Constellation-X Spectral Resolution
High spectral resolution is essential to make unique line identifications. The detailed X-ray line spectra are rich in plasma diagnostics that provide unambiguous constraints on physical conditions in celestial sources. A spectral resolving power of at least 300, for example, is required to separate the density-sensitive He-like triplet, while in the region near the iron K complex, a resolving power exceeding 2000 is necessary to distinguish the lithium-like satellite lines from the overlapping helium-like transitions.
The Spectroscopy X-ray Telescope (SXT) uses two spectrometer systems that operate simultaneously to achieve the desired energy resolution: (1) 2 to 4 eV resolution quantum microcalorimeter array with a 2.5 arcmin field of view, and (2) a set of reflection gratings for energies < 2 keV. The gratings deflect part of the telescope beam away from the calorimeter array in a design similar to XMM except that the direct beam falls on a quantum calorimeter instead of on a CCD. The two spectrometers are complementary, with the grating optimal for high resolution spectroscopy at low energies and the calorimeter at high energies. The gratings also provide coverage in the 0.25-0.5 keV band where the calorimeter thermal and light-blocking filters cause a loss of response. This low-energy capability is particularly important for high-redshift objects, for which line-rich regions will be moved into this lower energy band.
Simulations of helium-like iron (Fe XXV) emission from a ~ 20 million degree plasma. The top panel shows the spectrum from the Constellation-X calorimeter, which is currently under development. The bottom panel shows the spectrum from the microcalorimeter to be flown on Astro-E. The crosses indicate the statistical uncertainties for the observed counts in each individual energy bin. The Astro-E calorimeter provides a major advance over current capabilities and will resolve the resonance line from its satellites for the first time. The Constellation-X calorimeter will resolve the entire satellite complex with a resolution similar to that currently used to observe the Sun. Note also the increased throughput of the Constellation-X system.
Measurement of accurate radial velocities from X-ray emission lines is central to many astrophysical investigations such as mapping the velocity fields of clusters of galaxies, obtaining flow velocities in stellar flares, mapping the coronae of RS CVn and other binary systems, and measuring the kinematics of clumps of ejecta in supernova remnants. These require velocity sensitivity (the ability to centroid a line) better than 100 km s-1 below 1 keV. At the higher energy of the iron K line, the velocity sensitivity of order 40 km s-1 achievable with Constellation-X will provide radial velocity measurements enabling us to determine the mass distribution of black holes, neutron stars, and white dwarfs for a large sample of binary systems.
This combined ASCA and Ginga spectrum (the observations were not simultaneous) of the Seyfert II galaxy NGC 4945 illustrates the need for the hard X-ray telescope. The continuum source is heavily absorbed, probably by an optically-thick torus, and is only seen above 10 keV, exactly where telescopes like Chandra and XMM cut off. At lower energies, only emission scattered around the torus is seen. Constellation-X will for the first time be able to measure the underlying continuum and the scattering component simultaneously. This will enable the total energy output to be determined. By observing a large sample of such systems, the overall accretion geometry of AGN should be revealed. Observations will be possible for systems that are a factor of 100 times fainter than NGC 4945, thereby covering a wide range of luminosity and redshift.
