Satellites' X-ray Eyes Catch Magnetar's Tantrums

Neutron stars are the densest material objects in the Universe, with 1-2 times the mass of the Sun squeezed into a ball less than 20 miles across. After their birth from a violent supernova, neutron stars usually give rise to the extreme phenomena known as pulsars, where the interaction of the spinning star core and high magnetic fields causes jets of particles and radiation to shoot out like a rotating lighthouse beacon. But, adding to the rogues' galery of extreme and mysterious objects in the Universe, neutron stars can sometimes become something even more baffling, a magnetar.

Magnetars are certainly intriguing to astronomers and physicists, providing a laboratory for extreme physics that could never be realized in an experiment on Earth. The magnetic field of a magnetar is amazingly strong - up to 10 trillion times stronger than a refrigerator magnet, and 100-1000 times stronger than that of a pulsar. The field at the surface of a magnetar is stronger than the so-called quantum critical field, the magnetic field in which the physics must be described with quantum field theory rather than classical physics, allowing normally-forbidden processes to occur. Magnetars are truly rare objects as there are currently only about 20 known.

Astronomers know that the intense magnetic field of a magnetar acts like a brake, slowing down the rate at which the neutron star spins. Occasionally, this is disrupted by a sudden increase in spin rate, known as a 'timing glitch'. A timing glitch, which is often followed by a large outburst of radiation in X-rays and gamma rays that we can observe, is thought to be caused by a 'star quake', a release of shaking energy through the neutron star analagous to an earthquake.

Recently, a team of researchers led by Lucien Kuiper of the National Institute for Space Research in the Netherlands, and including Willem Hermsen of the University of Amsterdam, KIPAC postdoctoral researcher Peter den Hartog, and Johnson Urama of the University of Nigeria, carried out a detailed study of the emission from one particularly active magnetar, known as 1E 1547.0-5408, over a 27 month period following a 2009 outburst. This period of intense activity was fortuitously observed by the European Space Agency's INTEGRAL hard X-ray observatory satellite and by NASA's Swift and Rossi X-ray Timing Explorer (RXTE) satellites.  In addition to studying magnetars, den Hartog had previously been involved in investigating "missing link" pulsars with magnetic field strengths approaching those of magentars, using data from the Fermi Gamma-ray space telescope.

The team discovered a record-breaking timing glitch in 1E 1547.0-5408, and high-energy unpulsed X-ray emission was detected immediately after it, which suggests that the magnetar has a sort of atmosphere of charged particles surrounding the neutron star. Unlike pulsars, which emit their radiation only along certain magnetic field lines, magnetars apparently have such strong magnetic fields that particles can be accelerated to produce X-rays all around the neutron.

The team's data also revealed another unexpected discovery, a new transient high energy X-ray pulse that took about 300 days to decay to undetectable levels. This detection may prove key to unraveling the processes at play in the extreme environment of a magnetar. In a model put forward in 2009 by Andrei Beloborodov from Columbia University, a star quake could twist the magnetic field lines that are anchored to a star's surface. The lines would then gradually untwist in a particular way, releasing magnetic energy and producing high energy X-ray emission like that seen with the long duration pulse in 1E 1547.0-5408.

It is extremely important to study magnetars in a broad energy range of X- and gamma rays to grasp all the phenomena which take place in such extreme environments.  These results have shown that there are vastly different processes which play a role in different energy ranges, at different times. Only the long-term monitoring and synergy with different observatories made these results possible.  Further study of magnetar antics in X-ray and gamma-ray light will be needed to confirm whether the Beloborodov mechanism is the correct or full story, but by combing the data from this magnetar after its outburst, the team showed the intriguing possibility that it may be the case.

This work is described in a paper published in the Astrophysical Journal (2012, 748, 133).  It was supported in part by NASA grant NNX10AJ54G.

Science Contact:
Peter den Hartog
KIPAC
hartog@stanford.edu

Tidbit author: Jack Singal