ESA Science & Technology - Magnetar SGR 0418+5729 with a magnetic loop

This image shows an artist's impression of the magnetar SGR 0418+5729.

Magnetars are peculiar pulsars – the spinning remnants of massive stars – that are characterised by unusually intense magnetic fields. Astronomers discovered them through their exceptional behaviour at X-ray wavelengths, including sudden outbursts of radiation and occasional giant flares. These peculiar features of magnetars are caused by their extremely strong magnetic fields, which range between 10¹⁴ and 10¹⁵ Gauss (G) and are hundreds or thousands of times more intense than those of regular pulsars.

Since it was discovered in 2009, SGR 0418+5729 has been puzzling astronomers. Its dipolar magnetic field – the most direct manifestation of a magnetar's average magnetic field, which can be estimated from its spin-down rate – is about two  orders of magnitude lower than other magnetars. Astronomers believed that this magnetar concealed, in its interior, a very strong magnetic field. A study based on data from ESA's XMM-Newton X-ray Observatory has found evidence to confirm this, revealing that the magnetar's internal field may even exceed 10¹⁵ G, removing any lingering doubts about the object's true identity.

The strong internal magnetic field of this magnetar reveals itself only in a small feature emerging from its surface, where protons absorb some of the X-rays emitted by the magnetar. The protons are confined in this region by a strong and localised magnetic field, with lines that are probably shaped like a series of adjacent arcs, resembling the appearance of loops on the surface of the Sun.