Astronomers Puzzled By Chameleon-Like Behavior Of Reverse Pulsar
January 25, 2013

Astronomers Puzzled By Chameleon-Like Behavior Of Reverse Pulsar

April Flowers for - Your Universe Online

Pulsars are one of the most baffling classes of astronomical objects. Originally discovered as flickering sources of radio waves, pulsars were soon interpreted as rapidly rotating and strongly magnetized neutron stars about the size of a small city. Because of the oppositely directed beams of radiation emitted from their magnetic poles, pulsars are like cosmic lighthouses. The star spins and the beams sweep past the Earth, displaying a brief flash. The first was discovered in 1967, and since then approximately 2000 have been cataloged, yet a detailed understanding of the mechanism that powers the phenomenon still escapes the grasp of 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," explains Wim Hermsen from SRON--the Netherlands Institute for Space Research. "How exactly the particles are stripped off the neutron star's surface and accelerated to such high energy, however, is still largely unclear," he adds.

The new study, published in a recent issue of Science, is based on observations of the pulsar known as PSR B0943+10. These observations were made simultaneously using the European Space Agency's (ESA) XMM-Newton telescope — the most sensitive X-ray telescope in existence - hunting for X-rays and radio waves. Studying the emission in different wavelengths, the international research team was able to discern which of various possible physical processes take place near the magnetic poles of pulsars.

"The behaviour of this pulsar is quite startling, it's as if it has two distinct personalities. As PSR B0943+10 is one of the few pulsars also known to emit X-rays, finding out how this higher energy radiation behaves as the radio changes could provide new insight into the nature of the emission process," Dr Ben Stappers from The University of Manchester's School of Physics and Astronomy said in a statement.

This campaign of observation challenges all existing models for pulsar emission instead of narrowing down the possible mechanisms suggested by prior theories, reopening the question of how these intriguing sources are powered.

"Many pulsars have a rather erratic behavior: 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," says Hermsen. "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."

The switch between what are called the "radio-bright" and "radio-quiet" states correlate to the pulsar's dynamics, recent studies have shown. Pulsars rotate, and as they do, their spinning period slows down gradually. In some cases, the slow-down has been observed to accelerate and slow down again, seemingly in conjunction with the switch between "radio-bright" and "radio-quiet" states. A connection between a pulsar's immediate vicinity and, on a grander scale, its co-rotating magnetosphere, which may extend up to about 31,000 miles for objects like PSR B0943+10, is suggested by the existence of correlated variations in both the rotation and emission. The pulsar's global environment must go through a very rapid, and reversible, transformation for the radio emission to vary so radically on the short timescales that have been observed.

"Since the switch between a pulsar's bright and quiet states links phenomena that occur on local and global scales, a thorough understanding of this process could clarify several aspects of pulsar physics. Unfortunately, we have not yet been able to explain it," says Hermsen.

The team set out to search for an analogous pattern at a different wavelength — in X-rays — in order to understand this switching pattern. PSR B0943+10 is well known for its switching behavior at radio wavelengths and for its X-ray emission, which is brighter than might be expected for its age, making it a perfect candidate for this study.

"Young pulsars shine brightly in X-rays because the surface of the neutron star is still very hot. But PSR B0943+10 is five million years old, which is relatively old for a pulsar: the neutron star's surface has cooled down by then," explains Hermsen.

Astronomers, who believe that this emission comes from the magnetic poles — the sites on the star's surface where the acceleration of charged particles is triggered - know only a handful of old pulsars that shine in X-rays.

"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 adds.

Depending on whether the electric and magnetic fields at play allow charged particles to escape freely from the neutron star's surface, there are two main models astronomers used to describe these processes. In both models, it is believed that the X-ray emission follows that of radio waves. The emission that is observed in each case, however, is characterized by different spectral and temporal characteristics. By monitoring both X-ray and radio waves at the same time, the team hoped to be able to discern between the models.

Gaining observation time on the requested telescopes was a lengthy and involved 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," says Hermsen, "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."

The team performed their observations in late 2011, combining the X-ray monitoring of the XMM-Newton with simultaneous radio wave observations at the Giant Metrewave Radio Telescope (GMRT) in India and the recently inaugurated Low Frequency Array (LOFAR) in the Netherlands. The LOFAR was used during it's commissioning phase, during the testing of its science operations.

"The X-ray emission of pulsar PSR B0943+10 beautifully mirrors the switches that are seen at radio wavelengths but, to our surprise, the correlation between these two emissions appears to be inverse: when the source is at its brightest in radio waves, it reaches its faintest in X-rays, and vice versa," says Hermsen.

Data from the XMM-Newton showed that the source pulsates in X-rays only during the X-ray bright phase. This corresponds to the radio-quiet state of radio wavelengths. The X-ray emission during this phase appears to be the sum of two components. The first is a pulsating component comprised of thermal X-rays, seen to switch off during the X-ray quiet phase. The second is a persistent component consisting of non-thermal X-rays. Neither of the main prediction models for pulsar emission predicted such behavior.

"The data collected during our monitoring campaign are truly challenging our understanding of pulsars, since no current model is able to explain them," comments Hermsen. "In the second half of 2013, we plan to repeat the same study for another pulsar, PSR B1822-09, which exhibits similar radio emission properties, but is characterized by a different geometrical configuration. This will allow us to study these extreme objects under different viewing angles," he adds.

The unexpected results of this study will keep theoretical astrophysicists busy investigating possible physical mechanisms that could cause the sudden and drastic changes to the pulsar's entire magnetosphere and result in such a curious emission.

"The unpredictable behavior of this pulsar, revealed using the great sensitivity of the telescopes on board XMM-Newton, may require a radically new approach to study the fundamental processes that power these fascinating objects," comments Norbert Schartel, XMM-Newton Project Scientist at ESA.

Commenting on the study's findings, the project leader Wim Hermsen says, "To our surprise we found that when the brightness of the radio emission halved, the X-ray emission brightened by a factor of two! Furthermore the intense X-rays have a very different character from those in the radio-bright state, since they seem to be thermal in origin and to pulse with the neutron star's rotation period."

Dr Stappers says this is an exciting discovery. "As well as brightening in the X-rays we discovered that the X-ray emission also shows pulses, something not seen when the radio emission is bright. This was the opposite of what we had expected. I've likened the changes in the pulsar to a chameleon. Like the animal the star changes in reaction to its environment, such as a change in temperature."

Geoff Wright from the University of Sussex adds, "Our observations strongly suggest that a temporary "hotspot" appears close to the pulsar's magnetic pole which switches on and off with the change of state. But why a pulsar should undergo such dramatic and unpredictable changes is completely unknown."

The team will continue their investigations, looking at other objects which have similar behavior to understand what happens to the X-ray emission. Later in 2013, they will have another round of simultaneous X-ray and radio wave observations that will include the Lovell telescope at Jodrell Bank Observatory, of another pulsar.