Thermal Bremsstrahlung

Transcription

1 Thermal Bremsstrahlung ''Radiation due to the acceleration of a charge in the Coulomb field of another charge is called bremsstrahlung or free-free emission A full understanding of the process requires a quantum treatment, since photons of energies comparable to that of the emitting particle can be produced However, a classical treatment is justified in some (most of ) regimes, and the formulas so obtained have the correct functional dependence for most of the physical parameters. Therefore, we first give a classical treatment and then state the quantum results as corrections (Gaunt factors) to the classical formulas'' George B. Rybicky & Alan P. Lightman in ''Radiative processes in astrophysics'' p. 155 Processi di radiazione e MHD THERMAL BRESSTRAHLUNG Daniele Dallacasa

2 Bremsstrahlung Foreword Free electrons are accelerated (decelerated) in the Coulomb field of atomic (ionized) nuclei (free-free) and radiate energy If Electrons and nuclei are in thermal equilibrium at the temperature T, their emission Is termed thermal bremsstrahlung Electrons move at velocity v wrt the ions and during ''collisions'' are deflected. The electrostatic interaction decelerates the electron which emits a photon bringing away part of its kinetic energy N.B. e and p have also homologous interactions (e with e; p with p) but they do not emit radiation since the electric dipole is zero. Astrophysical examples: 1. HII regions (10 4 o K) 2. Galactic bulges & hot-coronae (10 7 o K) 3. Intergalactic gas in clusters/groups ( o K) Bremsstrahlung is the main cooling process for high T (> 10 7 K) plasma

3 b

4 Bremsstrahlung single event Radiated power (single particle) is given by the Larmor's formula: = 2e2 a 2 3c 3 acceleration is from Coulomb's law P = de dt b x e _ v F = ma Ze2 a = Ze 2 x 2 m x 2 where x= actual distance between electron (e-) and ion (Ze+) Ze + Remarks: 1. a m 1 P m 2 2. a x 2 P x 4 Radiated power is relevant 1. for electrons (positrons) only i.e. light particles 2. when the two charges are at the closest distance: (x b) The interaction lasts very shortly: Δ t 2b v where v = velocity

5 Bremsstrahlung Pulses and spectrum The total emitted energy in a single collision (consider x ~ b) P Δ t = 2 e 2 ( Z e 2 )2 2 b = 4 Z 2 e 6 1 x b 3 c 3 m x 2 v 3 c 3 m 2 b 3 v Radiation comes in pulses Δ t long Asymmetric profile ( decrease of the e velocity) Fourier analysis of the pulse spectral distribution of the radiated energy Flat up to a frequency ν max 1 v 2 Δ t 4 b related to the kinetic energy of the electrons (exponential decay beyond ν max ) P(t) P(ν) t ν max ν

6 Bremsstrahlung radiated energy Exercise: Determine the range of distances (order of mag) in which the released energy is significant e = statc m e = g v = 100km/ s Remember... Bohr's radius a o cm Typical ISM densities n ISM 1 cm 3 P(ν) ν max ν The radiated energy per unit frequency (assuming flat spectrum) from a single event can be written as P Δ t Δ ν P Δ t ν max = 2P(Δ t) Z 2 e 6 c 3 m e 2 1 b 2 v 2 1 b 2 v 2

7 Bremsstrahlung Unphysical case: a cloud with ion number density real plasma n Z and free electron number density n e In a unit time, each electron experiences a number of collisions with an impact parameter between b and b + db is 2 π n Z v bdb and the total number of collisions is 2π n e n Z v bdb Total emissivity from a cloud emitting via bremsstrahlung (single velocity!) J br (v, ν) = = 32π e6 dw d ν dv dt 3c 3 m e 2 v n e n Z Z2 bmin = 2 π n e n Z v bmin b max db b = 32 π e 6 3c 3 2 m e b max 16 1 v 3 Z 2 e 6 c 3 m e 2 n e n Z Z 2 ln( b max b min ) 1 b 2 v 2 b db =

8 Thermal bremsstrahlung electron velocities / total emissivity A set of particles at thermal equilibrium follow Maxwell-Boltzmann distribution: Then f (v)dv = 4 π( m e 2πk T )3/2 e m e v2 /2kT v 2 dv n e (v) = n e f(v)dv is the number density of electrons whose velocity is between v and v+dv and n e (v) replaces n e in J br (ν) When the plasma is at equilibrium, the process is termed thermal bremsstrahlung Total emissivity: must integrate over velocity (energy) distribution (0 v<?) J br (ν, T)= dw dt dv d ν = vmin Jbr (ν, v)f (v)dv = Plasma density (n e 2 ) = T 1/2 e h ν/kt n e n Z Z 2 g ff (ν, T) erg s 1 cm 3 Hz 1 T ~ particle energy high energy cut-off (T)

9 Bremsstrahlung impact parameter Evaluation of b max : at a given ν, we must consider only those interactions where electrons have impact parameters corresponding to v max >v and then b max v 4 ν is a function of frequency Evaluation of b min : a) classical physics requires that Δ v v Δ v aδ t = Ze2 m e b 2 2b v v b min c 2 Ze2 m e v 2 b) quantum physics and the Heisemberg principle Δ p Δ x h/2π Δ p=m e Δ v m e v h 2 π Δ x h b 2πb min q h min 2π m e v we must consider which b min has to be used

10 Bremsstrahlung classical/quantum limit which [classical/quantum] limit prevails: >1 when v 0.01c b min q h/(2 π m v) e b min c 2 Z e 2 /(m e v 2 ) = h 4 π Z e 2 c v c 137 Z v c but v = 3k T/m e then b min q 137 b min c Zc 3k m e T 1/2 Examples: HII region (T~10 4 ) Hot IGM cluster gas (T~10 8 )

11 Bremsstrahlung g ff (ν, T) = 3 π ln ( b max b min ) Gaunt factor The Gaunt Factor ranges from about 1 to about 10 in the radio domain Values around in the radio domain, slightly higher than 1 at higher energies

12 bremsstrahlung interlude: photon frequency.vs. temperature Consider an electron moving at a speed of v =1000 km s -1. In case it would radiate all its kinetic energy in a single interaction E = h = 1 2 m v 2 = kg 10 6 m s 1 2 = J the frequency of the emitted radiation is = E h = kg m 2 s kg m 2 s 1 = Hz in case this electron belongs to a population of particles at thermal equilibrium for which v = 1000 km s 1 is the typical speed, determine which is the temperature of the plasma T = E k = kg m 2 s kg m 2 s 2 K 1 = K k = m 2 kg s -2 K -1 h = m 2 kg s -1 m e = kg

13 bremsstrahlung interlude (2): the cut-off frequency The cut-off frequency in the thermal bremsstrahlung spectrum depends on the temperature only, and is conventionally set when the exponential equals 1/e. h kt = 1 = k h T = kg m 2 s 2 K kg m 2 s 1 T = T Hz It is interesting to determine the cut-off frequency for a warm plasma (HII region, T ~ 10 4 K) and for a hot plasma (IGM in clusters of galaxies, T ~ 10 8 K). In observations, the measure of the cut-off frequency is a way to determine the plasma temperature

14 Thermal bremsstrahlung total emissivity(2) Total emissivity does not depend on frequency (except the cut off) and the spectrum is flat J br (ν, T)= T 1/2 e h ν/kt n e n Z Z 2 g ff (ν, T) erg s 1 cm 3 Hz 1 Total emitted energy per unit volume: must integrate over velocity (energy) distribution(0 v<?) at a given frequency v the velocity of the particle v must be as such 1 2 m v2 h ν v min 2h ν m e and integrating over the whole spectrum: J br (T) = dw dt dv T 1/2 n e n Z Z 2 g ff (T) erg s 1 cm 3 g ff (T) average gaunt factor

15 Thermal bremsstrahlung cooling time Let's consider the total thermal energy of a plasma and the radiative losses: the cooling time is t br = E tot th J br T = 3/2 n e n p kt J br T we can assume 3n e kt = T 1/2 sec = T 1/ T 1/2 n 2 e g ff n e g ff n e g ff n e = n p yr N.B. The average Gaunt Factor is about a factor of a few Astrophysical examples: HII regions: Galaxy clusters n e ~ cm 3 ; T ~ 10 4 K n e ~ 10 3 cm 3 ; T ~ K (thermal) bremsstrahlung is the main cooling process at temperatures above T~ 10 7 K N.B.-2- all galaxy clusters have bremsstrahlung emission

16 bremsstrahlung practical applications(1) A galactic HII region: M42 (Orion nebula) distance : 1,6 kly ~ 0.5 kpc angular size ~ 66 ' ~ 1.1 courtesy of NRAO/AUI h cutoff t br compute: ~ kt = erg K erg /ev T = T ev = n e g ff T 1/2 j br T = T 1/2 n e 2 g ff erg s 1 cm 3 yr L br = j br V erg s 1

17 bremsstrahlung practical applications(2) A cluster of galaxies: Coma (z=0.0232), size ~ 1 Mpc Typical values of IC hot gas have radiative cooling time exceeding 10 Gyr all galaxy clusters are X- ray emitters

18 bremsstrahlung practical applications(3) Cluster of galaxies: a laboratory for plasma physics

19 bremsstrahlung self-absorption (1) Thermal free-free absorption: energy gained by a free moving electron. At thermal equilibrium (Kirchhoff 's law) S(ν) = j(ν, T) μ(ν, T) 3 ν 1 = B(ν, T) = 2h c 2 e h ν/kt 1 j(ν, T) = μ(ν, T)B(ν, T) [Kirchhoff ' slaw] for an isotropically emitting plasma cloud via thermal bremsstrahlung dw dt dv d ν = 4 π J (ν, T) br and therefore we can get μ br (ν, T) = J br (ν, T) 4 π B(ν, T) = = 1 4 π T 1/2 n e n Z Z 2 g ff (ν, T) c2 μ br (ν, T) T 1/2 ν 3 (1 e hν/kt ) strong dependency on frequency 2h ν 3 (1 e hν/kt )

20 bremsstrahlung self-absorption (2) at high frequencies μ(η, T) is negligible(ν 3 and e hv/kt ) at low frequencies (h ν kt) μ br (ν, T) T 1/2 ν 3 (1 e hν/kt ) 3( ν T 1/2 ν 1 1+h μ br (ν, T) = 4 e6 3m e hc ( 2π 3k m e kt ) = T 3/2 ν 2 )1/2 n e n Z Z 2 T 3/2 ν 2 g ff (T) g ff (T) 10 therefore μ br (ν, T) T 3/2 ν 2 n e n Z Z 2 g ff (T) 0.2 T 3/2 ν 2 n e n Z Z 2

21 bremsstrahlung brightness/spectrum (1) The total brightness from a free-free emitting cloud: let's consider emission + absorption B(ν, T) = 1 j br (ν, T) 4 π μ br (ν, T) (1 e τ (ν, T) )=B BB (ν, T)(1 e τ (ν, T) ) Therefore: according to radiative transfer B(ν, T) T 1/2 n e n Z Z 2 g ff (ν, T) T 3/2 ν 2 n e n Z Z 2 g ff (ν, T) (1 e τ(ν, T) ) T ν 2 (1 e τ(ν, T) ) and the opacity [ τ(ν, T) = μ(ν, T)l ; where l = mean free path] determines the spectral shape and the regime: 1. optically thick τ(ν, T) 1 2. optically thin τ(ν, T) 1

22 bremsstrahlung (2) T B ~ T e t brightness/spectrum T B ~ T e τ 1 (1 e τ ) = ( 1 1 e τ ) 1 B(ν, T) T ν2 τ 1 (1 e τ ) = (1 1+τ o(τ 2 )) τ = μ l o B(ν, T) T ν 2 τ μ(ν) = J (ν) [br] T ν where B inr J approximation 4 π B BB (ν, T) BB 2 (2 h ν 3 /c 2 ) (kt/h ν) T 1/2

23 bremsstrahlung real X-ray spectra (3)

24 bremsstrahlung evaluatingb min,b max b max v frequency dependent!...velocity dependent! 4 ν Thermal distribution range of velocities 3 Order of magnitude 2 kt = 1 2 m v2 v 3kT = kg m 2 s kg o K 1 T = T m/s For a minimum temperature of 10 4 K we get a typical velocity of 680km/ s. At Hz we get:b max v 4 ν m/ s s m b min(c) 2Ze2 m e v 2...

25 Relativistic bremsstrahlung minor remark: the Gaunt Factor is slightly different wrt the case of ''thermal br.'' The emissivity for relativistic bremsstrahlung as a function of v for a given velocity v is: j rel,br (v, ν) = 32 π e m 2 e c 3 v n n e Z Z2 ln ( 183 ) Z 1/3 N.B. T is not a relevant concept any more, v must be used The typical velocities of the particles are now relativistic and the energy (velocity) distribution is described by a power law: n e (E) n e,o E δ Integrating over velocities (energies) j rel,br (ν) ne h ν (E)n Z Z 2 de h ν the emitted spectrum is a power law! E δ de E δ+1 1 δ ν δ+1

26 bremsstrahlung the sun solar wind: n e ~ 4 7 cm -3 v swind ~ km s -1 T ~ K (not consistent with v!) flares: relativistic particles with power law energy distribution

27 Final remarks on bremsstrahlung emission: a photon is emitted when a moving electron deeply penetrates the Coulomb field of positive ion and then is decelerated the energy of the photon can never exceed the kinetic energy of the electron generally the ''classical'' description of the phenomenon is adequate, the quantum mechanics requires an appropriate Gaunt Factor only the individual interaction produces polarized radiation (fixed direction of E and B fields of the e-m wave wrt the acceleration) all the interactions produced in a hot plasma originate unpolarized emission (random orientation of the plane of interaction) we have either ''thermal'' or ''relativistic'' bremsstrahlung, depending on the population (velocity/energy distribution) of the electrons.