Below is a direct copy of the press release found on the ASTRON news site. The paper it's based on was published in Science today.
Chameleon pulsar baffles astronomers
An international team - led by Dutch astronomers (SRON,
NOVA and ASTRON) - has made a tantalizing discovery about the way pulsars emit
radiation. 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 (among others); the research results appear in today’s [date] issue of the journal Science.
Pulsars are small
spinning stars about 20 km in diameter -the size of a small city - with a mass
that roughly compares to the mass of our Sun. They have a strong magnetic field
that is approximately one million times stronger than the fields we can make in
laboratories on Earth. The pulsar emits a beam of radiation. As the star spins
and the radio beam sweeps repeatedly over Earth we detect a short pulse of
radiation, a bit like a lighthouse. Some pulsars emit radiation across the
entire electromagnetic spectrum, including both X-ray and radio wavelengths. Although
pulsars were discovered more than 40 years ago the exact mechanism by which
pulsars shine is still unknown.
It has been known for some time that some radio 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
(often within a single rotation period). 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 - could it be that the X-rays also flip?
Moments of flip
The scientists studied a particular pulsar called PSR B0943+10, one of the first pulsars discovered. The pulses from PSR B0943 + 10 change in form and brightness every few hours, and these changes happen within about a second. It is as if the pulsar has two distinct personalities. As PSR B0943+10 is one of the few pulsars also known to emit X-ray radiation, knowing how this pulsar behaved in X-rays during 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 operation,
the ESA-funded XMM-Newton. The observations took place over six separate sessions
of about six hours in duration. To identify the exact flips in the pulsar’s
radio behaviour the X-ray observations were tracked simultaneously with two of
the most powerful radio telescopes in the world, GMRT and LOFAR (developed and
built by ASTRON, the Netherlands
Institute for Radio Astronomy).
Two X-ray states as well
The results were totally surprising. The X-rays
did indeed change their behaviour synchronously with the radio emission,
as might have been expected. But 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 intensified. “To
our surprise we found that when the brightness of the radio emission decreased
to half the original brightness, the X-ray emission brightened by a factor of
two!” says project leader Wim Hermsen. 'And only then the X-ray emission is
pulsed. Lucien Kuiper, who scrutinised the data from XMM-Newton , concluded that
this strongly suggests that a temporary ”hotspot” close
to the pulsar’s magnetic pole switches on and off with the change of state.
Most striking was that this metamorphosis takes
place within seconds, after which the pulsar remains stable in its new state
for a few hours. Why a pulsar should
undergo such dramatic and unpredictable changes cannot be explained by current theory.
It strongly suggests a quick change of the entire magnetosphere”.
Tale of the unexpected
This unexpected chameleon-like behaviour of the radio pulsar PSR B0943+10 bolsters fundamental research into the physical processes which occur in the extreme conditions which occur in the magnetosphere of pulsars, 45 years after the discovery of neutron stars. Wim Hermsen and his colleagues have been granted new observation time on XMM-Newton. By combining their X-ray observations with observations from a number of radio telescopes (Westerbork, GMRT and Lovell telescopes) the astronomers will also be able to simultaneously observe pulsar PSR B1822-09. In radio wavelengths PSR B1822-09 shows similar flips as PSR B0943+10.Team
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 (link!) will appear in Science today.
