Using a sample of 11 X-ray selected BL Lacertae objects extracted from the EXOSAT High Galactic Latitude Survey we derive the LogN-LogS relation of BL Lacs and we compare it with the results obtained with other X-ray surveys. All EXOSAT BL Lacs are characterized by featureless optical spectra and have been detected in the radio band. Excellent agr…

Bl-Lacertae Objects as X-Ray Sources - the EXOSAT Observations

Maccagni, D.; Giommi, P.; Garilli, B. +2 more

Two New Bl-Lacertae Objects Discovered in the Error Boxes of Hard X-Ray Sources

Tagliaferri, G.; Beuermann, K.; Branduardi-Raymont, G. +6 more

Two new BL Lacertae objects, located within the error boxes of hard X-ray sources, have been discovered during a systematic program to identify a large sample of EXOSAT serendipitous sources. Both hard X-ray sources were detected during the HEAD 1 whole sky survey and have been associated to a Seyfert galaxy and to a Be star. The EXOSAT data are c…

The EXOSAT Spectral Survey of Bl-Lacertae Objects

Maccagni, D.; Tagliaferri, G.; Giommi, P. +3 more

The results of an analysis of the X-ray spectra of seventeen BL Lacertae objects measured in 66 observations are presented. The 0.1 - 10 keV spectra of these objects exhibit convex curvature in the log F - log plane. This curvature is intrinsic to the continuum . No evidence for high energy tails is found, at least below energies of 8 - 10 keV, no…

The X-Ray Variability of PKS:2155-304 - a Power Spectrum Analysis

Maraschi, L.; Tagliaferri, G.; Stella, L. +2 more

The bright BL Lac object PKS 2155-304 was observed with EXOSAT 9 times between 1983 and 1985 for a total of 80 hours. During some observations the source appeared stable, while during others, flare-like activity was observed, the intensity doubling in less than an hour. Here we present a power spectrum analysis of the 3 brightest observations. Sig…

Discovery of an X-Ray Selected Optically Highly Variable Quasar

Tagliaferri, G.; Branduardi-Raymont, G.; Giommi, P. +5 more

In the course of a programme of optical identification of serendipitous EXOSAT sources we have discovered a quasar (EX0033041-2613.1, referred to here as Gepu; z = 0.67) which varied by more than 3 magnitudes between November 1985 and February 1988. We present the results of optical spectroscopy and CCD photometry of the quasar which were obtained…