Mysterious magnetar boasts one of strongest magnetic fields in Universe

Mysterious magnetar boasts one of
strongest magnetic fields in
Universe
Artist's impression of a magnetar
Credit: ESA/ATG Medialab
(Phys.org) —A team of astronomers
including two researchers from UCL's
Mullard Space Science Laboratory has
made the first ever measurement of
the magnetic field at a specific spot
on the surface of a magnetar.
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 have now revealed a
strong, localised surface magnetic
field on one.
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. A
few years ago, this star was
discovered to have a relatively gentle
dipolar magnetic field compared to
other magnetars. 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.
Sometimes, the surface breaks and
the hidden magnetic field leaks out
(artist's impression) Credit: ESA/ATG
Medialab
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," says
Silvia Zane (UCL Mullard Space
Science Laboratory), 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.
Magnetars are far too small – only
around 20km across – and distant for
even the best telescopes to see any
details on their surfaces. They
appear just as dots of light in
astronomers' observations. So the
team had to look for indirect signs of
variation on SGR 0418+5729's
surface. To do this, they measured
how the magnetar's X-ray emissions
varied as the star rotates.
"SGR 0418+5729 rotates once every 9
seconds. We found that at one point
during the rotation, the magnetar's
X-ray brightness drops sharply. That
means something on or near one
part of the surface is absorbing the
radiation," adds Roberto Turolla (an
honorary professor at MSSL and co-
author of the study).
Closeup of the magnetic field leaking
out of a magnetar (artist's
impression) Credit: ESA/ATG Medialab
The team believes that a
concentration of protons over a small
area of the magentar's surface –
perhaps as little as a few hundred
metres across – is absorbing the X-
rays. The protons are confined to a
small volume near the surface by a
strong, localised magnetic field
emerging from the magnetar 's
interior, giving powerful evidence that
a strong and twisted internal
magnetic field lurks beneath the
surface.
"This exciting discovery also confirms
that, in principle, other pulsars with
relatively low external magnetic fields
might conceal a similar strong
magnetic field in the interior. As a
result, many pulsars may switch on
and become active flaring magnetars
for a while, so in the future we may
discover much more magnetars than
what we previously thought. This call
for a major revision of our current
ideas of magnetic field formation and
amplification in neutron stars,"
explains Zane.
The study is published in the journal
Nature .
More information: "A variable
absorption feature in the X-ray
spectrum of a magnetar," by A.
Tiengo et al is published in Nature ,
15 August 2013.
Provided by University College
London