A cosmic mystery, Part 1: Gamma Ray Bursts.

TWO COSMIC MYSTERIES. I have personally encountered two cosmic mysteries early in my career – like about 40 years ago. They both involved massive, unprecedented sources of energy. The mysteries continued for decades and occasionally the two encounters kept popping up in my memory. Only in the last few years have both these mysteries been solved – and both explanations involve black holes. This is Part 1. Part 2 will come later.

Figure 1. An Advanced Vela series satellite. Click on image to enlarge or to source, then back-arrow to return to blog article.

GAMMA RAY BURSTS. I flew into Los Alamos, New Mexico in 1972 to work at the famous scientific laboratory there (only an hour and a half drive from where I now live in Albuquerque). My job was to study data recorded by satellites (called Vela) that looped around the earth – data on high energy protons and electrons specifically. One kind of such particles are spewed out from a solar flare and are called solar cosmic rays. I spent one glorious year and several summers after that studying solar cosmic rays at Los Alamos.

My office was next to another guy named Ian, and he was trying to understand Gamma-ray bursts from unknown sources. These bursts were first recorded by Los Alamos detectors on these same Vela satellites just a few years before I arrived. Now Gamma rays are part of the light spectrum that extends from visible light to ultra-violet (UV) light to X-rays to Gamma rays as energy increases. Gamma-rays, which are high-energy X-rays, are the radiation sometimes used to treat cancer tumors.

Cosmic gamma-ray bursts came in two kinds: Short Gamma-bursts lasting less than 2 seconds, and Long Gamma-bursts last 30 seconds on average. Most (70%) of recorded Gamma-bursts are Long.

The other Ian at Los Alamos was a Scotsman of small stature who skipped around with an infectious smile on his face, always wanting to talk about the source of these enigmatic Gamma-ray bursts that lasted less than two seconds. These bursts came to be recognized as some of the highest-energy explosions known in the universe, but the actual mechanism that produces the Short Gamma-ray bursts was unknown until 47 years after my encounter with them at Los Alamos.

In 2017, two neutron stars collided and a gamma-ray burst was recorded on earth, as well as other emissions like X-rays, UV light, and visible light. Click HERE to read my previous blog on this topic.

Figure 2. An artist’s rendering of the merger of two neutron stars on 17 August 2017. Source: Robin Dienel, The Carnegie Institution for Science. Click on image to source.

NEUTRON STARS. A neutron star is an amazing thing. When a star like our sun runs out of nuclear fuel, it starts to collapse. The gravity force becomes so large that it crushes atoms like hydrogen and helium into nuclear proportions, and the end result is a gigantic “nucleus” made up only of neutrons. The density of a neutron star is so great that a teaspoon weighs a billion tons. So if two neutron stars collide, which is rare, the result is a super-massive explosion that can generate a Short Gamma-ray burst.

All of the above describes Short Gamma-ray bursts (lasting less than 2 seconds). Long Gamma-ray bursts (lasting on average 30 seconds) have a different source which is related to black holes. Hard to believe, but a black hole is even more mysterious than a neutron star.

BLACK HOLES. The following is adapted from Black Holes 101.
Black holes are among the most fascinating objects in our universe. A black hole is a region in space where the force of gravity is so strong that nothing can escape from it – not even light – the fastest thing known. This is why it’s called a black hole.

Figure 3. Illustration of the black hole named Cygnus X-1 formed when a large star caved in. This black hole pulls matter from the blue star beside it. Credits: NASA/CXC/M.Weiss. Click on image to source.

STELLAR BLACK HOLES form when massive stars die and collapse. They’re roughly 10 – 20 times the mass of our Sun. They’re scattered throughout the universe and there could be millions of these stellar black holes just in our own galaxy, the Milky Way.

Black holes remained largely unknown until the 20th century. In California, in 1938, J. Robert Oppenheimer and his students were creating the modern theory of black holes. Curiously, Oppenheimer was later appointed head of the Los Alamos Laboratory that built the first atomic bomb in the 1940s. But it wasn’t until 1971, when theory became reality. Astronomers, studying the constellation Cygnus, discovered the first black hole. This was just one year before I joined Los Alamos Laboratory.

The outside boundary of a black hole is called the event horizon, a point of no return, inside of which we cannot see anything. Once something crosses the event horizon, it collapses into the black hole’s singularity: an infinitely small, infinitely dense point where space, time, and the laws of physics no longer apply. Wow!

LONG GAMMA-RAY BURSTS. Part of the following is adapted from Wikipedia HERE. These have been linked in many cases to the collapse of a supernova to a black hole. When nuclear fusion no longer generates enough pressure to counteract gravity, a massive star explodes outward (see fuzzy orange boundary in Figure 4). At the same time, gravity causes part of the star to implode and rapidly collapse to form a black hole. It’s as if it crushes down to a neutron star first but gravity is so strong that the crushing continues down to nothing – what’s called a singularity. This is very hard to imagine and even harder to understand.

During the imploding collapse, energy may be released along the axis of rotation to form a Gamma-ray burst (see two green jets in Figure 4).

Figure 4. Artist’s illustration of a Long Gamma-ray burst occurring in an exploding supernova. Energy from the crushed black hole is beamed into two narrow, oppositely directed jets of Gamma-rays. Click on image to enlarge or to source.

Gamma-ray bursts are very energetic (bright) as observed from Earth despite their typically immense distances. This combination of energy and distance implies an extremely energetic source.

Gamma-ray bursts are thought to be highly focused explosions, with most of the explosion energy collimated into a narrow jet (Figure 4). Because their energy is strongly focused, the Gamma-rays emitted by most bursts are expected to miss the Earth and never be detected.

The energy of a Gamma-ray burst is comparable to the energy released in a bright supernova (the outward explosion of a massive star). In fact, very bright supernovae have been observed to accompany several of the nearest Gamma-ray bursts.

THIS IS THE END OF PART 1. I hope this brings a new sense of wonder about the universe. Human brains, so small and puny, were able to measure Gamma-ray bursts and understand their origin – although it took 50 years. I was impressed, all those years ago, by the thoughtful scientists at Los Alamos who discovered the Gamma-ray bursts. But now I’m absolutely enchanted by the explanation in terms of  some of the most fantastic objects in the cosmos: neutron stars and black holes. Wow!

But it gets better… see Part 2, coming soon.

POST-SCRIPT: this week the Science Channel on TV has been running episodes of How the Universe Works. At least two of these episodes were about black holes. My eyes were glued to the screen.

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The heavens declare the glory of God, and the firmament proclaims His handiwork. Day after day pours forth speech, and night after night shows forth knowledge. There is no speech nor spoken word [from the stars]; their voice is not heard; yet their voice [in evidence] goes out through all the earth, their sayings to the end of the world.        [Book of Psalms, chapter 19].