A magnetic monster’s dual personality

16 July 2012


An international team of scientists have discovered a rare hybrid star using ESA’s XMM-Newton and Nasa's SWIFT X-ray-telescopes.

Hybrid neutron star animation

While the first example of its kind was discovered two years ago, the team has now identified its long-sought twin. Featuring both the properties of magnetars and radio pulsars, the new hybrid star will help the team to understand more on the interior of neutron stars. The discovery was announced by ESA today, and published in The Astrophysics Journal.

“For many years magnetars and radio pulsars have been considered distant relatives in the neutron star family, separated by the unusually strong magnetic fields of the former,” said Dr Roberto Mignani (UCL Mullard Space Science Laboratory), co-author of the study and a member of the international team. “Now, we know that it is not the magnetic field alone which makes the difference, helping us to understand the distinction between these two neutron star classes."

Both magnetars and radio pulsars are neutron stars, created when the dead cores of massive stars collapsed in on themselves having burnt up all their fuel, exploding in dramatic supernova events.

Magnetars exhibit puzzling activity, consisting of frequent bursts of energetic X- and gamma-rays. Also known as Soft Gamma Repeaters and Anomalous X-ray Pulsars, it is believed that magnetars are powered by the most intense magnetic fields in the Universe, billions times more intense than the terrestrial one, 100 billion times more intense than that at the Sun's surface, and thousands of times larger than that of other, standard, radio pulsars.  

This project represents a definite step forward in our understanding of magnetic field evolution in neutron stars

Dr Silvia Zane

Radio pulsars are also spinning neutron stars – compact stellar remnants – and appear to pulse due to their rapid rotation. These pulses are seen when beams of radiation rotate through our line of sight from Earth, rather like the sweeping beam of a lighthouse. The radio pulsars, however, are surrounded by a magnetic field which is ~100-1000 times lower than that of magnetars. 

Named SWIFT J1822.3–1606 after the satellite that led to its discovery (SWIFT), and its source coordinates, the recently discovered star appears to be a hybrid of these two stellar breeds: the spinning stellar skeleton guises as a radio pulsar while hiding inside the star an intense internal magnetic field, much like a magnetar. With an internal field many times stronger than its external magnetic field, this new star appears to fit into a new class of ‘low-field magnetars’.

”This find proves that “magnetar activity” can occur also in neutron stars with external field in the radio-pulsar range, and opens new exciting perspectives in the study of the Galactic neutron star population, especially concerning the links between magnetars and “normal” radio-pulsars," said Dr Nanda Rea (CSIC-IEEC, Barcelona).

But where does the energy come from to power X-ray bursts and flares? In low-field magnetars, the energy comes from a component of the magnetic field which is confined in the star’s interior. As the field lines in the star’s interior unwind, energy is released as a burst of X-rays through fractures in the star’s ‘crust’.

"The problem is that these low-field magnetars are much more elusive than their ultra-magnetised companions,” said Dr Silvia Zane (UCL Mullard Space Science Laboratory). 

“Not only is their activity more sporadic, but their measurement requires a continuous and tenacious X-ray monitoring to catch the tiny slowdown of the star spin. Thanks to Swift and XMM-Newton, we have been able to nail down the second of these sources. This represents a definite step forward in our understanding of magnetic field evolution in neutron stars."

The new two low-field magnetars will help scientists to understand more on the initial distribution of magnetic field in Galactic neutron stars and its effect on the observed properties of neutron stars’ sources, an important field which is still poorly known. 

"The discovery of low-field magnetars strengthens the idea that magnetar-like behaviour may be much more widespread than believed in the past. Furthermore low-field magnetars would provide a first ever opportunity to unravel the inner properties of compact stars,” said Professor Roberto Turolla (UCL Mullard Space Science Laboratory and Padova University). “This would enable theorists to dig deeper and better understand these enigmatic objects." 


A new low magnetic field magnetar: the 2011 outburst of Swift J1822.3-1606