In its first year of operation, not only did Fermi re-draw maps of the gamma-ray sky in dazzling detail, the telescope also collected evidence in favor of Einstein's view that the fabric of space-time is predominantly smooth and continuous. Fermi captured the photo-finish of two photons racing away from the same gamma-ray burst—with one photon (purple) carrying a million times the energy of the other (yellow). The fact that the photons arrived at essentially the same time despite their energy disparity ruled out some versions of quantum gravity, an alternative to Einstein's theory of general relativity in which space-time is "frothy," and therefore more likely to delay the higher-energy photons.
Measuring all the light from all the stars that have ever shone seems an impossible task, like counting all the grains of sand in all the deserts or emptying the world's oceans, one drop at a time. Yet data from Fermi has enabled astrophysicists to do just that. This starlight, called extragalactic background light, or EBL, is like a vast fog filling the entire universe. Occasionally a gamma ray will interact with an EBL photon (the brilliant magenta spot at right), creating an electron–positron pair and taking that gamma ray out of commission. Astrophysicists compared gamma rays from blazars at several different distances, and from the way the rays from more distant blazars were dimmed, the scientists were able to estimate how much intervening EBL fog they had crossed.
On April 27, 2013, Fermi took part in a historic opportunity to study a gamma-ray burst of epic proportions. The majority of gamma-ray bursts, which are the most brilliant explosions the universe can cook up, are thought to signal the death of super-massive stars. The star observed in April did not want to go gently into its good night. Fermi's Large Area Telescope, or LAT, recorded one gamma ray from the burst with an energy of at least 94 billion electronvolts—about three times more energy than the LAT's previous record. Emissions in the billion-electronvolt range lasted for hours, and the burst shone in the LAT's sights for the better part of a day, setting a new record for the longest gamma-ray emission from a gamma-ray burst.
Blazars are another type of strong gamma-ray source catalogued by Fermi. They are one example of a phenomenon called active galactic nuclei—huge black holes in the centers of distant galaxies that swirl up dust and gas like cotton candy around a stick. In the case of blazars, the trapped matter heats up due to friction until some of it is blasted back out in two powerful jets, one aimed in our direction. Fermi became especially familiar with a blazar 7.2 million light years away, which, for a time, became the brightest object in the gamma-ray sky, as seen in the lower left quadrant of this Fermi all-sky map. The blazar, known as 3C 454.3, flares occasionally, although scientists still don't know why.
Wimpy-sized galaxies helped Fermi place more precise limits on WIMPy-type matter. By looking at 10 dwarf galaxies that orbit the Milky Way, including the one shown here, Fermi was able to offer insight into Weakly Interacting Massive Particles, or WIMPs, a prime suspect in the hunt for dark matter. Scientists used Fermi data to look for certain theorized types of WIMPs: ones that can't interact with normal matter but can interact with themselves to produce gamma rays. Studying the tiny, quiet dwarf galaxies—which don’t emit any other bright sources of gamma rays that could drown out a dark-matter signal—the researchers were able to place constraints on four different ways that WIMPs could produce gamma-rays. This can be used to limit the minimum mass of the dark matter particle.
Sixth on the list of Fermi's greatest hits but quite possibly first on the list of Fermi's biggest surprises is the portrait it painted of the Milky Way blowing two gamma-ray bubbles, one above and one below our galaxy's center (magenta). Each bubble is 25,000 light-years tall, and the whole structure may be only a few million years old. Energetic electrons inside the bubbles interact with lower-energy light to create the gamma rays, but the source of the electrons themselves remains a mystery. In another way, the bubbles represent one of Fermi's greatest strengths: The telescope's data are available to anyone who requests them, and these bubbles were discovered by a small team of Harvard scientists who took NASA up on the offer.
In 2010, scientists announced Fermi had detected gamma rays streaming from supernova remnants such as W44, shown here, long after the initial gamma-ray bursts heralding the deaths of these giant stars had faded away. The researchers suspect the gamma rays were created by the interaction of cosmic rays in the form of accelerated protons with particles from nearby gas clouds. This would not only explain the gamma rays, but would also pin down the origin of cosmic rays occurring within our galaxy.
Early this year scientists announced that they had found the characteristic signature of a cosmic-ray proton interacting with a much slower-moving proton in the cloud of dust and gas near two supernova remnants, including the Cassiopeia A supernova remnant, shown here. This provided undeniable proof, 100 years after cosmic rays were first discovered, that at least some of the cosmic rays originating within our galaxy are made up of particles boosted to high energies in the natural particle accelerator that is the chaotic environment of a supernova remnant.
The Crab Nebula, a supernova remnant long considered a steady, sedate member of the Milky Way neighborhood, shocked scientists when Fermi detected a gamma-ray flare five times stronger than any previously seen originating from the neutron star at the heart of the nebula. The Crab is one of the most studied objects in the night sky; Chinese astronomers first noted it in 1054, and it's also known as M1, the first object the comet-hunter Charles Messier listed in his catalogue of "what not to mistake for comets" in the late 1700s. The Fermi data caused scientists the world over to rethink the evolutionary path of the star at the heart of the Crab and other pulsars like it.
Fermi has detected more than 100 pulsars, the super-dense neutron stars born in the death throes of massive stars. These neutron stars spin incredibly quickly, with rotation rates ranging from less than 10 seconds to mere milliseconds, and have intense magnetic fields that focus their radiation into powerful beams. If the beams are oriented in such a way that they sweep over the Earth as the neutron star rotates, they gain the moniker “pulsar.” Fermi has catalogued an intriguing variety of pulsars during its five years, including pulsars that are only detectable in gamma rays, surprisingly powerful millisecond pulsars and pulsars that are members of binary systems.
Twice every 3.4 years, a fast-spinning neutron star known as pulsar B1259-63 dives through the disk of gas encircling the massive blue star it orbits, generating gamma rays as it goes. Fermi watched both trips through the disk during the most recent event, observing something quite surprising: The gamma rays flared much brighter on the trip back out, as shown by the intense magenta in the bottom foreground of this image. The question, still not answered, is why. With Fermi's mission planned for another five years, the gamma-ray telescope will definitely be watching during the pulsar's next trip through the giant star's dusty disk in 2014.
Some points of gamma-ray light detected by Fermi remain mysterious, and scientists are hard at work trying to match these sources with actual physical objects. Data from Fermi led scientists to ten more "black widow" pulsars—very old neutron stars that, after having spun down almost to a stop, gained new life by drawing matter from their companion stars and using the angular momentum of that matter to start spinning up again. What makes a black widow particularly vicious is that once it starts up for its second incarnation, it emits beams of energy that blast across the companion star. What a black widow doesn’t eat, it blows away. In this image, one beam from the tiny pulsar is about to strike its companion star, superheating the star's gas and causing it to boil away.
Fermi has revealed that a star-forming region located in the constellation Cygnus is a sort of cosmic pinball table for the fast-moving particles called cosmic rays. Cosmic rays are generally impossible to trace back to any one location because they are continually deflected by magnetic fields. Yet if they collide with interstellar gas, they produce gamma rays, which aren't affected by magnetic fields. The Cygnus star-forming region contains plenty of young, energetic stars with strong magnetic fields to waylay cosmic rays and enough gas and dust to create gamma rays, some of which have made a beeline for Fermi. The locations of gamma-ray production are circled in this image.
In addition to the stellar job Fermi has done observing some of the most distant, most powerful objects in the universe, the telescope has recently started moonlighting as another kind of observatory: a solar observatory. This image from March 7, 2012 shows the sun erupting in a solar flare so powerful that, for a short time, it was brightest object in the gamma-ray sky. The flare broke several records, including the highest-energy light ever detected during or just after a flare and the longest-lasting gamma-ray emissions from a flare. Fermi still has the boundaries of the cosmos to push back, but, during its next five years, it will be sure to keep an eye on the sun as well.
The Greek god Zeus is known for hurling lightning bolts down to Earth from the heights of Mount Olympus. Fermi has discovered that lightning bolts hurl antimatter back up into the sky. The antimatter comes in the form of positrons, which are paired with their matter twins, electrons. Scientists believe the matter-antimatter pairs are generated by thunderstorms in a two-step process: First, electrons trapped within the strong electric field at the tops of thunderclouds are driven upward by the field until they reach nearly the speed of light—fast enough to emit gamma rays, called terrestrial gamma-ray flashes, when they're deflected by air molecules. Then the gamma rays transform into the electron-positron pairs that Fermi detects, shown here in magenta.
First Fermi helped prove terrestrial gamma-flashes created antimatter. Then Fermi—or more specifically the Gamma-ray Burst Monitor onboard Fermi—helped show that terrestrial gamma-flashes also generate a strong burst of radio waves. Previously, these radio waves were associated with the lightning coming from the same thunderstorms generating the terrestrial gamma-flashes. Clearing up that misconception will help scientists understand what's really happening.