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

Abstract

We report the results of new observations of the two BL Lac objects 3C 66A and ON 325 performed with Beppo SAX 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 $F(2{-}10~{\rm keV}) = 2.0^{+0.3}_{-0.5}\times 10^{-12}$ erg cm -2 s -1 in January 1999 and $F(2{-}10~{\rm keV}) = (2.6 \pm 0.6)\times 10^{-12}$ 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 $1.2\pm 0.1$, steeper than that measured in the optical, equal to $0.86\pm 0.05$. The Beppo SAX observation of ON 325 was performed in December 1998 when its X-ray flux was $F(2{-}10~{\rm keV}) = 8^{+3}_{-4}\times10^{-13}$ 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 $1.48\pm0.12$ and $-0.57\pm0.63$. 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.