SIMULATIONS SHOWING HESI'S CAPABILITIES FOR IMAGING SPECTROSCOPY

Using our knowlege of RMC imaging, we have attempted to predict the imaging capability of the proposed instrument for a variety of flare morphologies, energy spectra, and temporal variation. We focus on four types of imaging: (1) Low photon-count, high-energy flares; (2) Rapidly changing, simple, intermediate-energy flares; (3) Complex, extended, low-energy flares; and (4) Multi-component, soft/hard spectrally diverse flares. In each case, we have run the flare model through the modulation simulator, which incorporates the proposed grid pitches, grid orientations, sampling rates, statistics, and background environment, to produce modulation time profiles for each of the collimators. When the number of photons/detector is relatively large (e.g. > 1000/s), the profile is simply the number of modulated photons as a function of telescope rotation angle. When the number of incident photons is small, the modulation profile is a series of probablistically-determined distribution of photon time tags. The modulation profiles are then fed into the CLEAN algorithm to deconvolve the images for comparison with the flare models. Iteratively, 10% of the response to a point source at the maximum of the dirty map was subtracted, until either no more flux was extractable. The method is efficient and simple for sources with a few point components, and is slow but reliable for sources with extended components. The variation of the point spread function across the field of view (an unavoidable facet of RMC imaging), is taken completely into account.

1. Imaging in Nuclear Lines (Using two 3-cm thick grid pairs)

Perhaps the most challenging task of HESI will be the imaging of nuclear line flares. This is most feasible for the 2.2 MeV line, which is narrow enough that the signal-to-noise ratio will be reasonably high in large flares.
Model:
Point source, 200 photons/detector, Monte-Carlo time tags
Time Interval: Multi-rotation integration (> 4 s)
Backgrounds: 0, 20, 50, 100, 200 photons (randomly distributed)
Results:
Reliable maps of single point sources can be made using two collimators for 200 flare photons in the presence of up to about 100 noise photons. When the noise becomes comparable to the flare photon rate, large spurious sources appear in the maps. The pitches of the two thick grids used in this simulation are 39" and 57", but the source maximum is located to within 10" in all cases.

2. Rapidly-Varying, Simple Hard X-ray Flares

Model:
Single point source appears and disappears in 0.5 s, overlapping with double source component of life 1.8 s
Time Resolution of maps: 100 ms
Backgrounds: negligible compared to flare rate
Results:
Reliable "snapshot" maps of double-point sources can be made with 100 ms time resolution, using all 12 collimators with the strawman relative orientations of slats (67.5 degrees). The sources must be strong enough that statistics and background are not significant factors.
Triple-point sources cannot be mapped with reliability at this time resolution, unless their relative locations are fortuitously placed so that visibilities happen to be optimal.

3. Spatially Complex Sources Mapped at Lower Energies (2-10 keV).

Model:
An SXT image of "Post Flare Loops" has been scaled down by a factor of 2.5 from the normal SXT 5" resolution so that the image shows 2" structure.
Time resolution: 2 s
Background: negligible for moderate flares at these energies
Results:
The modulation profiles, computed for 2 s intervals for all collimators, were fed into the standard CLEAN algorithm, After 500 iterations, the map was still incomplete, showing contours only down to the 15% level, but otherwise accurate. Convergence of maps of this type is very slow, because of the dominance of the visiblities at large spatial scale. (SXT maps typically show visibility amplitudes decreasing with spatial frequency as a power law with index ~1.5, which means that the modulation amplitudes for grids with pitch of 80" are about 60 times greater than the amplitudes for grids with pitch of 5"). It is expected that at higher energies, the sources will not be as extended, and the range of modulation from coarse to fine collimators will not be as large, so this test is more severe than necessary for imaging at 2 keV and above.

4. Spectral Resolution (2 keV-100 keV)

Model:
Masuda Flare with self-consisten spectra in the 10-100 keV range
Time resolution: 1 s
Background: 100 photons/detector/s in the 50-100 keV range, negligible at 10-20 keV
Results:
When the model is available, the map cube will be fed into the modulation program, and the modulation curves will then be piped into the Clean program. It is expected that the convergenve of Clean will be quite rapid, because of the contrast of the images. Images at a wide range of energies will be made, forming a clean map cube. Spectra at a number of important pixels will be extracted from the cube for comparison with the original model.
Last modified Nov 8, 1995. Ed Schmahl, ed@astro.umd.edu