High energy Astrophysics Mat Page Mullard Space Science Lab, UCL 12. Cosmic rays

This lecture: What are they? Discovery of cosmic rays Interaction with the atmosphere Interaction with the interstellar medium Origin of the cosmic rays Slide 2

Despite the name, cosmic rays are not electromagnetic radiation. They are high energy particles with an extra-terrestrial origin. They are protons, atomic nuclei, electrons and the anti-particle equivalents because these are all stable particles. What are cosmic rays? Slide 3

Story begins at the beginning of the 20 th Century. Rontgen had discovered X-rays (1895) Radioactivity ( , ,  –rays) had been discovered by Becquerel (1896) About 1900 it was discovered that electroscopes discharged over time even if they were kept in the dark away from radioactive materials. Rutherford determined that most of the ionisation was due to natural radioactivity in rocks or contaminated equipment. Discovery Slide 4

But in 1910 Wulf took an electroscope to the top of the Eiffel Tower –Ionisation should have decreased by factor of 16 if the terrestrial radioactivity was the cause – in fact ionisation decreased by only a factor of Hess and Kolhorster flew balloons to altitudes up to 9 km. –Above 1.5 km the level of ionization increased with respect to that observed on the ground ‘Cosmic radiation coming from space’ 1929 Bothe and Kolhorster used a pair of Geiger- Muller detectors with different sized slabs of gold and lead in between to determine that the cosmic rays were not  -rays but corpuscular with energies eV. Slide 5

Victor Hess after one of his first successful balloon flights. Slide 6

Most of the ‘cosmic rays’ observed at the earth’s surface are secondary events from interactions between primary cosmic rays and the Earth’s atmosphere. When high energy particles interact with material they do 3 things: –Ionize atoms –Destroy crystal structures and molecular chains –Interact with the nuclei of atoms Interaction with the atmosphere Slide 7

Ionization is similar to what happens in a hot gas – the cosmic ray interacts with an atom or ion and as a result an electron is ejected. –If the cosmic ray is a proton or more massive, with an energy of MeV or greater, it will produce many ionizations before being stopped. –If it is an electron it will lose energy much more rapidly because it will transfer a larger proportion of its momentum in each interaction. Slide 8

Chemical bonds can be broken if the electrons are given energy. –Plastics and other polymers make good particle detectors – tracks corresponding to the paths of cosmic rays form, the lengths of the tracks depend on the mass of the cosmic rays. Cosmic rays can affect molecules in our body and cause us to mutate! Tracks are formed in meteorites. –We can use meteorites of different ages to work out the history of cosmic rays over the lifetime of the solar system. –We can use cosmic ray tracks to work out the ages of meteorites Breaking of crystal structures and molecular bonds Slide 9

Cosmic ray kinetic energies exceed the rest mass of a proton (~ 1 GeV) –Cosmic rays can interact with atomic nuclei, basically by smashing individual nucleons to pieces. –Results in a shower of pions, which decay to  - rays and muons and end up as electrons and positrons, neutrinos and electromagnetic radiation. –Particle physics was based on cosmic ray interactions before particle accelerators. Nuclear reactions Slide 10

Decay products may also have enough energy to interact with further nucleons in the nucleus. Fragments of the nucleus are ejected. The nucleus and fragments will interact with other nuclei or decay if they are not stable. Elemental abundances are actually changed by cosmic rays. Ejected neutrons are absorbed by 14 N to make radioactive 14 C – this is the basis of radio carbon dating. Slide 11

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The highest energy cosmic rays can initiate large air showers: –nuclear interactions initiated by the highest energy cosmic rays can produce 10 6 relativistic particles arriving at the ground. –These can be detected by detectors with wide separations (eg 300 metres). –The atmosphere is like the first stage of a cosmic ray detector! Extensive air showers Slide 13

Small detectors Scintillation (crystal) detectors (really old…) Proportional or Geiger counters Cloud or bubble chambers Nuclear emulsions –Crystals in the emulsion are activated by electrons Slide 14

Larger area detectors Ultraviolet fluorescence imagers –Nitrogen excited and fluoresces after interactions with secondary particles Multi-mirror reflectors –detect Cerenkov radiation in the atmosphere Water tanks to detect secondary particles. –Cerenkov radiation emitted by secondary particles in the water and detected by imagers Slide 15

So now we can get to the astronomical interpretation of cosmic rays… The spectrum as measured at the top of the atmosphere, either by satellite experiments, or inferred by the products produced by interactions with the atmosphere, is approximately a broken power law with the break at eV. Chemical abundances in cosmic rays similar to solar, but with more light elements (B, Be, Li) Below about 1 GeV, the spectrum is strongly affected by the Solar wind At the top of the atmosphere Slide 16

Cosmic ray spectrum at top of atmosphere Slide 17

At > eV only expect 1 particle per km 2 per Century! Need vast detectors to detect these cosmic rays. Basically, detectors have to cover large areas on the ground or watch large areas of the sky. Before Pierre Auger Observatory, largest experiments were AGASSA (Japan) and HIRES (US) Each covers 10s of square kilometres. Slide 18

Pierre Auger observatory Largest observatory so far constructed in Argentina, covering 3000 km 2. Slide 19

Water tanks for Auger Slide 20

Fluorescence imagers for Auger Slide 21

Of course cosmic rays will interact with the interstellar medium just as they interact with our atmosphere. –Strong  -ray Galactic background. Cosmic rays also interact with the photon field - this results in a limit to how far cosmic rays can travel before their energy is given up. –Electrons lose energy faster than protons so travel smaller distance. –Cosmic rays with E> eV should not be able to travel further than about 30 Mpc before losing their energy through interactions with CMB. Interaction with interstellar medium and microwave background Slide 22

Shocks in supernovae are known to be a good source of cosmic rays – we see them in radio by synchrotron radiation. –Supernovae are probably the source of most of the cosmic rays we receive –However, its not thought possible to give particles > eV in a supernova shock. The origins of cosmic rays with > eV are less certain. So where are cosmic rays coming from? Slide 23

Wimpzillas –Massive weakly interacting massive particles that decay to produce cosmic rays –‘dark matter particles’ in Galaxy dark matter halo Z bursts –Collisions between high energy neutrinos –Possible if there are populations of high energy neutrinos Topological defects –Distortions in spacetime –E.g. Magnetic monopoles and cosmic strings –These distortions are not found in microwave background More extravagant possibilities: Slide 24

Massive black holes –We know massive black holes accelerate particles to high Lorentz factors AGN radio lobes –Shock fronts in lobes may make cosmic rays Magnetars (highly magnetised neutron stars) –If magnetars spin rapidly enough they can do the business –Could be many of these sources in star-forming galaxies The directions of cosmic ray sources should correspond to these sources – we should try to resolve the cosmic ray background. Less extravagant possibilities: Slide 25

Or  -ray bursts –We know they accelerate particles to really high energy –Large distances, spectrum should be cut off at 5x10 19 eV. Results 2007 from Auger Observatory: Correlation between high energy cosmic ray directions and nearby AGN! But… without any fanfare or press release, the Auger team now say that the significance has gone down with more data rather than up, i.e. this result is not yet verified (and may be wrong!). Slide 26

Some key points: Cosmic rays are protons, electrons and atomic nuclei with high energy. They interact with the atmosphere, by ionising it and by nuclear interactions with atomic nuclei, revealing subatomic particles. They also interact with the interstellar medium. The highest energy cosmic rays cannot come from outside the local Supercluster. Cosmic rays up to eV are predominantly from supernovae. AGN might be in the frame as responsible for some or all of the the highest energy cosmic rays. Slide 27