Astronomers using ESA’s Integral and XMM-Newton space observatories have
caught a fast-spinning ‘millisecond pulsar’ in a crucial evolutionary
phase for the first time, as it swings between emitting pulses of X-rays
and radio waves.
Pulsars are spinning, magnetised neutron stars, the dead cores of
massive stars that exploded as a dramatic supernova after having burned
up their fuel. As they spin, they sweep out pulses of electromagnetic
radiation hundreds of times per second, like beams from a lighthouse.
This tells us that the spin period of the neutron stars can be as short
as a few milliseconds.
Pulsars are classified according to how their emission is generated. For
example, radio pulsars are powered by the rotation of their magnetic
field, while X-ray pulsars are fuelled by the accretion of material
siphoned off from a companion star.
Theory holds that initially slowly rotating neutron stars with a
low-mass companion are spun up as matter accretes onto them from a
surrounding disc fed by the companion. X-rays are emitted as the
accreting material heats up as it falls onto the neutron star.
After a billion years or so, the rate of accretion drops and the pulsars
are thought to switch on again as a radio-emitting millisecond pulsar.
There is thought to be an intermediate phase during which they swing
back and forth between the two states several times, but until now,
there has been no direct and conclusive evidence for this transitional
phase.
Thanks to the combined forces of ESA’s Integral and XMM-Newton space
observatories, along with follow-up observations by NASA’s Swift and
Chandra satellites and by ground-based radio telescopes, scientists have
finally caught a pulsar in the act of changing between the two
evolutionary steps.
“The search is finally over: with our discovery of a millisecond pulsar
that, within only a few weeks, switched from being accretion-powered and
X-ray-bright to rotation-powered and bright in radio waves, we finally
have the missing link in pulsar evolution,” says Alessandro Papitto from
the Institute of Space Sciences in Barcelona, Spain, who led the
research published this week in
Nature
.
The object, identified as IGR J18245-2452, was first detected in X-rays
on 28 March 2013 by Integral in the globular cluster M28, which lies in
the constellation Sagittarius.
Observations by XMM-Newton determined the pulsar’s spin period to be 3.9
milliseconds, meaning that it rotates on its axis more than 250 times
every second, clearly identifying it as an X-ray-bright millisecond
pulsar.
But comparing its spin period and other key characteristics with those
of other known pulsars in M28 showed it matched perfectly those of
another pulsar that had been observed in 2006 – but only at radio
wavelengths.
“At that time, it appeared to be just another millisecond radio pulsar,
but now here it was shining in X-rays – this is clearly no ordinary
pulsar,” adds Dr Papitto.
The astronomers kept monitoring the object with X-ray telescopes, but
also started a series of radio observations, on the lookout for hints
that it might change personalities again.
What the astronomers didn’t expect was that the change in behaviour would happen within just a few weeks.
“We used to think the change would occur only once over the billion-year
evolution of these systems, yet within a month, the neutron star swung
back and forth between an X-ray and a radio pulsar state, showing the
switch can be made even on extremely short timescales,” says co-author
Enrico Bozzo of the University of Geneva, Switzerland.
Despite occurring on a far quicker timescale than previously imagined,
the characteristics of the transformation, which is thought to lie in
the interplay between the pulsar’s magnetic field and the pressure of
material falling onto it from its low-mass companion star, still fits
current theory.
When the inflow of material from the neighbouring star is more intense,
the high density of matter shuts off the radio emission, and the pulsar
is only visible through the X-rays emitted by the accreting matter as it
heats up while falling onto the pulsar.
Conversely, when the accretion rate decreases, the magnetic field of the
pulsar expands and pushes any remaining matter away from the pulsar,
allowing the radio emission to switch back on.
Looking back through archival data for this particular pulsar, the
astronomers have shown that these cycles may repeat on timescales of
just a few years.
“The discovery of this transitional pulsar completes a decades-long
quest for such an object and will help us to understand better the
evolution of pulsars,” says Erik Kuulkers, Integral Project Scientist at
ESA.
“Although it took a long time to make this first detection, we believe
that pulsars in such binary systems are fairly common, so we’re looking
forward to finding more,” adds Norbert Schartel, XMM-Newton Project
Scientist at ESA.
Read an in-depth version of this story on ESA SciTech:
Swinging between X-rays and radio waves: the missing-link pulsar
Swings between rotation and accretion power in a millisecond binary pulsar
by A. Papitto et al. is published in
Nature
26 September 2013.
The study is based on data from a number of space-based high-energy
observatories and ground-based radio telescopes: ESA’s Integral and
XMM-Newton and NASA’s Swift and Chandra space telescopes, and CSIRO's
Australia Telescope Compact Array and Parkes radio telescope, NRAO's
Robert C. Byrd Green Bank Telescope, and ASTRON's Westerbork Synthesis
Radio Telescope.
For further information, please contact:
Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: markus.bauer@esa.int
Alessandro Papitto Institut de Ciències de l’Espai (ICE), CSIC-IEEC (Spanish National Research Council - Institute for Space Studies of Catalonia)
Barcelona, Spain
Tel: +34 935 868355
Email: papitto@ice.csic.es
Enrico Bozzo
ISDC Data Centre for Astrophysics
University of Geneva, Switzerland
Tel: +41 79 3129209
Email: Enrico.Bozzo@unige.ch
Erik Kuulkers
ESA Integral Project Scientist
Tel: +34 918131358
Email: Erik.Kuulkers@esa.int
Norbert Schartel
ESA XMM-Newton Project Scientist
Tel: +34 91 8131 184
Email: Norbert.Schartel@esa.int
Source: ESA
