The Universe is filled with many cases where matter has evolved to extreme physical conditions. Examples include white dwarf stars and neutron stars where the gravitational force of collapse is balanced by the quantum forces exerted by electrons and nuclear matter, and black holes where the force of gravity has won out over all other forces and the mass density is so high that even light is gravitational bound. We can study these systems by the effects they have on the matter around them. Often galactic neutron stars and black holes have binary companion stars that are so close that stellar material is transferred to the compact object. Black holes are also highly likely to exist at the centers of most galaxies and have huge accretion disks around them. As the matter falls closer to the compact object it is heated to such extreme temperatures that X-rays are the primary form of emission.
This X-ray emission is then the carrier of information that offers clues to the underlying compact object. So far this emission has provided a growing number of probes into the structure of the matter in the vicinity of these singular objects.
From these observational probes we can hope to address such basic questions as:
The Rossi X-ray Timing Explorer (RXTE) mission, with its high throughput and very high time resolution is providing a wealth of new data and phenomena on neutron stars and black holes. The Russian Spectrum-Roentgen-Gamma (SRG) mission will have a U.S. supplied X-ray polarimeter that may open up a new window of investigation on these objects. However, answers to many of these questions will also simultaneously require higher spectral resolution. For example, high spectral resolution would allow Doppler shifts of X-ray lines from accretion disks to be directly measured, revealing the radial properties of the disk. Upcoming missions such as AXAF, XMM, and Astro-E will have high spectral resolution, but will be limited in their ability to study phenomena on short times scales due to insufficient throughput or instrument limitations with high counting rates. HTXS is being developed to avoid these limitations.
Missions: Astro-E, AXAF, GLAST, HTXS, SRG, XMM