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Chandra: Promises Made and Kept

by Wallace Tucker

May 20, 2011 ::

A promise made is a debt unpaid.    Robert Service
Chance favors the prepared mind.    Louis Pasteur

Top Ten

Not long ago a request came down from above for a list of Chandra's achievements that have "completely transformed the way we have viewed our world, solar system, sun, or universe."

In other words, how many discoveries of the century have you made this year?

In a bow to David Letterman, or the decimal system, or other lists of ten that you can easily summon up, Chandra Project Scientist Martin Weisskopf submitted a list of Chandra's top ten which would probably fall beyond the event horizon, never to be seen again. Not really, because it appears below, and being an environmentally conscious group, we will likely recycle the list several times before the next request requires generation of a new list which will be similar, but not identical to previous lists because real progress is being made.

Chandra's Top Ten (As of early 2011, and not necessarily in order of importance):

1. Deep field observations resolved the X-ray background and showed that it is dominated by accreting supermassive black holes including a large number of highly obscured black holes.


2. Images of clusters of galaxies established that energetic feedback by rotating supermassive black holes dramatically affects the evolution of intracluster gas and galaxies.


3. X-ray rings and jets around rotating neutron stars provide the most direct evidence of the transformation of rotational energy of these stars into jets and winds of high-energy particles.


4. X-ray and optical observations of the Bullet cluster of galaxies show the separation of dark and ordinary matter in a collision between galaxy clusters.


5. Observations of the rate at which massive galaxy clusters grow have provided confirmation that the expansion of the universe is accelerating, an effect attributed to the prevalence of dark energy, and have ruled out some alternatives to Einstein's General Theory of Relativity.


6. Observations of supernova remnants showed that supernova explosions are asymmetric and turbulent, requiring mixing of layers either during or prior to the explosions, and images of supernova shock waves provide evidence for acceleration of electrons to extremely high energies.


7. Detection of absorption by highly ionized oxygen atoms in X-ray spectra of a quasar behind the Sculptor wall of galaxies provided evidence for the Warm Hot Interstellar Medium thought to contain the missing baryons in the local universe.


8. Chandra observations of spectrally soft X-ray sources in early-type galaxies led to the conclusion that mergers, rather than accretion-driven explosions, are responsible for the Type Ia supernovas in these galaxies.


9. A number of multi wavelength studies of star clusters have provided an unprecedented look at the co-evolution of young stars and their disks in a wide variety of conditions.


10. Chandra was used to discover and/or contribute to an understanding of the X-ray emission processes from comets, the moons, of Jupiter, the Io plasma torus, the atmospheres of Venus and Mars.

The list, which could have easily been expanded to fifteen or more by including insight into the nature of stellar black holes (event horizon, rotation rate), the accretion process near black holes, the studies of the Galactic center region, starburst galaxies, etc., got me to wondering how well the promise of Chandra has been met, and whether we can possibly guess what the future holds. This led me to my personal archive to dig out copies of old proposals. The oldest was the proposal for a Large Orbiting X-ray Telescope (LOXT), which was submitted to NASA in May of 1970.

The LOXT was to have two telescopes, one designed for high resolution with the capabilities approximately that of Chandra, and the other for maximum efficiency with approximately the capabilities of XMM-Newton. What did we expect to be the major accomplishments of LOXT? The following list is in order of appearance in the proposal.

L1. Test the binary star model for X-ray stars.
L2. Determine the nature of strong, transient X-ray stars.
L3. Resolve the X-ray emission from and around the Crab pulsar & detect and search for other rotation powered X-ray emitting pulsars in supernova remnants.
L4. Study the dynamics of supernova shock waves and measure the abundances of the elements in supernova remnants.
L5. Detect X-rays from supernovas in their first month.
L6. Detect X-rays from stellar coronas, stellar winds and flare stars.
L7. Determine the populations of X-ray sources for different galaxy types & their association with galactic features such as spiral arms.
L8. Resolve the X-ray emission around M87, especially the optical jet.
L9. Detect and study X-ray emission from quasars and similar objects.
L10. Measure the spatial variations in the X-ray background radiations.
L11. Look for shadows cast by cool intergalactic matter.
L12. Search for the missing mass in the form of ionized gas in clusters of galaxies.
L13. Set limits on the mass density of the intergalactic medium through observations of the soft X-ray background radiations.

Given that this proposal was submitted before the launch of the UHURU X-ray satellite , when the total useful time from all the rocket flights was about 100 hours, and the evidence for black holes, or the binary nature of compact X-ray sources, or the existence of hot gas in galaxy clusters was still in the future, the overlap between this list and the latest Chandra list is remarkable. It is also noteworthy that Chandra has made significant advances on every one of the topics.

LOXT never made it through the budget gauntlet, but, largely because of the success of UHURU, the Einstein X-ray Observatory , a smaller version of the LOXT did survive, and the X-ray images made with its mirrors made the case for the Advanced X-ray Astrophysics Facility (AXAF), or Chandra. Perusing the various brochures being circulated in the halls of NASA and Congress in the mid-1980's, I came up the following list of prime scientific objectives for AXAF:

A1. Understanding the magnetic dynamos in stars.
A2. Probe the nature of the supernova process through observations of supernova remnants.
A3. Determine the size and thermal conductivity of neutron stars, and provide insight into the composition of matter at extreme densities.
A4. Confirm the existence of black holes on a stellar and galactic scale.
A5. Measure the distribution of dark matter on various size scales.
A6. Study the formation and the evolution of quasars.
A7. Establish the contribution of various classes of discrete sources to the X-ray background.
A8. Measure the rate of expansion of the universe.
A9. Measure the evolution of the heavy element content of the universe through observations of clusters of galaxies.
A10. Study heating and particle acceleration processes in stellar coronas, supernova remnants and cosmic jets.
A11. Study the relation of high-energy jets to apparently unrelated lower energy thermal phenomena such as star formation.
A12. Use an archive of thousands of serendipitous sources to discover new types of objects, ranging from brown dwarfs to quark stars to newly types of galaxies to cosmic strings.

The list contained most of the elements of the LOXT list, but the impact of the discoveries made by the Uhuru, Einstein, HEAO -A and other observatories, as well as the growing connections with other fields of astronomy and the physics of elementary particles is evident. Black holes are now on the list and it has been established that dark matter cannot be in the form of hot gas.

A list prepared just before launch for PR purposes was similar, but shorter and less technical, and included the use of clusters of galaxies to test cosmological models.

Comparing the list of actual Chandra accomplishments with what was promised shows that Chandra has more than fulfilled the promises made. The list also shows that most of the discoveries, except the evidence on precursors to Type Ia white dwarf supernova explosions, were anticipated in a general way. The potential for discoveries in the solar system were mentioned in the LOXT and AXAF documents, but weren't given much ink. The discovery of dark energy wasn't anticipated, but it was well understood that the rate of formation of galaxy clusters would provide an important cosmological probe. I think it is also true that in every case that the reality exceeded the anticipation - see the images of the Crab Nebula and the Perseus Cluster as prime examples.

It seems that astrophysical theorists deserve some credit for the close correspondence between what was promised and what was delivered. Maybe not so much for being visionary, but for their ingenuity in being able to adapt existing models and theories to the changing landscape revealed by observation.

That is not to say that there won't be any surprises in the future - with 95% of the energy density being in either in dark energy or dark matter, we are still very much in the dark! But the increasingly rapid and positive feedback between observational discoveries and theoretical modeling should give us a feeling of what to expect from Chandra in the next decade, and prepare the way for an ingenious use of the broad and deep data base that will be complemented by increasing multi-wavelength coverage of the areas observed by Chandra.

There is great joy in serendipitous discoveries, and they make for good stories, but most of them are not totally unexpected -again dark energy is a notable exception, but even there, the researchers were confident they would find something of cosmic significance. Most discoveries occur because chance does indeed favor the prepared mind.

With that in mind, there is a wealth of discoveries for which Chandra seems well prepared to make. I would give you my list, but I have already exceeded my allotment of words.

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