Constellation-X Bandpass

Many cosmic X-ray sources exhibit characteristic spectral features over a broad range of energy. These include

AGN, in which Compton reflection off surrounding cold material produces a tail to the continuum spectrum at energies > 10 keV, while fluorescence and recombination radiation from the same medium produces a range of discrete emission and absorption features down to low energies;
stellar flares, in which a hard, nonthermal, impulsive continuum can accompany the line-rich thermal emission produced by heating of coronal plasmas; and
supernova remnants, in which synchrotron radiation generated by cosmic ray electrons accelerated at the shock front can produce a hard extension to the thermal spectrum from shocked gas. Unambiguous measurement of such effects requires a bandpass which extends from ~ 0.2 keV to at least 40 keV.

The Constellation-X hard X-ray telescope (HXT) uses multilayer coatings on individual mirror shells to provide the first focusing optics system to operate in the 6 to 40 keV band. Compared to other non-focusing methods such as those used for RXTE, Constellation-X has twice the area, 640 times the energy resolution, 240 times the spatial resolution, and, above 10 keV, 100 times the sensitivity. No current or upcoming X-ray observatory provides similar high-energy sensitivity. Chandra and XMM, designated as the workhorses of X-ray astronomy in the next decade, will detect photons with energies only up to ~10 keV.

Compelling motivation for the HXT is provided by the existence of sources whose energy output peaks in the 10 - 40 keV range. The Seyfert 2 galaxy NGC 4945 is an example of such a source. The highly absorbed continuum X-ray emission from the AGN, which is obscured by an optically-thick torus, is visible only above 10 keV. For energies less than ~10 keV, where Chandra, XMM, and Astro-E are sensitive, only the X-rays that are scattered around the torus are seen. Measuring the continuum above 10 keV is crucial for understanding the true nature of the source. With its hard X-ray sensitivity, the HXT can detect sources with column densities as high as 10²⁵ cm^-2. For obscured AGN, which may make up the majority of the XRB, the HXT becomes background dominated at a flux of about 10^-14 ergs cm^-2 s^-1 (10 - 20 keV). In addition, about 55% of the 10 - 40 keV X-ray background is expected to come from sources with flux levels > 10^-14 ergs cm^-2 s^-1 and thus will be easily detectable for moderate HXT exposures. In summary, with the HXT, Constellation-X will be able to study AGN with the highest X-ray absorbing column densities and will go to much deeper flux levels, thus obtaining spectra for thousands of such objects.