Cosmic surprises

Print edition : September 11, 1999

The spectacularly vivid images obtained by the orbiting X-ray observatory Chandra create a sensation among astronomers.

THE cosmos abounds in mysteries, and the first images beamed on August 26 by the orbiting X-ray observatory, Chandra, a little over a month after it was launched aboard the space shuttle Columbia on July 23, have already thrown up two and created a sensa tion among astronomers. The images have been described as extraordinary. "This observatory is ready to take its place in the history of spectacular scientific achievements," said Dr. Martin Weisskopf, Chandra Project Scientist at the Marshall Space Fligh t Centre (MSFC) of the National Aeronautics and Space Administration (NASA).

Named after the Indian-born Nobel laureate Subrahmanyan Chandrasekhar, whose work laid the basis for much of the understanding of the cosmos today, this most powerful X-ray telescope inspires great expectations. And not without reason. For, the first ima ges from the Chandra X-Ray Observatory (CXO) have resulted in two remarkable discoveries - and these were only "First Light" observations using the telescope. "First Light" is something similar to the commissioning of a ship - the instrument, after being launched and checked, is readied for measurements. It is not the actual first light through the telescope. That occurred on August 13 when the telescope's aperture door was opened. "First Light" measurements are a means to standardise the instruments ag ainst signals from some well-known stellar objects. But in the case of Chandra these initialising exercises themselves brought a wealth of cosmic surprises.

One image traces the aftermath - the expanding shell of hot gas - of a supernova explosion that is believed to have occurred 320 years ago. The supernova remnant is in the constellation Cassiopeia in our galaxy and is called Casseiopeia A or Cas A. Scien tists say that it has been imaged in such "stunning detail" (in the X-ray region of the spectrum) that they can see evidence of what may be a neutron star or a black hole near its centre. Scientists have described it as the best X-ray image ever of a cos mic object. This discovery is expected to answer one of the many questions about Cas A that have bugged astronomers. A supernova remnant usually harbours in its centre a compact, highly dense star called the neutron star. Until now, no one had seen any c entral object in Cas A; it had eluded detection in optical, radio and even X-ray wavelengths. Chandra has, however, shown that it exists. For the present, the object has been named Leon X-1 in honour of Dr. Leon van Speybroeck, the key man behind the des ign of Chandra's highly sensitive X-ray-reflecting mirrors.

Supernovae are exploding stars, which spew out enormous amounts of energy and radiation and are seen as extremely bright objects. Despite its proximity (only about 9,100 light years away), for some reason the explosion had been missed by astronomers. It was discovered by Karl Jansky, the pioneer of radio astronomy, in the 1930s - it was one of the first radio-emitting stellar objects discovered. The nature of the explosion that resulted in the remnant Cas A, however, remained an enigma of sorts. Althoug h radio, optical and X-ray observations of the remnant indicate that it was a powerful explosion, the visual brightness of the outburst was perhaps much less than that of a normal supernova. It is believed that Cas A was produced by the explosion of an u nusually massive star that had already ejected most of its outer layers. Chandra may come up with clues to why Cas A is not quite like other known supernovae.

The spectacularly vivid images obtained by Chandra should allow scientists to trace the dynamics of the remnant and its collision with any material ejected by the star before it exploded. Extensive observations in radio, infrared, visible and X-ray wavel engths have revealed an incomplete shell of expanding gas with 300 glowing compact knots of material at temperatures of up to 28 million degree Celsius. Its outer shell is expanding at a speed of 800 km, creating shock waves rushing into inter-stellar sp ace at tens of millions of kilometres an hour. These violent sonic booms created by shock waves have created a vast 30-million-degree bubble of gas that is emitting X-rays for Chandra to see in fine detail. The first image has revealed the collision of t he debris from the exploded star with the matter around it. Chandra has also detected both the fast outer shock wave and the slower inner shock wave with great precision in addition to giving tantalising evidence of a neutron star associated with the exp losion.

Heavy elements in the hot gas produce X-rays of specific energies. The detectors aboard Chandra will also provide Cas A's precise X-ray spectra. These measurements make it possible to identify the heavy elements present and their quantities. This is nece ssary to understand how the elements essential for life were created and spread through the galaxy space by exploding stars. This should also help resolve another longstanding question: why is the Cas A a source of cosmic titanium-44, a radioactive isoto pe of the metal.

Infrared and X-ray spectral studies conducted until now have revealed the presence, as in all supernovae, of oxygen, neon, silicon, sulphur, argon, iron, calcium and even magnesium silicate, the stuff of inter-stellar dust. But the oddity about Cas A is that it seems to have an unusual abundance of Ti-44 as compared to nickel-56, another product of nucleosynthesis in a star. The presence of Ti-44 was discovered from the indirect evidence provided by the orbiting Compton Gamma Ray Observatory (CGRO) - of emission lines corresponding to scandium-44 decaying into calcium-44, the last leg of T-44's life cycle. If correct, Ti-44 holds the clue to the nuclear fusion in the core of the star that exploded. Chandra will look for Ti-44 from the sample of star ma terial just above the layers that formed the neutron star that seems to be present and will perhaps throw light on what Cas A's contribution will be in the making of future worlds of our galaxy.

While Chandra's image of Cas A corresponds with earlier X-ray pictures taken by the United States-German-United Kingdom X-ray satellite ROSAT, it has given an incredible amount of detail and that too just after 90 minutes of exposure, whereas ROSAT took days to make its image. This is an indication of the power of Chandra's on-board instruments, making it the largest and the most sensitive X-ray astronomy telescope in the world. Chandra is expected to do for X-ray astronomy what the orbiting Hubble Spac e Telescope (HST) has done to optical astronomy. Chandra's observations vis-a-vis Cas A are not yet over; they will continue even as the telescope's calibration and verification tests will be carried out over the next few weeks. One of the observa tions will be to confirm whether the spectrum, brightness and time variation of the point source at the heart of Cas A do correspond to a neutron star.

This X-ray image of the Cassiopeia supernova remnant is the official "First Light" image of the Chandra Observatory. Two shock waves are visible: a fast outer shock wave and a slower inner shock wave. The inner shock wave is believed to be the result of the collision of the ejecta from the supernova explosion with a circum-stellar shell of material, heating it to a temperature of 10 million degrees Celsius. The bright object near the centre may be the long-sought-after neutron star or black hole that remained after the explosion that produced Cassiopeia A.-GAMMA LIAISON

EVEN before Chandra focussed on Cas A, it had made an amazing discovery with regard to a distant X-ray source called PKS 0637-752, a luminous quasar in the farthest reaches of the universe, six billion light years away. The quasar has been emitting energ y with the power of 10 trillion suns, but from a region smaller than our solar system. It has little background X-ray noise or clutter and was also known to be a point source (against a dark background), making it an ideal target for sky calibration of t he on-board instrument for what is called "point spread function", a measure of the on-axis blur of the focussed radiation in the telescope. But, as Dr. Harvey Tannanbaum, Director of the Smithsonian Astrophysical Observa-tory's Chandra X-ray Centre in M assachusettes, observed after what CXO saw in the quasar, "nature often has surprises in store."

Instead of a neat little point source, Chandra found a massive jet spewing from the quasar. Although radio observations in the past have shown a jet, astrophysicists involved with Chandra did not expect to see it in X-ray because of the ultra-high resolu tion that has been achieved in radio observations by using multiple telescopes in conjunction. Dr. Tannanbaum said: "It is immediately apparent that we are not seeing a dot, that we are not seeing a simple point source." The jet appears to be at least 20 0,000 light years long, a huge expanse in inter-galactic space, which can hold the entire Milky Way.

This serendipitous discovery poses a fundamental question to theoretical and observational astrophysics: how can such a massive burst of matter and energy extend across so much of space? Chandra's image, combined with radio telescope observations, should provide insights into the processes by which supermassive black holes can produce such cosmic jets. It was after detecting the jet-spewing quasar that the CXO was aimed at Cas A, an extended object which gave rise to its own fresh set of mysteries.

According to the Chandra Project team, both images confirm that Chandra is in excellent health and that its instruments and optics are performing up to expectations. The experiments planned and designed for Chandra will commence once its orbital check-ou t and calibration phase is over.

SINCE X-rays are absorbed by the earth's atmosphere, space-based observatories have become necessary. Based on the recommendations of the U.S. National Academy of Sciences, Chandra, originally called the Advanced X-Ray Astrophysics Facility (AXAF), was c onceived in the 1970s as an advancement over the Second High-Energy Astronomy Observatory (HEAO-2, also called the Einstein Observatory), which was launched in 1978. Chandra has been billed as one of NASA's new Great Observatories for Astrophysics, compl ementing the already orbiting HST (which looks at visible and near infrared wavelengths) and the Compton Gamma Ray Observatory (which looks at even higher energies than Chandra) and the planned Space Infrared Telescope Facility (SIRTF) and ground-based r adio observatories.

With the arrival of the first images from Chandra, U.S. scientists have begun to claim that Chandra has put the country back in the lead in X-ray astronomy. Unlike the HST, which has a circular orbit, Chandra has a highly elliptical orbit with a perigee of 10,000 km and an apogee of 140,000 km (one-third of the distance to the moon). Its minimum life is about five years; it is, however, expected to last for at least 10 years.

The importance of X-ray astronomy stems from the fact that X-rays in the universe are not an oddity; they are present everywhere. Every major class of astronomical objects emits X-ray frequencies. It takes temperatures of around a million degrees to prod uce X-rays. X-ray sources have also been found to display tremendous time variability, indicating rapid dynamic events that produce X-rays. The Einstein Observatory found X-rays coming from quasars to normal stars to hot gas pervading the space between c lusters of galaxies and many extra-galactic sources. Weisskopf said: "Most of the mass in the universe seems to be tied up in hot X-ray emitting gas, even more than what is seen from stars in the galaxies." It was X-ray astronomers who, based on observat ions on Cygnus X-1, came up with evidence for the existence of black holes. (The black hole emitted X-rays.)

Dr. K.P. Singh, an X-ray astronomer with the Tata Institute of Fundamental Research (TIFR), said: "To understand really the most important elements of Chandra, one has to compare it with what we have had so far and what is likely to become available duri ng its lifetime." It was the advent of truly focussing X-ray telescopes that revolutionised X-ray astronomy. This began with the Einstein Observatory, which brought about a technological leap in imaging and sensitivity. Einstein Observatory was able to i mage with a blur (the angular resolution) of 4 arc-second. This increased the sensitivity to detect faint sources or features by a factor of about 1,000 through the simple act of focussing over much smaller detector areas than before. After that came ROS AT, which had even smoother mirrors. Its resolution was similar to Einstein, but because of a reflecting area that was four to five times larger than that of Einstein, it was much more sensitive. It also had a long operational life. The number of X-ray s ources rose to 100,000 with ROSAT, as compared to 10,000 with Einstein and 800 before.

From Chandra, an X-ray image of PKS 0637-752, a luminous quasar six billion light years away. It radiates with the power of 10 trillion suns from a region smaller than the solar system. The source of this energy is believed to be a supermassive bl ack hole.-GAMMA LIAISON

After ROSAT came ASCA (US-Japan), which had a poorer spatial resolution but pioneered the use of sensitive (single photon counting) CCD X-ray cameras and increased the spectral resolution by a factor of 10. Dr. K.P. Singh said that Chandra had much more improved X-ray mirrors with a designed resolution capability of 0.3 arc-second. Unprecedented, this improves the sensitivity beyond that of ROSAT by over a factor of 10, even though its effective area is only slightly larger. It also has a better bandwid th, which is important for spectral studies. According to him, the CXO will, therefore, be able to see much deeper into the universe than before and would be the best tool to study faint and distant populations. "It has the sensitivity to look for distan t and faint objects at a factor of 20 to 50 times better than anything that has been done before," Weisskopf said.

So what are the things that Chandra can do once it becomes an operational laboratory? According to Weisskopf, Chandra's ability to dissect energy spectrum and look carefully at line features and their characteristics will enable the study of plasmas in t he vicinities of black holes and neutron stars and other regions of high energy activity. The largest and most massive objects in the universe are galaxy clusters, an enormous collection of galaxies. These are held together by gravity and much of their m ass is in the form of an incredibly hot, X-ray emitting gas that fills the entire space between the galaxies. But neither the mass of the galaxies nor that of the hot gas is sufficient to hold a cluster together. So scientists believe that a dark matter which exerts this gravity, but cannot be seen, exists. So, what and where is this dark matter? X-ray observations with the help of Chandra can help map the location of the dark matter and help its identification. "Perhaps we, the geographers of the unive rse, will not discover what the dark matter is, but surely map where it is," Weisskopf said.

The other outstanding astrophysical question that Chandra may help answer is about the identity of the powerhouse that is driving the explosive activity in many distant galaxies, the centre of many of which are sources of incredibly enormous amounts of e nergy and radiation, particularly X-rays. Scientists theorise that massive black holes are at the centre of galaxies, gobbling up material. Detailed observations with the help of Chandra can probe even the faintest of these active galaxies and study not only their time variability, but also how such intense emissions are generated in the first place.

Weisskopf believes that some of the most astronomical discoveries that could be made with Chandra would relate to the so-called extended objects. Many types of X-ray sources are extended: the debris of exploding stars as shock waves heat the inter-stella r medium, dense regions of bright X-ray stars, the hot gas in galaxies, and clusters of galaxies, the largest known structures in the universe. "In my opinion, the pictures will be as spectacular as any of the pictures from Hubble," said Weisskopf.

The CXO will however, have to contend with competition even before it completes one year in orbit. Two observatories are ready for launch in early 2000 - the XMM , the European X-Ray Multi-Mirror telescope, and ASTRO-E, a U.S.-Japanese satellite. The XMM will have four times more the effective area of the CXO. Although its blurring will be more, the XMM will have a higher spectral resolution, according to Dr. K.P. Singh. It also will carry X-ray CCD cameras. ASTRO-E will have an effective area similar t o the CXO, but with a blur factor higher than that of the CXO and less than that of the XMM. It will have five independent telescopes which will carry CCDs; one will carry a new instrument called microcalorimeter (MCM), having a high spectral resolution at medium X-ray energies, thus having an edge over the CXO and the XMM. However, since the MCM needs to be cryogenically cooled, it will have a life of only two years. The other four telescopes will, however, have a minimum life span of five years.

The new millennium is, therefore, going to be the era of X-Ray astronomy in the true sense of the word. According to Dr. K.P. Singh, notwithstanding the other proposed X-ray observatories, Chandra will be the best for cosmologival studies and will remain at the forefront until the end of its life.

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