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 F(2-10 keV) = 2.0(-0.5)(+0.3) x 10(-12) erg cm(-2) s(-1) in January 1999 and F(2-10 keV) = (2.6 +/- 0.6) x 10(-12) 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 1.2 +/- 0.1, steeper than that measured in the optical, equal to 0.86 +/- 0.05. The BeppoSAX observation of ON 325 was performed in December 1998 when its X-ray flux was F(2-10 keV) = 8(-4)(+3) x 10(-13) 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 1.48 +/- 0.12 and -0.57 +/- 0.63. 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.