Astronomy is a painstaking discipline, requiring time and patience. Yet once
in while, a string of discoveries using different telescopes occur in the
same domain, each following hot on the heels of one another. Now, XMM-Newton
adds the latest chapter to the story of IC443, one of the most studied
supernova remnants.

Supernova remnants (SNR) are the shrouds of gas thrown into space when a
dying star explodes. Ejected matter is heated up as it interacts with the
interstellar medium, the gas between the stars, and reaches temperatures
so high that it generates X-rays. When certain stars explode, they are
completely annihilated, whereas in other cases their dense core remains but
it may be hidden.

Most of the emission from a supernova remnant is of thermal origin stemming
from the expansion of the explosion-generated blast wave advancing through
the interstellar medium. However, near the explosion centre a different kind
of radiation may be emitted from a hidden object. Rarer types of supernova
remnants described as ‘plerionic’ do not exhibit thermal emission and their
radiation clearly stems from a remaining core object, such as a pulsar (a
rotating neutron star) and its nebula.

These pulsars, or ‘stellar corpses’, are difficult to locate because they
are extremely small, with diameters of only 10 km, and because of the veil
of thermal emission surrounding them. One way to peer through the opaque
veil is to observe the high-energy radiation, such as X-rays, which these
objects emit. Discovering such objects in supernova remnants is important
because it gives astronomers more information about the original explosion.

The supernova remnant shell IC443 is situated in our Galaxy, some 5,000
light years away, in the opposite direction from the centre of our Galaxy.
It was long thought to emit solely thermal radiation. But in 1997 the
Japanese ASCA and German ROSAT observatories discovered hard X-ray emission
of a non-thermal nature, a first hint of the presence of a pulsar.

Very rapidly another satellite, BeppoSAX, detected distinct compact X-ray
sources in IC433, and the Very Large Array (VLA) radio telescope in New
Mexico (United States) also registered radio wavelength signals from the
same area. Finally, in the gamma-ray energy band, the EGRET instrument
aboard the Compton-GRO spacecraft also pinpointed a source which had all
the characteristics of a neutron star.

In autumn 2000, both ESA’s XMM-Newton and NASA’s Chandra X-ray observatories
focused on IC443. Last December, data from the American mission allowed
three young high school students working with their teacher, a Chandra
guest-observer, to proudly announce that they had located a young and
rapidly rotating neutron star, in other words a pulsar. Their conclusion
was supported by the VLA radio data.

A European team has now gone one step further, providing not only a detailed
morphology of the cloud of gas and dust, or nebula, surrounding the pulsar,
but also identifying it as the same source (numbered 3EG J0617+2238)
observed by EGRET in 1999.

“Thanks to XMM-Newton’s much longer exposure and to its higher effective
area — catching many more elusive X-ray photons than Chandra — we have
obtained spectra which provide the first exciting insights into the
pulsar’s environment,” explains Fabrizio Bocchino, of ESA’s Astrophysics
Division at ESTEC, Noordwijk, in the Netherlands.

“We have identified the plerionic extended diffuse emission powered by the
central pulsar. It was a difficult task because the pulsar nebula is deeply
immersed in the predominantly thermal emission. We succeeded in removing
this unwanted contribution and so observed the nebula in all its glory.
Extrapolating this to the EGRET energy band, we can now say that the X-ray
nebula is quite probably the counterpart of the EGRET source.”

ESA’s X-ray observatory has thus peered into the ‘coffin’ — the hot embers
surrounding this stellar corpse — and XMM-Newton is providing the first
close-up view of the core of the nebula. XMM-Newton has also been able to
measure some of the physical properties of the pulsar itself.

Moreover, the nebula shows the gradual softening of its spectrum at greater
distances from the pulsar, a characteristic feature of plerionic nebulae
which is caused by the short lifetime of high energy electrons compared
with those at lower energies.

The elliptical, ‘cometary’ shape, which indicates that the pulsar is moving
outwards from the centre of the supernova remnant, is also evident. The
X-ray and radio wavelength views of IC443 are different to what is normally
expected for a plerion.

“The new data on IC443 that we have obtained with XMM-Newton may help us
understand why there are so few supernova remnants with pulsars,” says
Fabrizio Bocchino. “Today we know around 250 SNRs, and only about 15 of
these contain pulsars, a surprisingly small fraction. Because pulsars are
like rotating lighthouses, we may not be in the correct plane to see their
beams or it may be the effect of absorption. But we now know that the signs
of a pulsar may be ‘outshone’ by the remnant thermal emission, as is the
case of IC443.”

Many supernova remnants remain to be observed by XMM-Newton and many more
pulsars may be discovered in this way.


The XMM-Newton observation of IC443, using the EPIC-MOS and pn cameras,
was carried out during the observatory’s calibration and performance
verification phase, in one 24,000 second exposure.

The results of this first analysis “The plerion nebula in IC443: the
XMM-Newton view” by F. Bocchino and A. Bykov (Ioffe Institute for Physics
and Technology, Russia) will be published in Astronomy and Astrophysics.

For more information please contact:

ESA Science Programme Communication Service

Tel: +31 71 5653183


* XMM-Newton home page


[Image 1:]
This Close-up view of the core of the IC443, obtained with the XMM-Newton
EPIC-MOS cameras (3-10 keV), shows the elliptical nature of the nebula.
The black line contours trace the VLA radio signal. (The field of view in
this image is 2.9 x 3.5 arcminutes.)

[Image 2:]
This hard X-ray (3-10 keV) view of IC443 was obtained by the EPIC-pn camera
on XMM-Newton. (The field of view is 25 x 27.5 arcminutes.) Superimposed is
the 95% error circle for the position of the pulsar as given by the EGRET
gamma-ray detector on the Compton-GRO spacecraft.

[Image 3:]
This image shows the soft X-ray (0.5-2 keV) emission from IC443 which was
recorded by XMM-Newton’s EPIC-pn camera. (The circled areas labelled Th2
and Bkg were used to calculate residual thermal emission. The field of
view is 25 x 27.5 arcminutes.)

[Image 4:]
This image of IC443 obtained with the ACIS camera on the Chandra X-ray
observatory shows a bright point of X-ray radiation which appears to be
surrounded by a wake (bluish haze) of hot X-ray emitting gas. (The field
of view is about 1 x 1.5 arcminutes.) Image courtesy NASA/Chandra.

[Image 5:]
An optical ground-based telescope view of IC443. The field of view of this
image is about 1.5 x 1.0 degrees. Image courtesy of Naoyuki Kurita.