9 research outputs found
Unifying models for X-ray selected and Radio selected BL Lac Objects
We discuss alternative interpretations of the differences in the Spectral
Energy Distributions (SEDs) of BL Lacs found in complete Radio or X-ray
surveys. A large body of observations in different bands suggests that the SEDs
of BL Lac objects appearing in X-ray surveys differ from those appearing in
radio surveys mainly in having a (synchrotron) spectral cut-off (or break) at
much higher frequency. In order to explain the different properties of radio
and X-ray selected BL Lacs Giommi and Padovani proposed a model based on a
common radio luminosity function. At each radio luminosity, objects with high
frequency spectral cut-offs are assumed to be a minority. Nevertheless they
dominate the X-ray selected population due to the larger X-ray-to-radio-flux
ratio. An alternative model explored here (reminiscent of the orientation
models previously proposed) is that the X-ray luminosity function is "primary"
and that at each X-ray luminosity a minority of objects has larger
radio-to-X-ray flux ratio. The predictions of the two scenarios, computed via a
Montecarlo technique, are compared with the observed properties of BL Lacs in
the two samples extracted respectively from the 1 Jy radio survey and the
Einstein Slew Survey. We show that both models can explain a number but not all
the observed features. We then propose a completely new approach, based on the
idea that the physical parameter which governs the shape of the SEDs, is (or is
associated with) the bolometric luminosity. Assuming an empirical relation
between spectral shape and luminosity we show that the observational properties
of the two surveys can be reproduced at least with the same accuracy as the two
previous models.Comment: 21 pages, when TeX-ed including tables (3) and figures (8). MNRAS
latex. mn.sty and psfig.sty included. Accepted for pubblication in MNRAS.
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