A pulsar in the Crab Nebula
The Crab Nebula (catalogue designations M1, NGC 1952, Taurus A) is a supernova remnant in the constellation of Taurus. The now-current name is due to William Parsons, 3rd Earl of Rosse, who observed the object in 1840 using a 36-inch telescope and produced a drawing that looked somewhat like a crab. Corresponding to a bright supernova recorded by Chinese astronomers in 1054, the nebula was observed later by English astronomer John Bevis in 1731. The nebula was the first astronomical object identified with a historical supernova explosion.
At an apparent magnitude of 8.4, comparable to that of Saturn's moon Titan, it is not visible to the naked eye but can be made out using binoculars under favourable conditions. The nebula lies in the Perseus Arm of the Milky Way galaxy, at a distance of about 2.0 kiloparsecs (6,500 ly) from Earth. It has a diameter of 3.4 parsecs (11 ly), corresponding to an apparent diameter of some 7 arcminutes, and is expanding at a rate of about 1,500 kilometres per second (930 mi/s), or 0.5% of the speed of light.
At the center of the nebula lies the Crab Pulsar, a neutron star 28–30 kilometres (17–19 mi) across with a spin rate of 30.2 times per second, which emits pulses of radiation from gamma rays to radio waves. At X-ray and gamma ray energies above 30 keV, the Crab Nebula is generally the brightest persistent source in the sky, with measured flux extending to above 10 TeV. The nebula's radiation allows for the detailed studying of celestial bodies that occult it. In the 1950s and 1960s, the Sun's corona was mapped from observations of the Crab Nebula's radio waves passing through it, and in 2003, the thickness of the atmosphere of Saturn's moon Titan was measured as it blocked out X-rays from the nebula.
The inner part of the nebula is a much smaller pulsar wind nebula that appears as a shell surrounding the pulsar. Some sources consider the Crab Nebula to be an example of both a pulsar wind nebula as well as a supernova remnant, while others separate the two phenomena based on the different sources of energy production and behaviour. For the Crab Nebula, the divisions are superficial but remain meaningful to researchers and their lines of study.
China’s ‘Monkey King’ Dark Matter Satellite Has Produced Some Unusual Results
The Earth happens to be located in one of the weirdest places imaginable: the Universe. Like, whatever this Universe is and whatever rules it follows, we puny humans still struggle to understand it. But we’re really trying to—and we can’t seem to agree.
China’s Dark Matter Particle Explorer satellite (DAMPE or “Wukong” in China) is reporting the results of a year-and-a-half of space-staring, measuring the mysterious, high-energy electrons blasting Earth from space. The experiment has directly detected something that some similar experiments have hinted at, but others haven’t: a sudden drop-off in the electrons hitting the satellite. Whatever is going on, it’s weird.
This change, called a spectral break, “confirms the evidence found by previousindirect measurements,” and “clarifies the behavior” of the high energy electrons, according to the paper published yesterday in Nature.
Science writes that the experiment is important as “China’s first mission dedicated to astrophysics” and “shows that the country is set to become a force in space science,” paraphrasing Physicist David Spergel at Princeton.
The drop is formed by hte red circles. Image: DAMPE Collaboration (via the arXiv)
DAMPE counts the electrons and their antiparticles, positrons, hitting Earth from space. If you peep a graph of the number of electrons hitting the satellite versus their energy, the quantity decreases as the energy increases. But starting around 0.9 tera-electron volt (TeV) electrons, those that have almost two million times the energy of everyday, lowest-energy electrons, DAMPE spotted hints of a mysterious drop-off. This “edge-like” change could possibly be the result of nearby pulsars or even dark matter.
But is this apparent edge real? It’s hard to say. Physicist Wim de Boer from the Karlsruhe Institute for Technology in Germany noted that the Calorimetric Electron Telescope (CALET) on the International Space Station doesn’t seem to observe this edge at high energy, nor does the experiment de Boer works on, the Alpha Magnetic Spectrometer, also on the ISS. But Spergel told Gizmodo the uncertainties in all of these experiments are too large to say anything conclusive.
There simply isn’t a lot of data for the electrons with the highest energies, and a lot of places where the experiments differ in the way they analyze the data or potential sources of error. As always, more data will lead to a better understanding of the situation.
As to whether there are hints of dark matter afoot, “I don’t think dark matter should be at these high energies,” or at least, that’s the popular belief, said De Boer—and for now, he doesn’t know how to interpret the break. But more in-depth analyses of the specific types of particles, like electrons versus their antiparticles, positrons, could also help tease out the details of the story.
We have many experiments in which scientists can’t agree on what they’re seeing within all that strange stuff hitting Earth from outer space—or even if they have enough data to disagree. But there’s still hope that some explanation for “dark matter,” whatever it may be, is out there.
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