BL Lac objects
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BL Lac objectsExtremely compact violently variable active galaxies that resemble quasars but lack both emission and absorption lines in their spectra. More than two hundred are known, the first to be identified was BL Lacertae, which was mistakenly classified as a variable star in 1928 and later (1968) shown to be the optical counterpart of a peculiar radio source. BL Lac objects are most easily identified from X-ray and radio surveys (most of those known are strong radio sources) but, like quasars, the peak of their emission lies in the infrared.
Light and radio waves from these objects are strongly polarized, indicating intense magnetic fields, and there are rapid variations in both strength and direction. BL Lac objects are also violently variable in luminosity at all wavelengths, sometimes flaring up five magnitudes (a factor of 100) in only a few weeks. This indicates that the size of their energy-producing regions cannot be more than a few light-weeks across. During periods of faint luminosity, weak emission lines may sometimes be observed and a redshift measured but, in general, distance measurements to BL Lac objects have been hampered by their absence of spectral lines. Many BL Lac objects are embedded in a faint ‘fuzz’ identified as the surrounding galaxy. This contains weak absorption lines, from which it is possible to measure a redshift. Like other radio-loud active galaxies, BL Lac objects appear to lie in elliptical hosts. They are most populous in the low-redshift Universe, so that their space distribution appears to be very different from other active galaxies such as quasars. It has been proposed that they are mostly active galaxies with relativistic jets pointing along the line of sight. See also radio galaxy.