UX Ari is a bright RS CVn binary system - a prototypical coronal source. A continuous emission measure model was constructed based on the published EUVE data (Dupree et al 1996, in ASP Conf. Ser, 109, 237); the peak emission occurred near logT=7. Simulations were run for both HETG and LETG with the same input spectrum to an equivalent exposure time of about 40 ks.
AXAF grating spectra of cool stars like this will detect a multitude of emission lines which are indicative of the physical conditions in the outer layers of the star - the temperature, density, and abundances - and are clues to the coronal heating mechanism and its evolution.
(click image to zoom)
Figure 1. This is a simulation of a UX Ari-like stellar coronal plasma as detected by the HETG (High Energy Transmission Grating) with the ACIS-S detector. The central part of the figure shows a detected photon intensity image in proper aspect on the 6-CCD detector array. Since the HETG has 2 grating sets rotated by a relative angle of 10 degrees, a shallow``X'' is formed on the detector. The positively sloped whisker is from the MEG (Medium Energy Grating), and the other from the HEG (High Energy Grating). A zoom on the zero-order region is shown above the image. Below are scatterplots of the photons in ``dispersion coordinates'' - the distance along and across a whisker. Colored points are first order photons. Whitepoints are higher order photons, which can in principle be distinguished based on their pulse-height. Simulations will directly determine how good order-sorting is.
(click figure to zoom)
Figure 2. Simulated data are used to develop software for extraction of one-dimensional spectra. Here are shown the MEG (top and bottom) and HEG (middle) first order counts versus wavelength for the same model and simulation as in Figure 1. The differences in resolution (and efficiency) are apparent between the top and middle panels, which cover exactly the same wavelength range. Since the HEG and MEG grating period ratio is very nearly a factor of two, the lower wavelength half of the MEG spectrum (top panel) covers the same wavelength region as the HEG spectrum (center panel).
Figure 3. This is an expanded view of the MEG first-order counts spectrum in the vicinity of some lines.
Figure 4. The same lines of the previous figure are shown, but for HEG first order. The higher resolution is apparent.
(click image to zoom)
Figure 5. This shows a simulation of the same coronal source but with the LETG (Low Energy Transmission Grating) spectrograph and HRC-S detector array. In the upper-right expanded view, the photons cross-dispersed by the grating support structure can be seen as rays from the zero-order. At the bottom is an expanded view of the photon distribution. Spectral lines show up as vertical bands. Since the HRC-S is a microchannel-plate detector, there is essentially no PHA discrimination, so higher order photons cannot be directly discriminated. These are shown in white points in the graphic.
(click figure to zoom)
Figure 6. Here is shown the LETG total counts versus wavelength for the same model and LETGS simulation as in the previous figure.
HETGS: One million input photons gave 124,000 ouput photons for an equvalent exposure time of 36 ks. In the extraction rectangle, there were 14,000 first order HEG photons and 50,000 MEG. The high-order contribution was about 4% of the first order, and about 50,000 photons went into the zeroth order.
LETGS: Four million input photons resulted in 85,000 output for a 39 ks equivalent exposure time. The zeroth order had 33,000 photons, and the first, 46,000, with high order contribution is about 10% of the first order.
The input spectrum was derived from an augmented SPEX line-list (Brickhouse, private communication) plus a Raymond-Smith continuum, for Solar abundances. A range of temperatures were chosen with weighting defined by the EUVE-modeled emission-measure distribution. The model Flux was 4E-11 ergs/cm^2/s in the range 0.1 to 10 keV.
Figure 7. The emission measure distribution for the UX Ari model spectrum, which specifies the contribution of each temperature over a range of plasma temperatures.
Figure 8. These three panels show the input model spectrum (top), the MEG first-order counts (center) and the HEG first order counts (bottom), vs. wavelength, as detected by ACIS-S.
It is interesting to examine the same model for different number of total counts. The following figures show such for the UX Ari model above with HETG and ACIS-S, for the MEG first order spectrum: 1000 counts; 3000 counts; 10000 counts; and 30000 counts.
Simulation courtesy of Dave Huenemoerder (MIT/ASC)
Home page | Staff
| Calibration | Analysis
[ Science Gallery | Software | Documentation | Links | Local Access ]