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. For all images and full captions see
below.
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.
Notes
Dr Ben Stappers co-leads the radio pulsar project with the LOFAR telescope.
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 will be
published in Science on Friday 25 January.
For more information contact:
Daniel Cochlin
Media Relations Officer
Faculty of Engineering and Physical Sciences
The University of Manchester
Tel: 0161 275 8387
Email: Daniel.Cochlin@manchester.ac.uk
Full captions to images
Artist's impression of a pulsar in radio-bright mode
This illustration shows a pulsar with glowing cones of radiation
stemming from its magnetic poles – a state referred to as 'radio-bright'
mode.
Pulsars were discovered in 1967 as flickering sources of radio waves and
soon after interpreted as rapidly rotating and strongly magnetised
neutron stars. There is a general agreement about the origin of the
radio emission from pulsars: it is caused by highly energetic electrons,
positrons and ions moving along the field lines of the pulsar's
magnetic field. When they are accelerated to very high energies,
particles radiate at radio wavelengths. The radio emission is
concentrated in cones that stem from the pulsar's magnetic poles, and we
see it pulsate because the rotation and magnetic axes are misaligned.
Many pulsars exhibit a rather erratic behaviour: in the space of a few
seconds, their radio emission becomes weaker or even disappears for a
while, then returns to the previous level after some hours. The
mechanisms causing this switch between what are usually referred to as
'radio-bright' and 'radio-quiet' states are still largely unknown.
Observations of the five-million year-old pulsar known as PSR B0943+10,
performed simultaneously with ESA's XMM-Newton X-ray observatory and
ground-based radio telescopes, revealed that this source exhibits
variations in its X-ray emission that mimic in reverse the changes seen
in radio waves. No current model is able to predict what could cause
such sudden and drastic changes to the pulsar's entire magnetosphere and
result in such a curious emission. Credit: ESA/ATG medialab
Artist's impression of a pulsar in X-ray-bright/radio-quiet mode
This illustration shows a pulsar with glowing 'hot-spots' that are
located at its magnetic poles, the likely sites of X-ray emission from
old pulsars. In particular, the illustration shows the pulsar in a state
characterised by bright X-ray emission, arising from the polar caps,
and relatively low radio emission from the cones that stem from the
pulsar's magnetic poles ('X-ray-bright/radio-quiet' mode).
Pulsars were discovered in 1967 as flickering sources of radio waves and
soon after interpreted as rapidly rotating and strongly magnetised
neutron stars. There is a general agreement about the origin of the
radio emission from pulsars: it is caused by highly energetic electrons,
positrons and ions moving along the field lines of the pulsar's
magnetic field. When they are accelerated to very high energies,
particles radiate at radio wavelengths. The radio emission is
concentrated in cones that stem from the pulsar's magnetic poles, and we
see it pulsate because the rotation and magnetic axes are misaligned.
Many pulsars exhibit a rather erratic behaviour: in the space of a few
seconds, their radio emission becomes weaker or even disappears for a
while, then returns to the previous level after some hours. The
mechanisms causing this switch between what are usually referred to as
'radio-bright' and 'radio-quiet' states are still largely unknown.
When they are young, pulsars also shine in X-rays because the surface of
the neutron star is still very hot. Old pulsars are much weaker sources
of X-rays, because the surface of the neutron star has cooled down.
Astronomers know of only a handful of old pulsars that shine in X-rays
and believe that this emission comes from the magnetic poles – the sites
on the neutron star's surface where the acceleration of charged
particles is triggered.
Observations of the five-million year-old pulsar known as PSR B0943+10,
performed simultaneously with ESA's XMM-Newton X-ray observatory and
ground-based radio telescopes, revealed that this source exhibits
variations in its X-ray emission that mimic in reverse the changes seen
in radio waves. No current model is able to predict what could cause
such sudden and drastic changes to the pulsar's entire magnetosphere and
result in such a curious emission. Credit: ESA/ATG medialab
The two states of pulsar PSR B0943+10 as observed with XMM-Newton and LOFAR
This illustration shows the two states of emission observed from pulsar
PSR B0943+10, which is well known for switching between a 'bright' and
'quiet' mode at radio wavelengths. Observations of PSR B0943+10,
performed simultaneously with ESA's XMM-Newton X-ray observatory and
ground-based radio telescopes, revealed that this source exhibits
variations in its X-ray emission that mimic in reverse the changes seen
in radio waves. No current model is able to predict what could cause
such sudden and drastic changes to the pulsar's entire magnetosphere and
result in such a curious emission.
In the upper part of the illustration, the artist's impression on the
left shows the pulsar with glowing cones of radiation stemming from its
magnetic poles – a state referred to as 'radio-bright' mode. Radio
emission from pulsars is known to arise from these cones, and we see it
pulsate because the pulsar's rotation and magnetic axes are misaligned.
The graphs on the right side show data from X-ray observations,
performed with XMM-Newton (upper graph), and from radio observations,
performed with the Low Frequency Array (LOFAR; lower graph). The upper
graph shows that, in the 'radio-bright' mode, the pulsar does not shine
brightly in X-rays. The lower graph shows a bright and pulsating
emission at radio wavelengths.
In the lower part of the illustration, the artist's impression on the
left shows the pulsar in a different state, with glowing 'hot-spots'
that are located at its magnetic poles. In particular, the illustration
shows the pulsar in a state characterised by bright X-ray emission,
arising from the polar caps, and relatively low radio emission from the
cones that stem from the pulsar's magnetic poles
('X-ray-bright/radio-quiet' mode). The graphs on the right side show
how, in this mode, the pulsar exhibits a brighter and pulsating X-ray
emission, whereas the radio emission is fainter but still pulsating. Credit: ESA/ATG medialab; ESA/XMM-Newton; ASTRON/LOFAR
Source: The University of Manchester
Jodrell Bank Centre for Astrophysics
