Erratic pulsar leaves astronomers baffled
New observations of a highly variable pulsar using ESA's XMM-Newton are perplexing astronomers.
Monitoring this pulsar simultaneously in X-rays and radio waves, astronomers have revealed that this source, whose radio emission is known to 'switch on and off' periodically, exhibits the same behaviour, but in reverse, when observed at X-ray wavelengths.
It is the first time that a switching X-ray emission has been detected from a pulsar, and the properties of this emission are unexpectedly puzzling.
As no current model is able to explain this switching behavior, which occurs within only a few seconds, these observations have reopened the debate about the physical mechanisms powering the emission from pulsars.
Few classes of astronomical objects are as baffling as pulsars -- which were discovered as flickering sources of radio waves and soon after interpreted as rapidly rotating and strongly magnetized neutron stars.
Even though about 2,000 pulsars have been found since the first was discovered in 1967, a detailed understanding of the mechanisms that power them still eludes astronomers.
"There is a general agreement about the origin of the radio emission from pulsars: it is caused by highly energetic electrons, positrons and ions moving along the field lines of the pulsar's magnetic field, and we see it pulsate because the rotation and magnetic axes are misaligned," Wim Hermsen from SRON, the Netherlands Institute for Space Research in Utrecht, The Netherlands, said.
"How exactly the particles are stripped off the neutron star's surface and accelerated to such high energy, however, is still largely unclear," he said.
Hermsen led a new study based on observations of the pulsar known as PSR B0943+10, which were performed simultaneously in X-rays, with ESA's XMM-Newton, and in radio waves.
By probing the emission from the pulsar at different wavelengths, the study had been designed to discern which of various possible physical processes take place in the vicinity of the magnetic poles of pulsars.
Instead of narrowing down the possible mechanisms suggested by theory, however, the results of Hermsen's observing campaign challenge all existing models for pulsar emission, reopening the question of how these intriguing sources are powered.
"Many pulsars have a rather erratic behaviour: in the space of a few seconds, their emission becomes weaker or even disappears for a while, just to go back to the previous level after some hours," Hermsen said.
"We do not know what causes such a switch, but the fact that the pulsar keeps memory of its previous state and goes back to it suggests that it must be something fundamental," Hermsen said.
Hermsen and his colleagues planned to search for an analogous pattern at a different wavelength -- in X-rays -- to investigate what causes this switching behaviour. They chose as their subject PSR B0943+10, a pulsar that is well known for its switching behaviour at radio wavelengths and for its X-ray emission, which is brighter than might be expected for its age.
Astronomers know of only a handful of old pulsars that shine in X-rays and believe that this emission comes from the magnetic poles -- the sites on the neutron star's surface where the acceleration of charged particles is triggered.
"We think that, from the polar caps, accelerated particles either move outwards to the magnetosphere, where they produce radio emission, or inwards, bombarding the polar caps and creating X-ray emitting hot-spots," Hermsen said.
There are two main models that describe these processes, depending on whether the electric and magnetic fields at play allow charged particles to escape freely from the neutron star's surface.
In both cases, it is believed that the emission of X-rays follows that of radio waves, but the emission that is observed in each scenario is characterized by different temporal and spectral characteristics. Monitoring the pulsar in X-rays and radio waves at the same time, the astronomers hoped to be able to discern between the two models.
Obtaining observing time on the requested telescopes turned out to be a rather lengthy procedure.
"We needed very long observations, to be sure that we would record the pulsar switching back and forth between bright and quiet states several times," Hermsen said.
"So we asked for a total of 36 hours of observation with XMM-Newton. This is quite a lot of time, and it took us five years before our proposal was accepted," he said.
The observations were performed in late 2011. The X-ray monitoring performed with XMM-Newton was accompanied by simultaneous observations at radio waves from the Giant Metrewave Radio Telescope (GMRT) in India and the recently inaugurated Low Frequency Array (LOFAR) in the Netherlands, which was used during its commissioning phase, while testing its science operations.
The XMM-Newton data also show that the source pulsates in X-rays only during the X-ray-bright phase -- which corresponds to the quiet state at radio wavelengths.
During this phase, the X-ray emission appears to be the sum of two components: a pulsating component consisting of thermal X-rays, which is seen to switch off during the X-ray-quiet phase, and a persistent one consisting of non-thermal X-rays. Neither of the leading models for pulsar emission predicts such behaviour.