X-RAY SPECTROSCOPY
AND ATOMIC DATA
Current Status

Ehud Behar

Technion / Columbia University

Collaborators

This work is a result of ongoing collaboration with the RGS consortium including teams from:

Columbia University, New York (Kahn et al.)

SRON, The Netherlands (Kaastra et al.)

MSSL, UK (Branduardi-Raymont et al.)

PSI, Switzerland (Güdel et al.)

… and laboratory measurements at LLNL California (Beiersdorfer et al.)

Outline

Introduction:

The Soft X-Ray Band with Chandra and XMM-Newton

Measurements in Collisional Plasmas (stars, normal galaxies, clusters)

Abundances

Temperature Structure

Beyond the Coronal Approximation

Transitions among excited levels (density and UV diagnostics)

Neighboring ion effects

Assessment of Fe-L Atomic Data

L-shells of Other Elements

Measurements in Photoionized Plasmas (active galaxies, x-ray binaries)

Wavelengths (inner-shell phenomena)

Column Densities and Abundances

Atomic Data Status and Supporting Lab. Measurements

Conclusions

Introduction

X-Ray Astronomy was born in 1962 with the discovery of Scorpius X-1.

Over the years x-ray observatories have revealed a diverse collection of x-ray sources ranging from nearby stars to distant galaxies.

However, it wasn’t until the recent launches (1999) of Chandra and XMM-Newton that x-ray line-resolved spectra have become available.

With this recent achievement, the x-ray branch of astronomy now joins other wavebands in using spectroscopy to perform quantitative investigations of cosmic objects.

Features of the X-Ray Band:
Highly Ionized Atoms

The conventional (soft) x-ray band (1 to 100 Å or ~ 0.2 – 10 keV) comprises emission lines from many K-shell and L-shell ions pertaining to many elements (C – Ni).

The x-ray band is uniquely compact, having several ions appear from each element and many lines present from each ion.

The wealth of lines and ions allows for elaborate plasma diagnostics such as temperatures, densities, ionization state, and elemental abundances.

A New Era in X-Ray Astrophysics:
Chandra and XMM-Newton

Chandra (NASA):

Launched July 23, 1999

1 telescope

2 CCD cameras

2 transmission grating

spectrometers (spectroscopy

mode is alternative to imaging)

XMM-Newton (ESA):

Launched December 10, 1999

3 telescopes

2 reflection grating spectrometers

1 Optical/UV monitor

The Difference High Spectral Resolution Makes (Capella)

SIS0 CCD spectrum with ASCA

(Brickhouse, Dupree, Edgar et al. 2000)

HETGS grating spectrum with Chandra

Stellar Coronae:
Hot, Collisional X-Ray Sources

Physical environment:

Hot (kT ~ 0.1 – 3 keV)

Density (n ~ 10¹⁰ cm^-3)

Optically thin

Ionization balance:

Standard electron-ion collisional processes: CI, RR, as well as EA and DR

Line excitation:

Electron impact

Coronal Steady-State Approximations

Excited level populations:

Individual-line emission-measure:

Ionization balance:

Stellar Coronae (cont.):
Temperatures and Abundances

Coronal Abundances (cont.)
Fe-L uncertainties most suspect

Uncertainties in the EM stem from uncertainties in P_ji * f_q

P_ji actually seems in pretty good shape; multi-line ions are very powerful in constraining models

Df_q suffers directly from the uncertainties in a(T_e) and S(T_e) – simple factor

T_max is less affected:
(Da / a = DS / S)

Galaxy Clusters:
Absence of the Cooling Flows

Peterson, Paerels, Kaastra et al. 2001

Dielectronic Recombination (DR): The weak link in the ioniz. balance

When the coronal approximation breaks down …

The 2p-3s lines in some
L-shell ions have the annoying habit of being populated by a variety of processes, not only CE, but also RE, and DR, and to a lesser extent also CI and RR
(Doron & Behar 2002)

Resonant absorption could (under special circumstances) affect these ratios:
e.g., NGC 4636
(Xu, Kahn, Peterson, et al. 2002)

Beyond the Coronal Regime (cont.) Density Diagnostics

Collisional depletion of the upper levels of forbidden lines

Most popular are the He-like triplets (Gabriel & Jordan 1969). The 1s-2s forbidden line is suppressed at high densities.

Critical density increases with Z.

Presence of UV Field

UV depletion of excited levels mimics density effects

Example: He-like triplets in z Pup

Provides measurement of distance from photosphere

Can we trust rates for transition
among excited levels?

Fe-L – Quick Summary

The 2p-3d and 2p-3s lines of Fe¹⁶⁺ - Fe²³⁺ dominate the soft x-ray spectrum of many sources.

Lines of different Fe-L ions have been measured to high accuracy with LLNL EBIT and are easily discernible with contemporary grating spectrometers.

Even within the simplified coronal approximation, Fe-L lines provide a powerful, robust tool for obtaining the (abundance-free) temperature structure (EM) of the source.

For years, Fe-L was deemed uncertain and considered the nightmare and scapegoat of many x-ray astronomers. Unjustly so!

The 2p – 3s line powers need to be treated more carefully.

Where possible, it still makes a lot of sense to use single-ion models independent of the ionization balance.

Testing of the rates for transitions among excited levels are encouraging.

There is Life After Fe-L …

Procyon with LETGS

Raassen, Mewe, Audard, et al. 2002

Si, S – L are inadequate in the commonly used databases

C¹ Ori (RGS): Noticeable Ni-L contribution

The world beyond Fe-L (e.g. Ar)

Ar IX & X,

EBIT LLNL measurements vs. HULLAC calculations

Collisional Plasmas:
Atomic Data Status

Spectroscopic Measurements in
Photoionized Plasma

Outflow velocities, mass loss (wavelengths)

Electron temperatures (RRCs)

Optical depth => column densities
via absorption (oscillator strengths) and emission

Elemental abundances
(column density behavior as a function of
ionization parameter x = L/n_er²)

Ionization balance (x), density (n_e) and position (r)
(photo-ionization and recombination rates, including autoionization processes)

Note: C ² is not a measurable astrophysical quantity!

1s-2p (Ka) Inner-Shell Absorption

L-shell ions have vacancies in their 2p sub-shell and absorb by means of 1s-2p resonance lines

Have been observed in many sources and for many elements

Span wide range in ionization => very useful for probing the ionization state(s) of the plasma

Autoionizing upper levels affect line shapes and ionization balance

1s-2p Inner-Shell Line Comparison

Inner-Shell Line Comparison

Need for Laboratory Experiments
Z Pinch (& Laser Plasma)

Absorption measurements; wavelengths and oscillator strengths

Particularly needed:
Inner-shell lines

Ionization balance and line emission measurements are also possible

High laboratory densities might still hinder direct application to astrophysics

Inner-Shell Absorption (cont.)
Fe M-shell 2p-3d UTA

Comparison between the HULLAC and FAC atomic codes shows fair agreement in line energies and oscillator strengths for Fe I – Fe XVI

(courtesy of
Adrian Turner)

Fe M-shell 2p-3d UTA (cont.)

Type II AGN (cont.)
Measuring Elemental Abundances

Vertical offsets in plot of N_H as a function of the ionization parameter x, as deduced from the measured ionic column densities N_i , reveal the relative elemental abundances

Need f^+q and x_max

Fe-L analysis to be completed (A. Kinkhabwala - tomorrow morning)

Low-T DR of Fe-L

Photoexcitation-Autoionization

New level-by-level calculations enable
re-evaluation of fluorescence yields
(Gorczyca, Kodituwakku, Korista et al. 2002)

Note that effect on ionization balance could be spectrum-dependent and complex

Conclusions

The acquired high-resolution spectra from Chandra and XMM-Newton gratings need to be matched with equally high-quality atomic data.

For x-ray collisional plasmas, most of the atomic data are satisfactory, allowing for high precision measurements of EM structures, abundances, densities, and UV effects.

PLEASE USE SPECIAL CAUTION with 2p-3s LINE INTENSITIES,
NON-Fe L-SHELL IONS, and the IONIZATION BALANCE.

For x-ray photoionized plasmas, atomic data allow for sound measurements of outflow velocities, temperatures, column densities, and abundances.

PLEASE USE SPECIAL CAUTION with
INNER-SHELL LINE WAVELENGTHS (especially low-Z),
the IONIZATION BALANCE (density & location),
and FLUORESCENCE YIELDS.


	This work is a result of ongoing collaboration with the RGS consortium including teams from:

	Columbia University, New York (Kahn et al.)
	SRON, The Netherlands (Kaastra et al.)
	MSSL, UK (Branduardi-Raymont et al.)
	PSI, Switzerland (Güdel et al.)

	… and laboratory measurements at LLNL California (Beiersdorfer et al.)


Introduction:
	The Soft X-Ray Band with Chandra and XMM-Newton
Measurements in Collisional Plasmas (stars, normal galaxies, clusters)
	Abundances
	Temperature Structure
	Beyond the Coronal Approximation
		Transitions among excited levels (density and UV diagnostics)
		Neighboring ion effects
	Assessment of Fe-L Atomic Data
	L-shells of Other Elements
Measurements in Photoionized Plasmas (active galaxies, x-ray binaries)
	Wavelengths (inner-shell phenomena)
	Column Densities and Abundances
	Atomic Data Status and Supporting Lab. Measurements
Conclusions


	X-Ray Astronomy was born in 1962 with the discovery of Scorpius X-1.
	Over the years x-ray observatories have revealed a diverse collection of x-ray sources ranging from nearby stars to distant galaxies.
	However, it wasn’t until the recent launches (1999) of Chandra and XMM-Newton that x-ray line-resolved spectra have become available.
	With this recent achievement, the x-ray branch of astronomy now joins other wavebands in using spectroscopy to perform quantitative investigations of cosmic objects.


	The conventional (soft) x-ray band (1 to 100 Å or ~ 0.2 – 10 keV) comprises emission lines from many K-shell and L-shell ions pertaining to many elements (C – Ni).

	The x-ray band is uniquely compact, having several ions appear from each element and many lines present from each ion.

	The wealth of lines and ions allows for elaborate plasma diagnostics such as temperatures, densities, ionization state, and elemental abundances.


	Chandra (NASA):
	Launched July 23, 1999
	1 telescope
	2 CCD cameras
	2 transmission grating
	spectrometers (spectroscopy
	mode is alternative to imaging)

	XMM-Newton (ESA):
	Launched December 10, 1999
	3 telescopes
	2 reflection grating spectrometers
	1 Optical/UV monitor


	SIS0 CCD spectrum with ASCA
	(Brickhouse, Dupree, Edgar et al. 2000)


	HETGS grating spectrum with Chandra


	Physical environment:
		Hot (kT ~ 0.1 – 3 keV)
		Density (n ~ 10¹⁰ cm^-3)
		Optically thin

	Ionization balance:
		Standard electron-ion collisional processes: CI, RR, as well as EA and DR

	Line excitation:
		Electron impact


	Excited level populations:

	Individual-line emission-measure:


	Ionization balance:


	Uncertainties in the EM stem from uncertainties in P_ji * f_q
	P_ji actually seems in pretty good shape; multi-line ions are very powerful in constraining models
	Df_q suffers directly from the uncertainties in a(T_e) and S(T_e) – simple factor
	T_max is less affected: (Da / a = DS / S)


	The 2p-3s lines in some L-shell ions have the annoying habit of being populated by a variety of processes, not only CE, but also RE, and DR, and to a lesser extent also CI and RR (Doron & Behar 2002)

	Resonant absorption could (under special circumstances) affect these ratios: e.g., NGC 4636 (Xu, Kahn, Peterson, et al. 2002)


	Collisional depletion of the upper levels of forbidden lines

	Most popular are the He-like triplets (Gabriel & Jordan 1969). The 1s-2s forbidden line is suppressed at high densities.

	Critical density increases with Z.


	UV depletion of excited levels mimics density effects

	Example: He-like triplets in z Pup

	Provides measurement of distance from photosphere


	The 2p-3d and 2p-3s lines of Fe¹⁶⁺ - Fe²³⁺ dominate the soft x-ray spectrum of many sources.
	Lines of different Fe-L ions have been measured to high accuracy with LLNL EBIT and are easily discernible with contemporary grating spectrometers.
	Even within the simplified coronal approximation, Fe-L lines provide a powerful, robust tool for obtaining the (abundance-free) temperature structure (EM) of the source.
	For years, Fe-L was deemed uncertain and considered the nightmare and scapegoat of many x-ray astronomers. Unjustly so!
	The 2p – 3s line powers need to be treated more carefully.
	Where possible, it still makes a lot of sense to use single-ion models independent of the ionization balance.
	Testing of the rates for transitions among excited levels are encouraging.


	Procyon with LETGS
	Raassen, Mewe, Audard, et al. 2002
	Si, S – L are inadequate in the commonly used databases


	C¹ Ori (RGS): Noticeable Ni-L contribution


	Outflow velocities, mass loss (wavelengths)
	Electron temperatures (RRCs)
	Optical depth => column densities via absorption (oscillator strengths) and emission
	Elemental abundances (column density behavior as a function of ionization parameter x = L/n_er²)
	Ionization balance (x), density (n_e) and position (r) (photo-ionization and recombination rates, including autoionization processes)

	Note: C ² is not a measurable astrophysical quantity!


	L-shell ions have vacancies in their 2p sub-shell and absorb by means of 1s-2p resonance lines
	Have been observed in many sources and for many elements
	Span wide range in ionization => very useful for probing the ionization state(s) of the plasma
	Autoionizing upper levels affect line shapes and ionization balance


	Absorption measurements; wavelengths and oscillator strengths

	Particularly needed: Inner-shell lines

	Ionization balance and line emission measurements are also possible

	High laboratory densities might still hinder direct application to astrophysics


	Comparison between the HULLAC and FAC atomic codes shows fair agreement in line energies and oscillator strengths for Fe I – Fe XVI



	(courtesy of Adrian Turner)



	Vertical offsets in plot of N_H as a function of the ionization parameter x, as deduced from the measured ionic column densities N_i , reveal the relative elemental abundances




	Need f^+q and x_max

	Fe-L analysis to be completed (A. Kinkhabwala - tomorrow morning)



	New level-by-level calculations enable re-evaluation of fluorescence yields (Gorczyca, Kodituwakku, Korista et al. 2002)

	Note that effect on ionization balance could be spectrum-dependent and complex


	The acquired high-resolution spectra from Chandra and XMM-Newton gratings need to be matched with equally high-quality atomic data.

	For x-ray collisional plasmas, most of the atomic data are satisfactory, allowing for high precision measurements of EM structures, abundances, densities, and UV effects.
	PLEASE USE SPECIAL CAUTION with 2p-3s LINE INTENSITIES, NON-Fe L-SHELL IONS, and the IONIZATION BALANCE.

	For x-ray photoionized plasmas, atomic data allow for sound measurements of outflow velocities, temperatures, column densities, and abundances.
	PLEASE USE SPECIAL CAUTION with INNER-SHELL LINE WAVELENGTHS (especially low-Z), the IONIZATION BALANCE (density & location), and FLUORESCENCE YIELDS.