Magnetars are a special subset of the isolated neutron star family, with X-ray and radio emission mainly powered by the decay of their immense magnetic fields.
An artist’s impression of a magnetar eruption. Image credit: NASA’s Goddard Space Flight Center.
On October 5, 2020, SGR 1935+2154 — a magnetar some 30,000 light-years away in the constellation of Vulpecula — changed speeds. And a few days later, it abruptly started emitting radio waves.
In the new study, Rice University’s Professor Matthew Baring and his colleagues used X-ray data from ESA’s X-ray Multi-Mirror Mission (XMM-Newton) and 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.
They 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).
“People have speculated that neutron stars could have the equivalent of volcanoes on their surface,” Professor Baring said.
“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.”
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, they 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,” Professor Baring said.
“In most glitches, the pulsation period gets shorter, meaning the star spins a bit faster than it had been.”
“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 spin-down glitches, including the October 2020 event.
While glitches can be routinely explained by changes inside the star, spin-down glitches likely cannot.
The team’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 their paper, the authors constructed a volcano-driven wind model to explain the measured results from the October 2020 glitch.
“The model uses only standard physics, specifically changes in angular momentum and conservation of energy, to account for the rotational slowdown,” Professor Baring said.
“A strong, massive particle wind emanating from the star for a few hours could establish the conditions for the drop in rotational period.”
“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.”
“What makes the October 2020 event unique is that there was a fast radio burst from the magnetar just a few days after the spin-down glitch, as well as a switch-on of pulsed, ephemeral radio emission shortly thereafter.”
“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 a spin-down glitch.”
“This timing coincidence suggests the spin-down glitch and radio emissions were caused by the same event, and we’re 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. It provides new insight we have not had before.”
The team’s paper was published in the journal Nature Astronomy.
_____
G. Younes et al. Magnetar spin-down glitch clearing the way for FRB-like bursts and a pulsed radio episode. Nat Astron, published online January 12, 2023; doi: 10.1038/s41550-022-01865-y
