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
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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
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'.
Artist's impression of XMM-Newton, ESA's Earth-orbiting x-ray observatory.