ByANDY FLEMING
This illustration shows the pulsar with glowing cones of
radiation stemming from its magnetic poles – a state referred to as 'radio-bright'
mode. Credit: ESA/ATG medialab.
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 organised 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 25th January 2012.
Researchers from Jodrell Bank 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 behaviour.
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 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.
Dr Ben Stappers from Manchester’s School of
Physics & Astronomy says: “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.”
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 behaviour at the same time as the radio
emission, as might have been expected, in the state where the radio signal is
strong and organised the X-rays were weak, and 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.”
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 next step for the researchers is to look at
other objects which have similar behaviour 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.
Original Source: University of Manchester/Jodrell Bank Observatory
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