Artistic impression of a jet from a Blazar
Claudia Raiteri
Blazars are extremely interesting, very bright, and variable extragalactic sources. The scientific literature on them is vast, making it useful to consult a review article on the main observational and interpretative results obtained over decades of study. In this spirit, Claudia M. Raiteri, an astronomer at the INAF-Turin Astrophysical Observatory, wrote the article “The variability of blazars throughout the electromagnetic spectrum,” recently published in the journal “The Astronomy and Astrophysics Review.”
Blazars are active galactic nuclei whose power comes from a supermassive black hole, with a mass hundreds of millions to tens of billions of times that of the Sun, which feeds on incandescent matter from a disk of dust and gas (called an accretion disk) rotating around it. From the poles of this gigantic black hole, two jets of plasma are launched into space, within which particles travel at nearly the speed of light, emitting radiation across the entire electromagnetic spectrum, from radio waves to gamma rays. While jets of this type are common in other active galactic nuclei, the peculiarity of blazars is that one of them is oriented in our direction, causing its emission to be altered by the relativistic Doppler effect. Specifically, this effect increases their luminosity and shortens the timescales of variability.
In fact, the flow we observe from blazars can change over very short timescales, on the order of minutes, and by analyzing the episodes of fastest variability, it is possible to estimate the size of the emitting region, which is very compact. There is observational evidence suggesting that the emission at various frequencies originates from different regions of the jet; for example, visible light would be produced in a region further inside than radio radiation.
The causes of blazar variability are a matter of ongoing investigation, and researchers disagree. Particle acceleration mechanisms are certainly at work within the jets, such as shock waves propagating along the jet, magnetic reconnection, and turbulence. However, geometric phenomena can also cause changes in flow: the jet can rotate due to the rotation of the black hole and its accretion disk, it can precess if its axis does not coincide with their rotation axis, and it can twist if magnetohydrodynamic instabilities develop within it.
Indeed, very high-resolution radio observations obtained with interferometric techniques show that blazar jets exhibit curvatures that change over time.
Furthermore, the discovery of periodic behavior may indicate that the central engine is actually composed of a binary system of supermassive black holes, a possible consequence of the collision and subsequent merger of galaxies.
Another possible phenomenon causing variability is microlensing, which occurs if the blazar is distant and its line of sight includes a galaxy whose stars can act as gravitational lenses.
The article finally addresses the variability of the polarimetric behavior of blazars, which can provide valuable information. Recently, the IXPE satellite has offered the possibility of obtaining polarimetric data in the X-ray band and comparing them with those observed at low frequencies, particularly in the radio and optical bands, revealing unexpected behaviors.
The article concludes by listing a series of open problems for our understanding of these peculiar objects.
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