This black hole like no other questions astronomers

the binary system MAXI J1820+070 is no stranger to the media spotlight for black hole discoveries. Futura has devoted several articles to its subject, including the previous one below. We know him in particular because of his emissions in the field of X-rays because it accretes matter of his star companion.

There is a whole theory of matter accretion by a stellar black holethat is, a star compact resulting from thecollapse gravity of a massive star and which has a closed event horizon, functioning as a membrane that can only be crossed in one direction. To leave the region defined by this membrane, which has no material reality, it would indeed be necessary to go beyond the speed of light.

In the early 1970s, two astrophysicists Russians, Nikolai Shakura and Rashid Sunyaev, laid the foundations of this theory of accretion. the gas forming a disc around a black hole is viscous. Thus, falling in a spiral towards the black hole, the contiguous rings of matter forming the disk rub against each other, which transforms the gravitational energy of accretion into heat heating the gas to the point of ionizing it. Temperatures get so high that the disk starts emitting X-rays with a spectrum and very characteristic properties, which moreover makes it possible to identify a stellar black hole.

Such an object does not radiate, not even according to the process discovered by stephen hawking, in the absence of matter accretion. Indeed, the Hawking radiation is inversely proportional to the mass of a black hole and for those of stellar origin at least, this implies a temperature much lower than that of the cosmic radiationso that it absorbs it for the same reasons that heat passes from a hot body to a cold body when exchanges of work do not occur in a thermodynamic system (which is not the case with a fridge).

Jets mysteriously tilted in relation to the accretion disk

A black hole accreting matter and in rotation can also emit jets of matter, matter resulting from its accretion disk only and not of the black hole itself. However, it turns out that by studying the characteristics of the jets and especially of the radiation from the accretion disk, which is not limited to X-rays, it is possible to have information on the mass of the black hole, its cinematic moment of rotation as well as that of its accretion disk.

And that’s where an international team of astrophysicists led by their colleagues from the University of Turku, Finland, came across an astonishing and paradoxical discovery regarding MAXI J1820+070, as explained in an article published in Science and a version of which is freely available on arXiv.

By analyzing the polarization in the visible of the radiation from MAXI J1820+070 with the DIPol-UF instrument fitted to the Nordic Optical Telescope located at the observatory of Roque de los Muchachos, La Palma in the Canary Islands, the researchers came to the conclusion that the axis of rotation of the black hole of the binary system was tilted by 40° with respect to the axis perpendicular to the shot of the accretion disk, as seen in the video above.

Astrophysicists do not understand the reason because usually, the laws of celestial mechanics tend rather to produce stellar black holes whose axis of rotation, along which the jets are also emitted, is almost perpendicular to the plane of the disk. of accretion.

One can suspect that the conditions of formation of the stellar black hole were not those which one ordinarily attributes to the birth of such a system. It would be necessary to know to what extent this system is an exception… or not, because if such is not the case, the estimates of the masses and the angular moments of the stellar black holes, drawn from the observation of the systems binaries X of this kind, must be biased since it is assumed that the axes of black holes and accretion disks are almost parallel.

The eruption of a stellar black hole filmed by Chandra

Article of Laurent Sacco published on 03/06/2020

MAXI J1820+070 is a stellar black hole in the Milky Way whose existence can be inferred because it copiously emits X-rays by accreting matter. It has been observed several times by Chandra at these wavelengths; the images taken by this satellite of the Nasa now allow to see a time lapse an eruption accompanied by the emission of falsely transluminal jets of matter.

The collaboration Event Horizon Telescope has already amazed us by unveiling the first image of a black hole, in this case M87*. Its members let us hope that we will soon have a similar image showing Sgr A* which is located at the heart of our Milky Way. Better, films of the activity around the supermassive black hole of our Galaxy would be possible.

In the meantime, NASA’s Chandra satellite has already provided us with X-ray images showing this activity in the case of a stellar black hole located only 10,000 light years from Solar system. It is not unknown and it has already been talked about because it is the compact star and source of X-rays called MAXI J1820+070. With its 8 solar masses, its gravitational field pulls matter from its companion star containing about half a solar mass. It is therefore surrounded by an accretion disk. Complex magnetohydrodynamic processes associated with thespace-time curve of a Kerr black hole in rotation and the plasma generated in this disc lead it to emit material jets. Instabilities associated with accretion are also the cause of flashes intermittentelectromagnetic waves.

A team of astrophysicists led by Mathilde Espinasse, from the University of Paris, studied MAXI J1820+070 during four observation campaigns in November 2018 then, February, May and June 2019. The images obtained and their analyzes are the subject of an article published in The Astrophysical Journal Letters but which can also be consulted freely on arXiv.

Jets of falsely transluminal black holes

The researchers made it a kind of time-lapse showing ejections of packages of particles in two diametrically opposed jets. These jets give rise to the existence of an optical illusion that has been well known for decades with in particular the quasars. A naive analysis of the observations indeed initially suggests that one sometimes observes particles in these jets which go at superluminal speeds therefore which seem to go faster than light. The M87 supermassive black hole jets* are moreover a good example of this phenomenon which does not actually violate the theory of special relativity since the particles actually go slower than light.

The illusion of transluminal speeds occurs when the jets of a black hole are almost parallel to the direction of observation from Earth. In the case of the images taken by Chandra, the jet emitted at the south pole of MAXI J1820 + 070 is approaching us while that emitted at the north pole is moving away. We then have the impression that the first reflects speeds of the order of 60% of the speed of light for matter, while the second jet seems to be formed of particles moving at 160% of the speed of light. The real speed of the particles in the two jets is actually greater than 80% of the speed of light, but still less.

A laboratory black hole to understand relativistic jets

Further details on these jets have been obtained by studying MAXI J1820 + 070 in the wave domain radios with in particular the famous Karl G. Jansky Very Large Array (the “Very large Karl-G.-Jansky network”) or VLA, a network of radio telescopes allowing you to perform opening synthesis, as if you had a single, very large instrument. It is moreover with the VLA that had initially been highlighted movements surface-mounted supraluminances with MAXI J1820 + 070.

The data provided allowed astrophysicists to estimate a thousand Halley’s cometsor about 500 million times the mass of the Empire State Building, the amount of material ejected by the black hole in a few hours in 2018 and which is finally found in the observed jets.

The material in the jets then slowed down as it entered the interstellar medium after creating the equivalent of shock waves from a supersonic aircraft. The combination of data in the radio and X domain will allow relativistic astrophysicists to better understand black hole jets and indirectly their effects on the evolution of galaxies when they are produced by supermassive black holes.