The Emission Line Project:
Benchmarking the Plasma Spectral Emission Codes

Nancy S. Brickhouse

Smithsonian Astrophysical Observatory

Chandra X-ray Observatory Center

With Jeremy Drake

Chandra Users Committee Meeting
Center for Astrophysics
June 16, 1999

I. Introduction

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The Emission Line Project is a collaborative effort, organized by the Chandra X-ray Observatory Center, to improve the plasma spectral models used to analyze and fit X-ray spectral observations.

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The first phase will take advantage of high quality spectra of three stellar coronal targets (Procyon, Capella, and HR 1099) that will be obtained for the purposes of calibrating the Chandra transmission gratings.

Calibration sources:

• Cover a large range in plasma temperature

  Procyon (F5 IV); log T $\sim$ 6.3

  Capella (G1 III + G8 III); log T $\sim$ 6.8

  HR 1099 (K1 IV + G5 V); log T $\sim$ 7.2

• Well-observed, (reasonably) well-understood

• Emission line-rich spectra

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These calibration data will be made public as soon as possible after their acquisition.

II. Overview of Plasma Spectral Emission Codes

• Codes developed in the 1970's to model X-ray spectra appeared adequate until spectral resolution of instruments improved
• Problems with modeling ASCA, EUVE, and DXS data due to:

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  Inadequate numbers of parameters?

  e.g. 1- or 2-T models

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  Inappropriate physical assumptions?

  e.g. optically thin plasma, ionization equilibrium

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  Inaccurate atomic physics?

• Efforts to address plasma modeling issues

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  Workshops

  HEAD NAPA Plasma Codes Workshop (1994)

  ASC Rates, Codes, & Astrophysics Workshop (1995)

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  Modernization of plasma spectral emission codes

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  Ongoing updates to atomic data inputs to plasma spectral emission codes

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  Ongoing theoretical and experimental efforts

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  Emission Line Project memo (1997)

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  CXC (and other) spectral analysis tools

III. Critical Evaluation of Atomic Data

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These data are mostly theoretical. Very few experimental measurements have been made, esp. for higher Z and charge.

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Wavelengths may be as inaccurate as a few %.

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Ionization and recombination rates may be worse than factors of 2.

Example: Dielectronic recombination rate coefficients

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Collisional excitation rates are claimed accurate to 10 to 30 %.

Example: H-like collision strengths

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Processes that are left out are expected not to matter very much.

Example: Lines emission from high n

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The accuracy and completeness of data needed depends on what you are trying to do.

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Ironically, the standards for lower resolution spectral analysis may be higher than for grating spectroscopy, e.g. to determine abundances with ACIS.

IV. Why Other Benchmarks Are Not Enough

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Theorists' opinions

Based on agreement among different methods

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Laboratory plasma physics experiments

e.g. Tokamaks have transport/diffusion issues

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Solar spectra

Inadequate resolution or bandpass

Poor calibration

High background

Limited to lower temperatures for quiet corona, time-variability for higher temperatures (flares)

Difficult to access old data sets, while original identifications not complete or accurate

Example: Global Fitting Analysis of SMM flare spectra with MEKAL $\rightarrow$ Best-fit $\chi^2_{red} \sim 33.$ (Phillips et al. 1999)

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Laboratory atomic physics experiments

Great for wavelengths and identifications

Limited to benchmarks for other atomic data

Example: resonance structure in collision strengths

Example: Maxwellian vs monoenergetic beam (EBIT)

V. Utility of Chandra Spectra

  $\mbox{$\bullet$}$The ``coronal approximation'' is a good first assumption.

Emission is naturally weighted to high density regions where collisional processes will be more equilibrated.

Provides ``easy'' predictive capability with which to test observed spectra.

Breakdown of classical assumptions have predictable consequences.

• Broad-band imaging spectral coverage records large wavelength range simultaneously

• Well-calibrated spectrometers

• Sufficiently high spectral resolution to avoid catastrophic blending

VI. ELP Functional Activities

• Expedite and facilitate collaborative investigations of issues related to atomic data and spectral models based on Chandra calibration/ELP spectra.

• WWW distribution, collection, organization of data, information, software

• Primary ELP Products

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  Chandra observations of Procyon, Capella, HR 1099

  Raw data; pipeline products; flux-calibrated spectra; relevant calibration products/quantities; uncertainties; problems list; ``unofficial'' updates

  Line Catalog consisting of measured wavelengths, fluxes, FWHM, identifications

  Benchmarks: comparison of observed line flux ratios with predictions from existing atomic theory (branching ratios, relative level populations from rate matrix solution)

• Associated ELP Products

  • Source models; benchmarks for ionization equilibrium

  • Tests of coronal equilibrium; Maxwellian velocity distributions; negligible optical depth

  • Tests of completeness of existing line lists and spectral models

• Organizational/logistical Support

  • Bring together atomic physics and astrophysics communities

  • Coordinate collaborations involving different groups

  • Disseminate relevant results, both atomic physics and astrophysics

  • Maintain a bibliography of relevant literature

VII. Opportunities for Participation and Involvement

• The Chandra ELP data are public.

• EUVE will make simultaneous observations to support the ELP, including the extension of benchmarks to longer wavelengths.

  Other measurements have been proposed on a proprietary basis (e.g. VLA).

• The core working group led by CXC is open to collaborations.

• A web site is under development.

• ``Dear Colleague Letter'' in draft, soliciting letter proposals

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  Level 1 Proposals: one-page letter plus vita, describing interest, research relevant to ELP

  Letters will be used to determine amount of interest, to develop distribution list, to help coordinate targetted efforts.

  Respondents will be kept up to date on progress and issues.

  As an umbrella project, the ELP will help researchers in fundamental X-ray spectroscopy through information exchange and affiliation.

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  Level 2 Proposals: one-page letter plus vita, work plan with budget summary to conduct relevant research in fundamental X-ray spectroscopy or atomic data evaluation and compilation

  Modest funding is available ( $\sim$ $150,000), out of which we expect to support 10 to 15 projects.

  Funds are intended to supplement other sources of revenue for projects.

  Level 2 proposals will be peer reviewed by a panel appointed by NASA.

  Recipients and CXC staff will consider future formation of a steering committee or advisory panel on ELP-related issues.