We’ve come a long way since gamma rays were discovered.
The late 1800s and early 1900s were a time of great scientific advancements. Scientists were just getting a handle on the different types of radiation. Radium featured prominently in the experiments, including one by French scientist Paul Ulrich Villard in 1900.
Radium decays readily, and scientists had already identified alpha and beta radiation coming from radium samples. But Villard was able to identify a third type of penetrating radiation so powerful even a layer of lead couldn’t stop it: gamma rays.
Now we have a gamma ray detector in space, and it’s showing us how the Universe sparkles with this powerful energy.
Gamma rays are the most energetic form of light in the Universe, and as a new animation shows, the sky practically sparkles with flickering gamma-ray sources. The animation contains a year’s worth of observations from the Large Area Telescope (LAT) on NASA’s Fermi Gamma-ray Space Telescope. Each yellow circle is a gamma-ray source, and the expansion and contraction show how the source brightens and dims. The yellow circle is the Sun’s following its seemingly sinusoidal path relative to Earth.
The animation represents an entire year of observations. Each frame in the animation represents three days. The reddish-orange band that runs through the middle of the animation is the Milky Way’s central plane, which is a consistent gamma-ray producer.
This animation shows the gamma-ray sky’s frenzied activity during a year of observations from February 2022 to February 2023. The pulsing circles represent just a subset of more than 1,500 light curves – records of how sources change in brightness over time – collected by the LAT over nearly 15 years in space. Credit: NASA’s Marshall Space Flight Center/Daniel Kocevski
What the image really shows us is black holes.
These pulsing lights represent supermassive black holes, or most of them do. 90% of these sources are what are called blazars. Blazars are active galactic nuclei, which themselves are basically black holes. We call them active galactic nuclei when the black hole is actively accreting matter and emitting relativistic jets. When the jets are aimed at Earth, we call them blazars. Blazars are the most luminous and energetic objects in the Universe. They emit gamma-ray photons and are highly variable in luminosity, which explains the expansion and contraction of the circle sources in the image.
The animation is based on an interactive library of gamma-ray sources called the Fermi LAT Light Curve Repository that’s maintained by an international team of astronomers. A paper announcing and explaining the repository was published in the Astrophysical Journal on March 15th, titled simply “The Fermi-LAT Lightcurve Repository.“
This is a static screen grab of the LAT repository <click image to visit.> User can double-click on a source to bring up more information. Image Credit: NASA/Goddard Space Flight Center.
“We were inspired to put this database together by astronomers who study galaxies and wanted to compare visible and gamma-ray light curves over long time scales,” said Daniel Kocevski, a repository co-author and an astrophysicist at NASA’s Marshall Space Flight Center in Huntsville, Alabama. “We were getting requests to process one object at a time. Now the scientific community has access to all the analyzed data for the whole catalogue.”
The repository contains data for 1525 gamma-ray sources, but only variable ones. Astrophysicists are interested in variable sources because studying them has led to many important discoveries. Fermi and LAT helped find the link between blazars and neutrinos, for example. “A high duty cycle and long-term monitoring of the gamma-ray sky has made the Fermi Large Area Telescope a pivotal tool in the study of time-domain and multimessenger astronomy,” the paper states.
Multimessenger astronomy is the combined study of energy, particles, and gravitational waves in the cosmos. By identifying variable gamma-ray sources in the cosmos, the repository can play an important role in multimessenger astronomy. “By continuously reporting the flux evolution and transition to high-flux states for many variable sources, the LCR is a valuable resource for triggering observations at other observatories,” the authors write.
The repository shows 10 years of observations, and there are likely more discoveries waiting to be uncovered in all that data. “Having the historical light curve database could lead to new multimessenger insights into past events,” said paper co-author Michela Negro, an astrophysicist at the University of Maryland and NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
Paul Villard, the French scientist who discovered gamma rays, was a bit of an intellectual loner. He was the sole author of most of his published papers and was unconcerned with fame. Villard was also fortunate that an inheritance liberated him from the need to teach, and he constructed his experimental equipment himself. These reasons are partly why his results didn’t generate a ton of interest at the time. Another reason is that gamma rays didn’t really fit into the established view of radiation and particles.
After Villard published his two papers on gamma rays in 1900, he stopped studying them. Several years passed before Villard’s discovery was named gamma rays, though Villard himself never named them that. In 1903, New Zealand physicist Ernest Rutherford named them gamma rays, and the name stuck. In the years that followed, other researchers made more progress in understanding gamma rays. Villard’s name has faded away, while his fellow scientists from the same time are more well-known.
It’s interesting to imagine what scientists like Villard would think of the current state of science. Could he possibly have imagined that we would have an orbiting telescope that measured cosmic gamma-ray sources and tied them to supermassive black holes in distant galaxies? Could he have guessed in his wildest dreams that people would sit in front of their own computers and access data and images from space telescopes without pause or payment? Could he have envisioned the role gamma rays would play in astrophysics?