Supernovae

1. Supernovae High Energy Astrophysics emp@mssl.ucl.ac.uk http://www.mssl.ucl.ac.uk/

2. Introduction Supernovae occur at the end of the evolutionary history of stars. The star must be at least 2 solar masses: the core at least 1.4 solar masses. Stellar core collapses under force of its own gravitation. Energy set free by collapse expels most of star’s mass. A dense remnant, often a neutron star, is left behind.

3. Binding energy and mass loss A=total no. nucleons Z=total no. protons E = binding energy Change from X to Y emits energy since Y more tightly bound per nucleon than X. b binding energy per nucleon A X Y Fe Y X

4. Nuclear binding (cont.) • M (A,Z) < ZM + (A - Z)M • M (A,Z) = ZM + (A - Z)M - (E /c ) • Life of a star is based on a sequence of nuclear fusion reactions. Heat produced counteracts gravitational attraction and prevents collapse. p n nucleus 2 p n b

5. From the core outwards, H => He, He => C, C => O, Ne, Si Outer parts of star expand to form opaque and relatively cool envelope (red giant phase). Eventually, Si=>Fe: most strongly bound of all nuclei (further fusion would absorb). All fuel in core exhausted, then star collapses. 2 < M < 8 solar masses 1.4 < M < 1.9 solar masses 8 < M < 15 solar masses M > 1.9 solar masses star Type I SN core star Type II SN core

6. If the star has less than 2 solar masses (or the core is less than 1.4 solar masses), it undergoes a quiet collapse, shrinking to a stable white dwarf. Type I: Small cores so C-burning phase occurs catastrophically in a C-flash explosion and star is disrupted. (may also happen to stable wd in binary if M>1.4SM). Type II: More massive, so when Si-burning begins, star shrinks very rapidly.

7. Energy release in Supernovae • Outer parts of star require >10 J to form SN… how does the implosion lead to an explosion? • Once the core density has reached 10 - 10 kg m , further collapse impeded by nucleons resistance to compression • Shock waves form, collapse => explosion, sphere of nuclear matter bounces back. 44 17 18 -3

8. Shock Waves in Supernovae • Discontinuity in velocity and density in a flow of matter. Causes permanent change in medium..., speed >> sound speed, between 30,000 and 50,000 km/s. • Shock wave may be stalled if energy goes into breaking up nuclei into nucleons. This consumes a lot of energy, although the pressure (nkT) increases because n larger.

9. Importance of neutrinos _ p + e => n + n inverse b-decay Neutrinos carry energy out of the star and provide momentum through collisions to throw off material. Or they heat the material so that it expands. They have no mass (like photons) and can traverse large depths without being absorbed.

10. Thus a stalled shock wave is revived by neutrino heating. Boundary at ~150 km: inside, matter falls into core outside, matter is expelled. After expulsion of outer layers, core forms either neutron star (M < 2.5 solar masses) or black hole (depends on gravitational field which causes further compression). Neutrino detectors set up in mines and tunnels - must be screened from cosmic rays. core

11. Neutrino detection consistent with that expected from SN in LMC in Feb 1987. This was probably type II SN because originator was massive B star (20 solar masses)… although also individual. • Neutrinos are rarely absorbed so energy changed little over many x 10 years (except for loss due to expansion of Universe)… thus they are difficult to detect. • Density of collapsing SN core is so high however that it impedes even neutrinos!!! 9

12. 45 Energy release up to 10 J in type I and II SN, accounts for >10,000 km/s initial velocity of expanding SNR shell. Optically, star brightens by more than 10mag in a few hours, then decays (weeks-months) Explosive nucleosynthesis => heavy nuclei nuclear reactions produce ~1 solar mass Ni which decays to Co and Fe over a few months. Rate energy release consistent with optical light curves (exponential decay with 50-100 day time constant). 56 56 56

13. Supernova Remnants Development of SNR (averages; end of phase) Phase I II III IV Mass swept up 0.2 180 3600 Velocity (km/s) 3000 200 10 Radius (pc) 0.9 11 30 Time (yrs) 90 22,000 100,000 *solar masses

14. 0 0 0 At time t=0, mass m of gas is ejected with velocity v and total energy E . This interacts with surrounding interstellar material with density r and low-T. System radiates (dE/dt) . Note E ~10 J 0 Shock front, ahead of ‘heated’ material R Shell velocity much higher than sound speed in ISM, so shock front radius R forms. ISM, r 0 41-45 rad 0

15. SNR Development - Phase I • Shell of swept-up material in front of shock does not represent a significant increase in mass of the system. • ISM mass previously within sphere radius R is still small. (1)

16. (2) • Since momentum is conserved: • Applying condition (1) to expression (2) shows that the velocity of the shock front remains constant : v(t) ~ v and that R(t) ~ v t 0 0

17. Supernova 1987a • Star exploded in February 1987 in Large Magellanic Cloud. • Shock wave is now about one eighth of a parsec away from the star, and is moving at 3,000 km/s.

18. Dusty gas rings light up • Two sets of dusty gas rings surround the star in SN1987A, thrown off by the massive progenitor. • These rings were invisible before – light from the supernova explosion has lit them up.

19. Shock hits inner ring The shock has hit the inner ring at 20,000 km/s, lighting up a knot in the ring which is 160 billion km wide.

20. Phase II - adiabatic expansion Radiative losses are unimportant in this phase - no exchange of heat with surroundings. Large amount of ISM swept-up: (3)

21. Thus (2) becomes : (4) Integrating: (5) Substituting (4) for m v in (5), 0 0 R(t) = 4v(t).t, or v(t) = R(t)/4t

22. Taking adiabatic shock wave into account: and • Typical feature phase II – the integrated energy lost since outburst is still small:

23. Crab Nebula Exploded 900 years ago. Nebula is 10 light years across.

24. Wisps and knots and around the Crab pulsar • Watch very carefully. The pulsar is the left of the pair. There are four separate sequences, each one successively closer in

25. N132D in the LMC • Ejecta from the supernova slam into the ISM at more than 2,000 km/s creating shock fronts. • Dense ISM clouds are heated by the SNR shock and glow red. Stellar debris glows blue/green

26. Phase III - Rapid Cooling • SNR cooled, => no high pressure to drive it forward. • Shock front is coasting • Most material swept-up into dense, cool shell. Residual hot gas in interior emits weak X-rays. = constant

27. Phase IV - Disappearance • ISM has random velocities ~10 km/s. • When velocity(SNR)=10 km/s, it merges with ISM and is ‘lost’. • Oversimplification!!! - magnetic field (inhomogeneities in ISM) - pressure of cosmic rays

28. Example - Cygnus Loop - passed the end of phase II - radiating significant fraction of its energy R ~ 20pc v ~ 115 km/s (from Ha) lifetime, = 2 x 10 seconds = 65,000 years now now t ~ 12

29. 3 0 -21 -3 0 Assuming v = 7 x 10 km/s and r = 2 x 10 kg m , from (5) we find that m ~10 solar masses Density behind shock, r, can reach 4r , (r is ISM density in front of shock. Matter entering shock heated to: ( = av. mass elements in gas) 0 0 0

30. (6) For fully ionized plasma (65% H; 35% He) Cygnus Loop: v ~ 10 m/s => T ~ 2 x 10 K (from (6)) But X-ray observations indicate T ~ 5 x 10 K implying a velocity of 600 km/s. Thus Ha filaments more dense and slower than rest of SNR. 5 now 5 6

31. Young SNRs • Marked similarities in younger SNRs. • Evidence for two-temp thermal plasma - low-T < 5 keV (typically 0.5-0.6 keV) - high-T > 5 keV (T = 1.45 x 10 v K) • Low-T - material cooling behind shock High-T - bremsstrahlung from interior hot gas -5 2

32. Older SNRs • A number of older SNRs (10,000 years or more) are also X-ray sources. • Much larger in diameter (20 pc or more) • X-ray emission has lower temperature - essentially all emission below 2keV. • Examples : Puppis A, Vela, Cygnus Loop - all Crab-type SNRs.

33. Crab Nebula • 1st visible/radio object identified with cosmic X-ray source. • 1964 - lunar occultation => identification and extension • Well-studied and calibration source (has a well known and constant power-law spectrum)

34. Crab Nebula Exploded 900 years ago. Nebula is 10 light years across.

35. No evidence of thermal component • Rotational energy of neutron star provides energy source for SNR (rotational energy => radiation) • Pulsar controls emission of nebula via release of electrons • Electrons interact with magnetic field to produce synchrotron radiation

36. Model of the Crab • You’re moving out from the pulsar which is spinning around, in your line of sight to start with. • But the angle of orientation then changes…

37. -22 Watts per sq m per Hz -32 Log n (Hz) 8 10 16 20 Spectrum of the Crab Nebula Log flux density also g-rays detected up to 2.5x10 eV Radio IR-optical X-ray 11

38. -8 nebula 18 m • Summarizing: B ~ 10 Tesla to produce X-rays n ~ 10 Hz (ie. peak occurs in X-rays) E ~ 3 x 10 eV t ~ 30 years • Also, expect a break at frequency corresponding to emission of electrons with lifetime = lifetime of nebula. Should be at ~10 Hz (l~3000Angstroms). This and 30 year lifetime suggest continuous injection of electrons. 13 e- syn 15