Chameleon pulsar baffles 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 research was led by Professor Wim
Hermsen from The Netherlands Institute for Space Research and the University of
Amsterdam and will appear in the journalScience on the
25th January 2013.
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 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 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 The University of Manchester's School of Physics and Astronomy
said: "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 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.
Source: University of Manchester
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Posted by Unknown
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