BL Lacertae object

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.
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.[1] 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 (quasars) with strong emission lines, BL Lac objects have spectra dominated by a relatively featureless non-thermal emission continuum over the entire electromagnetic range.[2] This lack of spectral lines historically hindered identification of the nature and distance of such objects.[2]

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 galaxies. These active nuclei appear to be hosted in massive 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.[3]

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 quasars are generally more luminous and have stronger emission lines than BL Lac objects.[4]

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 Comae Berenices, ON 325 and Markarian 501.

  1. ^ Padovani, Paolo; Giommi, Paolo (15 December 1995). "A Sample-Oriented Catalogue of BL Lacertae Objects". Monthly Notices of the Royal Astronomical Society. 277 (4): 1477–1490. arXiv:astro-ph/9511065v1. Bibcode:1995MNRAS.277.1477P. doi:10.1093/mnras/277.4.1477.
  2. ^ a b Falomo, Renato (2014). "An Optical View of BL Lacertae Objects". The Astronomy and Astrophysics Review. 22: 44. arXiv:1407.7615. Bibcode:2014A&ARv..22...73F. doi:10.1007/s00159-014-0073-z.
  3. ^ Marscher, A. P.; et al. (24 April 2008). "The inner jet of an active galactic nucleus as revealed by a radio-to-gamma-ray outburst" (PDF). Nature. 452 (7190): 966–969. Bibcode:2008Natur.452..966M. doi:10.1038/nature06895. hdl:2027.42/62749. PMID 18432239.
  4. ^ Urry, Megan (1995). "Unified Schemes for Radio-Loud Active Galactic Nuclei". Publications of the Astronomical Society of the Pacific. 107: 803. arXiv:astro-ph/9506063. Bibcode:1995PASP..107..803U. doi:10.1086/133630.