HETG

Follow-on Science Instrument

Contract NAS8-01129

 

Monthly Status Report No. 012

February 2003

HETG Science Theme: the Inter-Stellar Medium

 

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

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1.0 Distribution List for Monthly Status Report

 

 

MIT-External Electronic:

FD03/Mike Smith, MSFC                  Carl.M.Smith@msfc.nasa.gov

PS41/Steve Morris, MSFC                 Steven.D.Morris@msfc.nasa.gov

SD50/Martin Weisskopf, MSFC            martin.weisskopf@msfc.nasa.gov

 

MIT-External Hardcopy:

                                    None specified.

 

 

            MIT-Internal Electronic:

                                                Elaine Tirrell                egt@mit.edu

                                                Gail Monahan             gmonahan@mit.edu

 

MIT-Internal Hardcopy:

                                                Claude Canizares         Room 3-234 (via Gail Monahan)

                                                Deepto Chakrabarty    Room 37-501 (via Elaine Tirrell)

                                                Kathryn Flanagan        Room NE80-6103 (via Elaine Tirrell)

                                                File                              (via Elaine Tirrell)

 

 

Please send distribution requests and other comments on this document to dd@mit.edu .

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2.0 Schedule of Past and Future Events Relevant to HETG

 

 

Date	Past Events	Comment
Mar. 3(14)	Chandra Cycle 5 GTO(GO) proposals due	Many involved.

 

 

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3.0 Instrument Status and Science Support

 

 

3.1 Flight Events and HETG Instrument Status

 

The HETG continues to function with no outstanding issues – though there were no HETG observations in February 2003. (Several HETG observations occurred in early March before this writing(3/18), in particular our GTO observation of TV Crit on 3/7 – the data look fine.)

 

3.2 Science Support to CXC, SWG, etc.

 

No support to CXC etc. this month as we worked on GTO target selection for Cycle 5.

 

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4.0 GTO Science Program

4.1 Observations and Data status

Planning for GTO Cycle 5 submissions (submitted 3/3) was a main activity this month - for this reason the usual Appendix A GTO table is not included.

 

4.2 Science theme progress

The HETG GTO science efforts span a range of “science themes” given in the list below. This month a brief summary of our work in studying the Inter-Stellar Medium is presented and completes a first-pass through the themes.

 

GTO Science Theme	Abbreviation (for App’ix A)	Researchers (HETG in caps)	Date of last [next] reporting Assembler of theme material
“Cool” Stars	Cool Star	dph,nss,psw,bb	September, 2002. Dave Huenemoerder
“Hot” Stars	Hot Star	nss,dph,psw,bi	October, 2002. Norbert Schulz
X-ray Binaries & Accretion Disks	XRB	MJ-G,AJ,nss,hlm, man, jmm, psw	December, 2002. Norbert Schulz, Mike Nowak
Supernova Remnants	SNR	KAF,DD,JMM, AF,jh,gea,tp	May, 2002. Dan Dewey
Isolated Neutron Stars	iNS	MDS,hlm, nss	January, 2003. Mike Stage, Herman Marshall
Galaxies & Clusters of Galaxies	Gal., Clust.	TJ,mw,jh	August, 2002. Michael Wise, Tesla Jeltema
Active Galactic Nuclei and Jets	AGN, Jet	SG,RG,MJ-G,hlm, man,jl,sm,jg	June, 2002. Herman Marshall
Inter-Stellar Medium	ISM	AJ,nss	February, 2003. Adrienne Juett, Norbert Schulz
Inter-Galactic Medium	IGM	SG,RG,hlm	July, 2002. Taotao Fang

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Inter-Stellar Medium Research Progress

 

X-ray Absorption in the Interstellar Medium

 

The space between stars is an ultrahigh vacuum by our laboratory standards but none-the-less contains gas, molecules and even dust -- the interstellar medium. The source of this stuff is most likely the expulsion of matter into space by supernova explosions (wisps in the figure here) and possibly other phenomena, e.g., jets, stellar winds and iteractions, etc. The following discussion and summary of ISM work is taken from A. Juett et al. “X-ray Absorption in the Interstellar Medium”, a poster presented recently at the AAS/HEAD ’03 meeting.

 

 

Among the most prominent features in the high-resolution spectra of bright continuum sources (e.g., X-ray binaries) are the photoelectric absorption edges and lines due to metals in the interstellar medium (ISM). The K shell absorption edges for Z = 8--26 (O through Fe) are all located in the 0.5--8~keV (1.5—25Å) energy range accessible to Chandra and XMM-Newton. The depths of these edges allow us to measure the column densities and relative abundances of these elements. These measurements provide independent estimates of ISM abundances that can be compared with measurements in the optical and UV (see e.g., Sofia & Meyer 2001, and references therein).

 

The high-resolution capabilities of Chandra and XMM also allow for studies of the structure of the ISM edges. X-ray absorption by the ISM is due to gas, molecules, and dust. Wilms, Allen, & McCray (2000) provided an updated absorption model for use with CCD resolution spectra, but the authors stress that their model, and similar models, are not appropriate for high-resolution grating spectra. The main cause for this is the lack of high-resolution cross-section measurements for atomic and molecular (dust) edges. Photoelectric absorption of an X-ray photon leads to inner shell

excitations, leaving the atom in an excited state. When the element is in atomic form, there are various X-ray resonance complexes which can be resolved, specifically resonances from the 1s-2p transitions of various ions. Once the element is bound in a molecule, energy levels, and therefore the line energies, may shift due to the change in binding energy. In addition, elements bound into dust grains show modification of their absorption edge energy and structure due to elastic scattering off the molecular structure or crystal lattice of solids. This modification is the cause of X-ray absorption fine structure (XAFS; see Woo 1995 and references therein) and particularly the so-called XANES (X-ray Absorption Near Edge Structure; Kossel 1920) which introduces significant structure to the edge. Many of the cosmically abundant elements, e.g., C, O, Si, and Fe, have large measured depletions (the fractional amount of the element thought to be in dust grains) and should therefore show XAFS in high-resolution spectra.

 

References

Behar, E. & Netzer, H. 2002, ApJ, 570, 165

Henke, B. L., et al. 1993, Atomic Data & Nucl. Data Tables, 54, 181

Kortright, J. B., & Kim, S.-K. 2000, Phys. Rev. B, 62, 12216

Kossel, W. 1920, Z. Phys., 1, 119

Pradhan, A. K. et al. 2003, MNRAS, in press (astro-ph/0302238)

Schulz, N.S., Juett, A.M., Chakrabarty, D., and Canizares, C.R., "X-ray Absorption in the ISM", 2003, AN, vol. 324, p. 166

Sofia, U. J. & Meyer, D. M. 2001, ApJ, 554, L221

Wilms, J. Allen, A., \& McCray, R. 2000, ApJ, 542, 914

Woo, J. W. 1995, ApJ, 447, L129

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Summary of Inter-Stellar Medium GTO Observations and Activities

 

Recently we have used photoelectric absorption features in the Chandra/HETGS spectra of seven bright X-ray binaries to study the relative abundances of O, Fe, and Ne in the interstellar medium (ISM). Three of these sources are in our GTO program: 4U 1636-53, Cyg X-2, and GX 349+2. We find Ne/O abundance ratios up to twice as large as predicted by standard ISM models, and Fe/O ratios of 0.6--1 times the predicted ratio. The shape and wavelength of the photoelectric edges can be used to discriminate between atomic and molecular forms of these elements. For example, we find that the O edge contains complex structure and measured edge wavelengths range from 22.89--23.12 Å. Through these measurements, we hope to better understand the line of sight variations in the abundances and to produce a universal absorption edge model for use by the community.

 

 

Oxygen Absorption

 

At right is the counts spectrum of the O-K absorption in Cyg X-2. The absorption line at 23.5 Å is due to the O I 1s-2p transition in the ISM. The line at 23.36 Å is due in part to the instrument, and in part to the O II 1s-2p transition.

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Neon Absorption

 

The Ne-K edge shows variations in its shape, with some sources having a well defined edge and others not; the edge from GX 349+2 is shown at left. Ne/O abundance ratios range from 0.7--2.2 times the ISM abundances given by Wilms et al. (2000). Such variations suggest line-of-sight variations in abundances, although there are large uncertainties in these measurements We note that for many of these sources, the Ne edge in the MEG minus first order lies in a chip gap where the effective area is poorly determined. This effect is most important for the lowest signal-to-noise observations. Better observations without chip gaps at the Ne edge are needed to reconcile real Ne/O ratio variations from instrumental effects.

 

Most sources show at least one accompanying absorption line at 14.61 Å (as shown here), possibly from the 1s-2p transition of Ne ions, comparable to that seen in O. Behar & Netzer (2002) recently calculated wavelengths and oscillator strengths for the 1s-2p transitions of Li-like to F-like ions of Ne and other elements. Comparing our lines to those of Behar & Netzer (2002), we find that if the 14.61 Å line is the F-like transition (predicted at 14.631 Å), then there is a wavelength offset of 0.02 Å. Given this shift, the O-like transition would be expected at 14.506 Å, and there is evidence in some of the sources for a line-like feature near this wavelength.

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Iron (Fe-L) Absorption

 

 

 

At right is the flux spectrum of Fe-L absorption edge from 4U 1636-53. The Fe-L III edge at 17.5 Å is due to transitions from the 2p_{3/2} level, while the Fe-L II edge at 17.2 Å is due to transitions from the 2p_{1/2} level and varies from source to source. A third transition is also possible, the Fe-L I edge at 14.6 Å (weak and not in the range shown), due to transitions from the 2s level, but this is the weakest edge.

Because this structure is not accounted for in the commonly used optical constants and absorption models (Henke et al. 1993; Wilms et al. 2000), we fit the Fe edges here using a custom table model for XSPEC and ISIS that uses the optical constant measurements of Kortright & Kim (2000). This model will be made publicly available upon publication of our initial results (Juett, et al. 2003, in prep).

 

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Future Work in ISM

 

·      Continue analysis of O K, Fe L, Ne K with respect to determining the atomic fine structure.

·      Extend sample of observations towards lower column densities.

·      Observe Ne edges out of the chip gap.

·      Correlate NH parameters with optical, UV and radio data.

·      Analyze higher energy edges, Mg K, Si K, S K ..., at high column densities.

·      Generate a generic model spectrum for high resolution spectral analysis use by the community.

 

 

4.3 HETG-related Software: Development, Evaluation, and Support

 

To support this work scripts were made to do the standard CIAO processing and we have made use of the “contamarf” routine from Dave H and John D. In fitting, we use a table model, which works in ISIS and XSPEC, made for the Fe-L edges based on the Kortright & Kim laboratory optical constant measurements.

 

 

4.4 Presentations (February)

 

[No presentations in February: everyone working on GTO and GO proposals!]

 

 

4.5 Publications (February), see also: http://space.mit.edu/csr_pubs.html

 

T. Fang, K.R. Sembach, C.R. Canizares, “Chandra Detection of Local O VII He-alpha Absorption along the sight line toward 3C 273”, ApJ 586:L[TBD].

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5.0 Systems and Engineering Support

 

5.1 Documentation and “Design Knowledge Capture” and 5.3 Anomalies, Insert/retract, etc. Support

 

No activities in these areas in February.

 

5.2 Spares Retest and Test Instrumentation

 

X-GEF was up and running in February. Tests were carried out on pairs of the Vacuum Storage Gratings; date codes are 030205, ’06, ’25, ’26, ’27, and 030304. Two further tests, 030311 and 0303 13, were carried out on the Proton radiation exposed samples. Preliminary results from these tests will be summarized in the March ’03 report.

 

 

6.0 Management

 

6.1 Program Office & NASA Support

 

No direct MSFC-related actions in February.

 

6.2 MIT-internal management activities

 

MIT responded to RFP from SAO for ’05-and-beyond funding on time.

For HETG postdoc hiring: progress is being made interviewing candidates.

 

 

7.0 Open Issues, Problems, etc.

 

There are no open issues or problems regarding the HETG.