2: THE HIGH ENERGY SKY

Question 1

1. How can X-rays be focused?
2. What are the two ways that energy and spatial information is retrieved?

Answer 1

1. Using grazing incidence reflections.
2. a) Photons are most commonly recorded by a CCD array b) or a grating of etched metal disperses the X-rays from bright sources into a 1D spectrum

Question 2

Why can’t gamma rays use x-ray detection?

Answer 2

The rays are too high for them to be focussed.

Question 3

What gamma ray detectors are used instead?

Answer 3

Compton telescope, pair conversion detector, Cherenkov telescopes

Question 4

1. How does a Compton telescope work?
2. How can energies E1 and E2 be used?
3. What gives the incident direction?
4. What does the sum of E1 and E2 produce?

Answer 4

1. It uses Compton scattering to determine the direction of source photons.
2. We can use the energy of the scattered electrons E1 and energy deposited in layer 2 E2 to compute the scattering angle theta in layer 1.
3. Combined with angle phi of the path between layers, this gives the incident direction.
4. The sum of E1 and E2 gives energy of incident y-ray.

Question 5

How does a pair conversion detector work?

Answer 5

1. Layers of silicon interleaved with layers of tungsten or tantalum which trigger γ → e
   ± pair production
2. Follow the trajectory of e
   ± through silicon layers and reconstruct arrival direction of γ. Angular resolution of ∼ 1 arcmin achieved in Fermi
3. Below layers of silicon, detectors measure energies of e± to determine energy of original γ

Question 6

1. How to Cherenkov telescopes work?
2. When does a particle emit Cherenkov radiation?

Answer 6

1. An incoming γ-ray interacts with particles in the atmosphere and produces high-energy e± via pair
   production.
2. A particle emits Cherenkov radiation if it moves into a medium in which its speed is faster than the
   speed of light in that medium, c/n, where n is the index of refraction.

Question 7

What are soft and hard in terms of x-rays?

Answer 7

Soft - low energy x-rays
Hard - high energy x-rays

Question 8

Why are hard-spectrum sources visible to larger distances than soft-spectrum sources?

Answer 8

At lowest energies (<0.5 keV), x-rays suffer photoelectric absorption from neutral gas in our Galaxy

Question 9

An x-ray sky consists of 1) isotropically distributed discrete sources and 2) discrete sources mapping the milky way. What are these sources?

Answer 9

1. Local stars, Distant AGN, Galaxy cluster gas, nearby normal galaxies
2. x-ray binary systems, SN remnants

Question 10

The sky consists of gamma rays, which two emissions demonstrate this?

Answer 10

1. Isotropically-distributed diffuse emission with localised radio-loud AGN and gamma-ray bursts
2. High contrast emission following the Milk Way (diffuse emission and discrete sources - pulsars and binary systems)