As my colleagues and I began working on a century-old cosmic mystery, we found an unexpected astronomical laboratory in Terzan 5, a dense star cluster that is currently passing through our galaxy at very high speeds.
This stellar oddity has given us the opportunity to study the behaviour of cosmic rays – high-energy particles whose irregular paths in space have puzzled astronomers since their discovery in 1912.
By observing the radiation produced by Terzan 5’s cosmic rays, we have achieved a scientific first: measuring how quickly these particles change direction due to fluctuations in interstellar magnetic fields. Our research is published today in Nature Astronomy.
high speed radiation from outer space
Cosmic rays are something no one expected to exist. When radioactivity was first discovered in the 1890s, scientists thought all sources of radiation were on Earth.
But in 1912, Austrian-American physicist Victor Hess measured ambient radiation levels in a high-altitude balloon and found that it was much higher than at ground level, even during eclipses when the sun was blocked. This meant that the radiation was coming from space.
Today we know the mysterious radiation discovered by Hess as cosmic rays: atomic nuclei and elementary particles such as protons and electrons that have somehow been accelerated to close to the speed of light. These particles travel through interstellar space, and because of their high energy a small fraction of them can penetrate the upper atmosphere, as Hess found.
But we can’t easily tell where they come from. Cosmic rays are charged particles, which means their direction of travel changes when they encounter a magnetic field.
Static diagram of the cosmic ray universe
The magnetic deflection effect provides the basic technology for old cathode ray tube (CRT) monitors and televisions, which use it to direct electrons toward the screen to produce pictures. Interstellar space is filled with magnetic fields, and those fields are constantly fluctuating, deflecting cosmic rays in random directions — much like a broken CRT in an old TV that only shows static.
So cosmic rays spread out roughly evenly across the galaxy, rather than coming straight to us from their source like light. Here on Earth we see them coming roughly evenly from all directions in the sky.
Although we now understand this general picture, many of the details are missing. The uniformity of cosmic rays across the sky tells us that the directions of cosmic rays change randomly, but we have no good way of measuring how quickly this process occurs.
Nor do we understand the ultimate source of the magnetic fluctuations. Or we did not understand it until now.
Terzan 5 and displaced gamma rays
This is where Terzan 5 comes in. This star cluster is a prolific producer of cosmic rays, as it contains a large population of rapidly spinning, incredibly dense and magnetic stars called millisecond pulsars – which accelerate cosmic rays to extremely high speeds.
These cosmic rays do not reach Earth, this is because of fluctuations in magnetic fields. However, we can see a clear sign of their presence: some cosmic rays collide with photons of starlight and convert them into high-energy ones not imposed particles called gamma rays.
Gamma rays travel in the same direction as the cosmic rays that create them, but unlike cosmic rays, gamma rays are not deflected by magnetic fields. They can travel in a straight line and reach Earth.
Because of this effect, we often see gamma rays coming from powerful sources of cosmic rays. But in Terzan 5, for some reason the gamma rays do not align exactly with the positions of the stars. Instead, they appear to come from a region about 30 light-years away, where there is no obvious source.
A galaxy-scale ‘comet’
Ever since this displacement was discovered in 2011, it remained a subject of unexplained curiosity until we found an explanation for it.
Terzan 5 is close to the center of our galaxy today, but that wasn’t always the case. The star cluster actually moves in a very wide orbit that keeps it away from the plane of the galaxy most of the time.
Coincidentally, it is passing through the galaxy right now. Because this plunge happens at a speed of hundreds of kilometers per second, the star forms a blanket of magnetic fields around itself, like the tail of a comet passing through the solar wind.
Cosmic rays emitted by the cluster initially travel along the tail. We don’t see any gamma rays produced by these cosmic rays because the tail is not pointed directly at us – these gamma rays travel along the tail and away from us.
And this is where magnetic fluctuations come in. If the cosmic rays remained perfectly aligned with the tail, we would never see them, but magnetic fluctuations cause their directions to change.
Eventually, some of them start pointing toward us, emitting gamma rays that we can see. But this takes about 30 years, which is why the gamma rays don’t come from the cluster itself.
By the time enough of those stars come toward us and their gamma-ray bursts become bright enough for us to see them, they are already about 30 light-years away from the cluster’s magnetic tail.
Cosmic rays and interstellar magnetic fields
So thanks to Terzan 5, for the first time we have been able to measure how long it takes for magnetic fluctuations to change the direction of cosmic rays. We can use this information to test theories about how interstellar magnetic fields work and where their fluctuations come from.
This brings us closer to understanding the mysterious radiation coming from space, discovered by Hess more than 100 years ago.
,Author: Mark Krumholz, Professor, Astronomy and Astrophysics Research School, Australian National University)
,disclosure statement: Mark Krumholz receives funding from the Australian Research Council and supercomputer time from the National Computational Infrastructure (Australia), Pawsey Supercomputing Centre (Australia) and Oak Ridge Leadership Computing Facility (USA).
This article is republished from The Conversation under a Creative Commons license. Read the original article.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)
