Physics – Optics
Scientific paper
Feb 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000xmm..pres....3.&link_type=abstract
XMM Press Release INFO 3-2000
Physics
Optics
Scientific paper
The images were obtained between 19-25 January at the very start of the science payload commissioning process. The spacecraft viewed three regions of the sky: part of the Large Magellanic Cloud (LMC), the Hickson Cluster Group 16 (HCG-16), and the star HR 1099. These targets were chosen because they all present a variety of X-ray extended and point sources and are very interesting regions.
The expanding cauldon in the Large Magellanic Cloud
The Large Magellanic Cloud also known as the Nebula Major is about 20 thousand light years in diameter. Situated 160 thousand light years from Earth, it is one of two irregularly shaped galaxies that are easily seen with the naked eye in the southern hemisphere. These galaxies are satellites of the Milky Way and appear to be slowly spiralling into our own Galaxy. The first image obtained by the EPIC-PN X-ray cameras viewed the 30 Doradus region of the Large Magellanic Cloud. Also called the Tarantula Nebula, 30 Doradus is a cauldron of creation where exploding stars are releasing vast amounts of matter and where new stars are being born.
The image presents the million degree temperatures of the emitting medium, with blue indicating the hottest regions; green intermediate temperatures and red the coldest regions. The white and blue arc-like formation just off the centre is a new object, only part of which was known in the past. It has the appearance of a supernova remnant with its expanding glowing-hot gas producing X-ray emission as it collides with the interstellar medium.
On the lower right of the picture is the remains of a star that exploded as Supernova 1987A on 24 February 1987. It was the first supernova to reach naked-eye brightness since 1604 (Kepler's star) and remained visible to the naked eye for nearly nine months. The brightest source in the view, upper left, is another supernova remnant (N157D).
Martin Turner, Principal Investigator for the EPIC cameras... "These first pictures are tremendously exciting after so many years of work. They are all that we hoped they would be. In the LMC we can see the elements, which go to make up new stars and planets, being released in giant stellar explosions. We can even see the creation of new stars going on, using elements scattered through space by previous stellar explosions. This is what we built the EPIC cameras for and they are really fulfilling their promise"
Multiwavelength views of Hickson Group 16
The HCG-16 viewed by EPIC and by the Optical Monitor in the visible and ultraviolet wavelengths is one of approximately a hundred compact galaxy clusters listed by Canadian astronomer Paul Hickson in the 1980s. The criteria for the Hickson cluster groups included their compactness, their isolation from other galaxies and a limited magnitude range between their members. Most Hicksons are very faint, but a few can be observed with modest aperture telescopes.
Galaxies in Hickson groups have a high probability of interacting. Their study has shed light on the question of galactic evolution and the effects of interaction. Investigation into their gravitational behaviour has also significantly contributed to our understanding of "dark matter", the mysterious matter that most astronomers feel comprises well over 90% of our universe.
Observation of celestial objects from space over a range of X-ray, ultraviolet and visible wavelengths, is a unique feature of the XMM-Newton mission. The EPIC-PN view of the Hickson 16 group shows a handful of bright X-sources and in the background more than a hundred faint X-ray sources that XMM-Newton is revealing for the first time.
Juxtaposing the X-ray view of HCG 16 with that of the Optical Monitor reveals one of the great strengths of XMM-Newton in being able to routinely compare the optical, ultraviolet and X-ray properties of objects.
Many of the X-ray sources are revealed as elongated "fuzzy blobs" coincident with some of the optical galaxies. Routine access to ultraviolet images is a first for the mission, allowing astronomers to learn much more about individual objects. Obtaining a ratio of the brightness of individual sources seen with different filters ('filter spectroscopy') gives indications on their temperature and composition. Using XMM-Newton to search for variability from sources such as these will enable astronomers to hunt for those elusive black holes thought to lurk at the centres of many galaxies.
"The performance of our instrument is very much as we expected" says Keith Mason, Principal Investigator for the Optical Monitor. "We have worked for over a decade on this mission and it is very exciting having the first data. And the pictures really show all the value of the multi-wavelength approach of the XMM design."
HR1099 in an infernal waltz
HR 1099 is a sixth magnitude star located about a 100 light years from the Sun only just visible to the naked eye. Its formidable brightness in the EPIC-MOS image conceals in fact a binary pair. Whereas our Sun rotates in 30 days, these two stars are whizzing around each other in only 3 days.
The rapid motion causes a kind of infernal dynamo, twisting the stars' magnetic fields into contorted shapes. If one star resembles our own Sun, its partner is infinitely more active than our Sun. It is the scene of intense stellar flares and storms which astronomers believe are due to the release of magnetic energy as the fields untwist. Measuring the phenomena that are present greatly helps us understand the way our own Sun functions and its effects upon us. Again this X-ray image reveals many serendipitous sources, hitherto unknown.
Discovering the elements
The final two examples of the initial data collected by XMM-Newton take the form of spectra. They are provided by one of the two Reflection Grating Spectrometers (RGS). Just as in optical spectroscopy, different elements absorb and emit light at specific and unique points of the radiation spectrum. The RGS spread these out in the form of two "bananas", the two so-called 'spectral orders' of the instrument. Emission lines appear as distinct features in the rainbow of X-ray colours, acting as signatures that reveal a great deal of information to astronomers.
The RGS spectra of HR 1099 are examples. The graphs display peaks or lines that correspond to the various elements present in the source. One can distinguish for instance the presence of different types of iron, oxygen carbon and neon. From the analysis of this data one can deduce the temperatures, densities, abundances and velocities of the different materials. "Firstly, the nice separation of the two spectral order bands shows the resolution of the RGS CCDs is well up to expectations" comments Bert Brinkman, RGS Principal Investigator. "For the spectrometers as a whole, the resolution which is of prime importance, is exactly or marginally better than what we expected after the ground calibrations. The instruments promise a lot for the future." All the images reveal hitherto unknown X-ray sources. Faint point sources barely perceptible with previous X-ray space telescopes appear in all their splendour. Understanding everything they show is somewhat premature since the XMM-Newton instruments have yet to be calibrated.
XMM Project Scientist Fred Jansen says: "As these are the very first astronomical data and we are already observing lots of new science, XMM-Newton holds a very clear promise for an exciting and scientific future." ESA's Director of Science Prof. Roger Bonnet is equally impressed. "I am amazed by the quality of the pictures as compared to previous X-ray missions. We see on them a lot of new sources, especially in the parts of the spectrum which correspond to the hottest temperatures and we see that the Universe is hotter than we thought and that many new sources are appearing. We are very hopeful that many more objects will be discovered and that by extending the temperature measurements of the Universe to many objects, we will have a much better picture of the history and the hectic behaviour of stars at the end of their life".
The Calibration and Performance Verification phase for X
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