Recently detected gamma micro-radiation: a "call for new ideas"

The first detection of a highly energetic radiation by a microquasar has led astrophysicists to look for new theories to explain the extreme acceleration of particles. A microquasar is a black hole that engulfs debris from a nearby star and throws powerful jets of material.

"What is amazing about this discovery is that all current theories on particle acceleration have difficulty explaining the observations," said Hui Li, theorist of Theoretical Division of the Laboratory National Los Alamos, member of the team. "This certainly calls for new ideas on particle acceleration in microquasars and black hole systems in general."

The team's observations, described in the Oct. 4 issue of the journal Nature, strongly suggest that particle collisions at the ends of the jets of the microquasar produced powerful gamma rays. Scientists believe that the study of messages from this microquasar, called SS 433, can provide insight into the most extreme events occurring at the center of distant galaxies.

The team gathered data from the high-altitude gamma-ray observatory Chernenkov (HAWC), a mountain-top detector in Mexico that observes gamma-ray emission from supernova remnants, stars dense in rotation called pulsars and quasars. Los Alamos, funded by the Office of High Energy Physics of the Department of Energy, participated in the construction of HAWC, completed in 2015.

The team has now studied one of the most well-known microquasars, located about 15,000 light-years away.

Quasars are massive black holes that suck material from the centers of galaxies, rather than feeding on a single star. They actively emit visible radiation throughout the universe. But most are so far away that the majority of the detected quasars must have their jets directed towards the Earth, which makes them easier to spot, as if you are looking directly into a flashlight. On the other hand, the SS 433 jets are far from Earth, making them more difficult to observe. However, HAWC was able to detect a similar energetic light when viewed from the side, allowing the entire length of the jet to be visible. "

"The new discoveries improve our understanding of particle acceleration in microquasar jets, which also illuminates jet physics in much larger and more powerful extragalactic jets in quasars," said Hao Zhao, of the physics division of the Los Alamos National Laboratory.

HAWC, located approximately 13,500 feet above sea level near the Sierra Negra volcano in Mexico, captures the rapidly moving rain of particles with a detector consisting of over 300 water tanks, each of which 39, about 24 feet in diameter. When the particles reach the water, they produce a shock wave of blue light, called Cherenkov radiation. The cameras installed in the tanks detect this light, allowing scientists to compile the history of gamma rays.

The HAWC collaboration examined data collected over more than 1017 days of observation and found that the gamma rays came from the ends of the jets of the microquasar, rather than from the central part of the star system. On the basis of their analysis, the researchers concluded that jet electrons reach energies about 1,000 times higher than those obtainable with Earth-bound particle accelerators, such as the Large Hadron Collider.

The jet electrons collide with the low-energy microwave background radiation that enters the space, causing gamma-ray emission. It is a recently observed mechanism to extract high energy gamma rays from this type of system. It differs from what scientists have observed when the jets are directed towards the Earth.

Until now, the instruments had not seen such detailed information about the SS 433 because, according to the existing sky charts, this microquasar is hidden in a superb remnant of supernova that also emits gamma rays. But HAWC's wide field of vision and long base of reference look at the sky every night. This feature allowed the detector, like a long-exposure camera and wide-angle lens, to solve the peculiarities of the microquasar, even if it's obscured by the environment.

Research report: "Acceleration of very high energy particles fed by the jets of the microquasar SS 433,"

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