kronos Kronos Science Goals

The specific objective of the Kronos science program is to answer the following:

  1. What is the structure of accretion flows onto black holes and other compact objects? Accretion in AGNs and stellar-mass binaries can lead to the formation of various types of accretion disks. The temperature structure of the accretion disk indicates where the energy is released and how dissipative viscosity arises. Kronos maps the distribution of emissivity and temperature across the accretion flow as a function of time. Such measurements, and, particularly, the comparison between AGNs and Galactic sources, are necessary to understand the properties of the innermost region of accretion flow near the event horizon.

  2. What is the physical origin of radiation from black hole systems? Processes in the accreting systems can convert high-energy X-ray radiation into low-energy photons (e.g., X-ray reprocessing) or lower-energy photons into X-rays (e.g., Compton upscattering in a hot corona above the accretion disk). While both processes are expected, what are their relative roles in AGNs? Kronos multiwavelength monitoring of continuum variations over long periods will clarify this. Similarly, multiwavelength observations of blazars and microquasars reveal the relationship between the jet phenomenon and the accretion process in the central engine. Jet power is a major source of mechanical feedback into galaxy environments and affects galaxy formation and evolution.

  3. What is the structure and kinematics of the broad-line emitting region in AGNs? Broad emission lines are one of the defining characteristics of the UV/optical spectra of AGNs. The broad lines vary in flux in response to continuum variations; and through the technique of reverberation mapping, the velocity field and geometry of the broad-line region can be determined uniquely. This will clarify whether the line-emitting gas is part of the accretion flow, part of an associated mass outflow, or in some stable configuration around the central source. In any case, reverberation mapping of even a small number of AGNs provides a tremendous improvement in the accuracy to which all AGN masses are measured. Currently, our limited understanding of the broad-line region is the greatest source of uncertainty in AGN black-hole masses.

Kronos is designed to answer these questions by using proven timing-based methods exploiting the natural variability of accretion-driven sources:

  1. Reverberation mapping uses the detailed response of AGN broad emission lines to continuum variations in determining geometry and kinematics of the broad-line region.

  2. Eclipse mapping uses the progressive occultation of the accretion disk by the cool secondary in high-inclination binaries in mapping the surface brightness distribution of the accretion disk. Kronos will do this during outbursts, thus tracking the evolution of the accretion disk structure.

  3. Doppler tomography maps out accretion-disk structure through line-profile variations.

  4. Mulitwavelength monitoring of continuum variations in both blazars (AGNs whose spectra are dominated by jet emission) and non-blazars is used in determining causal relationships between variations at different photon energies, thus identifying physical mechanisms at work.

Doppler tomography and reverberation mapping are indirect imaging methods, much like seismology or computed axial tomography (CAT scans). The methods identified above have reached natural limitations with existing facilities. These limitations are imposed primarily by the temporal coverage achievable from low-Earth orbit or from the ground.

Based on more than a decade of experience with these methods, we know precisely the required data products and instrumental capabilities necessary for dramatic advances in understanding accretion- driven sources. Kronos provides a multiwavelength approach to dedicated use of the time domain as pioneered by the Rossi X-Ray Timing Explorer. Kronos differs from other multiwavelength observatories (e.g., XMM-Newton, Swift) in its spectroscopic capabilities. It differs from other spectroscopic missions (e.g., International Ultraviolet Explorer, Hubble Space Telescope, Far-Ultraviolet Spectroscopic Explorer by a high-Earth orbit that permits long, uninterrupted observations for time-series analysis and/or its higher sensitivity. The key element of Kronos is that it is dedicated to long-term monitoring programs. Whereas other missions return snapshots, Kronos returns movies.

The defining features of Kronos are:

  1. The Kronos observatory is designed specifically for long-term, high time-resolution observations of accretion-driven sources. The Kronos high-Earth orbit affords continuous on-source times of up to nearly the orbital period of ~14 days with only rare interruptions.

  2. The Kronos telescopes cover the most important wavelength regimes for accretion-driven sources. The throughput of the two telescopes - X-ray (0.3 to 10 keV), ultraviolet/ visible (UV/VIS; 100 to 170 nm, 270 to 540 nm) - ensures that the on-source times are determined by timescales of physical processes, not by photon rates.

  3. Kronos is primarily a key-projects mission. The highest science return will be achieved through very long observations of a limited number of carefully chosen representative sources.

  4. Kronos is designed for flexibility. Changes in observing schedule are easily implemented. Data are in the form of photon lists, which allows post-facto optimal binning in wavelength and time to achieve different science goals using the same data.

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Updated 9 October 2001