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The night sky looks serene, but the telescopes tell us that the universe is filled with collisions and explosions. Long and violent events signal their presence by spitting light and particles in all directions. When these messengers reach Earth, scientists can use them to map the action-laden sky, helping to better understand the volatile processes taking place in the deepest space.
For the first time, an international collaboration of scientists has detected a highly energetic light coming from the outermost regions of an unusual star system in our own galaxy. The source is a microquasar – a black hole that engulfs objects from a nearby star and projects two powerful jets of material.
The observations of the team, described in the October 4, 2018 issue of the journal Nature, strongly suggest that the electron acceleration and collisions at the ends of the jets of the microquasar produced powerful gamma rays. Scientists believe that studying the messengers of this microquasar 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 (HAWC), designed to examine the gamma-ray emission from astronomical objects such as supernova remnants, quasars and rotating dense stars called pulsars. The team has now studied one of the most known microquasars, named SS 433, located about 15,000 light-years from Earth.
Scientists have seen a dozen microquasars in our galaxy and only two of them seem to emit high energy gamma rays. Due to the proximity and orientation of SS 433, scientists have a rare opportunity to observe extraordinary astrophysics.
"The SS 433 is located in our neighborhood and, thanks to HAWC 's unique wide field of vision, we have been able to solve both microquasar particle acceleration sites," said Jordan Goodman, Distinguished Professor at the University of New York. University of Maryland, and lead spokesperson and US spokesperson for HAWC collaboration. "By combining our observations with multi-wavelength and multi-messenger data from other telescopes, we can improve our understanding of particle acceleration in the SS 433 and its extragalactic cousins." giants, called quasars. "
Quasars are massive black holes that suck material from the centers of galaxies, rather than feeding on a single star. They actively expel radiation, which can be seen from anywhere in the universe. But they are so far away that most known quasars have been detected because their jets are directed towards the Earth – as if a flashlight were directed directly to the eyes. In contrast, the SS 433 jets are far from the Earth and HAWC has detected a similar energetic light from the microquasar.
Regardless of their origin, gamma rays move in a straight line to their destination. Those who arrive on Earth collide with molecules in the atmosphere, creating new particles and lower energy gamma rays. Each new particle breaks into more things, creating a shower of particles as the signal moves to the ground.
HAWC, located about 13,500 feet above sea level near the Sierra Negra volcano in Mexico, is perfectly located to catch the rapidly moving particle rain. The detector is made up of more than 300 water tanks, each having a diameter of about 24 feet. When the particles reach the water, they move fast enough to produce a blue light shock wave called Cherenkov radiation. Special cameras installed in the tanks detect this light, allowing scientists to determine the origin of the gamma rays.
The HAWC collaboration examined 1,017 days of data 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. Based on their analysis, the researchers concluded that jet electrons reach energies about a thousand times greater than those achievable with earth-bound particle accelerators, such as the Large Hadron Collider 39, a city located along the Franco-Swiss border.
The jet electrons collide with the low energy microwave background radiation that enters the space, causing gamma emission. This is a new mechanism for generating high energy gamma rays in this type of system. It is different from what scientists have observed when the jets of an object are directed towards the Earth.
Ke Fang, co-author of the study and former postdoctoral fellow of the Joint Space-Science Institute, a partnership between UMD and NASA's Goddard Space Flight Center, said that this new measure is essential for understand what is happening in the SS 433.
"Looking at just one type of light from the SS 433, it's like seeing only the tail of an animal," said Fang, currently an Einstein member of Stanford University. "Thus, we combine all its signals, from low energy radio to X-rays, to new high-energy gamma ray observations, to determine the type of SS 433 beast."
Until now, the instruments had not observed the SS 433 emitting gamma rays as energetic. But HAWC is designed to be very sensitive to this extreme part of the light spectrum. The detector also has a wide field of vision that looks up at the sky all the time. The collaboration used these capabilities to solve the structural characteristics of the microquasar.
"The SS 433 is an unusual star system and every year, something new has appeared," said Segev BenZvi, another co-author of the study and assistant professor of physics at the university. University of Rochester. "This new observation of high energy gamma rays relies on nearly 40 years of measurement of one of the strangest objects in the Milky Way." Each measurement gives us a different piece of the family quasar puzzle in his outfit. "
Research report: "Acceleration of very high energy particles fed by the jets of Microquasar SS 433", A. U. Abeysekara et al., October 4, 2018, Nature
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