2004A&A...418..459W

The high-frequency-peaked BL Lac, MS0205.7+3509 was observed twice with XMM-Newton. Both X-ray spectra are synchrotron-dominated, with mean 0.2-10keV fluxes of 2.80±0.01 and 3.34±0.02x10^–12erg/cm²/s. The X-ray spectra are well fit by a power-law with absorption above the Galactic value, however no absorption edges are detected, implying a low metallicity absorber (Z_☉=0.04^+0.03_–0.01) or an absorber with redshift above one (best-fit z=2.1 for an absorber with solar abundances). In either case the absorbing column density must be ∼9x10²¹/cm². A new optical spectrum is presented, with a MgII absorption doublet detected at z=0.351, but no other significant features. The optical spectrum shows little reddening, implying a low dust to gas ratio in the absorber. MS0205.7+3509 must therefore be viewed through a high column density, low-metallicity gas cloud, probably at z=0.351 and associated with the galaxy that has been shown to be within ∼2" of the BL Lac.