Scientific Objectives

The high sensitivity, broad energy range, and the spectral resolution of SIXA will enable observations of time-resolved X-ray spectra for a variety of astronomical objects. Potential observing programs include stellar coronae, molecular clouds, cataclysmic variables and X-ray binaries; accretion discs and coronae of neutron stars and black hole candidates; supernova remnants, active galactic nuclei, clusters of galaxies and the diffuse cosmic X-ray background.

The energy range of SIXA (0.5-20 keV) contains many K-shell transitions. Detailed analysis of these spectral features will permit the determination of the physical characteristics of the emitting region and its environment. The ratio of the strength of the helium-like (Fe XXV) to the hydrogen-like (Fe XXVI) lines is a measure of the plasma temperature. A temperature measure that is independent of the equilibrium assumption is the ratio of the K-alpha and K-beta line intensities. All these can be separated with SIXA.

In accretion-powered X-ray sources (X-ray binaries and AGN's) the X-ray emitting plasma is often optically thick, with FWHM almost 1 keV. With the resolution of SIXA (200 eV), line profiles can be measured, giving an opportunity to study the line broadening mechanisms and the geometry of the emitting regions.

The SIXA detector will be suitable for studies in a wide variety of time scales. Galactic sources of variable X-ray emission include accretion disks, active stars with variable X-ray emission (X-ray flares), and mass accreting compact stars in close binary systems (X-ray bursts on white dwarfs and neutron stars). These exhibit variations in timescales from fractions of a second to days.

Stars of almost all spectral types possess X-ray coronae with luminosities in the range 10²⁶-10³⁴ erg s^-1 and with mean coronal temperatures 10⁶-10⁸ K. Spectroscopic study of stellar coronae can provide crucial information on a number of interesting physical processes, such as the generation of magnetic fields by dynamo action, the conversion of non-radiative energy into thermal energy and plasma heating, and the processes that cause mass and angular momentum loss in stars. With SIXA, a 5$\sigma$ detection can be made in $2\times 10^4$ s at distances d = $50 \times (L/10^{27}~erg~s^{-1})^{\frac{1}{2}}$ pc for the continuum and $d~=~5 \times (L/10^{27}~erg~s^{-1})^{\frac{1}{2}}$ pc for the Fe-line. Spectroscopy with a few minutes time resolution gives an opportunity to model flare dynamics of most active stars.

X-ray emission is a fundamental property of active galactic nuclei (AGN). Measurement of the X-ray time variability and spectra of these objects give us the closest look at the central energy source. X-ray active AGN's typically have strong soft excesses and canonical power-law spectra. In some cases, a broad iron line has been detected. The resolution and through-put of SIXA are sufficient for searching for e.g. the relativistic (two-horn) effects in line profiles.

Especially suitable targets for the geometry and field-of-view of SIXA are clusters of galaxies (especially those with cooling flows) and supernova remnants.