An international team of astronomers using X-ray data from NASA’s Swift, RXTE, Chandra and ESA’s XMM-Newton Space Observatories has discovered a magnetar called SGR 0418+5729 (SGR 0418 for short) – an exotic object in our Milky Way Galaxy with the lowest surface magnetic field ever found for this type of neutron star.
Composite image of the magnetar SGR 0418 (X-ray: NASA / CXC / CSIC-IEEC / N.Rea et al.; optical: Isaac Newton Group of Telescopes / WHT; infrared: NASA / JPL-Caltech)
Magnetars are a type of neutron star that occasionally generates bursts of X-rays. They usually have a very strong magnetic field on their surface, ten to a thousand times stronger than for an average neutron star.
A new study, published in the Astrophysical Journal , shows that the magnetar SGR 0418 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. This has important consequences for how we think neutron stars evolve in time, and for our understanding of supernova explosions,” said lead author Dr Nanda Rea of the Institute of Space Science in Barcelona, Spain.
SGR 0418 is located in our galaxy some 6,500 light years away. The team was able to make an accurate estimate of the strength of the SGR 0418’s 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 Dr 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 team 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. We hope to find many more of these objects,” concluded co-author Dr Josè Pons of the University of Alacant in Spain.
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Bibliographic information: Rea N. et al. 2013. The outburst decay of the low magnetic field magnetar SGR 0418+5729. ApJ 770, 65; doi: 10.1088/0004-637X/770/1/65
