1. High energy Astrophysics Mat Page Mullard Space Science Lab, UCL 1. Overview

2. This lecture: What is going to be covered in the course What is high energy astrophysics A bit of history The X-ray,  -ray, and radio skies Slide 2

3. Basic ideas of high energy astrophysics: –what we learn about –what sorts of observations we use –what sorts of sources we look at Radiation physics: –production, absorption and energy sources High energy sources: –AGN,  -ray bursts, accreting binaries, etc A little bit of the history –big discoveries, who, when and how Some exotic astrophysics –gravitational waves What is to be covered in the course: Slide 3

4. More details on the lecture plan Let me know at any time if there’s any background you would like me to cover. What is to be covered in the course: Slide 4

5. What is high energy astrophysics? “High energy” because: –Photons emitted above 100 eV (X-ray sources) –Large energy density in photon field or magnetic field, (e.g. X-ray binaries, pulsars) –Large amount of energy stored in gravitational field (e.g. black holes) Unusually high concentrations of energy Slide 5

6. High energy radiation Recall that electromagnetic radiation has properties of both waves and particles. it comes in discrete packets of energy called photons. The energy of each photon is given by E=h   where  is frequency  h is the Planck constant. Slide 6

7. High energy radiation X-rays and  -rays are at high frequency, so each photon has a lot of energy – so much energy that high energy astronomers more often label radiation by photon energy than by wavelength or frequency. The natural unit is the electron volt (eV), or more usually the kiloelectron volt (keV). An electron volt is the energy of an electron after it has been accelerated by a potential difference of 1 volt (1 keV = 1000 eV). X-ray photons have energies comparable to the electrons used in a cathode ray tube! Slide 7

8. Consequences of high photon energies - 1 Since each photon carries a lot of energy, few high energy photons carry the same energy as many low energy ones. This means that a source radiating a particular power in X-rays or  -rays emits far fewer photons than a source emitting the same power at optical or longer wavelengths. X-ray and  -ray astronomers are photon starved. Shot-noise (statistical uncertainty resulting from the sparsity of photons) is a serious problem in X-ray and  -ray astronomy. Slide 8

9. Consequences of high photon energies - 2 The photons carry so much energy that they can be detected individually, as if they were particles. The energy of each individual photon can be measured. X-ray and  -ray detectors are often called “photon-counting” detectors because they record each photon individually. Typically, the arrival time, position and energy of each photon are recorded and telemetered to the ground from X-ray observatories. The first astronomical X-ray detectors were basically Geiger counters! Slide 9

10. X-rays from stellar photospheres? Take the Sun. T=5800K XUIV R Slide 10

11. X-rays from stellar photospheres? Expected radiation at h  > 0.1 keV from the sun if it has a blackbody spectrum is < 10 -50 times its bolometric flux, i.e. an infinitesimal fraction of a Watt. Stellar photospheres are typically pathetic X-ray sources High energy observations tell us about more exciting things than stellar photospheres Even for stellar photospheres with temperatures of 30,000K, the energy fraction emitted at h  >0.1 keV is ~ 10 -11 Slide 11

12. History: observational techniques By comparison: Optical Astronomy – thousands of years Astronomical spectroscopy – since 1860 Radio astronomy – since 1933 X-ray Astronomy – since 1962 Photographic Astronomy – since 1840 Slide 12

13. History: astronomical sources By comparison: Stars and planets – thousands of years Quasars - 1963 Neutron stars - 1968  -ray bursts – 1973 Spiral galaxies – since 1845 Slide 13

14. X-ray,  -ray and radio skies Slide 14

15. X-ray,  -ray and radio skies Slide 15

16. X-ray,  -ray and radio skies Slide 16

17. X-ray,  -ray and radio skies Slide 17

18. X-ray,  -ray and radio skies Slide 18

19. X-ray,  -ray and radio skies Slide 19

20. X-ray,  -ray and radio skies Slide 20

21. X-ray,  -ray and radio skies Slide 21

22. X-ray,  -ray and radio skies Slide 22

23. e.g. the brightest X-ray source in the sky: Sco X-1 Slide 23

24. The X-ray sky is dynamic Slide 24

25. Some key points: High energy astrophysics is concerned with unusually high concentrations of energy. In “high energy” radiation the individual photons carry a large amount of energy. Consequently, photons tend to be sparse. Stellar photospheres are not expected to be significant high energy sources. High energy astrophysics is a recent branch of astronomy. The sky looks very different in X-rays and  -rays compared to its appearance at optical wavelengths – high energy observations reveal very different sources. The X-ray sky is changing constantly: we are seeing highly dynamic phenomena. Slide 25