Graphic shows a magnetar called SGR 0418+5729 |
Magnetars
-- the dense remains of dead stars that erupt sporadically with bursts of
high-energy radiation -- are some of the most extreme objects known in the
Universe. A major campaign using NASA's Chandra X-ray Observatory and several
other satellites shows magnetars may be more diverse -- and common -- than
previously thought.
When a
massive star runs out of fuel, its core collapses to form a neutron star, an
ultradense object about 10 to 15 miles wide. The gravitational energy released
in this process blows the outer layers away in a supernova explosion and leaves
the neutron star behind.
Most
neutron stars are spinning rapidly -- a few times a second -- but a small
fraction have a relatively low spin rate of once every few seconds, while
generating occasional large blasts of X-rays. Because the only plausible source
for the energy emitted in these outbursts is the magnetic energy stored in the
star, these objects are called "magnetars."
Most
magnetars have extremely high magnetic fields on their surface that are ten to
a thousand times stronger than for the average neutron star. New observations
show that the magnetar known as SGR 0418+5729 (SGR 0418 for short) doesn't fit
that pattern. It has a surface magnetic field similar to that of mainstream
neutron stars.
"We
have found that SGR 0418 has a much lower surface magnetic field than any other
magnetar," said Nanda Rea of the Institute of Space Science in Barcelona,
Spain. "This has important consequences for how we think neutron stars
evolve in time, and for our understanding of supernova explosions."
The
researchers monitored SGR 0418 for over three years using Chandra, ESA's
XMM-Newton as well as NASA's Swift and RXTE satellites. They were able to make
an accurate estimate of the strength of the external magnetic field by
measuring how its rotation speed changes during an X-ray outburst. These
outbursts are likely caused by fractures in the crust of the neutron star
precipitated by the buildup of stress in a relatively strong, wound-up magnetic
field lurking just beneath the surface.
"This
low surface magnetic field makes this object an anomaly among anomalies,"
said co-author GianLuca Israel of the National Institute of Astrophysics in
Rome. "A magnetar is different from typical neutron stars, but SGR 0418 is
different from other magnetars as well."
By
modeling the evolution of the cooling of the neutron star and its crust, as
well as the gradual decay of its magnetic field, the researchers estimated that
SGR 0418 is about 550,000 years old. This makes SGR 0418 older than most other
magnetars, and this extended lifetime has probably allowed the surface magnetic
field strength to decline over time. Because the crust weakened and the
interior magnetic field is relatively strong, outbursts could still occur.
The case
of SGR 0418 may mean that there are many more elderly magnetars with strong
magnetic fields hidden under the surface, implying that their birth rate is
five to ten times higher than previously thought.
"We
think that about once a year in every galaxy a quiet neutron star should turn
on with magnetar-like outbursts, according to our model for SGR 0418,"
said Josè Pons of the University of Alacant in Spain. "We hope to find
many more of these objects."
Another
implication of the model is that the surface magnetic field of SGR 0418 should
have once been very strong at its birth a half million years ago. This, plus a
possibly large population of similar objects, could mean that the massive
progenitor stars already had strong magnetic fields, or these fields were
created by rapidly rotating neutron stars in the core collapse that was part of
the supernova event.
If large
numbers of neutron stars are born with strong magnetic fields then a
significant fraction of gamma-ray bursts might be caused by the formation of
magnetars rather than black holes. Also, the contribution of magnetar births to
gravitational wave signals -- ripples in space-time -- would be larger than
previously thought.
The
possibility of a relatively low surface magnetic field for SGR 0418 was first
announced in 2010 by a team with some of the same members. However, the
scientists at that time could only determine an upper limit for the magnetic
field and not an actual estimate because not enough data had been collected.
SGR 0418
is located in the Milky Way galaxy at a distance of about 6,500 light years
from Earth. These new results on SGR 0418 appear online and will be published
in the June 10, 2013 issue of The Astrophysical Journal. NASA's Marshall
Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA's
Science Mission Directorate in Washington. The Smithsonian Astrophysical
Observatory controls Chandra's science and flight operations from Cambridge,
Mass.
For
Chandra images, multimedia and related materials, visit: http://www.nasa.gov/chandra
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