by A dead star’s rapidly rotating envelope has mysteriously slowed, and astronomers believe it’s due to an “anti-glitch” that caused a gigantic volcano-like explosion from its surface.
The dead star, a magnetar classified as SGR 1935+2154 located 30,000 light-years from Earth, suddenly slowed in October 2020 before unleashing a month-long barrage of radio waves. Well, a new study published Jan. 12 in the journal Nature Astronomy (opens in new tab)has uncovered the likely cause: a violent eruption from the surface of the star corpse.
Magnetars and their less magnetized cousins, pulsars, are special types of neutron stars that are ultradense remnants of the explosive death of stars. Pulsars and magnetars are compressed by intense gravitational forces to a width of just a dozen miles, while being as dense as the nucleus of an atom. They have unusually strong magnetic fields, often millions or trillions of times stronger than Earth’s. As the burned-out stars spin rapidly in space—making a complete revolution once every few seconds—like giant lighthouses, they emit a beam of intense electromagnetic radiation from their poles. The composition of their surfaces remains largely a mystery.
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“People have speculated that neutron stars might have the equivalent of volcanoes on their surface,” said lead author Matthew Baring (opens in new tab)an astrophysicist at Rice University said in a statement (opens in new tab). “Our results suggest that this could be the case, and that in this case the rupture most likely occurred at or near the star’s magnetic pole.”
Astrophysicists study magnetars by the intense radiation they emit in the form of X-rays, radio waves, and gamma rays. Because these rays are coming toward us whenever a magnetar swings around to face us, for example, examining the gaps between the X-ray pulses gives scientists a reliable measure of how fast a magnetar is spinning. Sometimes these gaps between pulse trains narrow – suggesting that a glitch caused by a sudden change in the dead star’s structure caused it to spin faster.
“For most perturbations, the pulsation period gets shorter, which means the star spins a little faster than before,” Baring said. “The textbook explanation is that the star’s outer, magnetized layers slow down over time, but the inner, unmagnetized core does not. This leads to a build-up of tension at the boundary between these two regions and a glitch signal, a sudden transfer of rotational energy from the faster-spinning core to the slower-spinning crust.
Many spin-up perturbations have been observed before, but SGR 1935+2154’s spin-down “anti-perturbation” – which caused the star to slow down rather than speed up – is the third of its kind ever observed , and has done so too, widely defying explanation.
Additionally, the sudden slowdown was followed by three Fast Radio Bursts (FRBs) — ultra-bright bursts of radio energy that can last just a few milliseconds — and a month-long string of radio pulses. Upon arrival on Earth, the radio emissions were picked up by China’s 500-metre spherical aperture telescope, marking the first time a fast radio burst has been traced back to its source.
To get a better handle on what might have caused the stellar body to slow its rotation, the researchers built a model that explained the origin of the anti-glitch as an enormous and sudden eruption on the star’s surface. Surface plasma from a region near one of the star’s magnetic poles spilled outward, creating a charged particle wind that disrupted the magnetar’s magnetic field, acting as a jerky brake against its rotation.
“A strong, massive particle wind emanating from the star for a few hours could set the conditions for the rotation period to drop,” Baring said.
Researchers believe the burst that caused the slowdown is also responsible for the intense radio emissions that were subsequently detected. But to be sure, they need to spot another magnetar throwing a cosmic tantrum.
“Given the rarity of magnetar spin-down disturbances and radio signals, their approximate synchrony suggests an association that provides crucial clues to their origin and triggering mechanisms affecting the broader magnetar and FRB populations,” the researchers wrote in the paper .
