NICER Observes Merger of X-Ray Spots on Distant Magnetar

Using NASA’s Neutron star Interior Composition Explorer (NICER), astronomers have observed the merging of multimillion-degree X-ray spots on the surface of a magnetar called SGR 1830-0645.

X-ray spots on the surface of the magnetar SGR 1830-0645. Image credit: Younes et al., doi: 10.3847/2041-8213/ac4700.

Magnetars are isolated neutron stars with extremely powerful magnetic fields — up to 10 trillion times more intense than a refrigerator magnet’s and a thousand times stronger than a typical neutron star’s.

These objects exhibit distinctive months-long outbursts of enhanced X-ray activity.

“The crust of a neutron star is immensely strong, but a magnetar’s intense magnetic field can strain it beyond its limits,” said Dr. Sam Lander, an astrophysicist at the University of East Anglia.

“Understanding this process is a major challenge for theorists, and now NICER and SGR 1830-0645 have brought us a much more direct look at how the crust behaves under extreme stress.”

On October 10, 2020, the BAT instrument aboard NASA’s Neil Gehrels Swift Observatory spotted a short, soft X-ray burst from SGR 1830-0645, a magnetar located about 13,000 light-years away in the constellation of Scutum.

The Swift/BAT detected repeated pulses that revealed the object was rotating every 10.4 seconds.

NICER measurements from the same day show that the X-ray emission exhibited three close peaks with every rotation.

They were caused when three individual surface regions much hotter than their surroundings spun into and out of our view.

NICER observed SGR 1830-0645 almost daily from its discovery to November 17, 2020, after which the Sun was too close to the field of view for safe observation.

Over this period, the emission peaks gradually shifted, occurring at slightly different times in the magnetar’s rotation.

The results favor a model where the spots form and move as a result of crustal motion, in much the same way as the motion of tectonic plates on Earth drives seismic activity.

Dr. Lander and his colleagues think these observations reveal a single active region where the crust has become partially molten, slowly deforming under magnetic stress.

The three moving hot spots likely represent locations where coronal loops — similar to the bright, glowing arcs of plasma seen on the Sun — connect to the surface.

The interplay between the loops and crustal motion drives the drifting and merging behavior.

“Changes in pulse shape, including decreasing numbers of peaks, previously have been seen only in a few ‘snapshot’ observations widely separated in time, so there was no way to track their evolution,” said NICER science leader Dr. Zaven Arzoumanian, an astronomer at NASA’s Goddard Space Flight Center.

“Such changes could have occurred suddenly, which would be more consistent with a lurching magnetic field than wandering hot spots.”

The team’s paper was published in the Astrophysical Journal Letters.

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George Younes et al. 2022. Pulse Peak Migration during the Outburst Decay of the Magnetar SGR 1830-0645: Crustal Motion and Magnetospheric Untwisting. ApJL 924, L27; doi: 10.3847/2041-8213/ac4700