Transcript Lecture 14-X-ray Astronomy

Astronomical Observational Techniques
and Instrumentation
RIT Course Number 1060-771
Professor Don Figer
X-ray Astronomy
1
Aims of Lecture
•
•
•
•
review x-ray photon path
describe x-ray detection
describe specific x-ray telescopes
give examples of x-ray objects
2
X-ray Photon Path
3
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!
4
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
X Rays
Visible Light
5
X Rays (and Gamma Rays “”) Can be
“Absorbed”
• By dense material, e.g., lead (Pb)
Sensor
6
Imaging System Based on Selective
Transmission
Input Object
(Radioactive Thyroid)
Lead Sheet with Pinhole
“Noisy” Output Image
(because of small number
of detected photons)
7
How to “Add” More Photons
1. Make Pinhole Larger
 “Fuzzy” Image
Input Object
(Radioactive Thyroid
w/ “Hot” and “Cold” Spots)
“Noisy” Output Image
(because of small number
of detected photons)
“Fuzzy” Image
Through Large Pinhole
(but less noise)
8
How to “Add” More Photons
2. Add More Pinholes
• BUT: Images “Overlap”
9
How to “Add” More Photons
2. Add More Pinholes
• Process to combine “overlapping” images
Before Postprocessing
After Postprocessing
10
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
EXIST
Balloon
J. Grindlay (Harvard)
USA
Survey
20X6 (HM)
5
2500
4
Instruments being studied
CdZnTe
5-600
1% @ 100 keV
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
11
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
12
X Rays and Grazing Incidence
X-Ray “Mirror”

X Ray at “Grazing Incidence
is “Deviated” by Angle 
(which is SMALL!)
13
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)
14
X-ray Collecting Mirrors
n.b., Distance from Front End to Sensor
is LONG due to Grazing
Incidence
15
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
16
X-Ray Mirrors
• Add more shells to increase collecting area
– Chandra has 4 rings (instead of 6 as proposed)
Nest of “Rings”
Full Aperture
• Collecting area of rings is MUCH smaller than for a FullAperture “Lens”!
17
X-ray Detection
18
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!
19
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
20
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
21
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 Xray 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 xrays obtained in the
RIT Rochester
Imaging Detector
Lab (RIDL) with a
silicon detector.
22
Fe55 Experiment with Silicon Detector
23
CCDs as X-Ray Detectors
24
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
25
Measured Attributes of Each X Ray
1.
2.
3.
Position of Absorption [x,y]
Time when Absorption Occurred [t]
Amount of Energy Absorbed [E]
•
Four Pieces of Data per Absorption are Transmitted to
Earth:
 x, y, t, E
26
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
27
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”
28
X-ray Telescopes
29
Chandra
Originally AXAF
Advanced X-ray
Astrophysics Facility
http://chandra.nasa.gov/
Chandra in Earth orbit (artist’s conception)
30
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
31
Chandra Mirrors Assembled and Aligned by Kodak
in Rochester
“Rings”
32
Mirrors Integrated
into spacecraft at
TRW (NGST),
Redondo Beach, CA
(Note scale of telescope
compared to workers)
33
Chandra ACIS CCD Sensor
34
X-ray Objects
35
First Image from Chandra: August, 1999
Supernova remnant Cassiopeia A
36
Example of X-Ray Spectrum
37
Example of X-Ray Spectrum
Gamma-Ray
“Burster”
GRB991216
Counts
E
http://chandra.harvard.edu/photo/cycle1/0596/index.html
38
Chandra/ACIS image and spectrum of Cas A
39
Light Curve of “X-Ray Binary”
40
http://heasarc.gsfc.nasa.gov/docs/objects/binaries/gx301s2_lc.html