SIMBAD references

The bright BL Lacertae object PKS 2155-304 was observed with the Extreme Ultraviolet Explorer (EUVE) in 1994 May with the primary objective of attempting to measure the EUV polarization using a novel technique. The light curves and spectra from the observation proved very useful, even though the method failed to produce useful polarimetric information owing to large, poorly understood, systematic instrumental effects. Light curves from this and previous observations showed variations of a factor of 2 over 1 month in 1993 and up to 25% variations on timescales of hours on several occasions (from four separate observations totalling 25 days spanning 3 yr). The soft X-ray spectra of the source were also derived from the EUVE observations and are comparable in shape to previous observations. The spectra do not show significant spectral features and are fitted acceptably with simple power-law spectra with an energy index α=3.0 for an assumed neutral hydrogen column density N_H=1.4x10²⁰ cm^–2. A flatter index of 2.0 fits the data if the N_H is reduced to 1.2x10²⁰ cm^–2, within the 2 σ uncertainty on N_H as derived from 21 cm observations. Previous models that involve extensive absorption troughs with a flat-index power law (as extended from the X-ray band) are also consistent with the data.