BL Lacertae Objects
BL Lacertae objects (BL Lacs) are a relatively rare subclass of AGN. Their special feature is very faint emission lines, or even a total lack of them. This means that the continuum emission from the relativistic jets is strong enough to completely overwhelm the thermal emission of the host galaxy.
The family of BL Lac objects has changed drastically in the last decades with the addition of new members. The first BL Lacs were very variable and strong radio emitters. Flaring continuum emission was even one of their selection criteria. Since then, many objects have been selected on the basis of their broad band spectral properties found through the cross-correlation of catalogues from several frequency bands. These new BL Lacs typically have little or no data from high radio frequencies, which makes their study at Metsähovi both an interesting and challenging task.
The Metsähovi source list includes 398 BL Lac objects. The brightest of them, e.g. AO 0235+164, S5 0716+714, OJ 287 and BL Lac, are measured often and their costant monitoring provides an unique set of continuous flux curves at 22 and 37 GHz. Most of the BL Lac sample are faint, and usually not detected at Metsähovi. Sometimes, however, we do measure a clear detection even for some of these faint sources. This suggests that they too may exhibit flaring, like their brighter peers, which enables us to detect them from time to time. Studying these radio-faint BL Lacs, and their unknown behaviour, is of special importance to the Planck mission. To get to Planck's primary target, the cosmic background radiation, all emission from foreground sources has to be removed from the maps. Metsähovi helps in providing real, observational evidence of the behaviour of the whole BL Lac population at high radio frequencies, to be used alongside theoretical predictions.
For details on the radio varibility research of BL Lacs in Metsähovi, see: 2007AJ....133.1947N
In addition to research at radio frequencies, we have extended our work on BL Lacs at Metsähovi to include multifrequency studies. The fundamental difference between radio-bright and faint sources is in their spectral energy distributions. The bright sources radiate most of their synchrotron emission in the radio to IR-frequencies, whereas the faint sources are strongest from optical to soft X-rays. The essential question is what is causing this difference in their energies. It has been suggested that this sequence would be governed by a single parameter, the bolometric luminosity. However, our studies so far suggest that the answer is more complex.
For details on the multifrequency studies of BL Lacs in Metsähovi, see: 2006A&A...445..441N