Mountain-Top Observatory

The night sky seems serene, but telescopes tell us that the universe is filled with collisions and explosions. Distant, violent events signal their presence by spewing light and particles in all directions. When these messengers reach Earth, scientists can use them to map out the actionpacked sky, helping to better understand the volatile processes happening deep within space.

For the first time, an international collaboration of scientists, including physicists from the University of Utah, has detected highly energetic light coming from the outermost regions of an unusual star system within our own galaxy. The source is a microquasar—a black hole that gobbles up matter from a nearby companion star and blasts out powerful jets of material. The team’s observations, described in the October 4, 2018, issue of the journal Nature, strongly suggest that electron acceleration and collisions at the ends of the microquasar’s jets produced the powerful gamma rays. Scientists think that studying messengers from this microquasar may offer a glimpse into more extreme events happening at the centers of distant galaxies.

The team gathered data from the High-Altitude Water Cherenkov Gamma-Ray Observatory (HAWC), which is a detector designed to look at gamma ray emissions coming from astronomical objects such as pulsar wind nebula, supernova remnants, and blazars. Now, the team has studied one of the most well-known microquasars, named SS 433, which is about 15,000 light years away.

“There are a lot of microquasars in our Milky Way galaxy, but this is the first and only one that we have detected emitting very-high-energy gamma rays,” said Anushka Udara Abeysekara, research assistant professor in the Department of Physics

& Astronomy at the University of Utah. “To understand the system in detail, we used multiple wavelengths of light, such as X-rays and radiowaves, to piece together the bigger picture.”

Quasars are massive black holes that suck in material from the centers of galaxies. SS 433 is a micro-version of a quasar, located inside our own galaxy. The system consists of an object, either a black hole or neutron star, sucking in matter from a single unstable star. The system sprays accelerated particles along the opposite sides of the system, creating two jets. The jets smash into the interstellar medium around the two objects and produce the high-energy gamma rays.

A separate collaboration focused on gamma rays, called VERITAS, of which Abeysekara and Dave Kieda, dean of the U’s graduate school, are affiliated, made more than 70 hours of observations of SS 433 but did not detect a strong enough signal to make a definitive claim. HAWC’s measurements cleared the threshold for scientists to claim that they had truly observed gamma rays. In July, Kieda, Abeysekara and the VERITAS array published a paper confirming high-energy gamma ray emissions from a supermassive black hole located in a distant galaxy.

“SS 433 is right in our neighborhood, and so, using HAWC’s unique wide field of view, we were able to resolve both microquasar particle acceleration sites,” said Jordan Goodman, a distinguished professor at the University of Maryland and United States lead investigator and spokesperson for the HAWC collaboration.

Wherever they originate, gamma rays travel in a straight line to their destination. The ones that arrive at Earth collide with molecules in our

atmosphere, creating new particles. Each particle then creates more material, resulting in a particle shower as the signal cascades toward the ground.

HAWC, located roughly 13,500 feet above sea level near the Sierra Negra volcano in Mexico, is perfectly situated to catch the fast-moving rain of particles. The detector is composed of 300 tanks of water, each of which is about 24 feet in diameter, and collects data 24 hours a day, seven days a week. When the particles strike the water, they produce a shock wave of blue light, called Cherenkov radiation. Special cameras in the tanks detect this light, allowing scientists to compile the origin story of the gamma rays. Kieda, Abeysekara, co-author Robert Wayne Springer, and two graduate students, Ahron Barber and co-author Michael Newbold in the Department of Physics & Astronomy at the U, have been with the HAWC collaboration from its beginnings. Abeysekara designed and developed the GPS timing and central control system of the instrument that ensure all parts of the instrument work nearly simultaneously.

“We can count how fast particles in a shower hit the sensors, and based on that timing, we can get the angle, the size, and the energy of the gamma rays. Then, we can trace it back to its origin in the universe,” said Abeysekara. “All of the data take

up several hard disks each day. The fastest way to transfer data is literally by the truckload. One of our collaborators at the National Autonomous University of Mexico packs his truck with crates full of hard disks down the mountain to Mexico City.”

The HAWC collaboration examined data taken over 1,017 days and saw evidence that gamma rays were coming from the ends of the microquasar’s jets, rather than the central part of the star system. Based on their analysis, the researchers concluded that electrons in the jets attain energies that are about 1,000 times higher than can be achieved using Earth-bound particle accelerators, such as the city-sized Large Hadron Collider. The jet electrons collide with the low-energy microwave background radiation that permeates space, resulting in gamma ray emission.

“The two jets of SS 433 are the most powerful jets ever observed in our galaxy. That makes them a brighter gamma-ray source compared with other microquasars,” said Abeysekara. “We need to keep observing the sky and increasing our sensitivity to find more microquasars similar to SS 433, if they exist, to improve our understanding of the jets.”

Adapted from a press release prepared by the University of Maryland.