HETG
Follow-on Science Instrument
Contract NAS8-01129
Monthly Status Report No. 003
May 2002
HETG Science Theme: Supernova Remnants
Prepared in accordance with DR 972MA-002
DPD #972
Prepared for
National Aeronautics and Space Administration
Marshall Space Flight Center, Alabama 35812
Center for Space
Research; Massachusetts Institute of Technology; Cambridge, MA 02139
Supernova Remnants Progress
Introduction
The study of supenova remnants provides information on i) the end point of a star's evolution, ii) the creation and dispersal of heavy elements (beyond H and He) in the universe, and iii) the events that produce a neutron star or black hole, and on iv) the acceleration of cosmic rays. These remnants generally have a hot plasma component which emits x-rays corresponding to the elements present, in particular the HETG can measure lines from O, Ne, Mg, Si, S, and Fe.
In addition, some SNRs exhibit "non-thermal" X-ray emission due to synchrotron or non-thermal bremsstrahlung radiation mechanisms. Separating these lines for measurement requires the spectral resolution of the HETGS. Because the sources are extended and not simple point sources the HETG resolution attainable depends on the details of the size and spatial structure of the source. Much of our research effort is involved in teasing high-resolution results from the SNR observations.
HETGS observations have been carried out as part of our GTO program on four supernova remnants: "E0102" in the SMC, "N132D" and "N103B" both in the LMC, and "Cas A" in our Galaxy. Information on the specific observations is given in the Appendix A table. For each remnant "true color" images of the SNR are shown at left (from http://hea-www.harvard.edu/ChandraSNR/ ) and the dispersed image on the ACIS-S array is shown to the right. We take each remnant in turn and summarize our analysis status and scientific results.
E0102 
This SNR was observed very early in Chandra's first year and its well defined ring structure shows up beautifully in the dispersed image above. Bright lines of O, Ne, and Mg dominate the spectrum with little of the "Iron forest" of lines appearing here. This clean, well characterized spectrum is used to advantage in calibrating the ACIS detector.
There is a wealth of data to be extracted from the E0102 observations some of which has been presented at various conferences since the data were obtained. At the moment we are just putting the finishing touches (elemental mass estimates) on a comprehensive paper (Flanagan et al. 2002) covering several analyses of the data - some key results are shown in the following figures taken from the paper.
The plot at left shows for the measured Oxygen line fluxes contour lines describing regions of allowed shock model parameters, temperature and ionization age (tau).
In the figure at right the location measured as the distance from E0102 center of an elements' H- and He-like ion emission shows a clear correlation with the ionization age, tau, at which that element has greatest emission. With the "younger" material on the inside this suggests the motion of a "reverse shock" through the medium from outer to inner radii.
This image of E0102 is the result of our prototype spatial-spectral analysis software and shows a model of the Ne X line emission on the sky which is color coded for regions of high velocity: red regions are moving away at 500 to 2200 km/s and the green regions are moving toward us at ~500 to 1300 km/s; a few regions, colored blue, are moving toward us at velocities in the 1400 to 2200 km/s range. The structure of velocity seen here is suggestive of an expanding cylinder or partially filled spherical shell seen a little off axis hinting at the 3-D spatial-velocity structure of the remnant.
N132D 
This remnant is larger and spatially more complex than E0102. There are many bright "filamanents" in the remnant - long, curvy but generally narrow regions. As the true color image of this remnant shows, there is quite a variation in this remnant from location to location. Using in-house "filament analysis" software it is possible to measure the intensity of O and Fe emission lines in each of over a dozen filament features in N132D.
This plot shows a point for each identified N132D feature depending on the ratio of O VIII to O VII and the ratio of Fe-17A to O VIII for that region. The scatter in the plot further displays the variation in plasma conditions through out the remnant.
N103B 
The smallest of our remnants in terms of size on the sky is N103B. Because of this smaller size it is possible to obtain a reasonable spectrum of N103B by essentially analyzing it with standard grating analysis techniques.
Most of the "usual suspects" show up in this N103B spectrum, however, the clear presence of Oxygen in this spectrum (at 19A, 22A), not clearly seen in previous CCD spectra, puts into question previous conclusions that this was a type Ia supernova. Most notable in comparison to E0102 and N132D are the very bright lines of Si which contain useful counts in both HEG and MEG spectra. We have fit these data with a multi-temperature plane-parallel shock model and are working to get a measure of overall elemental abundances. A preliminary spatial-spectral analysis of the Si lines was carried out and may be able to produce accurate maps of the Si resonance and forbidden line emission.
Cas A 
This remnant fills about one half of the S3 chip of ACIS and represents about the most extended source one could imagine observing with the HETG. Fortunately it is very bright and so there are significant numbers of counts from individual small bright knots and filaments. As in the case of N103B the remnant is very bright in the Si line and the HEG and MEG spectra contain useful information.
Our analysis to date has focused on one bright Si-rich narrow feature. Analyzing the dispersed images of this feature shows the Si lines to be clearly blue-shifted by a doppler velocity of order 2000 km/s.
The location of Si lines expected are labeled at the top of the figure, Si XIV, Si XIII. The measured locations are consistently shifted to the left due to a Doppler velocity of order 2000 km/s. This result is in agreement with previous Einstein/FPCS and recent XMM/EPIC-MOS observations (Markert et al. 1983, ApJ, 268, 13; Willingale et al. 2001, A&A 381, 1039) but is much more specific in its spatial region and accuracy of Doppler shift and maybe a useful calibrator for the XMM data sets.
Supernova Remnants Plans
There is much more that can be done with these data and we are also looking to get additional data as part of the GTO program (another observation of E0102 at a different roll angle to complement our initial data set is in the peer review competition for Cycle 4.) Some of these additional efforts are described here:
E0102
- finish estimate of abundances in E0102.
- measure line fluxes and ratios in localized regions, e.g., the SE arc.
- compare fluxes with power-law shock model values.
- repeat the Ne X spatial-velocity analysis including obsid 968 as well.
- investigate 3-D models for their agreement with the Ne X spatial-velocity results and the variation of
ionization stage with position.
- create s/w to jointly analyze the Cycle 1 and Cycle 4 E0102 observations.
N132D
- try including an Fe "pseudo continuum" to better measure Ne and Mg line fluxes in the various regions.
- plot a model grid of line ratios over the O7/O8 vs Fe/O8 scatter plot.
- establish limits on non-thermal X-ray emission using radio observations.
N103B
- present more detailed results at COSPAR meeting in October.
- further spatial-spectral analysis.
- establish limits on non-thermal X-ray emission using radio observations.
Cas A
- for the Si feature combine grating line flux values with ACIS zeroth-order continuum values
to get n_H/n_Si and feature masses.
- analyze additional features in Si and S for flux, temp, mass, and doppler velocities.
General
- write a paper on the analysis techniques used for these extended sources, a "spatial-spectral SNR paper".
- work with ISIS s/w team to determine useful spatial spectral s/w functions to be added to ISIS.