A NASA illustration of a neutron star surrounded by its accretion disk. A new study suggests that gamma-ray bursts from colliding neutron stars could release deadly radiation at a far wider angle than previously thought.
Gamma-ray bursts are among the most powerful events in the universe, ignited when stars die in massive explosions or when they merge in … massive explosions.
As these violent cosmic explosions occur, they act like cosmic lighthouses, releasing beams of some of the brightest light in the universe, along with a flood of neutrinos, those wispy, ghost-like particles that slip through the universe almost entirely undetected.
Clearly, you would not want to be exposed to one of these deadly, DNA-frying energy bursts. But physicists used to think gamma-ray bursts were dangerous only if you were in the narrow path of one of the jets coming from the explosion. Unfortunately, a new study updated on the arXiv database Nov. 29 (but not yet peer-reviewed) suggests that these eruptions are bad news all around and may send deadly rays at a far wider angle than previously thought.
Cosmic gamma-ray factories
Over the decades, astronomers have identified two kinds of celestial gamma-ray bursts (called GRBs for short): long ones lasting more than 2 seconds (up to several minutes) and short ones lasting less than 2 seconds. We’re not exactly sure what causes GRBs out in space, but it’s thought that the long ones are produced when the largest stars in our universe die off in supernova explosions, leaving behind neutron stars or black holes. A cataclysmic death like that releases blindingly huge amounts of energy in a relative flash, and voila! Gamma-ray bursts.
The short GRBs, on the other hand, are thought to originate from a completely different mechanism: the merger of two neutron stars. These events aren’t nearly as powerful as their supernova cousins, but they wreak enough havoc locally to produce a flash of gamma-rays.
Inside a jet engine
Still, when neutron stars collide, it’s an ugly thing. Each neutron star weighs several times the mass of Earth’s sun, but that mass is compressed into a sphere no wider than a typical city. At the moment of impact between two such objects, they are ferociously orbiting each other at a healthy fraction of the speed of light.
Next, the neutron stars merge to form either a larger neutron star or, if conditions are right, a black hole, leaving behind a trail of destruction and debris from the preceding cataclysm. This ring of matter collapses onto the corpse of the former neutron star, forming what’s known as an accretion disk. In the case of a newly-formed black hole, this disk feeds the monster at the heart of the pile of wreckage at a rate of up to a few suns’ worth of gas per second.
With all the energy and material swirling around and pouring into the center of the system, a complicated (and poorly understood) dance of electric and magnetic forces winds up material and launches jets of that matter up and away from the core, along the spin axis of the central object and into the surrounding system. If those jets break through, they appear as giant, brief searchlights racing away from the collision. And when those searchlights happen to point at Earth, we get a pulse of gamma-rays.
But those jets are relatively narrow, and as long as you don’t see the GRB head-on, it shouldn’t be that dangerous, right? Not so fast.
It turns out that jets form and move away from the site of the neutron star merger in a messy, complicated way. Gas clouds twist and tangle up on each other, and the flows of radiation and material away from the central black hole don’t come in a neat and orderly line.
The result is utter, destructive chaos.
In the new study, a pair of astrophysicists explored the details of these systems after the collision event. The researchers paid close attention to the behavior of massive clouds of gas as they trip over themselves in the stampede powered by the escaping jets.
Sometimes, these gas clouds collide with each other, forming shock waves that can accelerate and power their own sets of radiation and high-energy particles, known as cosmic rays. These rays, made up of protons and other heavy nuclei, get enough energy to accelerate to nearly the speed of light, so they can temporarily merge to produce exotic and rare combinations of particles, like pions.
The pions then quickly decay into showers of neutrinos, tiny particles that flood the universe but hardly ever interact with other matter. And because these neutrinos are produced outside of the narrow region of the jet blasting away from the GRB itself, they can be seen even when we don’t get the full blast of gamma-rays.
The neutrinos themselves are a sign that ferocious, deadly nuclear reactions are happening farther away from the center of the jets. We don’t yet know exactly how far the danger zone extends, but better safe than sorry.
So, in summary: Just don’t go anywhere near colliding neutron stars.
Paul M. Sutter is an astrophysicist at The Ohio State University, host of Ask a Spaceman and Space Radio, and author of Your Place in the Universe.