Star Eats Companion
Media Contact:
Maurizio Falanga
CEA-Saclay
mfalanga@cea.fr
issued by the European Space Agency
September 6, 2005 Noordwijk, Netherlands -- ESA's Integral space observatory, together with NASA's Rossi X-ray Timing
Explorer spacecraft, has found a fast-spinning pulsar in the process of
devouring its companion.
This finding supports the theory that the fastest-spinning isolated
pulsars get that fast by cannibalising a nearby star. Gas ripped from the
companion fuels the pulsar's acceleration. This is the sixth pulsar known
in such an arrangement, and it represents a 'stepping stone' in the
evolution of slower-spinning binary pulsars into faster-spinning isolated
pulsars. "We're getting to the point where we can look at any
fast-spinning, isolated pulsar and say, 'That guy used to have a
companion'," said Dr Maurizio Falanga, who led the Integral observations,
at the Commissariat ˆ l'Energie Atomique (CEA) in Saclay, France.
'Pulsars' are rotating neutron stars, which are created in stellar
explosions. They are the remnants of stars that were once at least eight
times more massive than the Sun. These stars still contain about the mass
of our Sun compactified into a sphere of only about 20 kilometres across.
This pulsar, called IGR J00291+5934, belongs to a category of 'X-ray
millisecond pulsars', which pulse with the X-ray light several hundred
times a second, one of the fastest known. It has a period of 1.67
milliseconds which is much smaller that most other pulsars that rotate
once every few seconds.
Neutron stars are born rapidly spinning in collapses of massive stars.
They gradually slow down after a few hundred thousand years. Neutron stars
in binary star systems, however, can reverse this trend and speed up with
the help from the companion star. For the first time ever, this
speeding-up has been observed in the act. "We now have direct evidence for
the star spinning faster whilst cannibalising its companion, something
which no one had ever seen before for such a system," said Dr Lucien
Kuiper from the Netherlands Institute for Space Research (SRON), in
Utrecht.
A neutron star can remove gas from its companion star in a process called
'accretion'. The flow of gas onto the neutron star makes the star spin
faster and faster. Both the flow of gas and its crashing upon the neutron
star surface releases much energy in the form of X-ray and gamma
radiation.
Neutron stars have such a strong gravitational field that light passing by
the star changes its direction by almost 100 degrees (in comparison light
passing by the Sun is deflected by an angle which is 200 thousands times
smaller). "This 'gravitational bending' allows us to see the back side of
the star," points out Prof. Juri Poutanen from the University of Oulu,
Finland. "This object was about ten times more energetic than what is
usually observed for similar sources," said Falanga. "Only some kind of
monster emits at these energies, which corresponds to a temperature of
almost a billion degrees."
From a previous Integral result, scientists deduced that because the
neutron star has a strong magnetic field, charged particles from its
companion are channeled along the magnetic field lines until they slam
into the neutron star surface at one of its magnetic poles, forming 'hot
spots'. The very high temperatures seen by Integral arise from this very
hot plasma over the accretion spots.
IGR J00291+5934 was discovered by Integral during a routine scan of the
sky on 2 December 2004, in the outer reaches of our Milky Way galaxy, when
it suddenly flared. On the day after, scientists accurately clocked the
neutron star with the Rossi X-ray Timing Explorer. Rossi observations
revealed that the companion is already a fraction the size of our Sun,
perhaps as small as 40 Jupiter masses. The binary orbit is 2.5 hours
long (as opposed to the year long Earth-Sun orbit). The full system is
very tight; both stars are so close that they will fit into the radius of
the Sun. These details support the theory that the two stars are close
enough for accretion to take place and that the companion star is being
cannibalised.
"Accretion is expected to cease after a billion of years or so," said Dr
Duncan Galloway of the Massachusetts Institute of Technology, USA,
responsible for the Rossi observations. "This Integral-Rossi discovery
provides more evidence of how pulsars evolve from one phase to another -
from an initially slowly spinning binary neutron star emitting high
energies, to a rapidly spinning isolated pulsar emitting in radio
wavelengths."
The discovery is the first of its kind for Integral (four of the first
five rapidly spinning X-ray pulsars were discovered by Rossi). This bodes
well in the combined search for these rare objects. Integrals's sensitive
detectors can identify relatively dim and distant sources and so, knowing
where to look, Rossi can provide timing information through a dedicated
observation extending over the entire two-week period of the typical
outburst.
These findings will appear in an upcoming issue of the Astronomy and
Astrophysics Journal
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