So This is How You Get Magnetars

Neutron stars are stellar remnants. Composed of dense nuclear material, they all have strong magnetic fields. But the magnetic fields of some neutron stars can be a thousand times stronger. They are known as magnetars, and we aren’t entirely sure how they generated such powerful magnetic fields. But a new study in Nature Astronomy reveals some clues.

The general thought has been that magnetars create their fields through some type of dynamo process. This is where a flow of magnetic material generates a magnetic field. Since the flow is driven by heat convection, it can power strong fields. Earth’s magnetic field is unusually strong for a planet of its size and is powered by the convection of iron in its core. However, the core of a neutron star is made of nucleons, not atoms, so it is difficult to determine a specific dynamo process for magnetars.

For this study, the team wanted to understand what are known as low-field magnetars. These are magnetars that have weaker magnetic fields than most magnetars, but still generate bursts of X-rays and gamma rays. Most magnetars are identified by their high-energy emissions, since it takes intense magnetic fields to create such powerful bursts. Low-field magnetars shouldn’t have a strong enough field to create such bursts, but they sometimes do. This would suggest that at times their magnetic fields become intense. The question is how.

To answer this question, the team ran computer simulations of several dynamo models, looking for one that best fit the observational data. They found that the best fit involved what’s known as the Tayler–Spruit dynamo. This dynamo is well known in stellar models and involves the differential rotation of a stellar core. Stars don’t rotate as a single rigid object. Instead, different latitudes of a star rotate at slightly different rates. This is likely caused by a fast-rotating core, which can produce the Tayler–Spruit dynamo.

The authors demonstrated that as a low-field magnetar forms, the supernova that created the magnetar transfers angular momentum to its core, thus creating a differential rotation. Through the Tayler–Spruit dynamo, this can create bursts of intense magnetic fields that power the X-rays and gamma rays we observe from these stars. This process is likely unique for low-field magnetars, as opposed to traditional magnetars that generate their magnetic fields in other ways.

Reference: Igoshev, Andrei, et al. “A connection between proto-neutron-star Tayler–Spruit dynamos and low-field magnetars.” Nature Astronomy (2025): 1-11.

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