BL Lacertae Object

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A BL-Lacertae-Object , also shorter BL-Lac-Object , is one of the many types of Active Galactic Nuclei (AGN) . BL Lacs are a little fainter than quasars and are among the most luminous known cosmic objects . Astronomers (less often) also refer to them as lacertides. BL Lac objects are closely related to the Optically Violently Variables (OVV), which are a subclass of the Quasi Stellar Objects (QSO's) .

discovery

Such an object was first discovered by Cuno Hoffmeister in 1929 in the constellation Lizard (Lacerta) and classified as a variable star with the usual designation  BL for this star class ; the name BL Lacertae resulted . Its brightness variations were irregular. It was not until much later that it became apparent that it was actually an extragalactic object that only appeared star-like. In 1974, Oke and Gunn were able to use the Doppler effect to deduce a distance of just under a billion light years from the spectrum.

In 1968, measurements with a radio telescope found that this object was also a strong radio source that was also variable. Several other objects of this type were found at the same time, showing the particularities of these objects.

properties

BL Lacertae objects are particularly characterized by three properties:

  • Continuous spectrum without lines
In astrophysics, when observing stars, the absorption and emission lines found in their spectrum usually tell something about their physical and chemical nature. However, the spectrum of BL Lac and other objects of this type does not show any such lines and therefore does not allow any conclusions to be drawn about the composition of the objects.
Only a small nebula around BL Lac could later be spectroscoped and turned out to be a giant galaxy, which was assigned a distance of 800 million light years due to its redshift of 0.0688 . So BL-Lacertae objects - similar to the quasars , but which have a line spectrum - are so bright that they can outshine an entire galaxy.
  • Brightness variation across the spectrum
All previously known BL-Lac objects have a non-periodically variable luminosity over their entire spectrum, whereby this spectrum can only be limited by the possibilities of measurement (from radio to X-ray range ). These changes can be on the order of hours and thus greatly limit the size of the BL Lacertae objects: they cannot be greater than the distance that light travels in this time. One must therefore assume that these are very compact objects, no larger than our solar system.
  • Polarization of the emitted light
The radiation emitted by the BL Lac objects is strongly polarized (up to 20%) across the entire spectrum , with both polarization and brightness being variable.
  • strong blue brightness and weak optical emission and absorption lines
In times of low luminosity, emission lines can be observed and thus indicate a missing gas torus, which is typical for elliptical galaxies. The AGN type BL Lac is only observed in host galaxies of the elliptical Hubble type .

Explanations

On the basis of the properties described above, a conclusive model for the description of the BL-Lac objects could be found: like the quasars, they are massive black holes in the center of active galaxies . Due to the non- thermal spectrum and the clear polarization , they cannot be ordinary stars. Today it is assumed that the source of the radiation are relativistic plasma streams ( jets ) which are collimated and ejected from the center of the object. The emitted radiation has the properties of synchrotron radiation .

The BL-Lac objects (like the quasars) do not get their extremely high luminosity like stars through nuclear fusion of hydrogen, helium etc. to form heavier elements, but through black holes in their core with masses that can reach billions of times that of the sun. The radiated energy is released when matter is drawn into the black holes and falls into them. In the event of a crash, the matter is partially converted directly into energy, which when released reaches approximately the value  mc 2 (with the speed of light  c ). This releases more than ten times the amount of energy than if the same amount of matter were fused inside the star. In addition, the energy is emitted in a much shorter time than the lifespan of stars, which drastically increases the luminosity.

Some BL-Lac objects radiate so strongly that they outshine all of the light emitted by the galaxy surrounding them. This means that it is not possible to specify a distance for these objects due to the redshift of the galaxy.

As with the quasars, thermal radiation is also emitted by the accretion disk , but this has its maximum at very short wavelengths.

Difference to quasars

A possible explanation for the phenomenon of the BL-Lacs would be that the synchrotron jet is aimed directly at our direction of observation and thus simply outshines the emission lines in the spectrum through its enormous radiation. The lack of emission lines makes it difficult to determine the redshift and hence the distance. This is supported by the fact that emission lines suddenly appear during the minima of BL-Lac objects , or the host galaxy suddenly becomes observable, as happened at the end of 2007 with the BL-Lac object S50716 + 714. This unified model favors that there is only one class of active galaxies, it just depends on the angle from which these objects are observed. If we look directly into the jet ("face on",  = 0 ° and  = 180 °) , it is a BL-Lacertae object (no emission lines visible in the spectrum), with an oblique angle of view onto the accretion disk (wide and narrow emission lines are observable) one speaks of a Seyfert galaxy type 1 , and when looking at the edge ("edge on",  = 90 °) , whereby the accretion disk is covered by the dust torus (only narrow emission lines observable), we speak of one Seyfert galaxy type 2.

As Quasar or QSO all AGNs be referred whose absolute brightness (the brightness of the object at a distance of 10 pc would have) over the limit of -23 m is (M> 23 m ). For all objects below (M <-23 m ) one speaks of an AGN .

The quasar phenomenon (M> -23 m ) probably only occurs in young galaxies that still have a lot of interstellar gas in the central region, with which the black hole in the center is fed, so to speak. When the gas is finally used up, the active galactic core (the black hole) becomes inactive and the galaxy becomes a perfectly normal star system, similar to our Milky Way. This would also be an explanation for the fact that the quasars can be found relatively frequently at great distances (past), but hardly in our closer cosmic environment. It may be a normal phase in the evolution of galaxies.

Individual evidence

  1. ↑ a b c BL Lac object. In: Andreas Müller, Lexicon of Astrophysics. 2007, Retrieved July 19, 2019 .
  2. ↑ Oke, JB; Gunn, JE: The Distance of BL Lacertae . In: Astrophysical Journal, vol. 189, p.L5 . August. bibcode : 1974ApJ ... 189L ... 5O . doi : 10.1086 / 181450 .
  3. ↑ P. Giommi, et al .: AGILE and Swift simultaneous observations of the blazar S50716 + 714 during the bright flare of October 2007 . In: A&A, Volume 487, Number 3, September 2008 . August. doi : 10.1051 / 0004-6361: 200810189 .
  4. ↑ Quasars, BL-Lacertae objects and AGNs or the "engine of the quasars". In: Klaus Wenzel, Federal German Working Group for Variable Stars eV (BAV), circular 2/2010. April 2010, accessed July 19, 2019 .