Cassiopeia A is a remnant that blew up as a supernova approximately 11,000 years ago.
This composite image shows the supernova remnant Cassiopeia A, a structure resulting from the explosion of a star in the constellation of Cassiopeia. The blues represent data from NASA’s Chandra X-ray Observatory, the turquoise is from IXPE, and the gold is courtesy of the NASA/ESA Hubble Space Telescope. Image credit: NASA / CXC / SAO / NASA / MSFC / Vink et al. / STScI.
Cassiopeia A, which is located about 11,000 light-years from Earth, is the most studied nearby supernova remnant.
When the original star ran out of fuel, it collapsed onto itself and blew up as a supernova, possibly briefly becoming one of the brightest objects in the sky.
Although astronomers think that this happened around the year 1680, there are no verifiable historical records to confirm this.
Cassiopeia A was the first object the Imaging X-ray Polarimetry Explorer (IXPE) — a collaboration between NASA and the Italian Space Agency — observed after it began collecting data.
One of the reasons the supernova remnant was selected is that its shock waves are some of the fastest in the Milky Way.
The shock waves were generated by the supernova explosion that destroyed a massive star after it collapsed. Light from the blast swept past Earth more than three hundred years ago.
“Without IXPE, we have been missing crucial information about objects like Cassiopeia A,” said Dr. Pat Slane, an astronomer at the Harvard & Smithsonian’s Center for Astrophysics.
“This result is teaching us about a fundamental aspect of the debris from this exploded star — the behavior of its magnetic fields.”
This graphic combines data from IXPE with an X-ray image from Chandra (blue) and a view in optical light from Hubble (gold) of Cassiopeia A. The lines in this graphic come from IXPE measurements that show the direction of the magnetic field across regions of the remnant. Green lines indicate regions where the measurements are most highly significant. These results indicate that the magnetic field lines near the outskirts of Cassiopeia A are largely oriented radially, i.e., in a direction from the center of the remnant outwards. The IXPE observations also reveal that the magnetic field over small regions is highly tangled, without a dominant preferred direction. Image credit: NASA / CXC / SAO / NASA / MSFC / Vink et al.
Magnetic fields push and pull on moving charged particles like protons and electrons. Under extreme conditions, such as an exploded star, they can boost these particles to near-light-speed.
Despite their super-fast speeds, particles swept up by shock waves in Cassiopeia A do not fly away from the supernova remnant because they are trapped by magnetic fields in the wake of the shocks.
The particles are forced to spiral around the magnetic field lines, and the electrons give off an intense kind of light called synchrotron radiation, which is polarized.
By studying the polarization of this light, scientists can reverse engineer what’s happening inside Cassiopeia A at very small scales — details that are difficult or impossible to observe in other ways.
The angle of polarization tells us about the direction of these magnetic fields.
If the magnetic fields close to the shock fronts are very tangled, the chaotic mix of radiation from regions with different magnetic field directions will give off a smaller amount of polarization.
Before IXPE, astronomers predicted X-ray polarization would be produced by magnetic fields that are perpendicular to magnetic fields observed by radio telescopes.
Instead, IXPE data show that the magnetic fields in X-rays tend to be aligned in radial directions even very close to the shock fronts.
The X-rays also reveal a lower amount of polarization than radio observations showed, which suggests that the X-rays come from turbulent regions with a mix of many different magnetic field directions.
“These IXPE results were not what we expected, but as scientists we love being surprised,” said Dr. Jacco Vink, an astronomer at the University of Amsterdam.
“The fact that a smaller percentage of the X-ray light is polarized is a very interesting — and previously undetected — property of Cassiopeia A.”
A paper on the findings will be published in the Astrophysical Journal.
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Jacco Vink et al. 2022. X-ray polarization detection of Cassiopeia A with IXPE. ApJ, in press; arXiv: 2206.06713
