ASCA and other contemporaneous observations of the blazar B 2 1308 + 326

The high redshift (z = 0.997) blazar B2 1308+326 was observed contemporaneously at x-ray, optical and radio wavelengths in June 1996. The x-ray observations were performed with ASCA. The ASCA results were found to be consistent with reanalysed data from two earlier ROSAT observations. The combined ASCA and ROSAT data reveal an xray spectrum that is best fit by a broken power law with absorber model with photon spectral indices of Γsoft = 3.4 −1.1 and Γhard = 1.63 −0.09 and a break energy at 1.1 +0.4 −0.4 keV in the rest-frame of the blazar. The break in the x-ray spectrum is interpreted, from the shape of the simultaneous broadband spectral energy distribution, to be the emerging importance of inverse Compton (IC) emission which dominates the ASCA spectrum. The faint optical state reported for these observations (mV = 18.3± 0.25) is incompatible with the high synchrotron flux previously detected by ROSAT. The IC emission detected by both ROSAT and ASCA was not significantly affected by the large change in the synchrotron component. Mg ii emission was detected with an equivalent width (Wλ) of ∼ 15 Å, significantly different from previously reported values. The small and variable Wλ in B2 1308+326 may be due to the highly variable continuum and not intrinsically weak lines in the source. A lower limit on the Doppler boost factor calculated from the contemporaneous data is consistent with expectations for highly polarised quasars and higher than expected for BL Lacs. Absorption at a level of NH = 3.0 −0.6 × 1020 cm−2 was detected which is in excess of the Galactic value of NH = 1.1 × 1020 cm−2, indicating the possible presence of a foreground absorber. A gravitational microlensing scenario cannot therefore be ruled out for this blazar. No significant variability on timescales of hours was detected in the optical or x-ray data. B2 1308+326 could be a typical radio-selected BL Lac in terms of peak synchrotron frequency and optical and radio variability but its high bolometric luminosity, variable line emission and high Doppler boost factor make it appear more like a quasar Send offprint requests to: D. Watson ([email protected]) than a BL Lac. It is suggested that B2 1308+326 be considered as the prototype of this class of composite source.