London: The first ever measurement of the magnetic field at a specific spot on the surface of a magnetar has been made by astronomers.
Magnetars are a type of neutron star, the dense and compact core of a giant star which has blasted away its outer layers in a supernova explosion.
Magnetars have among the strongest magnetic fields in the Universe. Until now, only their large scale magnetic field had been measured.
However, using a new technique and observations of a magnetar in X-rays, the astronomers including two researchers from University College London`s Mullard Space Science Laboratory have now revealed a strong, localised surface magnetic field.
Magnetars are very puzzling neutron stars. Astronomers discovered them through their unusual behaviour when observed in X-ray wavelengths, including sudden outbursts of radiation and occasional giant flares.
These peculiar features of magnetars are caused by the evolution, dissipation and decay of their super-strong magnetic fields, which are hundreds or thousands of times more intense than those of the more common type of neutron stars, the radio pulsars.
The magnetic field of a magnetar can have a complex structure. The most obvious, and easy-to-measure, component is the large scale external magnetic field, which is shaped (and behaves) much like a regular bar magnet`s. This is known as the dipolar field.
The study was carried out on a magnetar called SGR 0418+5729.
However, the star was showing the typical flaring and bursting activities seen in other magnetars, leading scientists to suggest that the star`s magnetic activity might be caused by a field hidden beneath its surface.
This new study, based on observations from ESA`s XMM-Newton X-ray space telescope, has finally found evidence that SGR 0418+5729 is indeed concealing a very strong magnetic field in its interior.
"This magnetar has a strong magnetic field inside it, but it is hidden beneath the surface. The only way you can detect that is to find a flaw on the surface, where the concealed magnetic field can leak out," said Silvia Zane, one of the co-authors of the study.
Such magnetic leaks would also explain the outbursts and flares of radiation observed from magnetars. The warped magnetic field trapped inside the star builds up stress below the magnetar`s surface, occasionally breaking its `crust` apart and releasing sudden flashes of X-rays.