Editing BL Lacertae object (section)

{{Short description|Type of active galactic nucleus}}
[[Image:BL Lacertae Pan-STARRS DR1.jpg|thumb|right|250px|[[BL Lacertae]] (center) is the prototype of BL Lacertae objects, with the surrounding host elliptical galaxy visible. The bright object at its left is a foreground star, while pixelated smudges are effects of data processing.]]
[[Image:fig1553.jpg|thumb|right|The optical spectrum of the BL Lac object PG 1553+11]]

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.<ref>{{cite journal|last2=Giommi|first2=Paolo|date=15 December 1995|title=A Sample-Oriented Catalogue of BL Lacertae Objects|url=http://ned.ipac.caltech.edu/level5/Padovani/Padovani_contents.html|journal=[[Monthly Notices of the Royal Astronomical Society]]|volume=277|issue=4|pages=1477–1490|arxiv=astro-ph/9511065v1|bibcode=1995MNRAS.277.1477P|doi=10.1093/mnras/277.4.1477|first1=Paolo|last1=Padovani|doi-access=free }}</ref> Because of these properties, the prototype of the class (BL Lac) was originally thought to be a [[variable star]]. When compared to the more luminous active nuclei ([[quasar]]s) with strong emission lines, BL Lac objects have spectra dominated by a relatively featureless non-thermal emission continuum over the entire electromagnetic range.<ref name=":0">{{Cite journal|last=Falomo|first=Renato|year=2014|title=An Optical View of BL Lacertae Objects|journal=The Astronomy and Astrophysics Review|volume=22|pages=44|doi=10.1007/s00159-014-0073-z|arxiv=1407.7615|bibcode=2014A&ARv..22...73F}}</ref> This lack of spectral lines historically hindered identification of the nature and distance of such objects.<ref name=":0" />

In the unified scheme of radio-loud active galactic nuclei, the observed nuclear phenomenology of BL Lacs is interpreted as being due to the effects of the [[relativistic jet]] closely aligned to the line of sight of the observer. BL Lacs are thought to be intrinsically identical to low-power [[Radio galaxy|radio galaxies]]. These active nuclei appear to be hosted in massive [[Elliptical galaxy|elliptical galaxies]]. From the point of AGN classification, BL Lacs are a [[blazar]] subtype. All known BL Lacs are associated with core dominated radio sources, many of them exhibiting apparent [[superluminal motion]].<ref>{{cite journal|date=24 April 2008|title=The inner jet of an active galactic nucleus as revealed by a radio-to-gamma-ray outburst|journal=Nature|volume=452|issue=7190|pages=966–969|bibcode=2008Natur.452..966M|doi=10.1038/nature06895|first1=A. P.|last1=Marscher|pmid=18432239|display-authors=etal|url=https://deepblue.lib.umich.edu/bitstream/2027.42/62749/1/nature06895.pdf|hdl=2027.42/62749|hdl-access=free}}</ref>

The blazar category encompasses all quasars oriented with the relativistic jet directed at the observer giving a unique radio emission spectrum. This includes BL Lacs as well as optically violent variable (OVV) quasars, however in general practice, "Blazar" and "BL Lac Object" are often used interchangeably. [[OVV quasar]]s are generally more luminous and have stronger emission lines than BL Lac objects.<ref>{{Cite journal|last=Urry|first=Megan|year=1995|title=Unified Schemes for Radio-Loud Active Galactic Nuclei|journal=Publications of the Astronomical Society of the Pacific|volume=107|pages=803|bibcode=1995PASP..107..803U|doi=10.1086/133630|arxiv=astro-ph/9506063}}</ref>

Some examples of BL Lac objects are BL Lacertae itself, [[OJ 287]], [[AP Librae]], [[PKS 2155-304]], [[PKS 0521-365]], [[Markarian 421]], [[3C 371]], W [[Coma Berenices|Comae Berenices]], ON 325 and [[Markarian 501]].