This webpage is being kept for archival purposes only.
It may be updated in the future, but it is not currently being maintained.
Relevance to NASA's Goals
EXIST was recommended as a high priority mission for the coming decade by the NASA Gamma Ray Program Working Group (GRAPWG), as summarized in the 1999 GRAPWG Report (pdf). The 2001 Decadal Survey Report recommended EXIST as a mission for this decade. EXIST was included in the most recent (2000) NASA Strategic Plan as a Mid-Term mission and would serve as the Black Hole Finder Probe in the Beyond Einstein Program as described in the current (2003) Structure and Evolution of the Universe Roadmap. A new Universe Exploration Roadmap is currently being developed and will be released in mid 2005.
What We Want to Know About Black Holes
Among the most important questions that EXIST will help us answer are:
- When and how were the first black holes (BHs) formed?
- How fast do massive BHs grow?
- How do massive BHs affect galaxy formation?
- What happens to stars that fall into the strong gravitational fields of massive BHs?
- How many stellar-mass and intermediate-mass BHs are there in the Galaxy?
- Does every galaxy have a massive BH?
- What fraction of the universe's total energy comes from BHs?
Why Should We Look in the Hard X-Ray Band?
The hard X-ray (HX) band, from ~5-600 keV, reveals an unusually rich range of astrophysical processes in both compact and diffuse sources. Black holes and the Gamma-ray bursts that are believed to accompany their birth emit most of their power in the HX band. The HX band is also key for study of the obscured universe, particularly the obscured massive BHs in the centers of galaxies, which are invisible in the soft X-ray and optical wavebands and can be confused with star formation and dust at infrared wavelengths.
HX sources include some of the most exotic and violent objects in the universe:
- Active Galactic Nuclei (AGN) - AGN are massive BHs in the centers of galaxies, ranging from roughly one million to one billion solar masses, that emit prodigous amount of radiation. Only a small fraction, perhaps 1%, of galaxies have AGN, although many and probably most galaxies have BHs in their centers.
- Gamma Ray Bursts (GRBs) - GRBs are flashes of gamma rays that last from less than one tenth of a second to several minutes.
During that time they are the brightest objects in the universe. Roughly one GRB occurs per day throughout the entire observable universe. They are believed to occur as a result of some supernovae and perhaps the collision of two neutron stars or white dwarfs.
- Stellar-mass BHs (sBHs) and Neutron Stars - In addition to their normal HX emission, X-ray binaries (and AGN?) may emit 511-keV positron-electron annihilation radiation during outbursts.
- Supernova Radioactive Decay Emission Lines - 44Ti emission can reveal
obscured SNe in the Galactic plane.
Science objectives are continuously refined as new observations and theoretical work advance our understanding of the hard X-ray universe. The links below may be slightly out of date but provide more detailed information.