PKS 2155-304 - Relativistically beamed synchrotron radiation from a BL Lacertae object

Abstract

The newly discovered BL Lac object, PKS 2155-304, has been observed with the medium and high energy detectors of the HE AO 1 A-2 experiment. We confirm the variability by a factor of 2 in less than a day reported earlier. Two spectra, obtained a year apart, while the satellite was in scanning mode, are well fitted by simple power laws with energy spectral index ~ 1.4. A third spectrum, of higher statistical quality, obtained while the satellite was pointed at this source, has two components, and we get an acceptable fit using a two power-law model, with indices = 2.0(-b 1.2,-0.6) and a 2 =-1.5 (+1.5,-2.3). An interpretation of the overall spectrum from radio through X-rays in terms of a synchrotron self-Compton model gives a good description of the data if we allow for relativistic beaming. Thus, from a consideration of the spectrum, combined with an estimate of the size of the source, we infer the presence of jets without directly observing them.