Eight Years of Ultraviolet Spectra of the Variable BL Lacertae Object PKS 2155-305

Abstract

The brightest known BL Lacertae object at ultraviolet wavelengths is PKS 2155-304, which was discovered as a result of its strong X-ray emission. Since 1979, 64 ultraviolet spectra of this object have been obtained with the International Ultraviolet Explorer (IUE) satellite. These have been analyzed in a consistent fashion using Gaussian extraction techniques that maximize the signal to noise. In general, the ultraviolet continuum of PKS 2155-304 is well fitted by a power-law model F_nu_ is proportional to ν^-α^, with an average spectral index of <α> ~ 0.89 +/- 0.12(σ) for the 40 fits to combined short- and long-wavelength spectra. If we compensate for the apparent miscalibration of some long-wavelength spectra (specifically the LWP images), the mean spectral index is somewhat flatter, <α> ~ 0.78 +/- 0.12. Over the years the ultraviolet intensity of PKS 2155-304 has varied by a factor of ~2 but the spectral index has changed very little and is uncorrelated with intensity in an absolute sense. The difference in spectral index between adjacent observations may be correlated with the difference in intensity, but more frequent sampling is required to confirm this. Time scales for ultraviolet variability, defined by t_var_=(d ln F_nu_/dt)^-1^, are as short as 10 days. In one case, a pair of short- and long-wavelength IUE spectra were obtained nearly simultaneously with a very soft X-ray measurement; the combined spectra, which represent the closest connection in frequency space across the inaccessible far- ultraviolet gap for any extragalactic object, show a steepening between the ultraviolet and soft X-ray bands. If the change in slope between the two bands is a simple sharp break, it must lie in the narrow frequency range ~8 x 10^16^-10^17^ Hz.