An astrophysical plane stuck in a “speed trap”

Science fiction author Arthur C. Clarke’s Seven Wonders of the World in a BBC television series in 1997. The only astronomical object he listed was SS 433. It had already attracted attention in the late 1970s due to its X-ray emission. It was later discovered that it was located at the center of a gas nebula that was named 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 about ten times that of the Sun, and a star with a similar mass but occupying a much larger volume, orbit each other for 13 days.

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

SS433’s jets can be detected in X-ray radio bands down to a distance of less than one light-year on either side of the central binary star, before becoming too faint to see. However, surprisingly, about 75 light-years away from their launch site, the jets were seen suddenly reappearing as bright X-ray sources. The reasons for this emergence have always been poorly understood.

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

Until recently, no gamma-ray emission from a microwave quasar had been detected. But that changed in 2018, when the High Altitude Water Cherenkov Gamma-ray Observatory (HAWC) succeeded, for the first time, in detecting very high-energy gamma rays from SS 433’s jets. This means that somewhere in the jets, there are particles that are being accelerated to… Maximum energies.

Despite decades of research, it remains unclear how or where particles inside astrophysical jets are accelerated.

Studying gamma-ray emission from quasars offers one crucial advantage: While SS 433’s jets are 50 times smaller than those of the nearest active galaxy (Centaurus A), SS 433 lies within the Milky Way a thousand times closer than the 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.

After the discovery of HAWC, the HESS observatory began a monitoring campaign for SS 433. This campaign yielded about 200 hours of data and the clear detection of gamma-ray emission from SS 433’s jets.

The HESS telescopes’ superior angular resolution compared to previous measurements allowed researchers to pinpoint the origin of gamma-ray emission inside jets for the first time, yielding interesting results:

While no gamma-ray emission has been detected from the central binary, the emission appears suddenly in the outer jets at a distance of about 75 light-years on either side of the binary star, according to previous X-ray observations.

However, what surprised astronomers most was the shift in the position of the gamma ray emission when viewed at different energies.

Gamma-ray photons with higher energies, above 10 TeV, are only detected at the point where the jets suddenly appear. In contrast, regions emitting gamma rays with lower energies are more visible along each stream.

“This is the first ever observation of an energy-dependent morphology in the gamma-ray emission of an astrophysical jet,” said Laura Oliveira Neto of the Max Planck Institute for Cairnphysics in Heidelberg, who was leading the HESS study. SS 433 as part of a doctoral thesis.