Tycho supernova is a particle accelerator

A research team has discovered that the Tycho supernova is a cosmic particle accelerator that produces cosmic rays that reach us
Tycho supernova remnant, a star that exploded in the constellation of Cassiopeia, whose light was first seen on Earth in 1572. Credit: NASA

The white dwarf that created the Tycho supernova has died in a violent explosion. Its legacy resembles a soft fluffy ball of pink cotton. In the latest image, released on February 28th, the supernova remnant is shown as a pink cloud bordered by a thin red line.

New study (ref.), astronomers have mapped the geometry of the magnetic fields near the shock wave with unprecedented detail. In this region, charged particles are accelerated to speeds similar to that of light before being expelled as cosmic rays that eventually reach Earth.

Discovery of the Phenomenon

The first time we observed this phenomenon was in 2011. At that time, it was Chandra, the X-ray observatory, that captured a pattern on the outer edge of Tycho. At the time, astronomers explained the pattern as points where the magnetic fields are trapped. Trapped electrons spiral in the magnetic fields towards higher energies, emitting X-rays.

While astronomers have long known that supernovae produce charged particles at extremely high energies, the details of how electrons are accelerated were not yet known. Now, researchers have studied the phenomenon in detail in Tycho, whose explosion released as much energy as the sun would emit in 10 billion years. Scientists say the latest findings explain how the Tycho supernova becomes a giant cosmic particle accelerator.

The process “involves a delicate dance between order and chaos” said Patrick Slane, senior astrophysicist at the Harvard-Smithsonian Center for Astrophysics and co-author of the study, in a statement. Slane’s team used data from NASA‘s Imaging X-ray Polarimetry Explorer (IXPE) space observatory. The three identical X-ray telescopes aboard IXPE studied Tycho twice in 2022. From late June to early July and from December 21 to 25.

The energy of electrons

Team studied X-rays produced by highly energetic electrons near the edge of Tycho while they raced through magnetic fields. The researchers explain that the red edge, where Tycho accelerates particles, is very thin. At that point, the electrons that emit X-rays lose their energy very quickly. Therefore, by moving significantly away from the edge, “they lose so much energy that they no longer produce X-rays” said Slane.

To finally map the geometry of the magnetic field, the team looked for signals that showed how polarized the X-ray radiation was. However, these signals are sensitive to magnetic field turbulence. When turbulence is high, the radiation is less directional and less intense, and IXPE cannot detect the signals. Fortunately, upon arrival of the IXPE data, the team found that the magnetic fields had high turbulence, “but not so high that we could not detect the polarization” he added.

By measuring the polarization of the X-rays, they discovered that it was 9% in the center of the Tycho residue and 12% higher at its edge. “These observations are the first ever. We probed the polarization of the emission from the most energetic electrons in the Tycho supernova, which behaves like a true particle accelerator” said Slane.

The detailed map of the magnetic field

Having calculated the polarization angle, Slane’s team was able to map the geometry of the magnetic field. The latter extends outward, in a radial manner. The researchers already knew this from previous radio observations, so the discovery was not a total surprise. But the IXPE space observatory helped them map the magnetic field in much more detail than previous observations.

Finally, they have understood why Tycho accelerates its charged particles at speeds close to that of light. “To do this, strong and turbulent magnetic fields are required,” said Slane in the same statement. “IXPE is showing us that there is large-scale uniformity, or coherence, also involved, which extends to the sites where the acceleration is taking place”. Using this data, the team has discovered that the radial structure remains intact up to the acceleration sites, which they did not know before. This insight will shed light on how Tycho accelerates charged particles to energies at least a hundred times higher than even the most powerful particle accelerators on Earth.

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