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Investigating Polarized Magnetar Flares

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artist's impression of a magnetar
An artist's impression of a magnetar with a cracked crust and a polar hot spot. [NASA/Goddard Space Flight Center Conceptual Image Lab]

Astronomers have observed polarised X-rays emitted by a magnetar, opening the door to understanding these extreme objects in more detail.

Magnetic Remnants

an illustration of a neutron star placed next to a map of New York to show the object's size

A magnetar is a sub-type of a stellar remnant called a neutron star. Magnetars and neutron stars are made solely of neutrons, and they are about 10–15 miles across. This illustration places a neutron star next to Manhattan for comparison. [NASA/Goddard Space Flight Center]

A magnetar is the remnant of the death a medium-sized star. It is a type of rapidly spinning neutron star with a typical rotation period of between one and twelve seconds. However, its magnetic field is one thousand times stronger than an ordinary neutron star. In fact, if you were to replace the Moon with a magnetar it would wipe the credit card details of everyone on Earth.

Their strong magnetic fields make magnetars prone to dramatic outbursts, emitting radiation across the electromagnetic spectrum. A magnetar’s X-ray bursts are particularly spectacular, with their X-ray flux suddenly leaping by a factor of up to a thousand.

artist's rendition of the Imaging X-ray Polarimetry Explorer (IXPE) spacecraft with Earth in the background

An artist’s impression of the Imaging X-ray Polarimetry Explorer (IXPE) spacecraft. [NASA]

Focusing on Flares

It hasn’t always been clear, however, where these flares are coming from. They could spring from the crust of the magnetar itself, or they could launch from material trapped in an atmosphere around it. A team led by Silvia Zane (University College London) recently went looking for answers using the Imaging X-ray Polarimetry Explorer (IXPE). Launched at the end of 2021, it is a joint mission between NASA and the Italian Space Agency.

As its name suggests, IXPE observes the polarisation of X-rays. Light is polarised if its waves all vibrate in the same plane. Zane’s team used IXPE to observe the polarisation of the magnetar known as 1RXS J170849.0-40091 to see if they could work out what was corralling the X-rays.

Belts, Caps, and More

plot of the polarization percentage and polarization angle of the source in different energy bands

Polarisation of the magnetar in different energy bands. The colors of the circles represent different detector units (DUs), and the orange stars and green crosses show the predictions for the “belt plus cap” and “cap plus cap” models, respectively. [Adapted from Zane et al. 2023]

They found that about 20% of the low-energy X-rays produced by 1RXS J170849.0-40091 are polarised, but this rises to around 80% for higher-energy X-rays.

The team ran simulations of various magnetar configurations to see if they could explain this result, settling on two possibilities. The first, which they call “belt plus cap,” has a warm region around the magnetar’s equator and a hotter atmosphere confined to a circular polar cap. The second, called “cap plus cap,” has the X-ray emission emanating from two circular spots. One is on the surface of the magnetar, and there’s a hotter atmospheric spot located in the opposite hemisphere.

Further work to untangle these two possibilities could have wider implications. Astronomers have previously suggested that magnetars are the main source of fast radio bursts, for example. Understanding why magnetars flare could help cement this link and assist astronomers in explaining these mysterious events.

Citation

“A Strong X-Ray Polarization Signal from the Magnetar 1RXS J170849.0-400910,” Silvia Zane et al 2023 ApJL 944 L27. doi:10.3847/2041-8213/acb703