Kronos Science Goals
The specific objective of the Kronos science program is to answer the
What is the structure of accretion flows onto black holes and other
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.
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.
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
- Reverberation mapping
uses the detailed response of AGN broad
emission lines to continuum variations in determining geometry and
kinematics of the broad-line region.
- 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
- Doppler tomography
maps out accretion-disk structure through
- 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
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
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
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:
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.
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.
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.
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.
Go to: [Kronos Home Page]
[OSU Astronomy Home Page]
Updated 9 October 2001