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BL Lacertae object

A BL Lacertae object or BL Lac object is a type of active galactic nucleus (AGN) or a galaxy with such an AGN, named after its prototype, BL Lacertae. In contrast to other types of active galactic nuclei, BL Lacs are characterized by rapid and large-amplitude flux variability and significant optical polarization. Because of these properties, the prototype of the class was originally thought to be a variable star. When compared to the more luminous active nuclei (quasars) with strong emission lines, BL Lac objects have spectra dominated by a relatively featureless non-thermal emission continuum over the entire electromagnetic range. This lack of spectral lines historically hindered identification of the nature and distance of such objects.

Host galaxies
, the closest BL Lac object to the Milky Way. Soon after the discovery of this unusual class of objects it was noted that the sources were surrounded by a faint nebulosity. In the late 1970s the use of modern detectors (such as CCD) allowed observers to probe with better accuracy the nature of the nebulosity. First images of the BL Lac object PKS 0548-322 by Michael John Disney in 1974 in various filters found it to be composed by a giant elliptical galaxy with a bright nucleus. Extensive surveys taken with the Hubble Space Telescope of 132 BL Lac objects comprising seven complete radio, X-ray, and optically selected samples in 2000 studied the morphologies of possible BL Lac host galaxies. The data concluded that in two-thirds of the BLL images taken, host galaxies are detected, including in nearly all with redshift z 0.5 were resolved because of relatively short exposure times. looks to be a significantly preferred brightness profile for 58 of the 72 resolved host galaxies at over ~99% confidence. The results of this survey conclude that there is an 8% limit to the number of disk systems in BL Lac objects and is therefore consistent with the assumption that all BL Lac host galaxies could be elliptical. These ellipticals are very luminous with a median absolute K-corrected magnitude of M_R\thicksim-23.7\pm-0.6 mag (rms dispersion). This is comparable to the brightest cluster galaxies. ==History==
History
John L. Schmitt first noticed the peculiar nature of BL Lac in 1968 when he matched it with a radio object, VRO 42.22.01. Within a year others observed that the radio flux varied, and that light was polarized. Peter Albert Strittmatter proposed the class of object in 1972 and added four objects. By 1976 there were 30 known objects. In 2017, a very high energy neutrino was detected by the IceCube project apparently coming from BL Lac object TXS 0506+056. ==References==
Source: Wikipedia ↗
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