Probably the most detailed analysis of the composition and dynamics of
the supernova remnant Cassiopeia-A has been presented at the symposium
‘New Visions of the X-ray Universe in the XMM-Newton and Chandra era’
which is taking place this week at the European Space Agency’s
Technology and Research Centre, ESTEC, Noordwijk in the Netherlands.

Cassiopeia A (Cas-A) is a young shell-shaped supernova remnant some 15
light years in diameter situated some 10 thousand light years away.
It is the remains of a massive star which, having exhausted all its
hydrogen fuel, exploded 320 years ago. The core of such a collapsing
star can give rise to a neutron star or black hole. Its external parts
are blown apart projecting stellar material, glowing in X-rays, into
the surrounding interstellar medium.

The stellar material contains many heavy elements which have been
forged from lighter elements in the progenitor star, and during the
explosion process. All the chemical elements in our human bodies have
their origins in such stellar explosions and the resulting primordial
broth.

Unprecedented accuracy

With its fine angular resolution, NASA’s Chandra had already provided
a view of the fine knotted structure of Cas-A. It highlighted clumps
and filaments of ejected matter. Today with its combination of
sensitivity and superior X-ray collecting power, ESA’s XMM-Newton has
provided element abundance maps of Cas-A of unprecedented accuracy.

Using the observatory’s EPIC-MOS cameras, Cas-A was targeted in late
2000 during XMM-Newton’s performance verification and calibration
phase. The total observation time was practically 24 hours,
collecting sufficient X-ray photons to allow a full spectral
analysis of each individual pixel in a 15 x 15 grid covering the
angular size of the remnant’s X-ray image.

The matrix of nine abundance maps shows the distribution and abundance
in Cassiopeia A of neon, magnesium, silicon, sulphur, argon, calcium,
the iron-L and iron-K ionisation states and nickel, with a colour
scale based on the solar values for these elements.

Large variations

The maps clearly highlight large abundance variations of the X-ray
emitting material across the face of the remnant. One notes however
that the distributions of silicon, sulphur, argon and calcium —
all oxygen burning products — are similar and distinct from the
carbon burning products, neon and magnesium.

Iron abundance is seen to vary over the remnant with little
correlation to other elements. The oxygen abundance, contrary to
other elements, is much higher than predicted by theory — perhaps
because it has a very different spatial distribution, concentrated
to the North and because the EPIC cameras have a poorer spectral
resolution at lower energies.

Fast moving iron

The study not only measured abundancies but also elemental velocities
by determining Doppler shifts — the displacement of spectral lines
when elements are moving. Iron, produced at the heart of the stellar
explosion and present now in the furthest reaches of the remnant,
was apparently expelled considerably faster to overtake other
elements that were formed nearer to the star’s surface. The dynamics
of the Cas-A X-ray emitting material indicates strong asymmetrical
effects in the supernovae explosion.

“Only XMM-Newton could give us all the data we required to produce
such reliable results,” says lead author Dr. Dick Willingale from
the University of Leicester, UK. “These new maps of Cas-A provide
fresh insights into the elemental composition, the distribution and
the plasma dynamics of the remnant. It also gives us clues as to its
progenitor — probably a star between 12 and 30 times the mass of our
Sun”.

The paper “X-ray spectral imaging and Doppler mapping of Cassiopeia A”
by R.Willingale (University of Leicester), J.A.M. Bleeker, K.J. van
der Heyden and J.Kaastra (SRON National Institute for Space Research,
Utrecht Netherlands) and J.Vink (Columbia University, New York) is to
appear in Astronomy and Astrophysics.

For more information:

Dr. Dick Willingale

Department of Physics and Astronomy

University of Leicester

Leicester LE1 7RH, UK

Tel : +44.116.252.3556

Email: rw@star.le.ac.uk

Dr. Fred Jansen

XMM-Newton Project Scientist

Tel: +31.71.565.4426

Email: fjansen@astro.estec.esa.nl

Monica Talevi

ESA Science Programmes Communications

Tel: +31.71.565.3223

Email: mtalevi@estec.esa.nl

USEFUL LINKS FOR THIS STORY

* XMM-Newton home page

http://sci.esa.int/home/xmm-newton/

IMAGE CAPTONS:

[Image 1:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=23&cid=12&oid=29047&ooid=29048]
Abundance maps for the elements included in the Cassiopeia A — spectral
analysis of the data provided by XMM-Newton. All are plotted on the
logarithmic scale indicated by the bar at the bottom. (Image courtesy
R. Willingale)

[Image 2:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=23&cid=12&oid=29047&ooid=29049]
The red, green, and blue regions in this Chandra X-ray image of
Cassiopeia A show where the intensity of low, medium, and high energy
X rays, respectively, is greatest. The red material on the left outer
edge is enriched in iron, whereas the bright greenish white region on
the lower left is enriched in silicon and sulphur. In the blue region
on the right edge, low and medium energy X rays have been filtered out
by a cloud of dust and gas in the remnant. (Image courtesy Chandra
X-ray Observatory Centre)