2024-02-02 10:45:19
Science-fiction author Arthur C. Clarke selected his own seven wonders of the world in a BBC television series in 1997. The only astronomical object he included was SS 433. It had already attracted attention in the late 1990s. 1970 due to its X-ray emission; It was later discovered to be at the center of a gas nebula, nicknamed the manatee nebula due to its unique shape similar to that of these aquatic mammals.
SS 433 is a binary star system in which a black hole, with a mass approximately ten times that of the Sun, and a star, with a similar mass but occupying a much larger volume, orbit each other with a period of 13 days. The black hole’s intense gravitational field tears material from the star’s surface, which accumulates in a disk of hot gas that feeds the black hole. As matter falls toward the black hole, two jets of charged particles (plasma) shoot out perpendicular to the plane of the disk, at a quarter of the speed of light.
SS 433’s jets are visible in the wavelength ranges from radio to X-rays, but only very close to their base. After a distance of less than a light-year on either side of the central binary system, the jets become too faint to be seen. However, surprisingly, about 75 light-years away from their place of origin, the jets abruptly reappear as bright sources of X-rays. The reasons for this reappearance have been a mystery.
Similar relativistic jets are also observed emanating from the centers of active galaxies (e.g., quasars), although these jets are much larger in size than the jets of SS 433. Because of this analogy, objects like SS 433 are classified as microquasars.
Until recently, gamma ray emission from a microquasar had never been detected. But this changed in 2018, when the HAWC (High Altitude Water Cherenkov Gamma-ray Observatory), for the first time, managed to detect very high energy gamma rays coming from SS 433. This means that at some point in the jets the particles are accelerated to reach extreme energies. Despite decades of research, it is still unclear how and where particles are accelerated in astrophysical jets.
Studying gamma-ray emission from microquasars offers a crucial advantage in solving this problem: while SS 433’s jets are 50 times smaller than those of the nearest active galaxy (Centaurus A), SS 433 lies within of the Milky Way, 1000 times closer to Earth. Being closer, the apparent size of the SS 433 jets in the sky is much larger and, therefore, their properties are easier to study with the current generation of gamma-ray telescopes.
Prompted by the HAWC detection, the HESS Observatory began an observation campaign of the SS 433 system. This campaign resulted in about 200 hours of data and a clear detection of gamma ray emission from the jets of SS 433. The best The angular resolution of the HESS telescopes compared to previous measurements allowed researchers to determine for the first time the origin of gamma-ray emission within the jets, yielding intriguing results:
While no gamma-ray emission is detected in the central binary region, in the outer part of the jets the emission appears abruptly at a distance of about 75 light-years on either side of the binary star, coinciding with previous observations. X-ray.
However, what surprised astronomers most was a change in the position of the gamma-ray emission when observed at different energies.
Gamma-ray photons with the highest energies, over 10 teraelectronvolts, are only detected at the point where the jets abruptly reappear. In contrast, regions emitting gamma rays with lower energies appear further along each jet.
“This is the first time that an energy-dependent morphology has been observed in the gamma-ray emission of an astrophysical jet,” says Laura Olivera-Nieto of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, who led the study. HESS of SS 433 as part of his doctoral thesis. “These findings really perplexed us, since the concentration of such high-energy photons at the site of the jet’s reappearance suggests a very efficient acceleration mechanism whose nature was until now unknown.”
The scientists carried out a simulation of the observed energy dependence in the emission of gamma rays and managed to estimate for the first time the speed with which the outer jets move. The difference between this speed and that with which the jets are launched suggests that the mechanism that accelerated the particles outward is a phenomenon called “strong shock”: a sudden transition in the properties of the medium. The presence of a shock would also provide a natural explanation for the mystery of the reappearance of jets as sources of X-rays, since the accelerated electrons also produce X-ray radiation.
“When these fast particles collide with a light particle (photon), they transfer part of their energy, and this is how they produce the high-energy gamma photons observed with HESS. This process is called the inverse Compton effect,” explains Brian Reville, head of the HESS. MPIK Astrophysical Plasma Theory group.
“There has been much speculation about the acceleration of particles in this unique system, but this is no longer the case: the HESS results determine the location of the acceleration, the nature of the accelerated particles, and allow us to characterize the motion of the jets on a large scale. launched by the black hole,” says Jim Hinton, Director of the Max Planck Institute for Nuclear Physics in Heidelberg and Head of the Department of Non-Thermal Astrophysics.
“Just a few years ago, it was unthinkable that ground-based gamma ray measurements could provide information on the internal dynamics of such a system,” adds co-author Michelle Tsirou, a postdoctoral researcher at the German Electron Synchrotron (DESY). its acronym in German).
However, nothing is known about the origin of the shocks in the places where the jet reappears. “We still do not have a model that can uniformly explain all the properties of the jet, since no model has yet predicted the presence of these shocks,” explains Olivera-Nieto. She now wants to dedicate herself to this task, a worthwhile goal, since the relative proximity of SS 433 to Earth offers a unique opportunity to study the emergence of particle acceleration in relativistic jets. It is hoped that the results can be translated to the thousand-fold larger jets of active galaxies and quasars, which would help solve the many enigmas related to the origin of the most energetic cosmic rays.
In green, radio observations reveal the Manatee Nebula with the microquasar visible as a bright spot near the center of the image. The solid lines show the contour of the X-ray emission from the central regions and the large-scale jets after their reappearance. The red colors represent the gamma ray emission detected by HESS at a) low (0.8-2.5 TeV, left), b) intermediate (2.5-10 TeV, center) and c) high (>10 TeV) energies. , right). The position of the gamma ray emission moves away from the microquasar as the energy decreases. (Images: Golap, M. Goss; NASA Wide Field Survey Explorer (WISE); X-rays (green outlines): ROSAT / M. Brinkmann; TeV (red colors): HESS Collaboration)
The HESS observatory
High-energy gamma rays can only be observed from the ground with a trick. When a gamma ray penetrates the atmosphere, it collides with atoms and molecules and generates new particles that rush toward the ground like an avalanche. These particles emit flashes lasting only a few billionths of a second (Cherenkov radiation), which can be observed with large, specially equipped telescopes. High-energy gamma-ray astronomy thus uses the atmosphere as a giant fluorescent screen. The HESS observatory, located in Namibia, at an altitude of 1,835 meters, officially became operational in 2002. It consists of a set of five telescopes. Four telescopes with 12-meter diameter mirrors are located at the corners of a square, with another 28-meter telescope in the center. The telescopes are capable of detecting cosmic gamma radiation in the range of a few tens of gigaelectronvolts (GeV, 10^9 electronvolts) to a few tens of teraelectronvolts (TeV, 10^12 electronvolts). For comparison: the visible light particles that humans observe have energies of two to three electron volts. HESS is currently the only instrument that detects high-energy gamma rays from the part of the sky visible from the southern hemisphere and is also the largest and most sensitive telescope system of its class.
The study is titled “Acceleration and transport of relativistic electrons in the jets of the microquasar SS 433.” And it has been published in the academic journal Science. (Source: Max-Planck-Institut Fur Kernphysik)
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