X-Ray Astronomy Program Working Group
Report to SEUS

February 22, 1999

Outline

• Membership of the Working Group
• Working Group Schedule and Process
• Key Science Topics
  • Chemical Composition and Evolution of the Universe
  • The Evolution of Structure in the Universe
  • Black Hole Astrophysics

• Recommended Program
• Visions for the Future
• XAPWG Road Map

Members of the XAPWG

• First meeting at Austin AAS 8 Jan 1999 - Short Organizational
  • Review charge from NASA
  • Outline strategy for report, develop themes, review draft white paper

• Second meeting GSFC 12,13 Feb 1999
  • Add to white paper
  • Select Science Questions/Develop Road map

• Third meeting TBD - Finalize Report to SEUS
  • Complete white paper

Key Science Topics (1)

• Chemical Composition and Evolution of the Universe
  • Fundamental Questions
    
    1. How are the elements synthesized?
       

    2. How are the elements distributed and recycled?
       

    3. How have these processes evolved over cosmic time?
  • The X-ray Advantage
    
    1. X-ray emission is often the primary radiation from the dominant component of an astrophysical system.
    2. Abundances can be measured in objects ranging from clusters of galaxies to the coronae of normal stars.
    3. The X-ray band from 0.1 - 100 keV is rich in both atomic transitions from nearly all charge states of cosmically abundant metals, as well as nuclear transitions from recently synthesized material.
    4. X-rays are only weakly absorbed by intervening material, and the signature of that absorption is contained in the measured spectrum.
    5. X-ray emitting plasmas often are optically thin, simplifying spectroscopic interpretation.
  • Key X-ray Observations
    
    1. X-ray imaging and spectroscopic observations of supernovae and remnants can directly measure the synthesis and distribution of heavy elements.
    2. The hot X-ray emitting gas in clusters, groups, and early type galaxies provides a fossil record of the star formation history that can be traced over cosmic time.
    3. X-ray spectroscopy of stellar coronae and winds shows how elemental abundances vary with stellar age and location within galaxies.
    4. Abundances in both hot and cold phases of the interstellar medium can be directly measured by X-ray emission and absorption.

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  Key Science Topics (2)

• The Evolution of Structure in the Universe
  (The Spatial Distribution of Matter)
  • Fundamental Questions
    
    1. How are baryons distributed in the universe and what is their relation to the dark matter?
    2. How does the distribution of dark matter change with cosmic time, and how does this test physical models of structure formation and evolution?
    3. What is the relationship between the formation of stars and galaxies, galactic structure, and the dynamics of the interstellar medium?
  • The X-ray Advantage
    
    1. Theory predicts, at the current epoch, most presently undetected baryons reside in a highly ionized IGM visible in the soft X-ray band. A census of presently observed baryons at low z shows that over 80% are only visible in X-rays.
    2. Large scale galactic structure, (the Galactic Bulge, Ridge, galactic winds, and halos), groups and clusters have virial temperatures > 10⁶K , and are uniquely observable in X-rays. For groups and clusters, X-rays are the dominant emission.
    3. X-rays readily identify distant clusters which are resolvable with moderate angular resolution at all redshifts. High z AGN are easily selected from their X-ray properties and optical follow-up.
    4. Large scale X-ray emitting structures reflect the gravitational potential, hence models provide accurate predictions.
    5. X-ray imaging provides an easy and efficient method for identifying young low mass stars.
  • Key X-ray Observations
    
    1. Studies of virialized clusters determine the dark matter distribution and measure the baryonic fraction of the Universe. X-ray spectral images of the hot baryonic component of the Universe provide a 3-D map of virialized, highly compressed, and shock heated structures.
    2. X-ray observations of large scale galactic structures facilitate measurements of the temperature, density, and abundance variations, allowing determination of their origin and dynamics.
    3. Large scale surveys provide an intermediate ( z ~ 1 ) map of structure for comparison to optical ( z ~ 0.2 ) and CMBR ( z ~ 1000 ) maps, testing models for the growth of structure.
    4. X-ray observations of SNR's and superbubbles (the by-product of wind-blown bubbles) are a unique way of measuring their contribution to the galactic energy budget.
    5. X-rays identify and map fossil star formation regions in our galaxy and nearby galaxies and provide a picture of recent star formation and its relation to large scale galactic structures.

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  Key Science Topics (3)

• Black Hole Astrophysics
  • Fundamental Questions
    
    1. How do black holes form and what determines their mass and spin? How do the masses and spins of black holes evolve with time?
    2. Is the turn-on of accretion luminosity from these galactic nuclei a tracer of galaxy formation?
    3. Which properties of accreting black holes are universal and which properties scale with mass?
    4. What is the physics of matter and radiation under extreme conditions (gravity, density, magnetic field)?
  • The X-ray Advantage
    
    1. X-rays are the predominant radiation produced near black hole event horizons and neutron stars, and from neutron star surfaces.
    2. X-rays can penetrate the gas surrounding black holes.
    3. X-ray luminosity traces black hole locations in both galactic and extragalactic environments.
    4. X-ray surface brightness is often higher (many million times) than in other bands, facilitating the study of much more compact objects.
  • Key X-ray Observations
    
    1. Time-resolved high-resolution iron line spectroscopy will map the innermost regions near black holes (reverberation mapping).
    2. Timing of rapid X-ray variability of black holes and neutron stars will measure the properties of strongly curved spacetime, and constrain the properties of matter under extreme conditions.
    3. X-ray spectroscopy will determine the geometries of accretion flows and the interaction of radiation with relativistic particles and flows near black holes and neutron stars.
    4. X-ray spectroscopy will measure the surface temperature and composition of isolated neutron stars and constrain the nuclear equation of state.

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  Recommended Program (1)

1. Complete the Current Program
   1. Launch and Operate the Chandra X-ray Observatory
   2. Continue Operating the Rossi X-ray Timing Explorer
   3. Participate in International Projects, for example:
      1. XMM
      2. Astro-E
      3. Spectrum X-Gamma

   4. Continue to Fund Advanced Technology for Constellation-X

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   Recommended Program (2)

2. Major Mission New Start: Constellation-X
   • Broad range of science addressed though an evolution of technologies that are currently achievable.
     1. High throughput for sensitivity and spectroscopy.
     2. High spectral resolution over the band of x-ray atomic transitions.
     3. High energy continuum spectrum and non-thermal processes.
        

   • Achieves high throughput via multiple satellites (4). Spreads development costs and reduces risk via multiple launches (2).
     

   • We strongly endorse this new start and urge NASA to provide the necessary advanced technology development support to permit full operation by the year 2007.
     

   • Infrastructure could be utilized though additional launches allowing newer technologies to be employed while maintaining a continuous high throughput capability.

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     Constellation-X Science Examples

     

     Torus Geometry

     Figure 1: X-ray emission from a supermassive black hole. Measurement of the iron K_a profile probes the immediate environment of the black hole. The high energy response helps measure the geometry and composition of the disk and torus.

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     Cycles of Matter

     Figure 2: Life cycles of matter in the Universe. Most of the processes in this cycle involve temperatures greater than 10⁶K\protect and can best be investigated via high resolution X-ray spectroscopy.

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     Virgo: Optical and X-Ray

     Figure 3: Optical and X-ray emission from the Virgo Cluster of galaxies. X-ray observations are crucial to map the mass content, metal abundances, and velocity profile of the cluster.

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   Recommended Program (3)

3. Missions within the Explorer Program
   
   • Many mission concepts that address exciting issues in Astrophysics fit within the boundaries of the Explorer Program (UNEX, SMEX, and MIDEX, as well as the Mission of Opportunity).
     

   • Small and medium missions act as pathfinders, surveyors, and also provide cutting-edge science capability. Explorer class missions are particularly well suited for sharply focused science objectives.
     

   • Complement larger X-ray missions, and observations at other wavelengths, providing context for better understanding of various phenomena.

   • Advances in spatial, spectral, polarimetric, and temporal resolution must be supported through the Explorer Program to help build the framework for future progress. No specific mission recommendations are made as the Explorer Program is carried out through an open science peer selection process.
     

   • We fully endorse this program and would like to see more frequent opportunities made available.

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   Recommended Program (4)

4. Supporting and Sustaining Programs
   • SR&T
     • New funds are needed to deal with complex and technical challenges . SR&T is fundamental to new instrumentation and training for young experimentalists.
       

   • MO&DA
     • Must sustain the operation and data analysis for existing missions. Relative to building new missions, the operation costs of existing missions that provide sustaining science are low. International collaborations are an important means of leveraging.
       

   • Theory
     • Need to provide theoretical understanding before missions are flown to assure that missions return valuable data. Need predictions of what will be seen to scope sensitivity and resolution requirements.

Visions for the Future (1)

• Extremely High Angular Resolution Goals
  • ~ 100 Micro-arcsecond: Examine stellar coronae, plasma interactions in close binary systems, and stellar wind interactions in early type stars and proto-stellar systems.
    

  • ~ 10 Micro-arcsecond: Examine stellar coronal morphology of nearby (100pc) stars.
    

  • ~ 0.2 Micro-arcsecond: Image the event horizons of black holes, study the infall of matter, and view the formation of jets.
    

  • Sub nano-arcsecond: Directly image black holes, neutron stars, white dwarfs, and their immediate environments in the Milky Way. At this resolution, ripples due to gravity waves will be visible.

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  Visions for the Future (2)

• Very Large Area/Large Field of View Goals
  • Modest angular resolution ( ~ 30 arcsecond), > 10 m² , ~ 1 deg² : Monitor time variability for faint sources on rapid time scales, study nearly extended objects and virialization process.
    

  • ~ 5 arcsecond, >> 10 m² , high resolution ( ~ 1eV ), ~ 100 arcmin² : Spectroscopy of faint objects;, either low luminosity relatively nearby sources, or bright but very distant sources. Map 3-D baryonic matter distribution.
    

  • Sub arcsecond, > 10 m² : Observe faint x-ray sources that might be associated with the most distant objects found. Measure flux and spectra for faint low luminosity black hole sources. Detect the earliest supermassive black holes as they ``turn on'' at high redshift.

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  Visions for the Future (3)

• Technology Needs
  • Optics
    
    1. ~ 10 Arcsecond, Light Weight, Extremely Large Area ( > 10 m² )
       

    2. Interferometry: Milli and Micro-Arcsecond Performance
       

    3. Sub Arcsecond, Large Area, Light Weight Optics
       

    4. Large Area Multilayers to Extend Imaging to High Energies ( ~ 100 keV )
       

    5. Large Area, Multiple Pinhole Masks for High Resolution Images

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  • Detectors
    
    1. Non-dispersive High Spectral Resolution (Cryogenic)
       1. Large Format, Multi Pixel, Sub-eV, Long Life
          

    2. Non-dispersive Modest Spectral Resolution (CCD)
       1. Low Energy, Large Format, Small Pixels, Fast, Radiation Hard
          

    3. Dispersive (Gratings)
       1. Low Energy, High Efficiency
          

    4. New Detection Techniques
       1. High Count Rate Capability with Timing
       2. High Energy Response with Many Pixels
       3. Low Energy, Solar blind, Windowless
       4. Polarization Sensitive Systems

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  • Systems
    
    1. Structures
       1. Deployment, On-orbit Assembly
          

    2. Station Keeping
       1. Separate Optics and Detectors
          

    3. Spacecraft
       1. Higher Reliability, Pointing Control, Power
          

    4. Cryogenics
       1. Lifetime, Operating Temperature, Low Power

The X-Ray Astronomy Program

1. Launch and Operate the Chandra X-ray Observatory.
2. Complete the Enabling Technology Program for Constellation-X.
3. Sustain X-ray Astronomy Participation in the Explorer Program.
4. Maintain Vigorous SR&T, MO&DA and Theory Programs.
5. Begin New Start for Constellation-X Leading to 2007 Operations.
6. Initiate an Advanced Technology Program to Meet the Needs of the Grand Challenges for X-ray Astronomy in the 21^st Century:
   1. ``Image'' a Black Hole
      Directly observe the effects of General Relativity at the event horizon.
   2. Find the ``First'' Black Holes
      Determine when supermassive black holes first turned on.

7. Initiate a Series of Missions with Increasing Area and Resolution (spatial, spectral, temporal and polarimetric) to Achieve the Challenge.

The Grand Challenge

• ``Image'' a Black Hole
  • Only in the x-ray band is there the potential to directly image the region around the event horizon of a black hole.
    

  • The innermost structure of a black hole is totally unknown and is of fundamental importance understanding black holes in general.
    • Directly measure all the relevant physical parameters of the central region without recourse to indirect arguments.
    • Measure, directly, the regions responsible for producing the x-ray radiation (which is at present completely unknown), e.g., jets, accretion, accretion modes, winds
      

  • Technically one has to obtain micro-sec angular resolution. The technical feasibility is similar to that needed for SIM and LISA.

• Find the ``First'' Black Holes
  • The first structures in the universe probably form at z ~ 30 .
    

  • The formation of a massive black hole will likely accompany the formation of the first large star clusters.
    

  • The first black holes should (like those at the current epoch) be X-ray luminous, since black hole physics will be the same at all epochs.
    

  • X-ray emission, unlike other wavelengths, will penetrate the dense environments expected in the early universe.
    

  • It therefore seems highly likely that the best way to find the first collapsed structures in the universe is by finding the first black holes in the X-ray band.

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  The Road Map

• Require very high angular resolution and very large area. We envision an interleaving of these two basic technological developments (along with advances in other areas - detectors, systems, etc.).
• The techniques for building lightweight and high quality optics for imaging can be applied to a pathfinder interferometric mission, which will require technology development in areas such as active tuning of optics, large structures, and station keeping.
• These techniques then enable even larger optical systems, until the sensitivity necessary to detect sources at z ~ 10 is achieved. Similarly, sub micro-arcsecond resolution, with sufficient sensitivity is, needed to image a black hole, will build upon previous missions.