Chameleon Pulsar baffles Astronomers

A pulsar that is able, without warning, to dramatically change the way in which it shines has been identified by an international team, including scientists from The University of Manchester. 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-organized pattern of radio pulses.

The research was led by Professor Wim Hermsen from The Netherlands Institute for Space Research and the University of Amsterdam and will appear in the journal Science on the January 25, 2012.

Researchers from Jodrell Bank Observatory, as well as institutions around the world, used simultaneous observations with the X-ray satellite XMM-Newton and two radio telescopes: the LOw Frequency Array (LOFAR) in the Netherlands and the Giant Meter Wave Telescope (GMRT) in India, to reveal this so-far unique behavior.

Pulsars are small spinning stars that are about the size of a city, around 20 km in diameter. They emit oppositely directed beams of radiation from their magnetic poles. Just like a lighthouse, as the star spins and the beam sweeps repeatedly past the Earth, we see a brief flash.

Some pulsars produce radiation across the entire electromagnetic spectrum, including at X-ray and radio wavelengths. Despite being discovered more than 45 years ago, the exact mechanism by which pulsars shine is still unknown. 

It has been known for some time that some radio-emitting pulsars flip their behavior 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 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 that change in form and brightness every few hours, with some of the changes happening within about a second.

Dr. Ben Stappers from The University of Manchester’s School of Physics and Astronomy said: “The behavior 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.”

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 behavior, 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 while 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, when the radio emission switched to weak the X-rays got brighter.

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.”

Stappers, who co-leads the radio pulsar project with the LOFAR telescope, 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 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.

This research was a global project spanning a number of countries. The research was led by Wim Hermsen (SRON Netherlands Institute for Space Research, UvA), Lucien Kuiper and Jelle de Plaa (SRON), Jason Hessels and Joeri van Leeuwen (ASTRON en UvA), Dipanjan Mitra (NCFRA-TIFR, Pune, India), Joanna Rankin (University of Vermont, Burlington, VS), Ben Stappers (University of Manchester, UK), Geoffrey Wright (University of Sussex, UK). The Pulsar Working Group and the Builders Group from the  LOFAR-telescope, which was at the time still in the commissioning phase,  gave support to these observations.

The results of this research, entitled: “Synchronous X-ray and Radio Mode Switches: a Rapid Transformation of the Pulsar Magnetosphere” are published in Science, January 25, 2013.