May 24, 2013
NASA X-Ray Observatory Reveals Magnetars More Common Than Thought
Lee Rannals for redOrbit.com — Your Universe Online
NASA's Chandra X-ray Observatory has revealed that some of the most extreme objects in the universe may be more common than previously thought.
Magnetars are the dense remains of dead stars that erupt sporadically with bursts of high-energy radiation. When a massive star runs out of fuel its core collapses to form a neutron star. Most neutron stars spin rapidly, but a small fraction have a relatively low spin rate while also generating occasional large blasts of X-rays. The only plausible source of the energy emitted in these outbursts is magnetic energy stored in the star, giving them the name "magnetars."
Chandra, along with other satellite observations, has helped scientists see that magnetars may actually be even more diverse and common than predicted.
"We have found that SGR 0418 has a much lower surface magnetic field than any other magnetar," Nanda Rea of the Institute of Space Science (ICE) in Barcelona, Spain, said in a statement. "This has important consequences for how we think neutron stars evolve in time, and for our understanding of supernova explosions."
The team monitored a magnetar known as SGR 0418+5729, which has a surface magnetic field to that of mainstream neutron stars. They monitored SGR 0418 for over three years using Chandra, ESA's XMM-Newton and NASA's Swift and RXTE satellites. They were able to use these instruments to make accurate estimates of the strength of the external magnetic field by measuring how its rotation speed changes during an X-ray outburst.
"This low surface magnetic field makes this object an anomaly among anomalies," said co-author GianLuca Israel of the National Institute of Astrophysics (IASF) in Rome. "A magnetar is different from typical neutron stars, but SGR 0418 is different from other magnetars as well."
The researchers modeled the evolution of the cooling of the neutron star and its crust. By doing this, they were able to estimate that SGR 0418 is about 550,000 years old, making it older than most other magnetars. The team believes after observing SGR 0418 that there are more elderly magnetars with strong magnetic fields hidden under the surface, making their birth rate 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 JosÃ¨ Pons of the University of Alacant (UA) in Spain. “We hope to find many more of these objects."
Scientists first became aware of the hidden magnetic field in SGR 0418 back in 2010. They reported in the journal Science Express about how SGR 0418 stands out against other magnetars, opening up the door to the latest research.