X-ray and optical observations of BL Lac objects: 3C 66A (B0219+428) and ON 325 (B1215+303)

We report the results of new observations of the two BL Lac objects 3C 66A and ON 325 performed with BeppoSAX and of nearly simultaneous photometric measurements in the bandpasses from I to B. 3C 66A was observed in January 1999 and in July 2001: its X-ray flux was erg cm-2 s-1 in January 1999 and erg cm-2 s-1 in July 2001. In both observations the X-ray spectrum can be fitted by a single power law with an energy index , steeper than that measured in the optical, equal to . The BeppoSAX observation of ON 325 was performed in December 1998 when its X-ray flux was erg cm-2 s-1. The X-ray spectrum cannot be fitted by a single power law, but it is found to be well represented by a broken power law with a break energy at around 4 keV and energy indices equal to and . Both sources are characterised by a synchrotron-dominated emission at energies lower than a few keV, with a sharp flattening toward higher energies due to the inverse Compton component. We obtained satisfactory modelling of the overall spectral energy distributions, from the optical to low energy X-rays, with log-parabolic laws.

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