The 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 attracted attention already in the late 1970s due to its X-ray emission and was later discovered to be at the center of a gas nebula that is dubbed 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, with a similar mass but occupying a much larger volume, orbit each other with a period of 13 days.
The intense gravitational field of the black hole rips material from the surface of the star, which accumulates in a hot gas disk that feeds the black hole. As matter falls in 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.
The jets of SS433 can be detected in the radio to X-ray ranges out to a distance of less than one light year on either side of the central binary star, before they become too dim to be seen. Yet surprisingly, at around 75 light years distance from their launch site, the jets are seen to 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 (for example quasars), though these jets are much larger in size than the galactic jets of SS 433. Due to this analogy, objects like SS 433 are classified as microquasars.
Until recently, no gamma ray emission has ever been detected from a microquasar. But this changed in 2018, when the High Altitude Water Cherenkov Gamma-ray Observatory (HAWC), for the first time, succeeded in detecting very-high-energy gamma rays 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 within astrophysical jets.
The study of gamma-ray emission from microquasars provides one crucial advantage: while the jets of SS 433 are 50 times smaller than those of the closest active galaxy (Centaurus A), SS 433 is located inside the Milky Way a thousand times closer to Earth. As a consequence, the apparent size of the jets of SS 433 in the sky is much larger and thus their properties are easier to study with the current generation of gamma-ray telescopes.
Prompted by the HAWC detection, the H.E.S.S. Observatory initiated an observation campaign of the SS 433 system. This campaign resulted in around 200 hours of data and a clear detection of gamma-ray emission from the jets of SS 433.
The superior angular resolution of the H.E.S.S. telescopes in comparison to earlier measurements allowed the researchers to pinpoint the origin of the gamma-ray emission within the jets for the first time, yielding intriguing results:
While no gamma-ray emission is detected from the central binary region, emission abruptly appears in the outer jets at a distance of about 75 light years on either side of the binary star, in accordance with previous X-ray observations.
However, what surprised the astronomers most, was a shift in the position of the gamma-ray emission when viewed at different energies.
The gamma-ray photons with the highest energies of more than 10 teraelectron-volts, are only detected at the point where the jets abruptly reappear. By contrast, the regions emitting gamma rays with lower energies appear further along each jet.
“This is the first-ever observation of energy-dependent morphology in the gamma-ray emission of an astrophysical jet,” said Laura Olivera-Nieto, from the Max-Planck-Institut für Kernphysik in Heidelberg, who was leading the H.E.S.S. study of SS 433 as part of her doctoral thesis.
Source: Max Planck Society