On February 22, 1971, a sounding rocket launched from Wallops Island, Virginia, with specialized sensors aimed at the Crab Nebula, a bright cosmic object 6,500 light-years away. In those days, before recovering physical tapes from an experiment, scientists first received scientific data on a tape recorder, a device that printed signals on paper. Astronomer Martin Weisskopf and his colleagues began the analysis on launch day by measuring the distance between the signals with a ruler and pencil.
“What makes science so beautiful and exciting is that for those few moments you see something that no one has seen before,” said Weisskopf, now an astronomer emeritus at NASA’s Marshall Space Flight Center in Huntsville, Alabama.
Decades later, Weisskopf proposed the development of an Earth-orbiting satellite with powerful instruments that could gather much more detailed measurements of the same kind about the Crab Nebula and other mysterious cosmic objects. This satellite became NASA’s Imaging X-ray Polarimetry Explorer (IXPE), which was launched on December 9, 2021.
Now, more than 50 years after the probe experiment, scientists have used IXPE to create a detailed map of the Crab Nebula’s magnetic field, revealing more of its inner workings than ever before. The new results, published in the journal Nature Astronomy (preprint available), help solve long-standing mysteries about the well-studied Crab Nebula and open new questions for future study.
The IXPE data show that the Crab Nebula’s magnetic field resembles that of the Vela Pulsar Wind Nebula, which is also donut-shaped. But at Krab, the scientists were surprised to find that the regions of magnetic field turbulence were more uneven and asymmetric than expected.
“This is a clear indication that even the more complex models developed in the past using advanced numerical techniques do not fully capture the complexity of this object,” said Niccolò Bucciantini, lead author of the study and an astronomer at INAF. Arcetri Observatory in Florence, Italy.
A favorite object of astronomers’ study, the Crab Nebula, is the result of a supernova documented in 1054. The explosion left behind a dense object called the Crab Pulsar, about the diameter of Huntsville, Alabama, or the length of Manhattan. the mass of about two Suns. The chaotic jumble of gases, shock waves, magnetic fields, and high-energy light and particles coming from a rotating pulsar is collectively called the “pulsar wind nebula.” These extreme conditions create a bizarre environment that is not yet fully understood.
Weisskopf and colleagues hoped to understand this extreme environment in a new way by measuring the polarization of X-rays from the Crab Nebula, which glows brightly in X-rays. X-ray polarization gives scientists clues to the direction the magnetic field is pointing in different parts of a space object, as well as how well the magnetic field is ordered. The geometry and turbulence of the magnetic field determine how the particles are catapulted towards the speed of light.
During the five minutes a sounding rocket experiment spent in the Earth’s atmosphere in 1971, it produced the world’s first measurement of X-ray polarization.
Scientists followed up in 1975 with a satellite called OSO-8, which also measured the X-ray polarization of the Crab Nebula. The rocket and the satellite produced generally the same result: The Crab Nebula has an average polarization of about 20%.
As project scientist for NASA’s Chandra X-Ray Observatory, which launched in 1999, Weisskopf continued his exploration of the Crab Nebula in new ways. With Chandra, “we took beautiful pictures of the nebula and the pulsar, and we could see jets and different structures,” he said. X-ray imaging of Chandra has revealed tuft-like structures moving through the nebula, helping scientists further understand the relationship between the pulsar’s energy and X-ray emissions.
Almost every recent large telescope has pointed to the Crab Nebula to better understand this mysterious supernova remnant. But only IXPE can study X-rays from the Crab in terms of polarization, which is a measure of the organization of electromagnetic fields.
“The crab is one of the most studied high-energy astrophysical objects in the sky.” So it’s extremely exciting that we were able to learn something new about this system by looking through the ‘polarized lens’ of IXPE,” said Michela Negro, a scientist at the NASA Goddard Space Flight Center affiliated with the University of Maryland, Baltimore, and collaborators. author of the study.
Across the entire nebula, IXPE found about the same average polarization as Weisskopf and colleagues in the 1970s. But with more sophisticated instruments, IXPE was able to refine the angle of polarization and examine differences in polarization across the entire object. Scientists see regions of high polarization in the outer regions of the nebula, light-years away from the pulsar, where the polarization is lower.
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