by Most of the light that passes through the Universe is invisible to human eyes. Beyond the average wavelengths that we can see, there is a whole cosmos that shines in high and low energy radiation.
But we humans are smart little animals and have managed to build instruments that can see the light that we can’t. One of them is NASA’s Fermi Gamma-Ray Space Telescope, an observatory suspended in low Earth orbit, monitoring the skies for gamma rays, the most energetic light in the universe.
Fermi continuously monitors the entire sky, observing gamma ray sources and how they change over time, providing astronomers with a map of the different gamma ray producers we can detect. This data is compiled into a catalog that scientists can use to probe the production of gamma radiation.
The animation depicts a year of fluctuating gamma radiation from 1,525 sources, represented by pulsing purple circles, collected between February 2022 and February 2023, with each image representing three days of observations. The larger the circle, the brighter the gamma radiation.
The yellow circle, meanwhile, represents the apparent trajectory of the Sun in the sky for this period.
“We were inspired to build this database by astronomers who study galaxies and wanted to compare visible and gamma light curves over long timescales,” says astrophysicist Daniel Kocevski of NASA’s Marshall Space Flight Center in Huntsville.
“We were receiving requests to process one object at a time. Now the scientific community has access to all the analyzed data for the entire catalog.”
Most of the flashing lights you see are from a type of galaxy known as blazars. It is a subset of quasar galaxies. A quasar is a galaxy with an extremely active core, which means the supermassive black hole is swallowing up matter at a breakneck pace. This material is heated by the extreme activity around the black hole, so it resonates through space. Quasars emit the brightest light in the Universe.
Some of these quasars have plasma jets launched from the galactic core. As the black hole feeds, some of the matter swirling around it is deflected and accelerated along magnetic field lines outside the event horizon. When it reaches the poles, this material is launched into space at high speed, often approaching the speed of light in a vacuum.
A blazar is a quasar whose jet is pointed towards or almost towards the Earth. Due to this orientation, the light appears even brighter across the spectrum. Blazars are known sources of gamma radiation, but their light fluctuates on fairly short time scales; their fluctuations can help astronomers study how these giants feed.
Combined with other data, it can also help answer questions about the Universe. For example, it is only recently that neutrino detections by observatories such as IceCube in Antarctica have been traced back to blazar galaxies.
Blazars account for over 90% of gamma ray sources in the new addition to Fermi’s gamma ray catalog. Other objects that emit gamma radiation include a type of neutron star called pulsars, the remains of ragged material left over from supernova explosions, and binary systems such as binary neutron stars.
And there’s the gamma-ray glow from the plane of the Milky Way galaxy, represented in the animation by a blobby orange band extending down the center of the image. There, a brighter color represents a more radiant glow.
Long-period observations will hopefully lead to a better understanding of some of the phenomena associated with gamma-ray sources. For example, tracing a neutrino to a brighter period of blazar activity could help narrow down the processes that produce these mysterious particles.
“Having the historical lightcurve database,” says astrophysicist Michela Negro of the University of Maryland, Baltimore County and NASA’s Goddard Space Flight Center, “could lead to new multimessenger information about past events”.
And we have an idea of how we might see the Universe if we had alien eyes.
The recently updated catalog is available free of charge at The Astrophysical Journal Supplement Series.
