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Frequently Asked Questions

IXO is a relatively new mission concept. Do you have questions? Contact Us.

Q: Why do we need X-rays to study the Universe?

A: High-energy phenomena—particularly in the X-ray band—characterize the evolution of cosmic structures on both large and small scales. On the smallest scales, X-rays provide the only electromagnetic spectral signatures from the regions of strong gravity near black holes and neutron stars. X-rays from these energetic processes penetrate absorbing gas carrying spectral and timing signatures that allow us to uncover the earliest massive black holes. On the largest scales, X-rays are indispensable for detecting the "missing" 50% of baryons in the local Universe, and for probing of both dark energy and dark matter. Thus, the X-rays qualify as a key window to study the Universe, complementing and augmenting studies at the other wavelengths.

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Q: What is IXO?

A: The International X-ray Observatory (IXO) is a new X-ray telescope with joint participation from NASA, the European Space Agency (ESA), and Japan's Aerospace Exploration Agency (JAXA). The launch date of IXO will depend on the outcome of the Astrophysics Decadal survey which will be announced in the summer of 2010. Based on the NASA, ESA, and JAXA budgets, the earliest would be ~2021. The mission will be designed to operate for a minimum of five years, with a goal of ten years. IXO will address many timely and key questions confronting astrophysics such as:

For more information, please refer to IXO Science Goals.

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Q: Why does IXO have to be in space?

A: X-rays do not penetrate the Earths atmosphere, therefore, instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, or satellites. A dedicated satellite with an X-ray telescope is specifically required to get good quality images and spectra, monitor X-ray sources and also to detect faint sources.

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Q: Which science will IXO address?

A: IXO will address a variety of science questions in a wide range of topics from stars to quasars, from galaxies and black holes to dark energy and planet formation. In particular, IXO will be able to gather the signals from black holes at the early stages of the Universe, and study their connection and co-evolution with primordial galaxies. Matter under extreme conditions, such as in the deep potential wells of supermassive black holes or in the core of neutron stars, will be one of the primary themes for IXO. Other major themes include the origin and content of our Universe, and the study of how elements are created and dispersed in the Universe via stars and cosmic explosions and accelerations. For more information, please refer to IXO Science Goals.

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Q: When did IXO get started?

A: This depends on your perspective. The IXO efforts have been underway for more than a decade as the independent projects Constellation-X and XEUS. However, as a unified vision, the project has been under development since May 2008.

See also: Before IXO: Con-X and XEUS

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Q: How do IXO capabilities compare to those of current X-ray Observatories?

A: The basic answer is that IXO has a much larger collecting area and improved spectral resolution than previous X-ray observatories. However, both of these properties are highly dependent upon energy. The detailed answer is included in the plots below, which show the effective area (collecting area of the mirror convolved with the quantum efficiency of the relevant detectors, etc.) and the spectral resolution of the various IXO focal instruments compared with current missions.

Effective Area Plot

Plot of the effective area of the IXO mirror and focal plane instruments compared to those of current X-ray observatories, showing the large improvement at all energies. Click the image for a larger view


Plot of the spectral resolution of the IXO calorimeter and grating (lower limit) compared to current X-ray observatories. The bottom panel shows the emission lines of the various elements in the energy range 0.1-3.0 keV (color) and their sum (black). Click the image for a larger view

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Q: Why does the effective area curve have bumps and edges?

A: The bumps and edges in the effective area curve appear because of resonance scattering and (or) absorption of X-rays which have energies near the M shell of the X-ray mirror's reflecting material (iridium).

The simplest Rutherford-Bohr model depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus. According to this model, the electrons can only circle in specific orbits. Each orbit is characterized by a certain discrete distance from the nucleus and specific energy.

When the X-rays hit the reflecting material, the oscillating electric field of the electromagnetic radiation interacts with the electrons bound in an atom. There are two types of possible interactions: the radiation either will be scattered by these electrons, or it will be absorbed and excite photoelectrons. At certain energies the absorption increases dramatically and gives rise to an absorption edge. Each edge occurs when the energy of the incident photon is just sufficient to cause excitation of a core shell electron of the absorbing atom to a continuum state, i.e. to produce a photoelectron. The energies of the absorbed radiation at these edges correspond to the binding energies of electrons in the K, L, M, etc., orbits of the absorbing atom.

Scattering occurs due to the electron transitions from the core level to the higher unfilled or half-filled orbital (e.g., s → p, or p → d).

As a result, the original photon is lost and other secondary photons with very different energies may be emitted by the atom. When this happens, the X-ray mirror has a relatively low reflectivity, but the detector has relatively high quantum efficiency. Because these are resonances, they appear as rather discontinuous jumps in the effective curve. If the effective area curve has taken into account not only the X-ray mirror, but also the calorimeter windows and absorbers, there will be even more bumps and edges. These bumps and edges are related to resonance scattering and (or) absorption of X-rays in those materials, such as aluminum, gold, or bismuth.

Effective Area Plot

The bumps and edges in the effective area curve appear because of resonance scattering and (or) absorption of X-rays which have energies near the M shell of the X-ray mirror reflecting material (iridium). Click the image for a larger view

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Q: How will IXO fit in with other planned multiwavelength facilities?

A: Virtually every phenomenon in the sky requires a panchromatic view to be fully understood. This is because the various wavelengths provide windows on different processes and components of the source, and only by considering all the pieces of information together can astronomers hope to capture the fundamental physical processes at work. With its large collecting area and improved spectral resolution, IXO will complement and augment planned future ground- and space-based observatories such as JWST, E-ELT, TMT, ALMA, and others, in shedding light on the most energetic processes at work in the heart of galaxies, on primordial black holes and their connection to forming structures, and on the mysterious "dark" components of the Universe. Indeed, IXO's science drivers are the same as those of the other Great Observatories, ensuring future synergy and commonality of scopes.

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Q: What (new) technology will IXO feature?

A: There are many new technologies that will make IXO a quantum jump from the current X-ray observatories. The development of new ultra-light X-ray optics makes possible increasing an order of magnitude in telescope sizes. Two different technologies are being pursued. An European technique employs the silicon wafers used to for micro-processor chips, while U.S. researches are exploring the slumped glass used for laptop screens.

The IXO detectors will all use new technologies that are under development. Micro-calorimeter arrays operate at 50mK (a hair above absolute zero) and sense the heat deposited when an X-ray is absorbed. These will be the workhorse devices that provide both images and high spectral resolution (similar to integral field spectrometers in the optical). The wide field imager will utilize active pixels to provide CCD spectral resolution, but will be immune to radiation damage and have fast readout times (and will also provide a high time readout for bright sources). New lightweight gratings will provide higher spectral resolution and higher efficiency than the Chandra and XMM gratings. And finally, new devices to sense the X-ray polarization from cosmic X-ray sources will open a whole new window on the X-ray Universe.

See also IXO detectors:

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Q: Why does IXO need all its instruments?

A: IXO has been conceived and designed to enable astronomers to address and, hopefully, answer many still open questions in modern astrophysics. See IXO Science Goals. To do this often requires many pieces of information gathered with different techniques—spectroscopy, timing, imaging, and polarimetry. This is why IXO will carry all these different instruments: each one of them will provide a unique viewpoint of the source and a crucial piece of the puzzle.

Detailed spectroscopy is the only way for high-energy astronomers to learn about the temperature, composition, and velocity of plasmas in the Universe. Moreover, the study of specific X-ray spectral features probes the conditions of matter in extreme gravity field, such as around supermassive black holes. To this end, IXO will be equipped with two high-resolution spectrometers, a microcalorimeter (XMS) and a set of dispersive gratings (XGS), which will provide high-quality spectra at hard and soft X-rays.

Flux variability adds a further dimension by linking the emission to the size of the emitting region and its evolution over time; the HTRS on IXO will allow these type of studies in a broad energy range and with high sensitivity.

To extend our view of the high-energy Universe to the hard X-rays and find the most obscured black holes, the WFI and the HXI together will image the sky up to 18 arcmin Field of View (FOV) with a resolution better than 1 keV (FWHM) at 40 keV.

And finally, IXO will open a new window of exploration with its X-ray polarimeter. Theoretical studies show that X-ray polarimetry is a powerful tool to explore compact sources, such as neutron stars and black holes. IXO will be the first observatory to enable such investigations.

See also:

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Q: Why is IXO the right mission to be the next major observatory after JWST?

A: Many pressing science questions that require observations of the hot Universe can only be made with X-ray telescopes. These include the origin and evolution of black holes, the cause of the accelerating Universe, the formation of large scale structure, the equation of state of neutron stars. All these and many other topical questions require an X-ray telescope with an order of magnitude better than any previous X-ray telescopes. The technology development over the past decade combined with the detailed mission studies means that the IXO mission can be ready for launch around 2021. It will join the other major observatories that cover other wavebands such as JWST, HST, Fermi and ALMA to bring a multi-wavelength solution to the many puzzles and problems confronting modern astrophysics.

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Q: How will IXO be launched?

A: IXO will be directly inserted into an 800,000 km semi-major axis halo orbit around the Sun-Earth L2 libration point. The main advantages of this orbit include its very stable thermal environment and the minimal shadowing from the Earth and/or Moon.

In its launch configuration IXO will have a mass about 6600 kg and will be approximately 10 meters long and 4 meters in diameter, so it can fit right into Atlas V or an Ariane V rocket. When deployed during the voyage to the L2 orbit, the observatory will be ~ 20 meters tall, which requires the use a deployable metering structure between the spacecraft bus and the instrument module, since no rocket fairing is large enough to fit such a long observatory.

IXO stowed configuration in fairing. Credit: NASA/GSFC. Click the image for a larger view

IXO deployable masts emerging from their stowage canisters. Credit: NASA/GSFC. Click the image for a larger view

Read more about IXO mission design »


  • Mechanical Overview of the International X-Ray Observatory, David W. Robinson, IEEE Aerospace Conference, 2009. Download PDF

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Q: Why are there multiple designs for the IXO spacecraft?

A: Conceptual development for an international scientific space mission begins with engineers and scientists from each agency grasping very early ideas and giving them form. Because IXO is an international cooperative project, the mission design has been independently studied by ESA as well as by NASA. Both designs are based on a common set of features including a single large X-ray mirror assembly and an extendible optical bench with a focal length of ~20 m, and a suite of focal plane instruments.

IXO science requires a low mass telescope with large effective area and high angular resolution. Currently such performance can be achieved via two innovative X-ray optics technologies pursued independently by each agency. The baseline technology for IXO is represented by the Silicon Pore Optics (SPO) developed by ESA, with the back-up technology as the Segmented Glass Optics (SGO) developed by NASA. Both technologies are compatible with the Flight Mirror Assembly Module and can be accommodated in both spacecraft designs.

Both NASA and ESA designs have the same configuration and modular approach, thus easing the international cooperation via the provision of independent spacecraft modules, such as the Flight Mirror Assembly or the Instrument Module. JAXA is presently running a study on the IXO Extendable Optical Bench based on the deployment system being space qualified for ASTRO-H.

Observatory Fore View IXO ESA

NASA observatory concept. Credit NASA. Click the image for a higher resolution.

ESA observatory concept. Credit ESA. Click the image for a higher resolution.

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Q: How is the international development of IXO being coordinated?

A: The IXO project concept was born in May 2008, when ESA and NASA established an international coordination group involving ESA, NASA and JAXA, with the aim of exploring a joint mission merging the ongoing XEUS and Constellation-X projects. As a result, a joint understanding was reached by the coordination group on a proposal to proceed towards the goal of developing an International X-ray Observatory (IXO).

Currently, IXO development is guided by four key advisory groups of scientists and engineers - the Science Definition Team, the Telescope Working Group, the Instrument Working Group, and the Study Coordination Group overseeing the coordination of all groups. In addition to advisory groups, more than 350 scientists from over 60 institutions around the world are actively involved with IXO. Many of them were active supporters of Constellation-X and XEUS projects.

More information on IXO advisory groups, along with the names of the group members and project leaders can be found on the Advisory Group page. The IXO mission concept was submitted to the U.S. Decadal Survey committee, which recognized IXO´s high scientific importance. And it is currently being evaluated by ESA's Cosmic Vision process.

In the JAXA science programme, IXO is actively being considered as the X-ray astronomy mission to follow ASTRO-H, with a preliminary Mission Requirements Definition activity progressing through the latter half of 2010.

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