An astrophysical plane caught in a “speed trap”

Science fiction author Arthur C. Clarke selected his Seven Wonders of the World into a BBC television series in 1997. The only astronomical object he included was SS 433. It had previously attracted attention at the late 1970s due to its X-ray emission. and was later discovered to be at the center of a gaseous nebula nicknamed the Manatee Nebula due to its unique shape resembling 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, of similar mass but occupying a much larger volume, orbit one of the ‘other over a period of 13 days.

The black hole’s intense gravitational field pulls material from the star’s surface, which accumulates in a disk of hot gas that fuels the black hole. As matter falls toward the black hole, two collimated jets of charged particles (plasma) are launched, perpendicular to the plane of the disk, at a quarter of the speed of light.

Jets from SS433 can be detected in the X-ray range up to a distance of less than a light year on either side of the central binary star, before they become too faint to see. Yet surprisingly, about 75 light years from their launch site, the jets abruptly reappear as bright X-ray sources. The reasons for this reappearance have long been poorly understood.

Similar relativistic jets are also observed emanating from the centers of active galaxies (e.g. quasars), although these jets are much larger than the galactic jets of SS 433. Because of this analogy, objects like SS 433 are classified as microquasars.

Until recently, no gamma ray emissions had been detected by a microquasar. But that changed in 2018, when the Cherenkov Gamma Ray Observatory (HAWC) first managed to detect very high energy gamma rays coming from the jets of SS 433. This means that somewhere in the jets, particles are accelerated to extreme energies.

Despite decades of research, it is still unclear how or where particles are accelerated in astrophysical jets.

Studying gamma-ray emission from microquasars offers a crucial advantage: while the jets of SS 433 are 50 times smaller than those of the nearest active galaxy (Centaurus A), SS 433 is located to the interior of the Milky Way a thousand times closer to Earth. . As a result, the apparent size of SS 433’s jets in the sky is much larger and their properties are therefore easier to study with the current generation of gamma-ray telescopes.

Prompted by the HAWC detection, the HESS Observatory launched an observation campaign of the SS 433 system. This campaign resulted in approximately 200 hours of data and a clear detection of gamma-ray emissions from the SS 433 jets.

The higher angular resolution of the HESS telescopes compared to previous measurements allowed researchers to identify for the first time the origin of the gamma-ray emission in the jets, yielding intriguing results:

Although no gamma-ray emission is detected from the central binary region, the emission appears abruptly in the outer jets at a distance of about 75 light-years on either side of the binary star, consistent with ray observations X previous.

However, what surprised astronomers the most was the change in position of the gamma-ray emission when observed at different energies.

Gamma photons, whose energies are the highest, above 10 teraelectron volts, are only detected at the point where the jets suddenly reappear. In contrast, regions emitting lower-energy gamma rays appear further along each jet.

“This is the first-ever observation of an energy-dependent morphology in the gamma-ray emission of an astrophysical jet,” said Laura Olivera-Nieto of the Max-Planck-Institut für Kernphysik in Heidelberg. , who led the HESS study on SS 433 as part of his doctoral thesis.

“These findings initially perplexed us. The concentration of high-energy photons at the reappearance sites of the X-ray jets means that effective particle acceleration must take place there, which was not expected .” The results were published in Science.

The scientists simulated the observed energy dependence of the gamma-ray emission and were able to obtain the first-ever estimate of the speed of the external jets. The difference between this speed and that with which the jets are launched suggests that the mechanism that accelerated the particles further is a powerful shock, an abrupt transition in the properties of the medium.

The presence of a shock would then also provide a natural explanation for the reappearance of the X-ray jets, because the accelerated electrons also produce X-ray radiation.

“When these fast particles collide with a light particle (photon), they transfer part of their energy. 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 astrophysical plasma theory group at the Max Planck Institute for Nuclear Physics in Heidelberg.

“There has been much speculation about the occurrence of particle acceleration in this unique system. This is no longer the case today: the HESS results actually identify the site of the acceleration, the nature of the particles accelerated, and allow us to probe the movement of particles. large-scale jets launched by the black hole”, emphasizes 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 about the internal dynamics of such a system,” adds co-author Michelle Tsirou, a postdoctoral researcher at DESY Zeuthen.

On the other hand, we know nothing about the origin of the shocks at the sites where the plane reappeared. “We still do not have a model that can uniformly explain all the properties of the jet, because no model has yet predicted this characteristic,” says Olivera-Nieto.

She wants to dedicate herself to this task next – a laudable goal, because SS 433’s relative proximity to Earth provides a unique opportunity to study the appearance of particle acceleration in relativistic jets. It is hoped that the results can be transferred to the thousand times larger jets of active galaxies and quasars, which would help solve the many puzzles regarding the origin of the most energetic cosmic rays.

The HESS observatory

High-energy gamma rays can only be observed from the ground with a trick. When a gamma ray enters the atmosphere, it collides with atoms and molecules and generates new particles that rush toward the ground like an avalanche. These particles emit flashes that last only a few billionths of a second (Cherenkov radiation), which can be observed on the ground using large, specially equipped telescopes.

High energy gamma astronomy therefore uses the atmosphere as a giant fluorescent screen. The HESS Observatory, located in the Khomas Highlands of Namibia at an altitude of 1,835 m, officially began operation in 2002. It consists of an array of five telescopes.

Four telescopes with mirrors with a diameter of 12 m are located at the corners of a square, with another 28 m telescope in the center. This makes it possible to detect cosmic gamma radiation in the range of a few tens of gigaelectronvolts (GeV, 10⁹ electronvolts) to a few tens of teraelectronvolts (TeV, 10¹² electronvolts).

For comparison: visible light particles have energies of two to three electron volts. HESS is currently the only instrument that observes the southern sky with high-energy gamma light and is also the largest and most sensitive telescope system of its type.