BL Lacertae object

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The optical spectrum of the BL Lac object PG 1553+11. Fig1553.jpg
The optical spectrum of the BL Lac object PG 1553+11.
The BL Lac object H 0323+022 (z=0.147) imaged at ESO NTT (R filter). The host galaxy and close companions are visible. H0323bl2.gif
The BL Lac object H 0323+022 (z=0.147) imaged at ESO NTT (R filter). The host galaxy and close companions are visible.

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]

Contents

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.

Host galaxies

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. BL Lac objects are luminous enough that only one quarter (6/22) of the images taken with z > 0.5 were resolved because of relatively short exposure times. [5] A de Vaucouleurs profile [6] 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 mag (rms dispersion). This is comparable to the brightest cluster galaxies. [5]

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. [7]

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. [8]

In 2017, a very high energy neutrino was detected by the IceCube project apparently coming from BL Lac object TXS 0506+056. [9]

Related Research Articles

Quasar Active galactic nucleus containing a supermassive black hole

A quasar is an extremely luminous active galactic nucleus (AGN), powered by a supermassive black hole, with mass ranging from millions to tens of billions of solar masses, surrounded by a gaseous accretion disc. Gas in the disc falling towards the black hole heats up because of friction and releases energy in the form of electromagnetic radiation. The radiant energy of quasars is enormous; the most powerful quasars have luminosities thousands of times greater than a galaxy such as the Milky Way. Usually, quasars are categorized as a subclass of the more general category of AGN. The redshifts of quasars are of cosmological origin.

An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much-higher-than-normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not produced by stars. Such excess non-stellar emission has been observed in the radio, microwave, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an "active galaxy". The non-stellar radiation from an AGN is theorized to result from the accretion of matter by a supermassive black hole at the center of its host galaxy.

3C 273 Brightest quasar from Earth located in the constellation Virgo

3C 273 is a quasar located in the constellation of Virgo. It was the first quasar ever to be identified.

Seyfert galaxy Class of active galaxies with very bright nuclei

Seyfert galaxies are one of the two largest groups of active galaxies, along with quasars. They have quasar-like nuclei with very high surface brightnesses whose spectra reveal strong, high-ionisation emission lines, but unlike quasars, their host galaxies are clearly detectable.

Radio galaxy Types of active galactic nuclei that are very luminous at radio wavelengths

Radio galaxies and their relatives, radio-loud quasars and blazars, are types of active galactic nuclei that are very luminous at radio wavelengths, with luminosities up to 1039 W between 10 MHz and 100 GHz. The radio emission is due to the synchrotron process. The observed structure in radio emission is determined by the interaction between twin jets and the external medium, modified by the effects of relativistic beaming. The host galaxies are almost exclusively large elliptical galaxies. Radio-loud active galaxies can be detected at large distances, making them valuable tools for observational cosmology. Recently, much work has been done on the effects of these objects on the intergalactic medium, particularly in galaxy groups and clusters.

Blazar Very compact quasi-stellar radio source

A blazar is an active galactic nucleus (AGN) with a relativistic jet directed very nearly towards an observer. Relativistic beaming of electromagnetic radiation from the jet makes blazars appear much brighter than they would be if the jet were pointed in a direction away from Earth. Blazars are powerful sources of emission across the electromagnetic spectrum and are observed to be sources of high-energy gamma ray photons. Blazars are highly variable sources, often undergoing rapid and dramatic fluctuations in brightness on short timescales. Some blazar jets exhibit apparent superluminal motion, another consequence of material in the jet traveling toward the observer at nearly the speed of light.

BL Lacertae Active galaxy in the constellation Lacerta

BL Lacertae or BL Lac is a highly variable, extragalactic active galactic nucleus. It was first discovered by Cuno Hoffmeister in 1929, but was originally thought to be an irregular variable star in the Milky Way galaxy and so was given a variable star designation. In 1968, the "star" was identified by John Schmitt at the David Dunlap Observatory as a bright, variable radio source. A faint trace of a host galaxy was also found. In 1974, Oke and Gunn measured the redshift of BL Lacertae as z = 0.07, corresponding to a recession velocity of 21,000 km/s with respect to the Milky Way. The redshift figure implies that the object lies at a distance of 900 million light years.

OVV quasar Type of highly variable quasar or subtype of blazar

An optically violent variable quasar is a type of highly variable quasar. It is a subtype of blazar that consists of a few rare, bright radio galaxies, whose visible light output can change by 50% in a day. OVV quasars have essentially become unified with highly polarized quasars (HPQ), core-dominated quasars (CDQ), and flat-spectrum radio quasars (FSRQ). Different terms are used but the term FSRQ is gaining popularity effectively making the other terms archaic.

VERITAS Ground-based gamma-ray observatory

VERITAS is a major ground-based gamma-ray observatory with an array of four 12 meter optical reflectors for gamma-ray astronomy in the GeV – TeV photon energy range. VERITAS uses the Imaging Atmospheric Cherenkov Telescope technique to observe gamma rays that cause particle showers in Earth's atmosphere that are known as extensive air showers. The VERITAS array is located at the Fred Lawrence Whipple Observatory, in southern Arizona, United States. The VERITAS reflector design is similar to the earlier Whipple 10-meter gamma-ray telescope, located at the same site, but is larger in size and has a longer focal length for better control of optical aberrations. VERITAS consists of an array of imaging telescopes deployed to view atmospheric Cherenkov showers from multiple locations to give the highest sensitivity in the 100 GeV – 10 TeV band. This very high energy observatory, completed in 2007, effectively complements the Large Area Telescope (LAT) of the Fermi Gamma-ray Space Telescope due to its larger collection area as well as coverage in a higher energy band.

Luminous infrared galaxies or LIRGs are galaxies with luminosities, the measurement of brightness, above 1011 L☉. They are also referred to as submillimeter galaxies (SMGs) through their normal method of detection. LIRGs are more abundant than starburst galaxies, Seyfert galaxies and quasi-stellar objects at comparable luminosity. Infrared galaxies emit more energy in the infrared than at all other wavelengths combined. A LIRG's luminosity is 100 billion times that of our sun.

Markarian 421 Blazar located in the constellation Ursa Major

Markarian 421 is a blazar located in the constellation Ursa Major. The object is an active galaxy and a BL Lacertae object, and is a strong source of gamma rays. It is about 397 million light-years to 434 million light-years (133Mpc) from the Earth. It is one of the closest blazars to Earth, making it one of the brightest quasars in the night sky. It is suspected to have a supermassive black hole (SMBH) at its center due to its active nature. An early-type high inclination spiral galaxy is located 14 arc-seconds northeast of Markarian 421.

Low-ionization nuclear emission-line region Type of galactic nucleus

A low-ionization nuclear emission-line region (LINER) is a type of galactic nucleus that is defined by its spectral line emission. The spectra typically include line emission from weakly ionized or neutral atoms, such as O, O+, N+, and S+. Conversely, the spectral line emission from strongly ionized atoms, such as O++, Ne++, and He+, is relatively weak. The class of galactic nuclei was first identified by Timothy Heckman in the third of a series of papers on the spectra of galactic nuclei that were published in 1980.

Hannys Voorwerp Astronomical object appearing as a bright blob close to spiral galaxy IC 2497 in Leo Minor, discovered by Hanny van Arkel

Hanny's Voorwerp, is a rare type of astronomical object called a quasar ionization echo. It was discovered in 2007 by Dutch schoolteacher Hanny van Arkel while she was participating as a volunteer in the Galaxy Zoo project, part of the Zooniverse group of citizen science websites. Photographically, it appears as a bright blob close to spiral galaxy IC 2497 in the constellation Leo Minor.

The Whole Earth Blazar Telescope (WEBT) is an international consortium of astronomers created in 1997, with the aim to study a particular category of Active Galactic Nuclei (AGN) called blazars, which are characterized by strong and fast brightness variability, on time scales down to hours or less.

Markarian 501 Galaxy with a spectrum extending to the highest gamma rays.

Markarian 501 is a galaxy with a spectrum extending to the highest energy gamma rays. It is a blazar or BL Lac object, which is an active galactic nucleus with a jet that is shooting towards the Earth.

Markarian galaxies Galaxy with a nucleus emitting exceptionally large amounts of ultraviolet

The Markarian galaxies are a class of galaxies that have nuclei with excessive amounts of ultraviolet emissions compared with other galaxies. Benjamin Markarian drew attention to these types of galaxies starting in 1963. The nuclei of the galaxies had a blue colour, associated to stars in the classes from O to A. This blue core did not match the rest of the galaxy. The spectrum in detail tends to show a continuum that Markarian concluded was produced non-thermally. Most of these have emission lines and are characterized by highly energetic activity. Markarian Catalogue entries are of the form "Markarian ####", and can frequently use the abbreviations Mrk, Mkr, Mkn; and rarely Ma, Mk, Mark.

Green bean galaxy Very rare astronomical objects that are thought to be quasar ionization echos

Green bean galaxies (GBGs) are very rare astronomical objects that are thought to be quasar ionization echos. They were discovered by Mischa Schirmer and colleagues R. Diaz, K. Holhjem, N.A. Levenson, and C. Winge. The authors report the discovery of a sample of Seyfert-2 galaxies with ultra-luminous galaxy-wide narrow-line regions (NLRs) at redshifts z=0.2-0.6.

S5 0014+81 Black Hole in the constellation Cepheus

S5 0014+81 is a distant, compact, hyperluminous, broad-absorption-line quasar, or blazar, located near the high declination region of the constellation Cepheus, near the North Equatorial Pole.

The Parkes Catalogue of Radio Sources, also known as the Parkes Southern Radio Source Catalog, consists of 8264 astronomical radio sources, mostly south of declination +27. The catalogue was mostly compiled by John Bolton and his colleagues for 20 years. Both the Molonglo 408-MHz survey and the 80-MHz Culgoora measurements of Slee et al have contributed to the usefulness of the catalogue. For now, the catalogue only contains sources originally found in the Parkes 2700-MHz survey. The catalogue contains radio sources that have a frequency range of 80 - 22,000 MHz.

AP Librae is a BL Lac object located at a distance of 700 million light years in the southern constellation of Libra. In the visual band it is one of the most active blazars known. AP Lib is surrounded by an extended source with a spectrum characteristic of a red-shifted giant elliptical galaxy. The derived visual magnitude of this region is 15.0, and it follows a radially decreasing brightness that is characteristic of an elliptical. Seven fainter galaxies are visible within an angular radius of 9′, suggesting it is the brightest member of a galactic cluster.

References

  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. 1 2 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.
  5. 1 2 Urry, Megan (2000). "The Hubble Space Telescope Survey of BL Lacertae Objects". The Astrophysical Journal. 523 (2): 816–829. arXiv: astro-ph/9911109 . Bibcode:2000ApJ...532..816U. doi:10.1086/308616.
  6. ↑ Sparke, L. S.; Gallagher, J. S. III (2007). Galaxies in the Universe: An Introduction. Cambridge University: Cambridge University Press. p. 244. ISBN   978-0-521-67186-6.
  7. ↑ Schmitt, John L. (May 1968). "BL Lac identified as a Radio Source". Nature. 218 (5142): 663. Bibcode:1968Natur.218..663S. doi: 10.1038/218663a0 .
  8. ↑ Stein, W. A.; O'Dell, S. L.; Strittmatter, P. A. (September 1976). "The BL Lacertae Objects" (PDF). Annual Review of Astronomy and Astrophysics. 14: 173–195. Bibcode:1976ARA&A..14..173S. doi:10.1146/annurev.aa.14.090176.001133.
  9. ↑ Overbye, Dennis. "It Came From a Black Hole, and Landed in Antarctica". NY Times. Retrieved 2018-07-16.
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