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 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…