X-ray spectroscopy of the BL Lac H1426+428

GT We propose to measure the parameters of the absorption feature at virgul600 eVoriginally detected by BBXRT in H1426+428. Sharp absorption features appear tobe common in the soft Xray spectra of BL Lac objects, as shown by severaldetections made with BBXRT: the lines are generally attributed to blueshiftedLyman alpha OVIII resonance absorption, implying rapid mass outflow from theactive nucleus. However the exact nature and origin of these features is stillunclear. The RGS will provide an accurate measure of the line energy and width,of the column density and the ionisation level of the gas, ultimately givingclues to the origin of the gas itself. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]