High energy Astrophysics Mat Page Mullard Space Science Lab, UCL 7. Supernova Remnants

6. Supernova Remnants This lecture: Why supernova remnants are important What we expect to observe What we actually observe –2 types How they emit, how they are powered. Slide 2

Supernovae have been observed since antiquity! –One observed in Centaurus in 185 AD by the Chinese Explosive end to a stars life Most energetic stellar events known The only extra-solar X-ray sources expected pre-1963 Prettiest stuff in X-ray astronomy! Supernova Remnants Slide 3

Enrichment of the interstellar medium. –Very important source of metals. –We are made out of supernova remnant! Important source of kinetic energy –Probably has a significant effect on star formation process. But why else are they important? Slide 4

2 types of supernova Type 1a: –Accreting white dwarf exceeds Chandrasekhar limit –Collapses to form a neutron star Type II: –Massive star runs out of nuclear fuel – no power from burning iron in core, outer shells of lighter elements –Collapses, shock wave ejects outer part of star Slide 5

Type 1a: –No hydrogen lines in optical spectrum of supernova –Theory predicts them to be rich in iron Type II: –Hydrogen lines in supernova spectrum –Theory predicts them to be rich in carbon, nitrogen and oxygen Slide 6

After the star has exploded… Expect three phases to the supernova remnant. Slide 7

Phase 1: free expansion The material flies away from the supernova at km/s. –Supersonic! (sound speed 10 km/s in cold ISM) Shock wave produced. Mass of material swept up by the shock wave small compared to mass of ejecta from stars –Shock carries on at constant velocity. Lasts until the swept up mass is similar to the original mass of ejecta (~200 years). Slide 8

Phase 2: adiabatic expansion As the ejecta sweeps outward, it sweeps up more and more of the ISM. The velocity of the shock decreases as more material is swept up –momentum is conserved. Mass of material in flow eventually exceeds considerably the mass of original ejecta. Phase lasts until material has cooled to a few 10 5 K, ~1000 years. Slide 9

Phase 3: radiative cooling The material is now cool enough to cool by ultraviolet line radiation. –Many ultraviolet transitions –Line cooling very efficient at this temperature Optical line emitting filaments of cool material form at this time. Phase lasts until remnant has faded into invisibility, ~10 5 years. Most of the energy is radiated during this phase. Slide 10

Slide 11

So what do we actually observe? Cassiopeia A (Chandra) Slide 12

Two types, most distinctive in radio: shell-like filled in Cas A Crab Slide 13

So why are there filled in remnants? Answer later! Slide 14

Back to the shells: Cassiopeia A Shell-like structure in radio and X-ray. But not uniform. There are two possible reasons: –The explosion was not perfectly symmetrical. –The interstellar medium is not uniform. Slide 15

Cass A, R band Slide 16

Slide 17

Slide 18

Slide 19

Thermal X-ray emission Slide 20

Spectrum: Tycho XMM Slide 21

Discovery of emission lines with the MSSL spectrometer on Ariel 5: Slide 22

Stellar post mortem The emission lines tell us two things: Temperatures and abundances With XMM and Chandra we can make maps in individual X-ray lines Tells us what elements are where –How were the layers of the dying star ejected, and what was it made of? Slide 23

Cas A Slide 24

G Oxygen line energy in blue. Slide 25

So why are some remnants filled in? The classic ‘filled remnant’ is the Crab. –Discovery of the crab pulsar solved the mystery. –The pulsar powers the crab nebula –(that’s why its brightest in the middle) –Similarly in other filled remnants Slide 26

Filled remnants The power source for the filled remnants is not the ejected material, but the rotation of the pulsar at the centre. The very strong magnetic field of the pulsar accelerates material to close to the speed of light. It then emits synchrotron radiation Slide 27

Slide 28

Powering the crab How do we know that the crab nebula is powered by the rotation of the pulsar? –We can work out the kinetic energy of the rotating neutron star (about 2x10 42 J) –We know how much energy the nebula is emitting (about W) –The pulsar is slowing. Period 33 ms, period increasing by 15 microseconds per year. Rate at which the rotational energy is being used up is approximately equal to the emission from the nebula Slide 29

Material cannot rotate faster than light – at some radius it has to break free from the magnetic field line, and radiate. Slide 30

Chandra image of the crab synchrotron nebula Slide 31

Some key points: Supernova remnants are important for the enrichment of the interstellar medium with metals, and as a source of kinetic energy They also help us to understand the supernova process Two types of remnant: filled and shell-like The shell-like emits X-rays from gas heated by the ejecta. The lines tell us what elements are inside The filled remnants are powered by the rotation of the central pulsar and emit synchrotron radiation Slide 32