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Updated: January 31, 2013 03:57 IST

Pulsar behaviour defies theories

Vasudevan Mukunth
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The artist's impression (top left) shows the pulsar with cones of radiation from its magnetic poles. The X-ray pulses from XMM-Newton (top right), and radio pulses from LOFAR and GMRT (bottom right) are emitted during weak and bright modes, respectively. The illustration (bottom left) shows the pulsar in a radio-weak mode. PHOTO: XMM-NEWTON/ESA
The artist's impression (top left) shows the pulsar with cones of radiation from its magnetic poles. The X-ray pulses from XMM-Newton (top right), and radio pulses from LOFAR and GMRT (bottom right) are emitted during weak and bright modes, respectively. The illustration (bottom left) shows the pulsar in a radio-weak mode. PHOTO: XMM-NEWTON/ESA

Unlike pulsars that emit either at X-ray or radio frequencies, a pulsar located 3,500 light years away has been found emitting both radio and X-ray radiation in sync

When a star heavier than our Sun by eight to 50 times runs out of hydrogen, it blows away its outermost layers of gas in a supernova explosion and undergoes a gravitational collapse.

The remnant of this is a core of neutrons called a neutron star — usually around 20 km across, densely compacted and highly magnetised.

When the neutron star is spinning, it emits strong radiation from its poles which can be detected only when it is pointed towards the Earth. Because of the spin, these emissions point at Earth periodically, making it appear as though the neutron star is pulsating. For this reason, spinning neutron stars are called pulsars.

In a paper published in Science journal on January 25, an international team of scientists working on the LOFAR telescope in the Netherlands and the GMRT telescope near Pune, India, announced a strange behaviour of a pulsar PSR B0943+10.

Located 3,500 light years from Earth, PSR B0943+10 has been known since the 1980s to emit radiation in the radio frequency. Pulsars usually emit radiation at radio or X-ray frequencies exclusively, although ones emitting purely gamma radiation have also been found.

Some other pulsars also switch modes, emitting brightly for some time and weakly for the some other time, within seconds. While this behaviour has been known for some time, a self-consistent theory to explain it still eludes astrophysicists.

However, for the first time ever, a pulsar, PSR B0943+10, has been observed switching between two different modes of radiation – X-ray and radio – within a second. The X-ray mode showed itself when the radio mode had weakened. Also, anomalous weak radio signals were observed when the pulsar was in the bright X-ray mode.

As Dr. Wim Hermsen, the lead author of the study, in an email to this Correspondent noted: “The details of our findings were totally surprising, and cannot be explained with the different presently prevailing theories.” Dr. Hermsen is associated with the Netherlands Institute for Space Research.

Dr. Dipanjan Mitra, from the National Centre for Radio Astrophysics (NCRA), Pune, led the Indian effort of the study, working with the Giant Meterwave Radio Telescope (GMRT). Alongside Dr. Joanna Rankin of the University of Vermont, Dr. Mitra continuously monitored the pulsar in 2009, and laid the foundation for the published paper.

Spotting the anomaly

The X-ray emissions were tracked using the European Space Agency’s XMM-Newton orbiting space telescope, and the radio signals were studied using the Low Frequency Array (LOFAR) near Exloo, the Netherlands, and GMRT. While LOFAR can detect only at a fixed frequency, GMRT can sweep a range of frequencies.

First, the radio and X-ray emissions were recorded simultaneously over several days. The times at which the “flips” occurred were first noted in the radio data. Then, the scientists studied the behaviour of the X-ray mode during these flip times.

They found that the radio and X-ray modes were perfectly synchronised with each other. While the radio signals were strong and highly organised, the X-ray signals were weak and disorganised, and vice versa. Also, each mode lasted typically for seven hours, although over the course of the study, they also observed that the X-ray mode sometimes lasted for only half-hour or so, making it unpredictable.

As Dr. Mitra said, “It’s as if the pulsar has two personalities. We knew about this behaviour in the radio, but now we see it in the X-rays as well.”

The team also found that the X-ray emission seemed thermal in origin. This suggests that a temporary “hotspot” appears close to the pulsar’s magnetic pole which switches on and off as the pulsar flips.The scientists will go on to analyse the data further to see if they have missed out anything, such as how much of an influence the pulsar’s plasma-atmosphere has on such emissions.

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