Scientists using NASA’s Chandra X-ray Observatory and other satellites have found that magnetars — the dense remains of dead stars that erupt sporadically with bursts of high-energy radiation — 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 ultra-dense object about 16 to 24 kilometres 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) does not 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.
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.
By modelling 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. We hope to find many more of these objects,” said Jose Pons of the University of Alacant in Spain.