17 Mar 2006

Known as anomalous X-ray pulsars (AXPs), this type of neutron star was first spotted pulsing low-energy X-rays into space during the 1970s by the Uhuru X-ray satellite. AXPs are extremely rare with only seven known to exist. Initially, AXPs were thought to be members of an X-ray binary system, with the X-ray emission produced by matter falling from a companion star onto the AXP.

An alternative theory is that each AXP is a neutron star, with sweeping beams of energy through space like a cosmic lighthouse. However, this scenario requires the AXP's magnetic field to be a thousand million times stronger than the strongest steady magnetic field achievable in a laboratory on Earth. Recent INTEGRAL observations show that this magnetic solution is correct.

The newly detected emission of hard X-rays and soft gamma rays, with energies > 10 keV, comes as a regular pulse every 6–12 seconds depending upon which AXP is being observed.

Discovered in three of the four AXPs studied, the emission has a distinctive energy signature that forces astronomers to consider that they are produced by super-strong magnetic fields.

"The amount of energy in the hard X-rays and soft gamma rays is ten to almost one thousand times more than can be explained by a kind of magnetic friction between the spinning AXP and surrounding space," said Wim Hermsen of SRON, the Netherlands Institute for Space Research, Utrecht, who together with SRON colleagues made the observations. This leaves magnetic field decay as the only viable alternative.

Neutron stars with super-strong magnetic fields are dubbed magnetars. Magnetars are the end product of a massive star that has exploded at the end of its life, leaving only the star's central core. Each magnetar is only around 15 kilometres in diameter yet contains more than one and a half times the mass of the Sun.
 
Magnetars are also responsible for the soft gamma-ray repeaters (SGRs), which explosively release massive quantities of energy when catastrophic reorganisations of their magnetic fields spontaneously take place. The big difference between an SGR and an AXP is that the process is continuous rather than explosive in an AXP and less energetic.

Exactly how that happens is the focus of future work. It is possible that SGRs, of which five are known, turn into AXPs once they have released enough of their energy into space.
 
All galactic AXPs are clustered towards the plane of the Milky Way, and some are associated with supernova remnants, indicating that they are the result of recent stellar explosions. An additional AXP is known to reside in the Small Magellanic Cloud satellite galaxy.