A pulsar that is able, without warning, to dramatically change the way in which it shines has been identified by an international team of astronomers. Using a satellite X-ray telescope combined with terrestrial radio telescopes the pulsar was found to flip on a roughly half-hour timescale between two extreme states; one dominated by X-ray pulses, the other by a highly-organised pattern of radio pulses.The emission of X-rays and radio waves by these pulsating neutron stars is able to change dramatically in seconds, simultaneously, in a way that cannot be explained with current theory. It suggests a quick change of the entire magnetosphere. In their research the team combined observations from the X-ray space telescope XMM-Newton and the radio telescope LOFAR.
It has been known for some time that some radio-emitting pulsars flip their behaviour between two (or even more) states, changing the pattern and intensity of their radio pulses. The moment of flip is both unpredictable and sudden. It is also known from satellite-borne telescopes that a handful of radio pulsars can also be detected at X-ray frequencies. However, the X-ray signal is so weak that nothing is known of its variability.
To find out if the X-rays could also flip the scientists studied a particular pulsar called PSR B0943+10, one of the first to be discovered. It has radio pulses which change in form and brightness every few hours with some of the changes happening within about a second.
"The behaviour of this pulsar is quite startling, it's as if it has two distinct personalities," said Dr Ben Stappers from The University of Manchester's School of Physics and Astronomy. "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."
Since the source is a weak X-ray emitter, the team used the most sensitive X-ray telescope in existence, the European Space Agency's XMM-Newton on board a spacecraft orbiting the Earth. The observations took place over six separate sessions of about six hours in duration.
To identify the exact moment of flip in the pulsar's radio behaviour the X-ray observations were tracked simultaneously with two of the largest radio telescopes in the world, LOFAR and the GMRT. What the scientists found was that whilst the X-rays did indeed change their behavior at the same time as the radio emission, as might have been expected, in the state where the radio signal is strong and organized the X-rays were weak, and when the radio emission switched to weak the X-rays got brighter.
"To our surprise we found that when the brightness of the radio emission halved, the X-ray emission brightened by a factor of two!," said project leader Wim Hermsen. "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."
"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," observed Geoff Wright from the University of Sussex. "But why a pulsar should undergo such dramatic and unpredictable changes is completely unknown."
The next step for the researchers is to look at other objects which have similar behavior to investigate what happens to the X-ray emission. Later this year there will be another round of simultaneous X-ray and radio observations of a second pulsar. These observations will include the Lovell telescope at Jodrell Bank Observatory.
For more information: Synchronous X-ray and Radio Mode Switches: a Rapid Transformation of the Pulsar Magnetosphere will be published in Science on Thursday 24 January. Journal reference: Science .
The Daily Gal;axy via University of Manchester