Transcript X-Ray photons - RIT - Center for Detectors

Astronomical Observational Techniques
and Instrumentation
Professor Don Figer
X-ray Astronomy
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Aims of Lecture
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review x-ray photon path
describe x-ray detection
describe specific x-ray telescopes
give examples of x-ray objects
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X-ray Photon Path
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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!
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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
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X Rays (and Gamma Rays “”) Can be
“Absorbed”
• By dense material, e.g., lead (Pb)
Sensor
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Imaging System Based on Selective
Transmission
Input Object
(Radioactive Thyroid)
Lead Sheet with Pinhole
“Noisy” Output Image
(because of small number
of detected photons)
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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)
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How to “Add” More Photons
2. Add More Pinholes
• BUT: Images “Overlap”
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How to “Add” More Photons
2. Add More Pinholes
• Process to combine “overlapping” images
Before Postprocessing
After Postprocessing
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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
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IBIS
INTEGRAL
P. Ubertini (IAS)
Italy
XB,AGN
02-
CdTe/CsI
20-10000
8% @ 100 keV
MURA
R|C
W
8X8 (FC)
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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
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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
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5200
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EXIST
Balloon
J. Grindlay (Harvard)
USA
Survey
20X6 (HM)
5
2500
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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
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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
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X Rays and Grazing Incidence
X-Ray “Mirror”

X Ray at “Grazing Incidence
is “Deviated” by Angle 
(which is SMALL!)
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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)
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X-ray Collecting Mirrors
n.b., Distance from Front End to Sensor
is LONG due to Grazing
Incidence
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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
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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”!
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X-ray Detection
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History
• Earliest experiments were done with Wernher von Braun’s V2
death rockets from WWII.
• In 1948, x-rays were discovered using these rockets. The total
power was found to be ~10-7 L.
• In 1962, Rossi & Giacconi et al. attempted to detect x-rays
from objects outside of the solar system, finding Sco X-1.
• Surprisingly, they found that Sco X-1 was emitting Px~103
LSco X-1. Sco X-1 is an low mass x-ray binary.
• An x-ray payload flew on Skylab.
• More recent missions include ROSAT, ASCA, BeppoSAX
(GRB afterglow).
• Current major mission is Chandra.
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Active Space Missions
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The Perfect X-ray Detector
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What would be the ideal detector for satellite-borne X-ray astronomy? It would
possess:
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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!
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X-ray Detectors
• Principle measurements
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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
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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
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gas
CZT (CdZnTe)
CdTe
silicon
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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 5.9 KeV (80%)
and K-beta photons with energy of 6.2 KeV (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.
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Fe55 Experiment with Silicon Detector
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CCDs as X-Ray Detectors
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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
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Measured Attributes of Each X Ray
• Position of Absorption [x,y]
• Time when Absorption Occurred [t]
• Amount of Energy Absorbed [E]
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Four Pieces of Data per Absorption are Transmitted to
Earth:
 x, y, t, E 
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Why Transmit [x,y,t,E] Instead of Images?
• Images have too much data!
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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
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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”
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X-ray Telescopes
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Chandra
Originally AXAF
Advanced X-ray
Astrophysics Facility
http://chandra.nasa.gov/
Chandra in Earth orbit (artist’s conception)
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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
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Chandra Mirrors Assembled and Aligned by Kodak
in Rochester
“Rings”
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Mirrors Integrated
into spacecraft at
TRW (NGST),
Redondo Beach, CA
(Note scale of telescope
compared to workers)
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Chandra ACIS CCD Sensor
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X-ray Obervations
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X-Ray Objects
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Solar System objects
single stars near main sequence
white dwarfs and cataclysmic variables
supernova remanants
magnetars
compact object binaries
black hole accretion disks
black hole binaries
galaxy clusters
active galactic nuclei
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X-Ray Radiative Processes
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free-free
inverse compton scattering
synchrotron
atomic
charge-exchange
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Example of X-Ray Spectrum
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Example of X-Ray Spectrum
Gamma-Ray “Burster” GRB991216
Counts
E
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http://chandra.harvard.edu/photo/cycle1/0596/index.html
Chandra/ACIS image and spectrum of Cas A
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Light Curve of “X-Ray Binary”
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There are High Mass X-Ray Binaries (HMXBs) and LMXBs.
GX 301-2 is a pulsar in orbit (P=41 d) with a B1I star.
Matter from the star can fall on, and spin up, pulsar.
Maximum is at periastron (x-ray heating?).
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http://heasarc.gsfc.nasa.gov/docs/objects/binaries/gx301s2_lc.html
X-Rays from Sun
• Sun emits x-ray flux far in excess of amount expected from blackbody
radiation.
• The source of x-ray flux is coronal heating produced by magnetic fields.
• (First “coronal” lines were observed in the optical, attributed to
“Coronium,” but later found to be due to FeXIV, implying 106 K!)
This picture of the Sun comes from the Soft X-Ray Telescope on the Yohkoh spacecraft, orbiting about 630
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kilometers above the Earth. NOTE: The Yohkoh solar observatory is currently offline. The image above was made in
December, 2001.
X-Rays from Earth (taken by Chandra!)
• The Earth’s atmosphere is seen to emit x-rays when solar wind
particles accelerate around the Earth’s magnetic field lines.
• The particles knock out inner electrons, inducing a cascade
and x-ray emission.
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X-Rays from The Moon (taken by Chandra!)
• X-rays from the Sun are absorbed by the surface of the Moon.
• Fluorescing oxygen, magnesium, aluminum, and silicon atoms
emit more x-rays.
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T Tauri Stars
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Young Stars: X-Ray Flux vs. Age
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Higher Rotation Produces Stronger B-field
and More X-Ray Flux in Stars
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Massive Stars
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Clusters of Massive Stars: Westerlund 1
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Clusters of Massive Stars: Arches Cluster
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Galactic Center
• Chandra observed the Galactic Center in a very long exposure,
yielding relatively little flux.
• The observation suggests that the black hole is not accreting at
a very high rate (~1% of Eddington).
This picture is a composite of mid-infrared data (purple), near-infrared data (white), and x-ray data (blue).
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