High Energy Astrophysics

What do we study at these high energies?

Challenges of High-Energy Detection

• Atmosphere is opaque to high energy radiation

– Good for our survival bad for detection

– Requires space based observations

• Very high energy photons are very penetrating

– Need thick detectors that photons wont penetrate

– Weight limitations because they must be space based

• Detection of weak sources against a fairly strong background

– Source detection on a photon-by-photon basis

– X-ray astronomers “name” their photons

Image formation at X-ray wavelengths

• X-rays and γ-rays tend to pass through or be absorbedrather reflected so they are difficult to focus

• Angle dependent sensitivity• Collimators• Grazing incidence mirrors• Coded Masks

Angle Dependent Sensitivity

• Many detectors facing different directions• Different signal strengths at each detector

helps fix direction• Difficult to calibrate• 2-5º resolution

Collimators

• An instrument that limits the field of viewand therefore cuts down on the background

Grazing Incidence Mirrors• Need grazing incidence

at these wavelengths sophotons are not absorbed

• Only recently possible• Excellent resolution - 1”• Can get high energy

resolution withdiffraction gratings

Wolter grazing incidence typesare generally used for the mirrordesign at these energies

Coded Mask Imaging

• Gamma ray technique drivenby the inability to focusgamma rays

• Combination of opaque platesand holes in front of thetelescope

• Detector records the shadowof the mask

• Allows determination ofposition and intensity of thesource

Detecting Photons at High Energies

• CCDs• Microchannel plates• Proportional counters• Scintillators• Microcalorimeters• Coded Mask imaging

High Energy CCDs

• Electron-hole pair creation in cooled siliconor germanium or room temp materials

• 1 e- per 3 eV of photon energy• High energy resolution, ~3-10%• Slower readout than proportional counters• Angular resolution can be good

Micro-channel Plates• Array of hollow tubes that release e- when struck by X-ray photons

• Records spatial and timing information

• Does not record energy information

• Above 5 eV some photons penetrate the channel walls

• Can achieve gains of 106-108 although ~100 typically

• Quantum efficiencies of 1-10% (higher energies are less efficient)coatings and optical incidence angles can increase this to 30-60%

Proportional Counters

• A windowed gas cell subdivided into low and high E-fieldregions

• Below 50 keV photons interact with gas molecules viaphotoelectric effect

• 1 e- per 30 eV• Energy resolution only 20-40%• Fast readout, good timing• Poor angular resolution• Background rejection is critical

– Energy selection– Rise-time discrimination– Anti-coincidence within the gas cell

Scintillators• Convert X-ray energy to

visible light• Usually use NaI and CsI

crystals• Poor efficiency: few percent

of energy becomes e-• Rugged, good for balloon

flights• 5 mm of NaI or CsI has 100%

detection efficiency between20-100 keV

Micro-Calorimeters• Energy from increase in heat from

photon absorption• Measure change in temperature at

<0.1K• Low heat capacity so small change in

energy gives large change intemperature

• Expect ΔE/E ~ 0.1-1%• Pixels are very small to keep heat

capacity low

Chandra• 0.1-10 keV• Sensitive to high energy X-rays and excellent spatial res. allowing for study of faint sources in crowded fields• Type 1 grazing incidence telescope, 30’ FOV• ACIS: 0.2-10 keV, CCDs with spectrometer

– E/dE = 9-50 @ 1 and 6 keV– Imaging 16’x16’, spectroscopy 8’x48’

• HRC: 0.1-10 keV, high resolution imager (0.5”) +low energy transmission grating, microchannelplate detectors,

International Gamma-RayAstrophysics Lab. (INTEGRAL)

• Energy Range : 3 keV - 10 MeV + Optical• High spectral and spatial resolution.• SPI: 20 keV - 8 MeV, Coded aperature mask. FOV 16°,

detector area. 500 cm2 (Germanium array) spectralresolution (E/dE) 500 @ 1 MeV, spatial resolution 2°.

• IBIS: 15 keV - 10 MeV, Coded aperature mask. FOV 9°X 9°, detector area. 2600 cm2 (CdTe array) & 3100 cm2

(CsI array), spatial resolution 12´.• JEM-X: 3- 35 keV, Coded aperature mask with 2 high

pressure microstrip gas chambers. FOV 4.8°, detectorarea. each 500 cm2, spatial resolution 3´

Rossi XTE

• Energy Range : 2 - 250 keV• Very large collecting area and all-sky monitoring

of bright sources• Proportional Counter Array (PCA): 2-60 keV

energy range, 6500 sq cm, time resolution 1 µsec• High Energy X-ray Timing Experiment (HEXTE)

15-250 keV energy range, 2 X 800 sq cm• All-Sky Monitor (ASM): 2-10 keV energy range,

30 mCrab sensitivity

Swift• Rapid response to γ-ray bursts

detected by the BAT telescope• 0.2-150 keV+UV/optical• BAT: 15-150 keV, Wide field-of-

view coded-aperture imager. FOV1.4 sr half coded, ~4’ positionaccuracy.

• XRT: 0.2-10.0 keV, CCD Imagingspectrometer. FOV 23.6´ X 23.6´,~5” position accuracy

XMM-Newton

• High throughput X-ray telescope with largeeffective area

• Energy Range : 0.1-15 keV• Three Wolter Type I grazing incidence mirrors• Spatial resolution 6" FWHM• EPIC: CCDs, FOV 33 ´ x 33 ´ Spec. res. (E/dE) ~

20-50• RGS: 0.35-2.5 keV CCDs, Spec. res. (E/dE) 200-

800, FOV 5´