An artist’s impression of a magnetar eruption. Credit: NASA’s Goddard Space Flight Center
The sudden slowdown of a star in 2020 provides an opportunity to test the “anti-glitch” theory.
On October 5th, 2020, a rapidly rotating remnant of a long-dead star located approximately 30,000 light-years away from our planet experienced a sudden change in speed. In a cosmic instant, its spinning slowed. And a few days later, it abruptly started emitting radio waves.
Rice University astrophysicist Matthew Baring and his team were able to test a new theory about the cause of the uncommon slowdown, or “anti-glitch,” of SGR 1935+2154, a highly magnetic neutron starA neutron star is the collapsed core of a large (between 10 and 29 solar masses) star. Neutron stars are the smallest and densest stars known to exist. Though neutron stars typically have a radius on the order of just 10 – 20 kilometers (6 – 12 miles), they can have masses of about 1.3 – 2.5 that of the Sun.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>neutron star referred to as a magnetar, due to timely measurements from specialized orbiting telescopes.
In a study published this month in Nature Astronomy, Baring and co-authors used X-ray data from the European Space AgencyThe European Space Agency (ESA) is an intergovernmental organization dedicated to the exploration and study of space. ESA was established in 1975 and has 22 member states, with its headquarters located in Paris, France. ESA is responsible for the development and coordination of Europe's space activities, including the design, construction, and launch of spacecraft and satellites for scientific research and Earth observation. Some of ESA's flagship missions have included the Rosetta mission to study a comet, the Gaia mission to create a 3D map of the Milky Way, and the ExoMars mission to search for evidence of past or present life on Mars.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>European Space Agency’s X-ray Multi-Mirror Mission (XMM-Newton) and NASAEstablished in 1958, the National Aeronautics and Space Administration (NASA) is an independent agency of the United States Federal Government that succeeded the National Advisory Committee for Aeronautics (NACA). It is responsible for the civilian space program, as well as aeronautics and aerospace research. Its vision is "To discover and expand knowledge for the benefit of humanity." Its core values are "safety, integrity, teamwork, excellence, and inclusion." NASA conducts research, develops technology and launches missions to explore and study Earth, the solar system, and the universe beyond. It also works to advance the state of knowledge in a wide range of scientific fields, including Earth and space science, planetary science, astrophysics, and heliophysics, and it collaborates with private companies and international partners to achieve its goals.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>NASA’s Neutron Star Interior Composition Explorer (NICER) to analyze the magnetar’s rotation. They showed the sudden slowdown could have been caused by a volcano-like rupture on the surface of the star that spewed a “wind” of massive particles into space.
The research identified how such a wind could alter the star’s magnetic fields, seeding conditions that would be likely to switch on the radio emissions that were subsequently measured by China’s Five-hundred-meter Aperture Spherical Telescope (FAST).
Matthew Baring is a professor of physics and astronomy at Rice University. Credit: Henry Baring
“People have speculated that neutron stars could have the equivalent of volcanoes on their surface,” said Baring, a professor of physics and astronomy. “Our findings suggest that could be the case and that on this occasion, the rupture was most likely at or near the star’s magnetic pole.”
SGR 1935+2154 and other magnetars are a type of neutron star, the compact remains of a dead star that collapsed under intense gravity. About a dozen miles wide and as dense as the nucleus of an atomAn atom is the smallest component of an element. It is made up of protons and neutrons within the nucleus, and electrons circling the nucleus.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>atom, magnetars rotate once every few seconds and feature the most intense magnetic fields in the universe.
Magnetars emit intense radiation, including X-rays and occasional radio waves and gamma rays. Astronomers can decipher much about the unusual stars from those emissions. By counting pulses of X-rays, for example, physicists can calculate a magnetar’s rotational period, or the amount of time it takes to make one complete rotation, as the Earth does in one day. The rotational periods of magnetars typically change slowly, taking tens of thousands of years to slow by a single rotation per second.
Glitches are abrupt increases in rotational speed that are most often caused by sudden shifts deep within the star, Baring said.
“In most glitches, the pulsation period gets shorter, meaning the star spins a bit faster than it had been,” he said. “The textbook explanation is that over time, the outer, magnetized layers of the star slow down, but the inner, non-magnetized core does not. This leads to a buildup of stress at the boundary between these two regions, and a glitch signals a sudden transfer of rotational energy from the faster spinning core to the slower spinning crust.”
Abrupt rotational slowdowns of magnetars are very rare. Astronomers have only recorded three of the “anti-glitches,” including the October 2020 event.
While glitches can be routinely explained by changes inside the star, anti-glitches likely cannot. Baring’s theory is based on the assumption that they are caused by changes on the surface of the star and in the space around it. In the new paper, he and his co-authors constructed a volcano-driven wind model to explain the measured results from the October 2020 anti-glitch.
Baring said the model uses only standard physics, specifically changes in angular momentum and conservation of energy, to account for the rotational slowdown.
“A strong, massive particle wind emanating from the star for a few hours could establish the conditions for the drop in rotational period,” he said. “Our calculations showed such a wind would also have the power to change the geometry of the magnetic field outside the neutron star.”
The rupture could be a volcano-like formation, because “the general properties of the X-ray pulsation likely require the wind to be launched from a localized region on the surface,” he said.
“What makes the October 2020 event unique is that there was a fast radio burst from the magnetar just a few days after the anti-glitch, as well as a switch-on of pulsed, ephemeral radio emission shortly thereafter,” he said. “We’ve seen only a handful of transient pulsed radio magnetars, and this is the first time we’ve seen a radio switch-on of a magnetar almost contemporaneous with an anti-glitch.”
Baring argued this timing coincidence suggests the anti-glitch and radio emissions were caused by the same event, and he’s hopeful that additional studies of the volcanism model will provide more answers.
“The wind interpretation provides a path to understanding why the radio emission switches on,” he said. “It provides new insight we have not had before.”
Reference: “Magnetar spin-down glitch clearing the way for FRB-like bursts and a pulsed radio episode” by G. Younes, M. G. Baring, A. K. Harding, T. Enoto, Z. Wadiasingh, A. B. Pearlman, W. C. G. Ho, S. Guillot, Z. Arzoumanian, A. Borghese, K. Gendreau, E. Göğüş, T. Güver, A. J. van der Horst, C.-P. Hu, G. K. Jaisawal, C. Kouveliotou, L. Lin and W. A. Majid, 12 January 2023, Nature Astronomy.
DOI: 10.1038/s41550-022-01865-y
The study was funded by the National Science Foundation, Japan’s RIKEN Advanced Science Institute, and Taiwan’s Ministry of Science and Technology.
Source: SciTechDaily
