Clemson physicists, international team unveil map of the universe’s most powerful marvels
CLEMSON, South Carolina — If gamma rays were visible to the naked eye, the night sky would look more like a series of blinding explosions than starry constellations.
In a new catalog of the universe, a team of international physicists has documented exactly where these high-energy gamma-ray sources exist. Led by Clemson University physics and astronomy professor Marco Ajello and former Clemson postdoctoral scholar Alberto Domìnguez – who is now at Universidad Complutense de Madrid in Spain – the data gained from the catalog will open doors for astrophysical studies that weren’t realized before their release.
The catalog was published on Sept. 27 in The Astrophysical Journal Supplement Series. Known as 3FHL, it is the result of an ongoing mission of the Large Area Telescope (LAT), an instrument on board the Fermi Gamma-Ray Space Telescope. Launched in 2008 by NASA, the spacecraft has been collecting data on high-energy astrophysical objects and their environments. In the new catalog, 1,556 sources of extreme energy are documented, 214 of which were previously undiscovered.
The most powerful and energetic of all forms of light, gamma rays are the signature burst of extreme, violent energy released near black holes or neutron stars, or with the breakdown of stars. Generated by the interaction of swiftly moving particles, gamma rays prove that such forceful objects are able to accelerate particles near the speed of light.
“At these extreme energies, the universe is not very well known,” Ajello said. “We know that our galaxy is able to accelerate particles in its environment. We see them swirling around all the time, and they hit other particles and gases, creating gamma-ray emission. But because of the Fermi-LAT telescope, we now have the deepest census of the universe for one of the very first times.”
For this new catalog, the telescope detects energy sources above 10 billion electron-volts, which is 10 billion times the energy of the light we can see with our own eyes. Every three hours, Fermi-LAT takes an image of the sky. To make the 3FHL catalog, those images were compiled into one large projection.
“It’s like if you took the surface of the Earth and you open it on a sheet of paper. You get the same kind of projection if you look at a sphere in the sky,” Ajello said. “Pointed toward the center of our own galaxy, we get this disc. All of these red and yellow emissions are very bright sites of gamma rays picked up by the telescope.”
At the center of the projection — and, therefore, the center of our galaxy — are two large bubbles in the shape of a figure-eight. Discovered in 2010, these “Fermi bubbles” are colossal balloons of high-energy gamma rays, believed to be leftovers from the enormous feast that supermassive black hole, Sagittarius A, had on gas millions of years ago — a gas that was thousands of times more massive than the sun.
Other emissions found in the catalog can be divided into two categories: gamma-ray sources belonging to our galaxy and gamma sources that don’t.
“In our galaxy, we believe most sources of particle acceleration occur every time you have a supernova explosion,” Ajello said. “In a supernova, a massive star collapses under the force of its own gravity and explodes, launching stellar material at incredible speeds. It’s very complicated, but supernovae leave behind neutron stars or black holes, and they also drive a shock to the interstellar medium, the place where particle acceleration happens.”
For the high-energy gamma sources belonging to other galaxies, the small dots on the 3FHL catalog will have to be studied.
“Typically, these are gamma ray accelerators, called blazars, which are pretty fancy objects,” Ajello said. “They are supermassive black holes at the center of galaxies and they’re able to accelerate particles in narrow beams, called jets. We see gamma-ray emission when the jets are pointing at us, like looking down the barrel of a gun.”
The jets can extend for large distances that are light-years away from their galactic centers. Though not fully understood, physicists believe that a blazar’s magnetic field can maintain the jets’ trajectory until they inevitably collide with the intergalactic medium, crashing back in on themselves.
“It’s like if you were shooting water from a hose, and the water was just so fast that it could reach the stratosphere. But if you spray a hose against a wall, you get some spray-back,” Ajello said. “How this happens, we aren’t sure yet, but we believe the supermassive black hole is rotating, dragging its own magnetic field that’s treading space and collimating [aligning] the jets. So the particles are accelerated and streamed away along a path.”
More than 80 percent of the newly discovered gamma sources are supermassive black holes, like blazars, some of which are so far from the Milky Way that their light has taken 11 billion years to reach us.
It’s difficult to know with certainty whether every high-energy gamma ray source that exists has been found by the Fermi-LAT satellite, simply because new telescopes can always be built with more sensitivity than existing ones. The first catalog of this kind was based on the Energetic Gamma-Ray Experiment Telescope (EGRET) atop NASA’s Compton Gamma Ray Observatory satellite, which ran from 1991 to 2000. For comparison, the Fermi-LAT spacecraft can detect more overall sources of gamma rays than the EGRET mission could record in photons, which are light particles.
Because the catalog is an image of the entire universe, it provides a map for physicists to focus their out-of-this-world research.
One experiment, in particular, is the Cherenkov Telescope Array (CTA), slated to begin in 2018. Using a fleet of telescopes from the ground, the experiment will aim to study some of these high-energy gamma sources in the universe.
“Telescopes on Earth offer a very narrow field of view of a very small portion of the sky,” Ajello said. “You need to know where to point the telescopes, because otherwise you wouldn’t see anything. Now, the CTA team has over 1,500 places in the sky where they can point their telescopes to study these objects in detail. Without our catalog, they wouldn’t be able to effectively understand how these objects work.”
But the catalog makes other revelations, too, by detailing the exact place in the universe where light is no longer visible.
“There is a pretty well understood phenomenon in astrophysics, called pair creation, where if you have two particles of light – two photons – they can hit each other, create two new particles, called an electron and positron, and then the photons can destroy themselves,” Ajello said.
The Einstein theory is only possible if gamma rays interact with the stellar photons of stars, because these two particles contain enough energy to create two new, massive particles. Luckily, there is an abundance of stellar photons and gamma rays throughout the universe. However, there is also a region of space where these high-energy emissions are no longer perceivable – a point called the horizon.
“The gamma-ray light beyond the horizon does exist, but the photons are absorbed as they travel toward us. We can think of it as if there’s a gas between us and these objects, and if the gas becomes too dense, we won’t be able to see beyond it,” Ajello said. “The difference in this case is that the gas that absorbs light is also made of light. So if we could measure how much light there is between us and one of these gamma sources, we could use it to understand the formation of stars across the universe.”
The Fermi-LAT team is taking a step toward that understanding, because for the first time, the 3FHL catalog has pinpointed exactly where the gamma ray horizon lies.
Ajello and his researchers at Clemson will continue examining the gamma-ray sources they have discovered – supernovae, blazars and other cosmic accelerators – as well as the gamma ray horizon.
“We are interested in the sources that are able to accelerate particles at the largest energies. Some accelerators are very good, but others are spectacular – and we want to understand the spectacular ones,” Ajello said. “How do they do it? How can they reach such large energies? Is it a property of the magnetic field configuration?”