We’ve come a long way since gamma rays were discovered.
The late 1800s and early 1900s were a time of great scientific advances. Scientists were in the process of getting to grips with the different types of radiation. Radium played a prominent role in the experiments, including one by French scientist Paul Ulrich Villard in 1900.
Radium decays easily, and scientists had already identified alpha and beta radiation from radium samples. But Villard was able to identify a third type of penetrating radiation, so intense even a layer of lead couldn’t stop it: gamma rays.
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Now we have a gamma ray detector in space and it’s showing us how the universe is sparkling with this powerful energy.
Gamma rays are the most energetic form of light in the universe, and the sky is practically twinkling with flickering sources of gamma rays, a new animation shows. The animation includes 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 the source getting brighter and dimmer. The yellow circle shows the Sun following its apparent sinusoidal orbit relative to Earth.
The animation represents a full year of observations. Each frame in the animation represents three days. The reddish-orange band running through the center of the animation is the Milky Way’s central plane, which is a persistent gamma-ray producer.
This animation shows the frenzied activity of the gamma-ray sky during an observation year from February 2022 to February 2023. The pulsating circles represent just a subset of more than 1,500 light curves — records of how the brightness of sources change over time — recorded by the LAT were collected over almost 15 years in space. Credit: NASA Marshall Space Flight Center/Daniel Kocevski
What the image really shows us are black holes.
These pulsing lights represent, or most of them represent, supermassive black holes. 90% of these sources are so-called blazars. Blazars are active galactic cores that are essentially black holes themselves. We call them active galactic nuclei when the black hole is actively accumulating 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, explaining the expansion and contraction of the circular sources in the image.
The animation is based on an interactive library of gamma-ray sources called the Fermi LAT Light Curve Repository, maintained by an international team of astronomers. On March 15th, an article was published in the Astrophysical Journal announcing and explaining the repository, simply titled “The Fermi-LAT Lightcurve Repository”.
This is a static screenshot of the LAT repository
“We were inspired to compile this database by astronomers who wanted to study galaxies and compare visible and gamma-ray light curves over long time periods,” said Daniel Kocevski, co-author of the repository and an astrophysicist at NASA’s Marshall Space Flight Center in Huntsville, Alabama . “We received requests to process one object at a time. Now the scientific community has access to all analyzed data for the entire catalog.”
The repository contains data for 1525 gamma-ray sources, but only variables. Astrophysicists are interested in variable sources because their study has led to many important discoveries. For example, Fermi and LAT helped find the connection between blazars and neutrinos. “High utilization and long-term monitoring of the gamma-ray sky have made the Fermi Large Area Telescope a pivotal tool in the exploration of time-domain and multimessenger astronomy,” the paper says.
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 may play an important role in multimessenger astronomy. “By continuously reporting flow evolution and transition to high-flow states for many variable sources, the LCR is a valuable resource to trigger observations at other observatories,” the authors write.
The repository shows observations spanning 10 years, and there are likely more discoveries waiting to be uncovered in all of this data. “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 an intellectual maverick. He was the sole author of most of his published work and was not interested in fame. Villard was also fortunate that an inheritance freed him from teaching and he constructed his own experimental apparatus. These reasons are partly why his findings did not arouse much interest at the time. Another reason is that gamma rays didn’t really fit into the established picture 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 called gamma rays, although Villard himself never called 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 further advances in understanding gamma rays. Villard’s name has faded while his peers from the same era are better known.
It is interesting to imagine what scientists like Villard would think about the current state of science. Could he possibly have imagined that we would have an orbiting telescope measuring cosmic gamma-ray sources and linking them to supermassive black holes in distant galaxies? Could he have imagined in his wildest dreams that people would sit in front of their own computers and access data and images from space telescopes without a break or payment? Could he have imagined the role gamma rays would play in astrophysics?
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