The hard synchrotron X-ray spectrum of the TeV BL Lac 1ES 1426+428

Abstract

We have observed 1ES 1426+428 with INTEGRAL detecting it up to $\sim$150 keV. The spectrum is hard, confirming that this source is an extreme BL Lac object, with a synchrotron component peaking, in a $\nu F_\nu$ plot, at or above 100 keV, resembling the hard states of Mkn 501 and 1ES 2344+514. All these three sources are TeV emitters, with 1ES 1426+428 lying at a larger redshift (z=0.129): for this source the absorption of high energy photons by the IR cosmic background is particularly relevant. The observed hard synchrotron tail helps the modeling of its spectral energy distribution, giving information on the expected intrinsic shape and flux in the TeV band. This in turn constrains the amount of the poorly known IR background.