Artist impression of a magnetar with a ‘magnetic loop'. This is the
interpretation of data collected by ESA’s XMM-Newton space telescope of
the magnetar known as SGR 0418, which boasts one of the strongest
magnetic fields in the Universe. In order to maintain such a strong
magnetic field, the magnetar must have a twisted internal magnetic
field, which manifests itself as a small region on the star’s surface,
somewhat similar to the localised magnetic fields anchored in sunspots
on the Sun. Copyright: ESA/ATG Medialab
Scientists using ESA’s XMM-Newton space telescope have discovered that a
curious dead star has been hiding one of the strongest magnetic fields
in the Universe all along, despite earlier suggestions of an unusually
low magnetic field.
The object, known as SGR 0418+5729 (or SGR 0418 for short), is a magnetar, a particular kind of neutron star.
A neutron star is the dead core of a once massive star that collapsed in
on itself after burning up all its fuel and exploding in a dramatic
supernova event. They are extraordinarily dense objects, packing more
than the mass of our Sun into a sphere only some 20 km across – about
the size of a city.
A small proportion of neutron stars form and live briefly as magnetars,
named for their extremely intense magnetic fields, billions to trillions
of times greater than those generated in hospital MRI machines, for
example. These fields cause magnetars to erupt sporadically with bursts
of high-energy radiation.
SGR 0418 lies in our galaxy, about 6500 light years from Earth. It was
first detected in June 2009 by space telescopes including NASA’s Fermi
and Roscosmos’ Koronas-Photon when it suddenly lit up in X-rays and soft
gamma rays. It has been studied subsequently by a fleet of
observatories, including ESA’s XMM-Newton.
“Until very recently, all indications were that this magnetar had one of the weakest surface magnetic fields known; at 6 x 1012 Gauss,
it was roughly a 100 times lower than for typical magnetars,” said
Andrea Tiengo of the Istituto Universitario di Studi Superiori, Pavia,
Italy, and lead author of the paper published inNature.
“Understanding these results was a challenge. However, we suspected that
SGR 0418 was in fact hiding a much stronger magnetic field, out of
reach of our usual analytical techniques.”
Magnetars spin more slowly than neutron stars, but still complete a
rotation within a few seconds. The normal way of determining the
magnetic field of a magnetar is to measure the rate at which the spin is
declining. Three years of observations of SGR 0418 had led astronomers
to infer a weak magnetic field.
The new technique developed by Dr Tiengo and his collaborators involves
searching for variations in the X-ray spectrum of the magnetar over
extremely short time intervals as it rotates. This method allows
astronomers to analyse the magnetic field in much more detail and has
revealed SGR 0418 as a true magnetic monster.
“To explain our observations, this magnetar must have a super-strong, twisted magnetic field reaching 1015 Gauss across small regions on the surface, spanning only a few hundred metres across,” said Dr Tiengo.
“On average, the field can appear fairly weak, as earlier results have
suggested. But we are now able to probe sub-structure on the surface and
see that the field is very strong locally.”
A simple analogy can be made with localised magnetic fields anchored in
sunspots on the Sun, where a change in configuration can suddenly lead
to their collapse and the production of a flare or, in the case of SGR
0418, a burst of X-rays.
“The spectral data provided by XMM-Newton, combined with a new way of
analysing the data, allowed us to finally make the first detailed
measurements of the magnetic field of a magnetar, confirming it as one
of the largest values ever measured in the Universe,” adds Norbert
Schartel, ESA’s XMM-Newton Project Scientist.
“We now have a new tool to probe the magnetic fields of other magnetars,
which will help constrain models of these exotic objects.”
Notes for Editors
“A variable absorption feature in the X-ray spectrum of a magnetar,” by A. Tiengo et al is published in Nature, 15 August 2013.
For further information, please contact:
Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: markus.bauer@esa.int
Andrea Tiengo
Istituto Universitario di Studi Superiori and Istituto Nazionale di Fisica Nucleare Pavia, Italy and Istituto di Astrofisica Spaziale e Fisica Cosmica/INAF Milan, Italy
Phone: +39-0382-375865 or +39-02-23699-468
Email: andrea.tiengo@iusspavia.it
Norbert Schartel
XMM-Newton Project Scientist
Tel: +34 91 8131 184
Email: Norbert.Schartel@sciops.esa.int
