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.