Years of observation of the microquasar SS 433 have allowed to identify the details of the processes responsible for the production of high-energy radiation and better understand its distant massive cousins: quasars, the Institute of Nuclear Physics PAS reports.

During the observation of the microquasar SS 433 at the High-Altitude Water Cherenkov Gamma-Ray Observatory (HAWC), gamma radiation with energies above 25 TeV was registered for the first time. Careful analysis of the data has led to surprising conclusions regarding the places and mechanisms responsible for the production of this radiation. The research results have just been presented in the prestigious scientific journal Nature (DOI: https://doi.org/10.1038/s41586-018-0565-5).

The Institute of Nuclear Physics PAS in Kraków informed PAP about the research. The institute`s employees participated in the research project.

According to the Institute of Nuclear Physics PAS release sent to PAP, quasars are among the most unusual and at the same time brightest objects of the Universe. The driving force of a quasar is a supermassive black hole in its centre, surrounded by an accretion disk formed by falling matter.

Quasars are sources of extremely intense electromagnetic radiation, covering almost the entire spectrum, from radio waves to high-energy gamma rays. But, being a type of galactic nuclei, quasars are by definition distant objects. The nearest quasar Markarian 231, powered by a pair of supermassive black holes spinning madly around each other, is in the nucleus of a galaxy 600 million light years away. That distance unfortunately is not conducive to carrying out high-resolution observations that could help understand the nature of the processes taking place there.

Fortunately, scientists can resort to observations of… miniature quasars. According to the Institute of Nuclear Physics PAS, what a quasar does on a galactic scale, a microquasar does on the scale of a star system.

Markarian 231`s black holes are gigantic: the smaller one has a mass 4 million times that of the Sun, the larger one`s mass is as much as 150 million times as great. The microquasar closest to us, SS 433, located in the background of the Aquila constellation, is a binary system with radically smaller dimensions. There is a very dense object here, probably a black hole with a mass of several suns, which is the remnant of a supernova explosion. It devours matter from an accretion disk powered by a stellar wind from a nearby spectral type A supergiant (a similar star, clearly visible in the night sky, is Deneb, the brightest star in the Cygnus constellation). This picturesque pair, spinning around each other at an impressive rate of 13 days and surrounded by the W50 nebula, is only 18 thousand light years from Earth.

„Both quasars and microquasars can generate jets: very narrow and very long streams of matter, emitted in both directions along the axis of rotation of the object” – explains Prof. Sabrina Casanova, quoted in the release. „The jets are created by particles accelerated to speeds not infrequently close to the velocity of light. In terms of speed, the jets from SS 433 are not particularly impressive: they reach only 26% of the velocity of light.”

Prof. Sabrina Casanova emphasises that something else is more important: „Most of the observed quasars have jets that are to a greater or lesser degree directed towards us. This orientation makes it difficult to distinguish details. On the other hand, SS 433 was kind enough to direct its jets not towards us, but almost perpendicular to the direction in which we look. Therefore, not only do we have the object almost at hand, it is also optimally positioned when it comes to observing details such as the sites from where the radiation originates” – the researcher says.

In our galaxy, SS 433 is one of just a dozen or so microquasars, and one of the few that emit gamma radiation. For 1017 days, this radiation was recorded at the HAWC observatory, operating at an altitude of over 4,100 m above sea level, on the slope of the Mexican Sierra Negra volcano. Its detector consists of 300 tanks of water, equipped with photomultipliers sensitive to fleeting light flashes known as Cherenkov radiation. It appears in the tank when a particle moving at a velocity faster than the velocity of light in water falls into it.

A key fact is that some of the flashes come from particles generated as a result of high energy collisions of gamma quanta with the Earth`s atmosphere. Appropriate analysis of the flashes in the tanks makes it possible to identify the reason for their existence. In this way, every day the HAWC indirectly registers gamma photons with energies from 100 gigaelectronvolts (GeV) to 100 teraelectronvolts (TeV). These energies are up to a trillion times greater than the energy of photons of visible light and several times larger than the energy of protons in the LHC accelerator.

During the observations of SS 433, conducted at the limit of HAWC`s resolution capability, scientists managed to register photons with energies above 25 TeV, three to ten times higher than those reported in the entire history of microquasar studies. To the surprise of the researchers, the brightest object in the system in the range of high-energy gamma radiation was not SS 433 itself, but the places on both sides of it where the jets break off and collide with the matter rejected by the supernova.

„That`s not the end of the surprises” – adds Dr. Francisco Salesa Greus from the Institute of Nuclear Physics PAS, , quoted in the release. „Gamma photons with energies of 25 TeV have to be produced by particles of even higher energies. These could be protons, but then they would have to have enormous energies at a level of 250 TeV. The data we collected, however, showed that this mechanism, even if it actually works, is not able to generate the right amount of gamma radiation in the case of SS433” – the scientist explains.

In further work, the data from HAWC were compared with SS 433 measurements in the remaining spectral ranges from other observatories. Ultimately, it was established that high-energy gamma quanta – or at least the majority of them – have to be emitted by electrons in the jet during their collisions with the low-energy microwave background radiation that fills the entire cosmos. The above mechanism, described for the first time in the paper in Nature, could not have been detected in observations of quasars with jets directed towards Earth. SS 433 has thus helped to reveal not only its own secrets, but also the secrets of the brightest lanterns of the Universe.

PAP