ASCA and other contemporaneous observations of the blazar B2 1308+326

The high redshift (z = 0.997) blazar B2 1308+326 was observed contemporaneo usly at x-ray, optical and radio wavelengths in June 1996. The x-ray observ ations were performed with ASCA. The ASCA results were found to be consiste nt with reanalysed data from two earlier ROSAT observations. The combined A SCA and ROSAT data reveal an xray spectrum that is best fit by a broken pow er law with absorber model with photon spectral indices of Gamma (soft) = 3 .4(-1.1)(+5.1) and Gamma (hard) = 1.63(-0.09)(+0.10) 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 spec tral energy distribution, to be the emerging importance of inverse Compton (IC) emission which dominates the ASCA spectrum. The faint optical state re ported for these observations (m(V) = 18.3 +/- 0.25) is incompatible with t he high synchrotron flux previously detected by ROSAT. The TC emission dete cted by both ROSAT and ASCA was not significantly affected by the large cha nge in the synchrotron component. Mg II emission was detected with an equivalent width (W-lambda) of similar to 15 Angstrom, significantly different from previously reported values. Th e small and variable W-lambda in B2 1308+326 may be due to the highly varia ble continuum and not intrinsically weak lines in the source. A lower limit on the Doppler boost factor calculated from the contemporaneous data is co nsistent with expectations for highly polarised quasars and higher than exp ected for BL Lacs. Absorption at a level of N-H = 3.0(-0.6)(+2.3) x 10(20) cm(-2) was detected which is in excess of the Galactic value of NH 1.1 x 10 (20) cm(-2), indicating the possible presence of a foreground absorber. A g ravitational microlensing scenario cannot therefore be ruled out for this b lazar. No significant variability on timescales of hours was detected in th e optical or x-ray data. B2 1308+326 could be a typical radio-selected BL Lac in terms of peak synch rotron frequency and optical and radio variability but its high bolometric luminosity, variable line emission and high Doppler boost factor make it ap pear more like a quasar than a BL Lac. It is suggested that B2 1308+326 be considered as the prototype of this class of composite source.