High Resolution X

-ray Spectra

Randall K. Sm

ithChandra X

-ray Center

The Glory and the G

randeur

Introduction

It has been said that Chandra and XM

M/N

ewton have

finally brought high-resolution spectra to X-ray astronom

y,but this is not strictly true.Chandra &

XM

M/N

ewton have m

ade high-resolutionspectra com

mon, but the first high-resolution spectra to be

published was:

A 7 ksec exposure of Capella

with the Einstein Solid State

Spectrometer (SSS) (H

olt et al.1979)

IntroductionO

f course, it could be argued that the Einstein SSS is notreally high-resolution; it is only ~ CCD

resolution. Thefirst published grating spectrum

was:

Wavelength

020

1030

-10-20

-30

Log Counts

A 42 ksec observation

of Capella w

ith theEinstein O

bjectiveG

rating Spectrometer

(OG

S) by Mew

e et al.(1982)

IntroductionClose on their heels w

ith an even higher resolutionspectrom

eter was:

Vedder &

Canizares (1983) with a 59 ksec observation of

(yes!) Capella w

ith the Einstein Focal Plane CrystalSpectrom

eter (FPCS) :

IntroductionA

fter the Einstein mission, EX

OSA

T was launched.

Although it prim

ary claim to fam

e is timing, it did m

anageto get a high resolution spectrum

of:

Capella, using the Transmission G

rating Spectrometer

(TGS) in a 85 ksec observation (Lem

en et al. 1989)

IntroductionCom

ing into the more m

odern era (i.e., when I started in

this business), there was a joint analysis of EU

V/X

-rayem

ission from:

Capella, using a 21 ksec with the A

SCA Silicon Im

agingSpectrom

eter (SIS) and a 120 ksec with the EU

VE

spectrometer (Brickhouse et al. 2000)

Capella

IntroductionO

nce Chandra was

launched, two new

gratingsbecam

e available. Here is

spectrum of Capella

observed for 95 ksec with

the Low Energy

Transmission G

rating(LETG

) using the High-

Resolution Camera (H

RC)as a detector. (Brinkm

an etal. 2000)

Introduction

And, of course, the H

igh-Energy Transmission G

rating (HETG

)also observed Capella, w

ith the Advanced CCD

Imaging

Spectrometer (A

CIS) for 89 ksec (Canizares et al. 2000)Capella

IntroductionW

ith the launch of XM

M/N

ewton, the Reflection G

ratingSpectrom

eter (RGS) w

as finally able to achieve the onetrue goal of all spectroscopists: a 52 ksec observation of

Audard et al. 2001

CAPELLA

!Capella.

Grating Basics

Chandra; notice the position of the removable gratings:

So, where do these gratings go? H

ere is a diagram of

Grating Basics

Here is a the H

ETG, pre-installation. The m

any grating facets areall carefully positioned to catch the light from

the 4 nested mirrors.

Grating Basics

The LETG, pre-installation. N

ote the support structures, which

will later show

up as scattered light in deep LETG observations:

Grating Basics

Canizares et al. (2000)

The two arm

s of the HEG

and MEG

gratings can be clearly seenhere. N

otice the larger effective area of the MEG

; the explodedregion show

s the effect of the HEG

’s higher resolution.

Grating Basics

Canizares et al. (2000)

X-ray gratings w

ork identically to optical gratings; both follow the

grating equation:sin b = m

l/pw

here l is the w

avelength, b is the dispersion angle (measured

from the zero-order im

age), p is the spatial period of the gratingitself, and m

is the order (1st, 2nd, etc).

Grating Basics

High-resolution (grating) spectra on Chandra cover a huge range of

wavelengths: from

1.2-170Å, over tw

o orders of magnitude. N

ote thatalthough “w

avelength” is the natural unit for grating data, many (older) X

-ray astronom

ers use energy (keV) units anyw

ay.

LETG/H

RC-S observation of NG

C6624

If ACIS is used, the CCD

resolution can distinguish between the

different orders; on the HRC, this m

ust be modeled.

The spatial and spectral elements are tightly coupled. If the zero-order

image is slightly displaced (i.e., if the source is piled-up), the +/- order

wavelengths w

ill be offset from each other. If so, it calls for reprocessing.

sin b = ml/p

Grating Basics

The light blue lines show the extraction region in the cross-

dispersion direction. Notice the enhanced background on the

rightmost chips; this can be (largely) elim

inated by using theenergy resolution of the CCD

s. Also, one chip has failed; this

also happened on RGS2, but fortunately at a different position.

The star AB

Dor observed

with the

XM

M RG

S1

Grating Basics

So, when you dow

nload grating data, what do you actually have?

4 spectra; +1,2 from each

of two RG

Ss.X

MM

/RGS

Not recom

mended

HRC/H

ETG

2 spectra; ±∑n i orders

HRC/LETG

6 spectra; ±1,2,3 ordersfor the LEG

ACIS/LETG

12 spectra; ±1,2,3 ordersfor both H

EG and M

EGA

CIS/HETG

For Chandra data, these are stored in one file with m

ultiple spectra,a PH

A Type II file; for X

MM

data, each spectrum is in a different

PHA

Type I file.

Grating Basics

Eff. Area @

1keV (cm

-2)

150

25 34 10

200~0.06

RGS 1st

2500.05

LEG 1st

5400.023

MEG

1st

10000.012

HEG

1st

Resolution@

1keVD

l

(Å)

Grating

The characteristics of the different gratings in theirprim

e arrangement:

Grating Basics

Consider a line at 1.0 keV (12.398Å

), on top of a 0.5 keV blackbody

So, what can w

e do with grating data that w

e can’t with CCD

s?

Grating Basics

Now

consider the same spectrum

observed by each of the four gratings:

If this data were fit assum

ing a pure Gaussian response, the scattering w

ings of theRG

S would m

ean we’d m

easure only 1/2 the true power!

Processing

This talk really isn’t about actual data processing; if you findyourself needing to reduce Chandra and/or XM

M/Newton

grating data, I recomm

end:

http://cxc.harvard.edu/ciao/threads/spectra_letghrc.html

LETG/H

RC

http://cxc.harvard.edu/ciao/threads/spectra_letgacis.html

LETG/A

CIS

http://cxc.harvard.edu/ciao/threads/spectra_hetgacis.html

HETG

/ACIS

http://heasarc.gsfc.nasa.gov/docs/xmm

/abc/node8.html

RGS

The end product will be either m

ultiple Type I or a Type IIPH

A file(s); basically, a histogram

of counts as a function ofw

avelength, along with the instrum

ental response.

Processing

1.Identify the dispersed source(s). In som

e cases, multiple sources m

ay bepresent; both CIA

O and SA

S software can identify and reduce m

ore than onesource, but the user m

ust select which source to extract.

2.Calculate the dispersion distance for each event, and (if available) resolve theorder by com

paring the dispersion wavelength to the m

easured CCD energy.

3.Extract the spectrum

into a PHA

file, suitable for use in Sherpa or XSPEC.

4.Create the detector response for the source. This depends on the position of thesource on the detector, the aspect solution, and the operating m

ode of thetelescope. The variations are not generally large, so using another observation'sresponse is O

K for quick w

ork.

The XM

M/N

ewton RG

S is a reflection grating, while the Chandra LETG

andH

ETG are transm

ission gratings. How

ever, the basic processing steps aresim

ilar:

SAS does steps 1-3 w

ith one comm

and, rgsproc. Chandra breakssteps 1-3 into subprogram

s: tg_create_mask, tg_resolve_events,

and tgextract.

ProcessingK

now your D

ata: RGS event files

unix% dm

list rgs_evt2.fits cols

ProcessingK

now your D

ata: HETG

/ACIS event files

unix% dm

list acis_evt2.fits cols

Processing

unix% dm

list hrc_evt2.fits cols

Know

your Data: LETG

/HRC event files

ProcessingFor bright sources on A

CIS-S, the background is likelynegligible. H

owever, on the H

RC-S or with the RG

S it isn’t:

Source/Background regions for the HR

C/LETG

Proper background subtraction is still a topic of some debate!

Processing

Using a detector w

ith even moderate energy resolution can

substantially reduce the background, because one can cross-correlate the w

avelength measured from

the dispersed positionw

ith the energy measured in the CCD

using the relation:

XM

M RG

S1spectra of thestar A

B Dor

EkeV = 12.398 / l

Å

Processing

What if you w

ant both high-resolution spectra AN

D tim

ing?

1) You’d better have a bright source

2) With Chandra, the A

CIS detectors can be run in continuousclocking m

ode while either of the gratings are in place. The

time resolution is about 3 m

s.3) W

ith XM

M/N

ewton, the RG

S detectors can be run in CCm

ode, but as of this writing (M

ay 2003) this mode is not

available to users for technical reasons.4) The H

RC has good time resolution, but due to an

electronics problem, the tim

e tags for each event suffer froman off-by-one problem

and actually refer to the previousevent. It is possible to run the H

RC in a “timing” m

odew

here this is mitigated, but not w

ith the full grating array.

Processing

Unfortunately, not every X

-ray source is as bright as Capellaor N

GC6624. W

hat can you do to enhance the S/N?

In this case, you have 4 choices:

1)C

o-add plus/minus orders of the sam

e grating ÔCan broaden lines if zero-order is offset.

2)C

o-adding different gratings, such as HEG

andM

EG or R

GS1 and R

GS2 Ô

Complicates line shape

function.3)

Co-add separate observations Ô

Instrumental

background can vary, plus the same issues of zero-

order offsets.4)

Co-add separate observations and instrum

ents ÔAll of the above

Grating A

nalysisunix%

sherpasherpa> data acis_pha2.fitssherpa> param

prompt off

sherpa> rsp[hm1]

sherpa> rsp[hp1]sherpa> rsp[m

m1]

sherpa> rsp[mp1]

sherpa> hm1.rm

f = acisheg1D1999-07-22rm

fN0004.fits

sherpa> hm1.arf = acisf01318H

EG_-1_garf.fits

sherpa> hp1.rmf = acisheg1D

1999-07-22rmfN

0004.fitssherpa> hp1.arf = acisf01318H

EG_1_garf.fits

sherpa> mm

1.rmf = acism

eg1D1999-07-22rm

fN0004.fits

sherpa> mm

1.arf = acisf01318MEG

_-1_garf.fitssherpa> m

p1.rmf = acism

eg1D1999-07-22rm

fN0004.fits

sherpa> mp1.arf = acisf01318M

EG_1_garf.fits

sherpa> instrument 3 = hm

1sherpa> instrum

ent 4 = hp1sherpa> instrum

ent 9 = mm

1sherpa> instrum

ent 10= mp1

sherpa> ignore allsets all sherpa> notice allsets w

ave 14.9:15.4 sherpa> source 3,4,9,10 = poly[b1] + delta1d[l1] + delta1d[l2] + delta1d[l3]sherpa> l1.pos = 15.014sherpa> l2.pos = 15.079sherpa> l3.pos = 15.2610 sherpa> freeze l1.possherpa> freeze l2.possherpa> freeze l3.possherpa> fitsherpa> lp 4 fit 3 fit 4 fit 9 fit 10sherpa> im

port(``guide'')sherpa> m

dl2latex\begin{tabular}{lllllll}M

odelNam

e & Line M

odel & Position &

Flux & Flux Error &

Fit Data &

Label \\ &

& A

ngstrom &

ph/cm$^2$/s &

ph/cm$^2$/s &

\\l1 &

delta1d & 15.014 &

0.00308923 & 6.7101e-05 &

3,4,9,10 & \\

l2 & delta1d &

15.079 & 0.000270431 &

2.81612e-05 & 3,4,9,10 &

\\l3 &

delta1d & 15.261 &

0.00125857 & 4.79625e-05 &

3,4,9,10 & \\

Load data

Define

instrument

response

Define source

Display results

Grating A

nalysis

Results from Sherpa

fits; notice the lowbackground in theH

EG data, and the

higher resolution.

Guide

GU

IDE is a collection of scripts w

hich access the atomic

database ATO

MD

B. GU

IDE provides a num

ber of informational

functions:

Output ionization balance fractions for

a given ionionbal

Convert fit parameters into a latex table

mdl2latex

Describe atom

ic parameters of a line

describe

List strong lines at a given temperature

strong

Print finding chart of wavelengths

identify

GU

IDE w

orks in Sherpa or Chips; initialize it with im

port(“guide”)

WebG

UID

E

WebG

UID

E

WebG

UID

E

Grating A

nalysis

When faced w

ith the task of understanding a high-resolution spectrum, especially a line-

dominated one, it is likely that sim

ple equilibrium m

odels will not w

ork adequately, despitepossibly giving low

reduced c2 values--usually because the counts/bin is low

and so theerrors are overestim

ated, not because the fits are good.

In this case, some new

strategies are needed. Here are som

e suggestions for starting points.

First: Determ

ine if your plasma is dom

inated by photoionization or collisional ionization.For exam

ple, the initial analysis of ASCA

\ data of Cygnus X-3 used a collisional m

odel,even though the em

ission is due to photoionization (see Liedahl & Paerels 1996)

Make a list of all processes that could be affecting line em

ission. For example, in helium

-like ions, the three dom

inant lines are the resonance, the forbidden, and theintercom

bination lines. These can be excited by direct excitation, radiative recombination,

dielectronic recombination of hydrogen-like ions, innershell ionization of lithium

-like ions,cascades from

higher levels, or by photoexcitation or photoionization. Once created, the

lines can be absorbed or scattered by like ions or by different ions. In many cases, sim

plephysical argum

ents can be used to limit or exclude various processes, reducing the

parameter space that m

ust be searched.

Grating A

nalysisSecond: A

ttempt to determ

ine the true continuum level w

ith confidence. Thecontinuum

in a hot plasma is not necessarily dom

inated by bremsstrahlung:

Weak em

ission lines will also blend in to m

ake the continuum seem

larger, which

will lead to a system

atic underestimate of line fluxes, and therefore elem

ental andionic abundances. A

fter you have found an acceptable spectral model, search for

regions in the model w

ith no or few lines, and com

pare the model to the data in this

region. If the model continuum

overestimates the continuum

here, it is likely itoverestim

ates it everywhere due to unresolved line em

ission

AstroA

tom

Conclusions

• Reprocessing grating data is no longer absolutely required, but hasgotten far easier and provides a sense of confidence about the data.• Co-A

dding and/or binning grating data should be avoided when

possible. Remem

ber that, statistically, nothing is gained by it,although it m

ay be much faster to fit it and easier to see the results.

• A num

ber of new facilities for atom

ic data analysis have beencreated for Sherpa and X

SPEC How

ever, remem

ber to check thecaveats on this data before trusting it totally! For the A

TOM

DB,

they are at http://asc.harvard.edu/atomdb/doc/caveats.htm

l• G

lobal fitting of generic equilibrium m

ay be useful for guiding theanalysis, but any project should begin w

ith a physics-basedapproach, follow

ed ideally by a line-based analysis and finally bychecking regions w

hich should well-understood (such as line-free

areas or those dominated by a single line).