M.Phys. & M.Math.Phys.

High-Energy Astrophysics

Garret Cotter [email protected]

High-Energy Astrophysics MT 2016 Lecture 1 High-Energy Astrophysics: Synopsis

1) Supernova blast waves; shocks. 2) Acceleration of particles to ultra-relativistic energies. 3) Synchrotron emission; total power and spectrum 4) Synchrotron self-absorption, population ageing, radio galaxies. 5) Accretion discs: structures and luminosities. 6) Accretion discs: spectra, evidence for black holes. 7) Relativistic jets; relativistic projection effects; Doppler boosting. 8) Cosmic evolution of AGN; high-energy background radiation and cosmic accretion history of black holes. 9) Bremsstrahlung; inverse-Compton scattering; clusters of galaxies; Sunyaev- Zel’dovich effect. 10) Cosmic rays and very-high-energy gamma rays; Cherenkov telescopes. Today’s lecture: Supernova blast waves and shocks.

• Taylor-Sedov solution for powerful explosions.

• Introduction to strong shocks.

• Derivation of strong shock jump conditions.

• Shocks in accretion onto compact objects and supernova explosions.

• Practical accretion: binary systems. Crab supernova remnant, Hubble Space Telescope image Supernova blast waves Supernova blast waves – Taylor-Sedov Solution 1/3 In 1950, G.I. Taylor used dimensional analysis to estimate the relationship between the energyIn 1950, G.I. inputTaylor used dimensional analysis to estimate the relationship of an extremely powerful explosion and the growth of between the energy input of an extremely powerful explosion and the growth the resulting fireball.of the resulting fireball. He showedHe sh thatowed that the rel the relevantevant pa parameters—explosionrameters—explosion energy energy E , blast radius r , time since detonation t and external density ρ—could E, blast radius r,be combined to time sincefor detonationm a dimensionless tquantityand external density ⇢—could be combined to form a dimensionless quantity r5⇢ . t2E This is one of the classic triumphs of dimensional analysis.

This is one of the classic triumphs of dimensional analysis. Supernova blast waves – Taylor-Sedov Solution 2/3

The crucial point here is that Taylor did not in a hand-wavy style "assume constants are of order one”. He reduced the problem to a self-similar scaling solution, and then used experimental results to get the value of the constant - about 1.1 in air. This was done via high-speed photographs of small detonations in the lab. The power of dimensional analysis is that even though the experiments were done on very small scales, we can be sure the scaling will still hold for explosions that are many, many orders of magnitude greater.

A full (and lengthy) solution was later found by the legendary Russian theorist Sedov - this is given in a link on the course web page. But Taylor’s method stands today as an example of how to deal with problems where some part or parts are known to be analytically intractable.

Taylor went on to used a declassified photograph of the fireball of the first atomic bomb test to calculate the yield of the bomb. He arrived at an accurate value of ≈ 20 kilotons (a total mass-energy conversion of about one gram). Taylor’s publication caused some consternation at the time, since the yield was still classified. Trinity atom bomb test, July 16 1945. 25 ms after detonation. The fireball is about 140 m in radius. Supernova blast waves – Taylor-Sedov Solution 3/3

Let us use this technique to examine the Crab supernova remnant.

E = ρr 5t−2

The electron density in the interstellar medium is about 1 cm−3 i.e. 10 6 m−3. The Crab supernova remnant today has a radius of about 3 pc and the time since the explosion is 2016 − 1054 = 962yr. We arrive at

E ≈ 1042 J

Which is equivalent to the total mass-energy conversion of 1025 kg or 0.001% of a solar mass.

Also significant is that the speed at which the blast wave propagates into the interstellar medium is highly supersonic. We next move on to the physical changes which occur at strong supersonic shocks. Prandtl-Glauert singularity Shocks

• A shock occurs when a disturbance moves through a medium faster than the sound speed in the medium, i.e., sufficiently fast that a pressure wave cannot precede the disturbance.

• The conditions in the medium—temperature, density, bulk velocity—thus change almost instantaneously at the shock. The material which is hit by the shock receives no forewarning.

• The classic introductory example is the case where energy is injected into the ambient medium suddenly by a supersonically-moving solid object: the sonic boom. The air ahead of a supersonic aircraft doesn’t have time to move out of the way before the shock hits it; temperature and density are suddenly (almost discontinuously) increased as the shock passes.

• This is similar to the blast wave of a supernova. The huge energy injection heats “cold” interstellar gas suddenly as the blast moves outward. Supernova SN1993J in M81: radio maps April 1993—June 1998

Initial expansion speed 20 000 km s−1 To get a better understanding of what’s going on—and to allow an easier calculation of the change in conditions at the shock—we conventionally transform into the instantaneous rest frame of the shock itself.

This may seem a bit contrived but a simple analogy can be used to illustrate the process... SHOCK

High bulk speed (v > sound)

Low density Low bulk speed Little internal motion −> low T High density

Much internal motion > high T Derivation of strong shock “jump conditions” In restframe of shock

SHOCK

upstream downstream unshocked ("virgin") gas shocked gas

v d r v u ru d T T= 0 d Assumptions:Assumptions:

MonatomicMonatomic gas gas o or,r, e e.g.,.g., fully fully-ionized-ionized h hydrogenydrogen plasma. plasma. •• • TheThermalrmal (inte (internal)rnal) energy energy of of unsho unshockedcked gas gas is is negligi negligible;ble; appr approximateoximate • pressurepressure and and tempe temperaturerature of of the the unsho unshockedcked gas gas to to z zero.ero.

ConseConserverve mass: mass:the the mass mass per per unit unit area area fl flowingowing across across the the sho shockck is is conse conserved.rved.

ρ⇢uvu = ρ⇢dvd

ConseConserverve momentum: momentum:the the sho shockck does does not not accele acceleraterate in in its its rest rest f frame.rame. The The difdifferenceference bet betweenween upstream upstream and and d downstreamownstream r ram-pressuresam-pressures m mustust be be pr providedovided bbyy the the gas gas pressure pressure d downstream.ownstream.

2 2 ρ⇢uv2 = Pd + ρ⇢dv 2 u u d d d ConseConserverve energy: energy: PPdVdV wworkork is is being being done done on on the the gas gas at at the the sho shock.ck. The The r rateate (per(per unit unit area) area) at at which which this this w workork is is done done is is vvddPPdd

1 2 1 2 3 vu( 1ρuv 2) vd( 1ρdv 2 + 3Pd) = vdPd vu( ⇢uv )− v ( ⇢ v + P )=v P 22 uu d 22 d dd 22 d d d Eliminating PPdd andandusing using ρ⇢uuvvuu == ρ⇢ddvvdd wwee can canget get

⇢2 ⇢ ρd 3 ρd 3 3 d v3u − 5 vu ++44vu ==00 ⇢2 u ⇢ ρu ρu

This rear rearrangesranges t to...o...

ρ⇢d ρ⇢d ( − 4)4)(( − 1) = 0 ρ⇢u ρ⇢u

v = v ...which has hasthe the t trivialrivial solution solution vuu = vdd,, b butut which whichotherwise otherwise gi givesves us the Strong shock jump conditions Strong shock jump conditions ρd = 4; vu = 4 ρ⇢ud = 4; vvdu =4 ⇢u vd

This is a highly-simplified case but is a reasonable approximation in This is a highly-simplified case but is a reasonable approximation in many astrophysical situations because of the extreme conditions. many astrophysical situations because of the extreme conditions. For a full discussion of shocks for arbitrary Mach numbers, For a full discussion of shocks for arbitrary Mach numbers, temperatures etc. see e.g. Blundell & Blundell. temperatures etc. see e.g. Blundell & Blundell. Shocks: accretion onto stellar-mass compact objects

• Violent accretion onto stellar-mass compact objects happens in binary star systems: the accreting mass has to come from somewhere, and if there is a binary companion then it can “donate” fuel for the accretion. • In accreting binary systems, the compact object can be a white dwarf, neutron star or black hole. The donor stars are usually either high-mass (and hence young) or are low- mass stars, but systems in which a massive star is accreting onto a white dwarf are not observed. • Jargon alert: Although all accreting binary systems emit X-rays, the phrase “X-ray binary” is commonly reserved only for those systems in which the compact object is a WD or NS. Systems in which a low-mass star is fuelling accretion onto a WD are termed “cataclysmic variables” or “dwarf novae”. (Question for those who have taken the Stars & Galaxies course: what is often the fate of such objects?) • X-ray binaries are then classified as high-mass or low-mass depending on the mass of the donor star. This distinction is important because it broadly divides the accretion into the two mechanisms we have discussed: material falling in gradually through an accretion disc, and material falling straight to the surface of the compact object.

Roche lobe overflow

This occurs when the envelope of the donor star expands beyond the equipotential surface between it and its compact companion (the Roche Lobe). The atmosphere of the donor star falls fairly smoothly down into the potential of the compact object and forms an accretion disc. This process can happen even with low-mass donor stars once they expand off the main sequence. Stellar wind capture

This occurs where the donor star is very high mass (typically M > 15M ⊙), and the stellar wind of the high-mass star can be captured by the compact object. In these systems the luminosity depends on the mass outflow rate from the donor. Shocks: implications for accreting objects

We must of course be aware that high-mass donor stars can also have sufficiently large envelopes that they fill their Roche lobes, so these two pictures represent the extreme cases. The details of binary star evolution will be given in Prof Podsiadlowski’s course.

A further complication in white dwarf and neutron star accretion is the magnetic field of the compact object. The extremely strong fields found in collapsed stars can have a profound influence on the flow of accreting material: in extreme cases this is a method of preventing an accretion disc forming. We will consider accretion discs in a later lecture. For the time being, let’s simply consider material falling radially onto a compact object.

Note that we cannot convert all of the bulk kinetic energy of the upstream gas into thermal energy downstream. So, when “cold” material falls at great speed onto a compact object, it is decelerated and heats up in a shock a little way above the surface. Shock

Matter falling in radially

White dwarf Shocks:ShoShocks:cks: calculationcalculation calculation (on problem set)

WeWWee cancan can useuse use thethe the idealideal ideal gasgas gas lawl lawawPPP===nknnkkBBBTTTtototo determinedete determinermine thethe the conditionsconditionsconditions ofof of thethe the shockedsho shockedcked gas.ga gas.s. ForF Foror aa a gasgas gas withwith with meanmean mean particlepa particlerticle massmass mass mmmandandand densitydensity density ⇢ρ⇢thethethe idealideal ideal gasgas gas lawl lawaw cancan can bebe be writtenw writtenritten asas as ⇢ρ⇢ PPP === kkkBBBTTT mmm WhichWhichWhich forf foror ourour our strongstrong strong shocksho shockck conditionsconditions conditions cancan can bebe be usedused used toto to obtainobtain obtain

22 33mvmvuu TTdd== 1616 kkBB

ForFForor materialmate materialrial fallingf fallingalling ontoonto onto thethe the surfacesur surfaceface ofof of aa a whitewhite white dwarf,d dwarf,warf, thethe the temperaturestempetemperaturesratures whichwhich which arisea ariserise areare are sufficientsufficient sufficient toto to generategene generaterate hardhard hard X-rayX X-ray-ray emission.emission.emission. Next Lecture

How to use shocks to accelerate particles to ultra-relativistic energies