Magnetars And Gamma Ray Bursts
How magnetic fields and photons synergistically produce the highest-energy flashes in the universe
Almost a year ago, on April 15th 2020, a wave of gamma rays blasted through our solar system, having traveled 11.42 million light-years across space from a source in the Sculptor galaxy. In other words, an event that actually occurred 11.42 million years ago, hundreds of trillions of kilometers away, finally reached us. Fortunately, by the time the gamma rays got here they were sufficiently diffused that they didn’t cause us any harm. But if we’d been up close and personal, we’d have been destroyed because gamma rays are the most powerful emissions in the universe.
Let’s start with the basics: what’s a gamma ray?
Gamma rays are highly energetic photons. Photons are what we commonly call light. When we open our eyes we see the world around us because photons created in the heart of the sun reach the Earth and then bounce off objects around us. Some of those photons end up going into our eyes where they hit the cells in our retinae and this triggers nerves to send messages that our brains then interpret as shapes and colors. Our color vision goes from violet to red as the cells in our eyes react to different frequencies of light. But we see only a tiny sliver of the photons our sun emits. If we imagine, by way of analogy, an old-fashioned 12-inch ruler then photons can be as small as the smallest 1/16th mark at the very beginning, and can stretch out as long as the entire ruler. Our eyes see a spread of only about 0.0035% across that imaginary ruler, or about four-hundredths of an inch. Nearly all the photons in the universe are invisible to the human eye.
When thinking about photons, which travel at the speed of light and which have no mass, it’s important to realize that they carry energy. The shorter the wavelength of the photon, the greater the energy. Very short wavelength photons can carry a huge amount of energy. The shortest-wavelength photons of all are called gamma rays. These are highly energetic photons whose energy can be so great that single photons can damage human cells.
So now, what’s a magnetar?
A magnetar is a neutron star that generates an intense magnetic field.
Magnetism is the force that causes an iron magnet to attract iron filings and it’s what makes fridge magnets stick to the metal door. It’s the result of the way electrons line up in a material: the more electrons line up in the same way, the stronger the magnetic field. Our Earth creates a huge magnetic field because the center of the Earth is comprised of molten iron, and as it turns inside the Earth its movement creates a magnetic field. That invisible field stretches out into space and it’s what makes life possible on our planet, because the magnetic field deflects harmful charged particles and stops most of them from hitting the ground. Were it not for our magnetic field, the earliest forms of life would have been blasted to pieces by all the charged particles like electrons and protons that are emitted by our sun.
The Earth’s iron core is like a huge dynamo and it generates a strong magnetic field. But the Earth’s core is feeble compared to other kinds of dynamo in the universe. Our sun’s magnetic field is much stronger than that of the Earth. And the strongest dynamos in existence are magnetars, which create magnetic fields so powerful they make our sun’s magnetic field look totally insignificant by comparison.
So what have photons and magnetars and their magnetic fields got to do with gamma rays?
To answer these questions we have to watch a star being born, go through its life, and then die.
Imagine a huge cloud of hydrogen gas with a diameter greater than our solar system, rotating very slowly around a central point. Over hundreds of thousands of years, gravity will cause the cloud to contract toward the center point, which will rotate faster and faster as the gas contracts due to conservation of angular momentum. Eventually there will be so much hydrogen pulled into a relatively small space that gravity will form it into an almost-perfect sphere and by that time it will be rotating quite quickly, perhaps turning on its axis once every few hours.
As the individual hydrogen atoms get crushed together in the newly-formed sphere, a tiny fraction fuse to form helium and whenever this happens a little energy is released. This energy can be enough to make adjacent hydrogen atoms fuse too, which releases more energy, and so the process speeds up until the sphere of hydrogen has so much fusion going on that it emits light. We call these big rotating balls of fusing hydrogen stars. Our sun is a star. But some stars form from larger clouds of gas and so they become larger stars. Larger stars fuse their hydrogen quite rapidly and when all the hydrogen is fused into helium, the helium begins to fuse. The star can keep fusing atoms into larger and larger groups until iron is fused in the center of the star. But that’s where the game stops. Even a very large star doesn’t have enough gravity to force iron to fuse into something bigger. So as the star creates more iron, there’s less and less energy being released.
Unfortunately for the star, it was only the energy being released from all that fusion that prevented the star’s own gravity from pulling all the atoms closer together. As the star runs out of fuel, it reaches a point where the outward force of the energy released by fusion becomes less than the inward pull of gravity. Very rapidly, in just a few seconds, the star begins to shrink as its gravity pulls all the atoms much closer together. If the star is very big, it will collapse into a black hole. But if it’s not big enough, as the atoms are all crushed together they’ll reach a point where the star’s gravity isn’t strong enough to crunch them down any further. At that moment the atoms will “bounce” off the center like trillions of balls bouncing off a hard surface. Those atoms will “explode” out into space, releasing tremendous amounts of energy as they go. This event, which causes the star to shine for a few weeks more brightly than a billion suns, are called supernova.
When a supernova explodes, a lot of the star’s mass is ejected out into space where it becomes a huge fast-moving cloud of atoms and other particles. But the core of the star, now a lot less massive, remains. And that core is so densely packed together that instead of having protons and electrons and neutrons, all the matter is compressed into neutrons alone. Only in this mode can all those atoms remain so densely crunched together. A neutron star is so dense that a teaspoon of material from a neutron star would, in Earth’s gravity, weigh as much as Mount Everest.
But what has all this to do with magnetrons? Well, neutron stars aren’t just lumps of solid neutrons. Deep inside, the constituents of the neutron star form a kind of soup, and that soup can rotate inside the neutron star just like the Earth’s iron core rotates inside our own planet. That rotating internal neutron star “soup” generates a magnetic field just like the one created inside our Earth — only billions of times more powerful.
Like the Earth, a neutron star has a crusty surface, though it’s almost perfectly flat due to the immense gravity of the super-dense star. The surface is tugged and pushed and pulled by the magnetic field generated by the core. Because the magnetic field is so strong, and because the crust of the star is so dense, enormous forces can build up over time even though the diameter of the neutron star is only around 20km. Just as happens with stresses on the Earth’s crust, eventually something snaps. Here on Earth that can create an earthquake that can move land twenty or thirty feet in an instant. But on a neutron star that snapping can cause a huge shift in the surface and release massive magnetic flux lines that billow out hundreds of millions of kilometers into space. As this happens, a hugely energetic plasma shoots out into space containing trillions of fast-moving electrons and protons, the neutron star equivalent of a coronal mass ejection from our own sun.
As this stream of plasma is moving almost at the speed of light, this causes highly energetic photons to be emitted in the direction of travel. Electrons in the plasma then collide with the photons, imparting to them even more energy, which boosts those photons further up the energy scale so they become gamma rays.
It’s calculated that the energy released in this kind of gamma ray burst, which lasts a few thousands of a second, is more energy than our own sun emits in 100,000 years.
We only see these bursts when they are traveling in our direction. Sometimes we see them as flashes that repeat every few seconds, because the neutron star is spinning incredibly rapidly and so it will turn the burst in our direction for a fraction of a second as it rotates. In this way we’re exposed to pulses of radiation that recur at a regular interval.
We’ve detected gamma ray bursts intermittently over the last fifty years but until recently the instruments hit by them were overwhelmed and shut down into “safe mode” to prevent their electronics from being fried, so scientists were unable to locate the source of the bursts because attempting to triangulate to detect the source of the bursts was impossible. It’s only with recent more robust instruments and with international collaborations that the origins of such bursts are finally becoming known.
It’s probable that as our instruments continue to improve, and as astronomers become more focused on magnetars, we’ll discover that this phenomenon is quite common. And we’ll learn more about this astonishing universe in which we, for a brief moment of time, exist.