XMM-Newton peers into a stellar coffin
14 June 2001Astronomy 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.
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