1

Astronomical Observational Techniques and Instrumentation

RIT Course Number 1060-771 Professor Don Figer

X-ray Astronomy

2

Aims of Lecture• review x-ray photon path• describe x-ray detection• describe specific x-ray telescopes• give examples of x-ray objects

3

X-ray Photon Path

4

Atmospheric Absorption• X Rays are absorbed by Earth’s atmosphere

– lucky for us!!!

• X-ray photon passing through atmosphere encounters as many atoms as in 5-meter (16 ft) thick wall of concrete!

5

X RaysVisible Light

How Can X Rays Be “Imaged”?• X Rays are too energetic to be reflected “back”, as is possible

for lower-energy photons, e.g., visible light

6

X Rays (and Gamma Rays “”) Can be “Absorbed”

• By dense material, e.g., lead (Pb)

Sensor

7

Imaging System Based on Selective Transmission

Input Object(Radioactive Thyroid)

Lead Sheet with Pinhole “Noisy” Output Image(because of small number

of detected photons)

8

How to “Add” More Photons1. Make Pinhole Larger

Input Object(Radioactive Thyroid

w/ “Hot” and “Cold” Spots)

“Fuzzy” Image Through Large Pinhole

(but less noise)

“Noisy” Output Image(because of small number

of detected photons)

“Fuzzy” Image

9

How to “Add” More Photons2. Add More Pinholes

• BUT: Images “Overlap”

10

How to “Add” More Photons2. Add More Pinholes

• Process to combine “overlapping” images

Before Postprocessing After Postprocessing

11

Coded Aperture Mask• A coded aperture mask can be used to make images by relating intensity

distribution to the geometry of the system through post-processing, as described in the thyroid example.

• Six coded aperture mask instruments are operational in space: XTE-ASM, HETE-WXM, SWIFT-BAT and the three instruments on INTEGRAL.

• EXIST is being considered as a Black Hole Finder Probe in NASA's Beyond Einstein program.

Instrument1 Principle Investigator Science Operational Period

Detector/ E-range

(keV) E-resolution

Pattern3 Config4

Mask mat.

FOV5 (degrees)/ Ang. Res. (arcmin)

Det. Area (cm2)

No. of

Modules6

Instruments flying

ASM RXTE

H. Bradt (MIT) USA

XB,AGN,GRB 95- PSPC 2-10 25%@6 keV

ORA R|S|1 Al/Au

6X90 (HM) 3X15

90 3

WXM HETE

E. Fenimore (LANL) & N. Kawai (RIKEN) USA & Japan

GRB, X-ray bursts

00- PSPC 2-30 15%@6 keV

ORA(33%) R|S/C|1 Al/Au

90X90 (ZR) 40

400 2

IBIS INTEGRAL

P. Ubertini (IAS) Italy

XB,AGN 02- CdTe/CsI 20-10000 8% @ 100 keV

MURA R|C W

8X8 (FC) 12

2500 1

SPI INTEGRAL

V. Schoenfelder (MPE) Germany

Galactic diffuse emission

02- Ge 20-8000 0.2% @ 1.33 MeV

HURA H|C W

16 (FC) 120

500 1

JEM-X INTEGRAL

N. Lund (DSRI) Denmark

XB,AGN 02- PSPC 3-35 13%@10 keV

ORA(25%) H|C W

4.8 (FC) 3

500 2

BAT SWIFT

N. Gehrels (GSFC) USA

GRB, hard X-ray survey

04- CdZnTe 15-150 6 keV throughout

URA H|C

120 17

5200 1

Instruments being studied

EXIST Balloon

J. Grindlay (Harvard) USA

Survey CdZnTe 5-600 1% @ 100 keV

20X6 (HM) 5

2500 4

1Instruments between parentheses experienced difficulties in flight 2 XB - galactic X-ray binaries, GRB - gamma ray burst phenomenon, ASM - all-sky monitoring, SSM - selected-sky monitoring, AGN - active galactic nuclei, Technology - instrument technology, GC - the galactic center 3 Pattern: FZP = Fresnel Zone Plate, ORA = Optimized RAndom pattern, URA = Uniformly Redundant Array, HURA = Hexagonal URA; p = pseudo-noise subset, h = hadamard subset, tp = twin prime subset; triadic = near-URA with 0.33 open fraction 4 Configuration: R = rectangular, H = hexagonal, C = cyclic, S = simple, 1 = 1-dimensional 5 HM=full width at half maximum, ZR=full width at zero response, FC=full width of fully coded field of view 6 Number of modules

12

Would Be Better to “Focus” X Rays• Could “Bring X Rays Together” from Different Points in

Aperture– Collect More “Light” Increase Signal– Increases “Signal-to-Noise” Ratio

• Produces Better Images

13

X Rays and Grazing Incidence

X Ray at “Grazing Incidenceis “Deviated” by Angle

(which is SMALL!)

X-Ray “Mirror”

14

Why Grazing Incidence?• X-Ray photons at “normal” or “near-normal” incidence

(photon path perpendicular to mirror, as already shown) would be transmitted (or possibly absorbed) rather than reflected.

• At near-parallel incidence, X Rays “skip” off mirror surface (like stones skipping across water surface)

15

X-ray Collecting Mirrorsn.b., Distance from Front End to Sensor is LONG due to Grazing Incidence

16

X-Ray Mirrors• Each grazing-incidence mirror shell has only a very small

collecting area exposed to sky– Looks like “Ring” Mirror (“annulus”) to X Rays!

• Add more shells to increase collecting area: create a nest of shells

“End” View of X-Ray Mirror

17

X-Ray Mirrors• Add more shells to increase collecting area

– Chandra has 4 rings (instead of 6 as proposed)

• Collecting area of rings is MUCH smaller than for a Full-Aperture “Lens”!

Nest of “Rings” Full Aperture

18

X-ray Detection

19

The Perfect X-ray Detector• What would be the ideal detector for satellite-borne X-ray astronomy? It would

possess:– high spatial resolution – a large useful area, – excellent temporal resolution – ability to handle large count rates– good energy resolution – unit quantum efficiency – large bandwidth– output would be stable on timescales of years – internal background of spurious signals would be negligibly low– immunity to damage by the in-orbit radiation environment – no consumables– simple design– longevity– low cost– low mass – low power – no moving parts – low output data rate– (and a partridge in a pear tree)

• Such a detector does not exist!

20

X-ray Detectors• Principle measurements

– flux– position– energy– time of arrival

• Specific types of detectors– gas proportional counter: x-ray photoionizes gas and induces voltage

spike in nearby wires– CCD: x-ray generates charge through absorption in silicon– microchannel plates: x-ray generates charge cascade through

photoelectric effect– active pixel sensor (i.e. CMOS hybrid detector array): x-ray generates

charge through absorption in silicon– single photon calorimeters: x-ray heats a resistive element an amount

equal to the energy of the x-ray

21

X-ray Detector Materials• Light sensitive materials must be sensitive to x-ray energies

(~1-100 keV)– must allow some penetration– must have enough absorption or photoionization

• Good materials– gas– CZT (CdZnTe)– CdTe– silicon

22

X-ray Image of Fe55 Source• For precise measurements of conversion gain (e-/ADU) or

Charge Transfer Efficiency (CTE), one often uses an Fe-55 X-ray source.

• Fe55 emits K-alpha photons with energy of 5.9KeV (80%) and K-beta photons with energy of 6.2KeV (20%). Impacting silicon, these will free 1616 and 1778 electrons, respectively.

Image of Fe55 x-rays obtained in the RIT Rochester Imaging Detector Lab (RIDL) with a silicon detector.

23

Fe55 Experiment with Silicon Detector

24

CCDs as X-Ray Detectors

25

CCDs “Count” X-Ray photons• X-Ray events happen much less often

– fewer available X rays – smaller collecting area of telescope

• Each absorbed x-ray has much more energy– deposits more energy in CCD– generates many electrons

• Each x-ray can be counted– attributes of individual photons are measured independently

26

Measured Attributes of Each X Ray1. Position of Absorption [x,y]2. Time when Absorption Occurred [t]3. Amount of Energy Absorbed [E]

• Four Pieces of Data per Absorption are Transmitted to Earth:

, , ,x y t E

27

Why Transmit [x,y,t,E] Instead of Images?• Images have too much data!

– up to 2 CCD images per second– 16 bits of data per pixel (216=65,536 gray levels)– image Size is 1024 1024 pixels 16 10242 2 = 33.6 Mbps– too much to transmit to ground

• “Event Lists” of [x,y,t,E] are compiled by on-board software and transmitted– reduces required data transmission rate

28

Image Creation• From event list of [x,y,t,E]

– Count photons in each pixel during observation• 30,000-Second Observation (1/3 day), 10,000 CCD frames are obtained

(one per 3 seconds)• hope each pixel contains ONLY 1 photon per image

• Pairs of data for each event are plotted as coordinates– Number of Events with Different [x,y] “Image”– Number of Events with Different E “Spectrum”– Number of Events with Different E for each [x,y] “Color Cube”

29

X-ray Telescopes

30

Chandra

Chandra in Earth orbit (artist’s conception)http://chandra.nasa.gov/

Originally AXAFAdvanced X-ray Astrophysics Facility

31

Chandra Orbit• Deployed from Columbia, 23 July 1999• Elliptical orbit

– Apogee = 86,487 miles (139,188 km) – Perigee = 5,999 miles (9,655 km)

• High above LEO Can’t be Serviced• Period is 63 h, 28 m, 43 s

– Out of Earth’s Shadow for Long Periods– Longer Observations

32

Chandra Mirrors Assembled and Aligned by Kodak in Rochester

“Rings”

33

Mirrors Integrated into spacecraft at

TRW (NGST), Redondo Beach, CA

(Note scale of telescope compared to workers)

34

Chandra ACIS CCD Sensor

35

X-ray Objects

36

First Image from Chandra: August, 1999Supernova remnant Cassiopeia A

37

Example of X-Ray Spectrum

38

Example of X-Ray Spectrum

Gamma-Ray“Burster”GRB991216

http://chandra.harvard.edu/photo/cycle1/0596/index.html

Counts

E

39

Chandra/ACIS image and spectrum of Cas A

40

Light Curve of “X-Ray Binary”

http://heasarc.gsfc.nasa.gov/docs/objects/binaries/gx301s2_lc.html