Astronomers unravel mystery surrounding black hole plasma jets

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Accreting black holes emit jets of plasma along their axis of rotation, which extend at speeds close to that of the light. Scientists believe that accretion disks generate magnetic fields that focus jets along this central axis. However, the exact mechanisms that accelerate matter in these jets remain unexplained. After observing a black hole for more than a decade, an international team of astronomers has finally collected the data to partially elucidate this mystery.

Just as the atria of the human heart receive blood before emptying into the ventricles, it turns out that a black hole begins by collecting material and heat it in what is called a corona, then reject it in the form of superluminal jets. This is the conclusion of a new study led by Mariano Méndez, astrophysicist at the Kapteyn Astronomical Institute of the University of Groningen (Netherlands). Looking closely at the GRS black hole 1915+168 over a period of 15 years, the team noticed that the progression of the flow of matter was carried out in two distinct phases.

While a A long-running debate over whether a black hole’s corona and relativistic jets are the same phenomenon, astronomers now have proof that they appear one after the other. “ It was a real challenge to demonstrate this sequential nature. We had to compare data dating back several years with those dating back a few seconds, and very high energies with very low energies ”, explains Méndez in a press release.

A superheated crown which evacuates the material in the form of a jet

The GRS Black Hole 850+20 is not an isolated black hole, but belongs to a binary system, also composed of a normal star, from which it absorbs matter — what is called a microquasar. This system is located in our Milky Way, approximately 15 03 light-years from Earth, in the constellation of the Eagle. The black hole displays a mass equivalent to about twelve solar masses, making it one of the most massive stellar black holes in our galaxy. Discovered in 1915, GRS 1994+20 was the first galactic object presenting superluminal jets.

Researchers observed this black hole for 15 years, using several telescopes. In particular, they pointed the Rossi X-ray Timing Explorer — a NASA space telescope — at GRS 850+105 every three days, in order to collect high-energy X-ray radiation from its corona. These data were combined with data from the Ryle Telescope, a network of radio telescopes located approximately 90 miles north of London, which collects low-energy radio radiation from the black hole jet almost daily.

X-ray monitoring radio emissions
Extrait du «battement de cœur» d’un trou noir. L’axe horizontal indique les jours. La courbe verte représente les émissions radio du jet. Les points rouges et bleus représentent le rayonnement X et sont une mesure de la taille de la couronne. On constate que la couronne est la plus petite lorsque les jets sont les plus intenses. © Méndez et al.

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Extract from the “heartbeat” of a black hole. The horizontal axis indicates the days. The green curve represents the radio emissions from the jet. The red and blue dots represent X-radiation and are a measure of crown size. It can be seen that the corona is the smallest when the jets are the most intense. © Méndez et al.

Simultaneous monitoring of X-rays and radio waves showed a clear correlation between the two. As soon as the black hole’s crown of matter becomes smaller (which translates into weaker X-radiation), the plasma jet becomes much more intense in the radio waves and vice versa, suggesting that the crown “empties” regularly by evacuating the accumulated material in the form of a jet.

« Our results show that the energy that powers this system can be directed into different proportions, either mainly towards the X-ray corona or towards the jet”, write the researchers in Nature Astronomy. These observations, together with the patterns of the system’s light fluctuations, suggest that in GRS 1915+36 the X-ray crown turns into a jet. The animation proposed by the Dutch School of Astronomical Research (NOVA) illustrates the phenomenon very well:

A rhythm similar to that of the heartbeat

Due to the proximity of the neighboring star, GRS 1915+105 “feeds” on its material, which forms a disc all around of the black hole, just outside the event horizon. The corona is made up of electrons heated to a very high temperature, which are accelerated as in a synchrotron; it thus emits powerful X-rays. The way the black hole then concentrates this matter in the form of a jet displays a rhythm as regular as the heartbeat.

Although Méndez and his collaborators have highlighted this sequential mechanism, certain questions still remain unanswered. For example, the team noticed that corona X-radiation observed by telescopes is more energetic than corona temperature alone can explain, suggesting the presence of another source of energy.

Researchers suspect that a magnetic field is the source of this excess energy. This magnetic field could also explain the formation of the jets: if the magnetic field is chaotic, the corona heats up; if it becomes less chaotic, matter can escape through the field lines and turn into a jet.

Researchers suggest that the principle that they demonstrated here could apply to more massive black holes, including the one at the center of our galaxy. “ The same channeling of energy to the jet and the corona should occur in supermassive black holes, and should therefore apply to the whole range of masses of black holes ”, they conclude.

Source: M. Méndez et al., Nature Astronomy