2006A&A...456..131P

The large majority of BL Lacertae objects belonging to the 1Jy sample, the class prototype for radio-selected sources, are thought to emit most of their synchrotron power in the far infrared band. Ironically, this spectral region is very sparsely sampled, with only a minority of the objects having IRAS data (most of them being upper limits or low-quality detections). We aim at filling this infrared gap by presenting new, simultaneous ISOCAM and ISOPHOT observations over the 7-200µm range (observer's frame) for half the sample. A precise measurement of the position of the synchrotron peak frequency, ν_peak, can provide direct information about particle acceleration mechanisms and constrain the inverse Compton radiation that will be detected by up-coming new γ-ray missions. We have observed seventeen 1Jy BL Lacertae objects with the camera and the photometer on board the Infrared Space Observatory (ISO) satellite. Given the intrinsic variability of these sources, the data were taken by concatenating the pointings to ensure simultaneity. The ISOPHOT data reduction was done employing a novel correction, which mitigates the effect of chopping for faint sources. Using our new ISO data, complemented by nearly-simultaneous radio and optical observations for ten and four objects respectively, and other multi-frequency data, we have built the spectral energy distributions of our sources (plus a previously published one) and derived the rest-frame ν_peak. Its distribution is centred at ∼10¹³Hz (∼30µm) and is very narrow, with ∼60% of the BL Lacs in the 1-3x10¹³Hz range. Given our set of simultaneous infrared data, these represent the best determinations available of the synchrotron peak frequencies for low-energy peaked BL Lacs. A comparison with previous such estimates, based on non-simultaneous optical and near infrared data, may indicate strong ν_peak variations in a number of sources, possibly associated with large flares as observed in the high-energy peaked BL Lac MKN 501.