Astronomical Observational Techniques and Instrumentation

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. 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

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 •  “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)

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.

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 “Mirror”  X Ray at “Grazing Incidence is “Deviated” by Angle  (which is SMALL!)

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 Mirrors n.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 Ray • 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:

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 Originally AXAF Advanced X-ray Astrophysics Facility http://chandra.nasa.gov/ Chandra in Earth orbit (artist’s conception)

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, 1999 Supernova remnant Cassiopeia A

37. Example of X-Ray Spectrum

38. Example of X-Ray Spectrum Gamma-Ray “Burster” GRB991216 Counts E http://chandra.harvard.edu/photo/cycle1/0596/index.html

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