AMHERST, Mass. – Scientists may have explained a phenomenon that seems to contradict the laws of physics. For the last decade or so, astronomers have been puzzled by the “weird behavior” of some jet-like X-ray features observed around bubbles of charged particles ejected from very fast-moving pulsars. These jets shoot out at super high speed into interstellar spaceat odd, unexpected angles, says Daniel Wang at the University of Massachusetts Amherst.
He explains, “If you drive very fast, your hat should be blown backwards off your head, not sideways. These jets do not follow the interstellar backflows, they shoot out to the sides – a really weird phenomenon.”
Wang presented a paper with a new observation he made with NASA’s Chandra X-ray Observatory this week at the virtual annual conference of the American Astronomical Society. He says his new evidence and analysis supports one particular theory of how such jets manifest their strange behavior.
Pulsars are rotating magnetized neutron stars that emit winds of charged particles intermixed with magnetic field at almost the speed of light, Wang explains. So, each pulsar blows a pulsar wind that forms a bubble of particles into the ambient interstellar medium. When a pulsar moves at very high speed, a supersonic shock forms on the front side of this bubble, forcing it to stretch backward – at least that is what one would expect, he adds.
But observations have shown that in some cases, highly energetic particles manage to shoot out from the sides of this shock bubble in streams of particles into the interstellar space. It is these strongly elongated streams that generate the weird X-ray jets that seem to ignore the basic laws of physics.
Wang’s group established the link of such an X-ray jet with a pulsar known as B2224+65 for the first time about 10 years ago, based on two observations made with the NASA Chandra X-ray Observatory in 2000 and 2006. These observations showed that both this “misaligned” jet and the pulsar move in a synchronized fashion. This was “a major surprise,” Wang recalls. Since then, similar jet-like features have been observed near several other fast-moving pulsars, but how these jet-like ejections form from pulsar wind remains unclear, he adds.
Now Wang has compared and analyzed all three Chandra observations of B2224+65. He shows that this pulsar also has a weak “counter-jet” and an even fainter and barely detected X-ray trail that actually coincides with the expected backflow of pulsar wind particles. Further, he tracked the change of the jet structures over time. Comparing these with recent theories and simulations, he discusses the implications of these results.
Wang says a theory that rests on energy-dependent confinement of particles by magnetic field seems to explain these jet phenomena well. “Magnetic field is everywhere, and it can guide particles like an invisible hand,” he adds. Imagining the pulsar wind bubble as a balloon, the magnetic tension confines the pulsar’s charged particles inside. But this confinement can be fragile and does not work well for high-energy particles, he adds.
Another property of magnetic field is that it is directional – “at some points in the bubble’s shock front, the interstellar and pulsar wind magnetic fields with opposing pole directions can meet and annihilate or cancel each other out”, Wang says. “It’s like sticking a pin in the balloon and producing holes that allow confined particles to burst out,” he explains. “The leaked particles are not totally free, though, and they stream mostly along interstellar magnetic field lines.”
He adds, “These observations are consistent with this leaking balloon picture, but the details still need to be worked out. It’s still uncertain how the leak appears to be sporadic and why the jets are so bright compared to the backflow.”
“Perhaps the leak can be unstable and fluctuate, as the bubble moves through the interstellar space,” he points out, and “There is also evidence for these leaked higher-energy particles to be accelerated when they collide with the interstellar magnetic field.”
“In short,” Wang writes, “magnetic field appears to play a central, though ‘invisible,’ role in determining the X-ray properties of the pulsar wind. It shows that the particle confinement is not perfect, and that a magnetic field accelerates particles to very high energies and enables them to radiate X-ray emission,” he says, adding that results seem to confirm this picture. More details of the work can be found in an article published in the Research Notes of the AAS and were given in the AAS meeting (#436.01; Jan. 14, 2021, 4:10 PM - 4:20 PM).