Magnetars, the ultra-dense remains of dead stars with extremely strong magnetic fields, are not as uncommon as astronomers once believed based on a campaign from NASA's Chandra X-ray Observatory and several other satellites that revealed an elderly magnetar, indicating the objects may be more diverse and more common that once thought.
When a giant star runs out of fuel to burn, its core collapses and forms a rapidly spinning neutron star. The ultra-dense neutron stars spin several times a second. But occasionally neutron stars spin slowly - only once every few seconds - yet still emit huge 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," according to NASA. The extremely strong magnetic fields of magnetars are as much as 1000 times stronger than those of regular neutron stars.
The big news from the Chanda mission is that an observed magnetar known as SGR 0418 does not have a typical magnetar-strength magnetic field, but rather a comparatively weak magnetic field similar to an average neutron star.
"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 in Barcelona, Spain, said in a press statement. "This has important consequences for how we think neutron stars evolve in time, and for our understanding of supernova explosions."
GianLuca Israel of the National Institute of Astrophysics in Rome said SGR 0418 is an "anomaly among anomalies."
"A magnetar is different from typical neutron stars, but SGR 0418 is different from other magnetars as well."
Analysis of SGR 0418 pins it at about half million years old and the researches think there are likely other elderly magnetars out in the cosmos.
"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."
The study of the magnetar is published in the Astrophysical Journal.
In the photo above, the image on the left from NASA's Chandra X-ray Observatory shows SGR 0418 as a pink source in the middle. Optical data from the William Herschel telescope in La Palma and infrared data from NASA's Spitzer Space Telescope are shown in red, green and blue.
On the right is an artist's impression showing a close-up view of SGR 0418. This illustration highlights the weak surface magnetic field of the magnetar, and the relatively strong, wound-up magnetic field lurking in the hotter interior of the star. The X-ray emission seen with Chandra comes from a small hot spot, not shown in the illustration. At the end of the outburst this spot has a radius of only about 160 meters, compared with a radius for the whole star of about 12 km.