There’s a ‘cosmic monster‘over our heads, a billion light years from us, and swallows up immense quantities of matter which then, in part,’ vomits’ towards the Earth. And the blazar Bl Lacertae which, with his hole
black supermassive at the center of a huge galaxy, it is in fact “swallowing an immense amount of matter, which is partly ejected at speeds close to that of light in the form of two jets, one of which points almost exactly towards us” reports the Institute National Astrophysics (Inaf). By studying this cosmic monster, Il Whole Earth Blazar Telescope (Webt) – a collaboration of astronomers from all over the world in which researchers and researchers of the National Institute of Astrophysics are actively part – has thus discovered that the radiation of its jet oriented towards the Earth shows strong cyclical fluctuations in brightness, which increases and decreases over a few hours.
According to the team, this “absolutely particular behavior is due to the fact that the jet is not exactly straight, but a kink has formed produced by instabilities created in the jet at a distance of about 16 light years from the black hole which rhythmically deflected the very powerful flux of radiation towards us and thus produced the oscillation of brightness detected. “The work, published in the journal Nature and led by Svetlana Jorstad of the University of Boston, was also attended by researchers from Italian universities and amateur astronomers with some non-professional observers in our country. Discovered in 1929 in the constellation of the Lizard (in Latin Lacerta) and initially classified as a variable star, Bl Lac is the ‘progenitor’ of a class of Agn (active galactic nucleus) with similar characteristics: objects of type Bl Lacertae.
The blazar is a particular active galactic nucleus (Agn) fed by material that falls into a black hole supermassive located at the center of a galaxy. Scientists explain that about 10% of Agn have a pair of jets that are projected into interstellar space at close to the speed of light. INAF adds that “we speak of a blazar when one of the jets points almost directly towards the Earth, which makes it appear much brighter due to a relativistic focusing effect. The jets produce radiation ranging from radio waves to the visible , up to X and gamma rays, the intensity of which varies rapidly over time “. ‘These variations are usually random, with no recognizable pattern,’ the researchers add.
The Webt project, started 25 years ago and coordinated by Claudia Maria Raiteri and Massimo Villata of the National Institute of Astrophysics (Inaf), aims to monitor the variability of visible light, but also that in the radio and near infrared bands, in the blazars observed in gamma rays by the Agile satellites of the Italian Space Agency and Fermi of NASA. “Our observations have led to the discovery of cycles of variation in the visible brightness, of that in gamma rays, and in the degree of polarization of the BL Lacertae blazar (BL Lac for short) on a time scale of about 13 hours. strong emission recorded at various wavelengths in the second half of 2020 “says Claudia Raiteri of INAF of Turin. BL Lac is powered by a black hole with a mass approximately 170 million times that of the Sun and is located approximately 1 billion light years from Earth. The cycles of brightness changes are called “quasi-periodic oscillations” or Qpo. Qpo are most often observed in other systems composed of pairs of black holes with masses between 10 and 50 times that of the Sun that emit X-rays.
In the case of the observations of the brightness of BL Lacertae, the team proposed that “a crease forms in the jet that twists the magnetic field of the jet itself, thus causing the oscillation of the brightness. In addition, another characteristic of the observed radiation, namely its polarization changes with a trend over time similar to that of brightness. ” “The polarized light comes from the jet and the polarization can only change if the magnetic field changes its configuration in the region that produces the light. The magnetic field in the jet must be twisted to cause the oscillations. BL Lac observations also show a strong correlation between visible light and variations in gamma rays without any delay, which places the origin of gamma rays in the region where visible light changes “reports the INAF.
“If the magnetic field of a jet has a spiral structure, the jet and the field can become unstable and twist, creating a crease. As the particles in the jet flow through the crease, the amount of radiation emitted increases and decreases rhythmically, producing the Qpo “. “However, to further improve the agreement between the observations and the theory, the researchers inserted the contribution linked to turbulence processes in the code that describes the dynamic behavior of the BL Lac jet, obtaining excellent results also in reproducing the trend of the polarization detected “adds INAF.
In support of this scenario, the images of the jet’s radiation in high-frequency radio waves, obtained by the team with data from the Very Long Baseline Array (Vlba), highlight a new bright spot that is displaced with respect to the axis of the jet. , with a direction of polarization that is favorable to the development of a fold in the jet itself.
“The objective of the Webt is to identify the physical mechanisms that cause the variability of the blazars through the organization of intense multi-frequency observation campaigns and the related analysis and interpretation of the data. The results obtained on Bl Lac were possible thanks to the exceptional temporal continuity of the observations that the Webt is able to obtain, due to the collaboration of dozens of astronomers and amateur astronomers who observe at different longitudes, thus giving themselves the change in the observational task during the 24 hours of the Earth’s rotation “says Massimo Villata of INAF of Turin and president of the Webt since 2000.
(by Andreana d’Aquino)
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