a n introduction to x tr no maryland my -ray - nasa

X-ray A

stronomy School 2003

An Introduction to X

-rayA

stronomy

Keith A

rnaudN

ASA

Goddard

University of M

aryland

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• Preamble

• A brief and idiosyncratic history

• A few

notes on X-ray data analysis

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X-ray A

stronomy

Emission in the energy range 0.1 - 100 keV

(0.12-120 Angstrom

s).The atm

osphere is opaque at these energies so all X-ray astronom

yis done using satellites, rockets, and, at the highest energies only,balloons. So :

• Our detectors have to w

ork right the first time - w

e can’t goand fix them

. Any problem

s have to be understood remotely

and calibrated.

• There are relatively few X

-ray astronomy experim

ents sopublic data archives are very im

portant.

In this school we w

ill concentrate on the 0.1-10 keV energy range

covered by Chandra and XM

M-N

ewton.

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X-ray Processes

X-rays are produced in hot and violent processes. A

lmost all point-

like X-ray sources are variable - som

e extremely variable. This has

two im

portant consequences :

• Monitoring observations are m

ore important in the X

-rayband than any other.

• There are few good calibration sources.

The X-ray band includes the K

-shell transitions (ie n=2 to 1) of allelem

ents heavier than He. The continuum

shape also providesim

portant clues to the emission processes.

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X-rays from

the Sun

The first astronomical X

-ray experiments w

ere performed in 1948

and 1949 using captured WW

II V2 rockets. X

-rays were detected

from the Solar corona by H

erb Friedman and collaborators at the

US N

aval Research Lab (in Washington D

C).

It is still not fully understood how the corona is heated to X

-rayem

itting temperatures.

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X-ray Binaries

The breakthrough experiment w

as performed in 1962 by Bruno

Rossi, Riccardo Giacconi, and collaborators at A

merican Science

and Engineering (AS&

E) in Cambridge, M

A. A

fter two failures of

the Aerobee rocket, they successfully launched a detector to look

for X-ray em

ission from the m

oon.

As the rocket spun the field-of-view

passed over an unexpectedlybright X

-ray source. This was designated Scorpius X

-1. A follow

-up cam

paign identified the X-ray source as a binary w

ith acom

pact (neutron star) primary.

Further rocket experiments in the 1960s found other X

-raybinaries as w

ell as identifying X-ray em

ission from several SN

R,from

M87, Cygnus-A

and the Coma cluster of galaxies.

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Giacconi et al., 1962

Sco X-1

X-ray background

The First Extra-Solar X-ray D

etection

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All-Sky Surveys

The satellite experiments U

huru (US) and A

riel-V (U

K) perform

edthe first all-sky surveys. These used collim

ated proportionalcounters w

ith resolutions of degrees so the images of the sky w

erenecessarily crude. H

owever, these surveys detected m

any galacticbinaries, SN

R, clusters of galaxies, and active galactic nuclei.

HEA

O-1 (U

S) performed a m

ore sensitive sky survey and made a

precise measurem

ent of the intensity and shape of the X-ray

background (XRB). There w

as a long debate about whether the

XRB w

as due to hot gas distributed through the universe or was

the sum of m

any lower flux point sources. The latter is now

known

to be the case although it is still an interesting question whether the

XRB can be com

pletely explained by the sum of individual

sources. (Dan Schw

artz was PI of the A

3 experiment)

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Uhuru (“Freedom

”)

Bruno Rossi

Marjorie Tow

nsend

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Ariel V

launch

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stronomy School 2003 H

ot gas in thePerseus Cluster

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X-ray Telescopes

X-ray focussing optics w

ere used first to observe the Solar coronaand then transferred to general astronom

y with H

EAO

-2 (US),

launched in 1978 and renamed the Einstein O

bservatory. Thetelescope im

aged X-rays in the energy range 0.5-4.0 keV

. Many

classes of astronomical objects w

ere detected in X-rays. This w

asthe first X

-ray astronomy m

ission with a guest observer program

and a “public archive” (which I used for m

y PhD thesis).

Its successor, over a decade later, was RO

SAT (G

ermany-U

S-UK

),w

hich performed an all-sky im

aging survey in the 0.2-2.5 keVrange follow

ed by longer pointed observations at specific targets.This generated a vast public database (w

hich still has lots ofpotential) - and is a fertile source of targets for Chandra and X

MM

-N

ewton.

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EinsteinO

bservatory

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X-ray D

etection of the Moon

ROSA

T PSPC

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Large proportional counter arraysParallel w

ith the development of X

-ray telescopes were m

issionsdesigned to collect large num

bers of X-rays from

relatively brightsources to perform

detailed spectroscopic and timing investigations.

EXO

SAT (ESA

) was launched in 1983 into a deep orbit w

hichallow

ed long continuous observations. It discovered Quasi Periodic

Oscillations in X

-ray binaries.

Ginga (Japan-U

K) w

as Japan’s third X-ray astronom

y satellite andw

as launched in 1987. Important results w

ere on Black Hole

Transients and the detection of Fe lines and Compton reflection in

active galactic nuclei.

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EXO

SAT lightcurve

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Ginga

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The current culmination of this line is the Rossi X

-ray Timing

Explorer (RXTE), launched at the end of 1995 and still operating,

which has detected kH

z oscillations in Galactic binary sources

providing possible tests of GR effects in the vicinity of neutron stars

and black holes.

RXTE also carries an all-sky m

onitor which has produced long-term

lightcurves for the brighter sources.

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High-throughput telescopes

ASCA

(Japan-US), launched in 1993, used high-throughput but

relatively coarse resolution telescopes that operated in the energyrange 0.5-10 keV

. The importance of these telescopes w

as less inthe im

aging and more in reducing the background - w

hich usuallyscales w

ith the volume of the detector in space experim

ents. ASCA

detected broad (100,000 km/s) Fe lines from

close to the black holein active galactic nuclei.

BeppoSAX

(Italy-Netherlands), launched in 1996, covered a very

wide bandpass (0.1-300 keV

) using a range of instruments. Its m

ostim

portant result was the discovery of gam

ma-ray burst afterglow

s.

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ASCA

observations of Fe K lines in A

GN

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Beppo-SAX

detection of GRB afterglow

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Great O

bservatoriesThis brings us up to the present and the tw

o major X

-rayastronom

y facilities launched in 1999 - Chandra and XM

M-

New

ton.

Chandra boasts the best (and most expensive) telescope ever built,

giving a sub-arcsecond resolution. Imaging is provided by CCD

and microchannel plate im

agers. High resolution spectroscopy by

two gratings that can be placed in the optical path behind the

mirrors.

While Chandra is a successor to RO

SAT, X

MM

-New

ton follows

the path of ASCA

in providing greater mirror area but at low

erresolution. X

MM

-New

ton has 3 mirrors, 2 of w

hich havereflection gratings, providing sim

ultaneous high resolutionspectroscopy and im

aging. There is also an optical monitor

telescope.

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Chandra

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Leon van Speybroeck (1935-2002)

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XM

M-N

ewton

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Chandra view of the G

alactic center

Wang et al.

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40 years of X-ray astronom

y have provided a billion times

improvem

ent in sensitivity and a quarter of a million tim

esim

provement in resolution.

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Chandra vs. XM

M-N

ewton

Chandra is best for…

• Anything requiring better than 5 arcseconds spatial resolution.

• High resolution spectroscopy for energies < 0.5 or > 2 keV

.

XM

M-N

ewton is best for…

• Imaging or im

aging-spectroscopy which does not require a

resolution of 5 arcseconds or better.• H

igh resolution spectroscopy for energies 0.5 < E < 2 keV.

• High resolution spectroscopy on extended objects that are

larger than 10 arcseconds and smaller than 1 arcm

inute.

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X-ray data

X-ray detectors are photon-counting in contrast to those in m

ostother w

avebands which m

easure incoming flux. In consequence,

basic X-ray data usually com

prise lists of events and their attributes.

X-ray datasets are usually photon-lim

ited, particularly for newer

missions such as Chandra and X

MM

-New

ton. Images, spectra, and

lightcurves created from the event lists m

ay well have a few

or evenno photons in m

any bins. The data analysis techniques (andstatistics) developed in other w

avebands may not transfer to X

-rayastronom

y.

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X-ray data II

The basic data file usually comprises tim

e-tagged events, eachw

ith a position (in detector and sky coordinates) and an energy(often called channel, PH

A or PI for historical reasons). Thus each

event can be thought of as occupying a position in a 4-D space.

The event may have other attributes of interest - eg for CCD

s thepattern of pixels from

which the charge for this event w

asaccum

ulated. It is often possible to increase S/N by selecting on

these secondary attributes.

After filtering the events as required w

e project them onto 1-D

or2-D

subspaces and bin them up to give im

ages, energy spectra, orlightcurves (tim

e series).

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X-ray data III

Each of these binned datasets requires its own calibration products.

• Image analysis uses :

• exposure maps - the m

irror and detector sensitivity acrossthe field-of-view

(taking into account any changes inaspect ie pointing direction).

• point spread function (PSF) - the probability that a photonof given energy and position is registered in a given im

agepixel.

• Energy spectral analysis uses :

• response matrices - the probability that a photon of given

energy is registered in a given channel.

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The 3 Most Im

portant Things for X-ray

Data A

nalysis are :

1.Calibration

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The 3 Most Im


Data A

nalysis are :

1.Calibration

2.C

alibration

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The 3 Most Im


Data A

nalysis are :

1.Calibration

2.C

alibration

3.Calibration

(The rest is “just” software, organization, and analysis.)

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The Calibration is Never G

ood Enough

There is always a system

atic error term associated w

ith your dataanalysis. If you have the m

isfortune to have very high S/N then this

systematic term

may dom

inate.

You usually can’t add the system

atics in quadrature to the statisticaluncertainties because the system

atic uncertainties are usuallycorrelated.

Don’t over interpret data w

ithout thinking very hard about the qualityof the calibration !

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Thank You

Any Q

uestions ?

a n introduction to x tr no maryland my -ray - nasa

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