NASA's Imaging X-ray Polarimetry Explorer (IXPE), the space agency's 'X-ray Eyes On The Universe', has unravelled the mysteries of a famous supernova which was discovered 450 years ago. The supernova remnant is called Tycho, and is named after its discoverer Tycho Brahe, a Danish astronomer who observed the bright glow of a new "star" in the constellation Cassiopeia in 1572. A supernova is a titanic stellar explosion.
What is polarisation? What is X-ray polarisation?
An international team of scientists used IXPE to study polarised X-rays from the Tycho supernova remnant. Polarisation is a property of light that depicts the direction of their oscillations and holds clues to the environment from which the light originates. Polarised light oscillates or vibrates in only one direction.
X-ray polarisation is a process in which the paths of electrons moving close to the speed of light are bent by a magnetic field, as a result of which the electrons start spiralling, and emit photons in the process, the electromagnetic field of which vibrates in one direction, or gets polarised.
The new study explores how the conditions in the shock waves created by supernovae accelerate particles to near the speed of light.
What information did IXPE reveal?
For the first time, IXPE revealed the geometry of the magnetic fields close to the shock wave, which is still propagating from the initial explosion. The shockwave forms a boundary around the ejected material. In order to understand how particles are accelerated in the vicinity of the supernova remnant, it is important to understand the magnetic field geometry.
The study describing the findings was recently published in The Astrophysical Journal.
In a statement released by NASA, Dr Riccardo Ferrazzoli, a researcher at the Italian Institute for Astrophysics in Rome, and lead author on the paper, said, Tycho, one of the historical supernovae, was observed by humanity in the past, and had a lasting social and even artistic impact.
Ferrazzoli also said that it is exciting to be here, 450 years after the first appearance of the Tycho supernova in the sky, to see that object again with new eyes and to learn from it.
Significance of the study
Scientists can determine the average direction of the magnetic field of light waves that generated X-rays from a high-energy source like Tycho. Electrons moving in the magnetic field produced the polarised X-rays in a process called "synchrotron emission". By determining the polarisation direction of the X-rays, scientists can map the direction of the magnetic fields at the location where the X-rays were generated.
Using this information, scientists can answer some of the biggest questions in astrophysics, such as how Tycho and other objects accelerate particles closer to the speed of light than the most powerful particle accelerators on Earth.
Patrick Slane from the Center for Astrophysics at Harvard in Cambridge, Massachusetts, said the process by which a supernova remnant becomes a giant particle accelerator involves a delicate dance between order and chaos.
Slane explained that strong and turbulent magnetic fields are required, but IXPE is showing scientists that there is also a large-scale uniformity involved, extending right down to the sites where the acceleration is taking place.
IXPE builds on the groundwork paid by NASA's Chandra X-ray Observatory, which has repeatedly observed the Tycho supernova remnant. Using information from IXPE, scientists can better understand the process by which supernova remnants accelerate cosmic rays and highly energetic particles that permeate the Milky Way galaxy.
Scientists have been able to map the shape of Tycho's magnetic field with exceptional clarity using IXPE. Previous observatories have observed Tycho's magnetic field in radio waves. However, IXPE measured the shape of the field on scales smaller than one parsec, or roughly 3.26 light years. This is the closest distance researchers have ever come to observing the source of the highly energetic "cosmic rays" emitted by one of these distant phenomena.
The information is important to scientists because they can explore how particles are accelerated in the wake of the initial explosion's blast wave.
The similarities between Tycho and the Cassiopeia A supernova remnant, which has also been studied by IXPE, are that the overall directions in both supernova remnants appear to be radial, stretched out along a direction extending outward. The difference is that Tycho yielded a much higher degree of X-ray polarisation compared to Cassiopeia A. This suggests that Tycho may possess a more ordered, less turbulent magnetic field.
The Type A supernova is classified as a Type Ia, which is a type of supernova that occurs when a white dwarf star in a binary system obliterates its companion star. The dwarf star captures some of the mass of its companion star, and triggers a violent explosion.
According to the study, the Tycho supernova blast released as much energy as the Sun would put out over the course of 10 billion years, and this brilliance rendered the Tycho supernova visible to the naked eye on Earth in 1572, when it was discovered by Brahe and other stargazers, potentially including an 8-year-old William Shakespear, who went on to describe the Tycho supernova in an early passage of "Hamlet".