Chinese scientists’ new map of the Milky Way turns theories of cosmic radiation on its head

Kazumasa Kawata of the University of Tokyo’s Cosmic Ray Research Institute in Kashiwa, Japan, said the findings would “provide new insights into the distribution, interaction processes and origins of the highest-energy cosmic rays in our galaxy.”

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Since cosmic rays were discovered by Austrian physicist Victor Hess in 1912, scientists have built detectors in space and on Earth to search for these mysterious, powerful particles from space, Kawata wrote in the journal Physics.

The highest-energy cosmic rays ever detected carry over a quintillion electron volts—equivalent to the kinetic energy of a baseball moving at 100 miles per hour (160 km/h), or millions of times more energetic than the particles created in the largest particle collider on Earth.

Today, scientists still don’t have a clear answer to where cosmic rays actually come from. Because they consist mainly of protons, their electrically charged nature means they don’t move in a straight line, instead being deflected by the universe’s magnetic fields. This means that key information about their birthplace is lost by the time they arrive on Earth, Kawata writes.

However, if cosmic rays collide with gas clouds on their way out, they generate electrically neutral gamma ray light particles that are unaffected by magnetic fields. These light particles are about one-tenth more energetic than their parent cosmic rays.

So by studying the number, distribution, and energy spectrum of these gamma rays, scientists can gain insight into the origins of cosmic radiation.

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Because of the limited sensitivity of detectors, past observations of the Milky Way’s gamma-ray nebula were mostly made by space telescopes at energies below one trillion electron volts.

When LHAASO is completed in 2021, it will become the world’s largest and most sensitive observatory for detecting gamma rays and cosmic rays at ultrahigh energies. A major component of LHAASO is called the Square Kilometer Array, or KM2A, comprising about 5,200 surface electromagnetic detectors and nearly 1,200 underground muon detectors.

The researchers first ruled out dozens of point—rather than diffuse—gamma-ray sources in the galaxy. They then made one of the most comprehensive and precise measurements of the gamma-ray distribution over a wide energy range of 0.1-1 PeV and over a large part of the galaxy, including both the inner and outer galactic planes.

To their surprise, the number of diffuse gamma rays measured by LHAASO was two to three times higher than predicted cosmic ray collisions with the interstellar gas.

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Also contrary to expectations, the energy spectrum of the gamma emission can be expressed by a single power law without any discontinuity, contradicting a popular theory that ultrahigh-energy cosmic rays will be trapped by the galaxy’s magnetic fields for a long time, somewhere between 10 000 to 10 million years before they finally escaped the basin.

“The discrepancy suggests that either additional gamma-ray sources are hidden in our galaxy, or cosmic ray densities change depending on location in our galaxy,” said Kawata, who worked on a China-Japan collaboration project called Tibet AS-gamma experiment for many years.

Kawata pointed out that it would be important for the world’s major cosmic ray detectors to mutually confirm the results of their observations in the future.

For example, a recent map of the Milky Way galaxy published by the US-led IceCube collaboration, which studies cosmic neutrinos from thousands of meters below the South Pole, provided strong evidence for interactions between cosmic rays and interstellar gas.

By putting these pieces together, scientists should be able to gain great insight into the mysterious origins of cosmic radiation, he said.

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