Star eats companion (Forwarded)

ESA News
http://www.esa.int

6 September 2005

Star eats companion

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
à 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.

For more information:

Maurizio Falanga, lead author of the results
CEA-Saclay, France
E-mail: mfalanga @ cea.fr

Chris Winkler, ESA Integral Project Scientist
E-mail: christoph.winkler @ esa.int

These findings will appear in an upcoming issue of the Astronomy and
Astrophysics Journal and also at
http://www.arxiv.org/abs/astro-ph/0508613

More about...

* ESA's gamma-ray astronomy mission
http://www.esa.int/SPECIALS/Integral/index.html
* Integral factsheet
http://www.esa.int/esaSC/SEMD9G1A6BD_index_0.html

Related articles

* Observations: Seeing in the gamma-ray wavelengths
http://www.esa.int/esaSC/SEM3A2T1VED_index_0.html
* Observations: Seeing in X-ray wavelengths
http://www.esa.int/esaSC/SEMTA2T1VED_index_0.html
* Observations: Seeing in microwave wavelengths
http://www.esa.int/esaSC/SEM4B2T1VED_index_0.html
* Three satellites needed to bring out 'shy star'
http://www.esa.int/esaCP/SEMSOI6DIAE_index_0.html
* A gamma-ray burst bonanza
http://www.esa.int/esaCP/SEMIVX8YFDD_FeatureWeek_0.html
* Integral's first look at the gamma-ray Universe
http://www.esa.int/esaCP/ESADW18708D_index_0.html
* Integral -- tracking extreme radiation across the Universe
http://www.esa.int/esaCP/ESAI0BTHN6D_index_0.html

Related links

* Related CEA release
http://www.cea.fr/fr/actualites/articles.asp?id=668
* Related Univ. of Oulu release
http://www.oulu.fi/ajankohtaista/uutiset/pulsarit.html
* Netherlands Institute for Space Research
http://www.sron.nl

IMAGE CAPTIONS:

[Image 1:
http://www.esa.int/esaCP/SEMWSAA5QCE_index_1.html]
This is an artist's impression of a pulsar, approximately 20 kilometres
in diameter, accreting material from a companion star. The strong
gravity from the dense pulsar attracts material from the companion. The
flow of gas from the companion to the pulsar is energetic and glows in
X-ray light.

Credits: NASA/Dana Berry

[Image 2:
http://www.esa.int/esaCP/SEMWSAA5QCE...html#subhead1]
This animation depicts a pulsar, approximately 20 kilometres in
diameter, accreting material from a companion star. The strong gravity
from the dense pulsar attracts material from the companion. The flow of
gas from the companion to the pulsar is energetic and glows in X-ray
light. The angular momentum of the accretion causes the pulsar to spin
faster and faster. Eventually the pulsar accretes the entire star and is
left as a fast-spinning isolated pulsar.

Credits: NASA/Dana Berry

[Image 3:
http://www.esa.int/esaCP/SEMWSAA5QCE...html#subhead3]
Integral, ESA's International Gamma-Ray Astrophysics Laboratory, is
gathering some of the most energetic radiation that comes from space.
The spacecraft was launched in October 2002 and is helping to solve some
of the biggest mysteries in astronomy.

Gamma rays are even more powerful than the X-rays used in medical
examinations. Fortunately, Earth's atmosphere acts as a shield to
protect us from this dangerous cosmic radiation. However this means that
gamma rays from space can only be detected by satellites.

At time of launch, Integral was the most sensitive gamma-ray observatory
ever put into space. It detects radiation from the most violent events
far away and from processes that made the Universe habitable.

Credits: ESA