Kronos Time Coverage, or “It’s About Time!”

The time-series methodologies used by Kronos can be likened to making movies: a movie is just a sequence of still pictures of something that is changing. A movie shows us the pictures in such rapid succession that we perceive the changes as a fluid sequence. For a movie to work, several things must happen:

(1)  the camera must be sensitive enough to make an good exposure on a time scale faster than the changes are occurring and faster than the time between exposures;

(2)  the time between exposures (i.e., time resolution, or “frame rate”) must be short compared to the time on which perceptible changes are actually occurring; and

(3)  the duration of the movie sequence must be long enough that the viewer sees the changes from start to finish.

These are precisely the conditions we need for the Kronos methodologies to work.

Sampling Requirements

Consider the case of multiwavelength monitoring of AGNs, one of the main science programs of the Kronos mission. The sampling parameters (time resolution and duration) are set by the requirements of reverberation mapping of the AGN broad emission lines. These requirements are well-understood by a combination of previous monitoring programs and detailed numerical simulations that have been carried out by the Kronos Science Team, as shown elsewhere on this website. For the AGNs that will be targets by Kronos, high-fidelity maps should be obtainable with a series of one-hour exposures twice per day (i.e., time resolution of 0.5 day) over a duration of 200 days, or less if the structure of the line-emitting region is simple. Unlike existing observatories, Kronos is designed to meet these sampling criteria, as shown graphically below.

In this diagram, we show the time coverage for three state-of-the-art monitoring campaigns undertaken by the International AGN Watch, color-coded to show different wavelength regions: magenta indicates high-energy observations (X-ray or gamma-ray), sky-blue indicates extreme ultraviolet, blue indicates ultraviolet, and green indicates visible wavelengths. The horizontal axis is time, in days, and the colored regions show the time during which data were being collected in each wavelength band. For each of the experiments shown, the first observation occurs on day 0.

The top set shows the sampling pattern for the best-sampled program ever, that on NGC 7469 in 1996. Simultaneous data were obtained by RXTE (Nandra et al. 1998, ApJ, 505, 594), IUE (Wanders et al. 1997, ApJS, 113, 69) and a network of ground-based telescopes (Collier et al. 1998, ApJ, 500, 162). While the time resolution of this program was good, the duration of the program was too short to meet Kronos science goals and the data were not strictly simultaneous across the wavebands.

The second set shows the sampling pattern obtained in a 1993 campaign on NGC 5548 using a ground-based network of telescopes to obtain optical spectra and IUE and HST to obtain ultraviolet spectra (Korista et al. 1995, ApJS, 97, 285) and the Extreme Ultraviolet Explorer (EUVE) to obtain EUV data (Marshall et al. 1997, ApJ, 479, 222). In this case, for Kronos science, the sampling was too coarse in every waveband except the EUV, the duration was too short in every waveband but the optical, and the sampling was irregular and patchy in every waveband.

The third set shows a more recent campaign on the narrow-line Seyfert 1 galaxy, featuring ultraviolet observations with HST (Collier et al. 2001, ApJ, in press), RXTE (Pounds et al. 2000) and ASCA (Turner et al. 2001, ApJ, in press) X-ray observations, and optical observations with a network of ground-based telescopes (Shemmer et al. 2001, ApJ, in press). This intensive program was simply too short to address Kronos science questions.

For comparison, the bottom set shows the time sampling that could be obtained with Kronos: the observations are simultaneous in all wavebands, continuous, except for short perigee gaps (due to Earth occultation of most targets) once every orbit (13.7 days), and with duration of up to seven months or so (limited by solar avoidance angles) or longer for some sources.

Unlike previous campaigns, Kronos will yield both high time resolution and long-duration data.

Kronos: it’s about time!

High-Earth Orbit

Many astronomical satellites, including HST and RXTE, are in low-Earth orbit (i.e., at altitudes lower than 1000 km or so) with orbital periods of 100 minutes or so. The advantage of low-Earth orbit is that, for a given satellite mass, it requires the smallest launch vehicle. The disadvantage of these orbits is that the Earth occults most astronomical targets for about ½ of each orbit. Moreover, communications are difficult and complicated since low-Earth orbiting satellites are only accessible to ground communications stations for at most for 10 minutes at a time as the satellite passes overhead. Also, low-Earth satellites pass through the Earth’s radiation belts, which negatively affects detector performance (some of HST’s light detectors have to be turned off entirely during passages through the most intensive radiation field, the South Atlantic Anomaly).

A key feature of the Kronos mission is its high-Earth orbit. Such an orbit is absolutely necessary for some Kronos science, such as eclipse-mapping of accretion disks in cataclysmic variables (or “dwarf novae”). The goal of Kronos is to map the accretion-disk structure over a series of eclipses during a dwarf nova outburst cycle to determine how the disk evolves. The diagram below illustrates the impossibility of doing this from low-Earth orbit.

The black line shows a model optical light curve of a dwarf nova outburst, in this case the system Z Cha. On top of the general increase in brightness of the system we see a regular pattern of eclipses (orbital period P = 107 minutes): detailed observations of the eclipse structure tells us about the structure of the accretion disk that is being eclipsed. The vertical bars show when this object would be observable by HST, i.e., for about 59 minutes out of every 96-minute orbit. The yellow bars indicate orbits when HST passes through the South Atlantic Anomaly and some detectors must be turned off; the blue bars represent “clean” orbits. The magenta blocks show when the system would also be observable by Chandra X-Ray Observatory. What we see here is that HST misses most of the orbits, and gets all of the eclipse very rarely. We also see that the overlap between HST and Chandra observations is very poor.

Essentially, each eclipse is a frame in a movie: HST misses most of the frames, but Kronos gets them all. Kronos makes movies. The number of eclipses that could be observed by HST is small on account of simple orbital mechanics.

Kronos: it’s about time!

Natural Time Scales of Accretion-Driven Sources

As demonstrated in the Tables below, Kronos will adequately cover nearly all important time scales for both extragalactic and Galactic accretion-driven systems. Time scales in green boxes are achievable with Kronos monitoring; those in yellow boxes are partially achievable, and those in red boxes are not achievable.

Important Time Scales for Active Galactic Nuclei

Time Scale

AGN (107MSun)

Light crossing time of inner disk

200 seconds

Light crossing time of outer disk

20,000 seconds

Light crossing time of hard X-ray source

3 days

Light crossing time of broad-line region

3 - 50 days

Free fall time

200 days

Sound crossing time of accretion disk

10 years


Important Time Scales for Stellar-Mass Accretion-Powered Systems

Time Scale



Frame dragging

0.1 - 1 second

Not applicable

Oscillation periods

Not applicable

10 - 30 seconds

Dynamical time scale of disk

0.001 sec to 10 hrs

seconds to 1 hour

White dwarf spin period

Not applicable

30 sec to 5 hrs

Orbital periods

3 hrs to 6.5 days

1.5 hrs to 2 days

Eruption rise times

~1 day

1 day to 1 week

Viscous time scale

0.01 sec to 10 days

1 day to 1 week

Eruption decay times

~30 days

1 - 2 weeks

Eruption duration

~200 days

Days to weeks

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Updated 18 February 2002