Cannibal stars like their food hot, XMM-Newton reveals (Forwarded)

ESA News
http://www.esa.int

23 March 2006

Cannibal stars like their food hot, XMM-Newton reveals

ESA's XMM-Newton has seen vast clouds of superheated gas, whirling around
miniature stars and escaping from being devoured by the stars' enormous
gravitational fields -- giving a new insight into the eating habits of the
galaxy's 'cannibal' stars.

The clouds of gas range in size from a few hundred thousand kilometres to
a few million kilometres, ten to one hundred times larger than the Earth.
They are composed of iron vapour and other chemicals at temperatures of
many millions of degrees.

"This gas is extremely hot, much hotter than the outer atmosphere of the
Sun," said Maria Díaz Trigo of ESA's European Science and Technology
Research Centre (ESTEC), who led the research.

ESA's XMM-Newton x-ray observatory made the discovery when it observed six
so-called 'low-mass X-ray binary' stars (LMXBs). The LMXBs are pairs of
stars in which one is the tiny core of a dead star.

Measuring just 15*20 kilometres across and comparable in size to an
asteroid, each dead star is a tightly packed mass of neutrons containing
more than 1.4 times the mass of the Sun.

Its extreme density generates a powerful gravitational field that rips gas
from its 'living' companion star. The gas spirals around the neutron star,
forming a disc, before being sucked down and crushed onto its surface, a
process known as 'accretion'.

The newly discovered clouds sit where the river of matter from the
companion star strikes the disc. The extreme temperatures have ripped
almost all of the electrons from the iron atoms, leaving them carrying
extreme electrical charges. This process is known as 'ionisation'.

The discovery solves a puzzle that has dogged astronomers for several
decades. Certain LMXBs appear to blink on and off at X-ray wavelengths.
These are 'edge-on' systems, in which the orbit of each gaseous disc lines
up with Earth.

In previous attempts to simulate the blinking, clouds of low-temperature
gas were postulated to be orbiting the neutron star, periodically blocking
the X-rays. However, these models never reproduced the observed behaviour
well enough.

XMM-Newton solves this by revealing the ionised iron. "It means that these
clouds are much hotter than we anticipated," said Díaz. With
high-temperature clouds, the computer models now simulate much better the
dipping behaviour.

Some 100 known LMXBs populate our galaxy, the Milky Way. Each one is a
stellar furnace, pumping X-rays into space. They represent a small-scale
model of the accretion thought to be taking place in the very heart of
some galaxies. One in every ten galaxies shows some kind of intense
activity at its centre.

This activity is thought to be coming from a gigantic black hole, pulling
stars to pieces and devouring their remains. Being much closer to Earth,
the LMXBs are easier to study than the active galaxies.

"Accretion processes are still not well understood. The more we understand
about the LMXBs, the more useful they will be as analogues to help us
understand the active galactic nuclei," says Díaz.

Notes to editors:

The findings appear in Astronomy & Astrophysics (445, 179*195, 2006). The
original article, 'Spectral changes during dipping in low-mass X-ray
binaries due to highly-ionized absorbers', is by M. Díaz Trigo and A.N.
Parmar (ESA, Noordwijk, The Netherlands), L. Boirin (Observatoire
Astronomique de Strasbourg, France), M. Méndez and J.S. Kaastra (SRON,
National Institute for Space Research, Utrecht, The Netherlands).

For more information:

Maria Diaz Trigo, ESA, ESTEC, Noordwijk, The Netherlands
E-mail: mdiaz @ rssd.esa.int

Arvind Parmar, ESA, ESTEC, Noordwijk, The Netherlands
E-mail: arvind.parmar @ esa.int

Norbert Schartel, ESA XMM-Newton Project Scientist, ESAC, Madrid
E-mail: norbert.schartel @ sciops.esa.int

More about...

* XMM-Newton overview
http://www.esa.int/esaSC/120385_index_0_m.html

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IMAGE CAPTIONS:

[Image 1:
http://www.esa.int/esaCP/SEM7T6OVGJE_index_1.html]
Artist's impression of a vast cloud of superheated gas whirling around an
asteroid-sized cannibal star, part of a low-mass X-ray binary star system.

The clouds, discovered by ESA's XMM-Newton space observatory, are composed
of iron vapour and other chemicals at many millions of degrees and are
located where the 'river' of matter from the companion star strikes the
disc. The clouds periodically block the X-ray emission from the cannibal
star and cause an X-ray 'blinking'.

Credits: ESA

[Image 2:
http://www.esa.int/esaCP/SEM7T6OVGJE...html#subhead1]
Artist's impression of XMM-Newton, ESA's Earth-orbiting x-ray observatory.

Credits: ESA