Synchrotron Radiation I

Transcription

1 Synchrotron Radiation I Examples of synchrotron emitting plasma Following are some examples of astrophysical objects that are emitting synchrotron radiation. These include radio galaxies, quasars and supernova remnants. High Energy Astrophysics: Synchrotron Radiation I /06

2 A 20 cm radio image from the VLA of the radio galaxy IC See Killeen, Bicknell & Ekers, ApJ 25, 80. This images shows the jets and lobes of the radio emission corresponding to a relatively nearby giant elliptical galaxy. High Energy Astrophysics: Synchrotron Radiation I 2/06

3 The inner part of the radio galaxy M87 from a 2cm VLA image by Biretta, Zhou & Owen. High Energy Astrophysics: Synchrotron Radiation I /06

4 VLA 4.9 GHz image of the z.2 quasar C204. The linear size is 59 h kpc Note the periodic knotty structure in jets that may be the result of internal shocks and the very bright core. See High Energy Astrophysics: Synchrotron Radiation I 4/06

5 The Supernova Remnant Cassiopeia A Cassiopeia A gets its name from radio astronomers, who rediscovered it in 948 as the strongest radio source in the constellation of Cassiopeia. About 5 years later optical astronomers found the faint wisps, and it was determined that Cas A is the remnant of an explosion that occurred about 00 years ago. The radio emission is synchrotron emission. High Energy Astrophysics: Synchrotron Radiation I 5/06

6 The X-ray image of the Cassiopeia A supernova remnant on the left was obtained by the Chandra X-ray Observatory using the Advanced CCD Imaging Spectrometer (ACIS). Two shock waves are visible: a fast outer shock and a slower inner shock. The inner wave is believed to be due to the collision of the ejecta from the supernova explosion with a shell of material, heating it to a temperature of ten million degrees. The outer shock wave is a blast wave resulting from the explosion. High Energy Astrophysics: Synchrotron Radiation I 6/06

7 2 What can we learn from synchrotron emission from an astrophysical object? Estimates of total radiative luminosity Total minimum energy (particles plus field) in the radio emitting particles Direction of the magnetic field Constraints on energy density of emitting particles and estimates of magnetic field Total amount of matter converted into energy (relevant for radio galaxies and quasars) Accretion rate onto central black hole (radio galaxies and quasars) Constraints on the mass of the black hole (radio galaxies and quasars) Information relating to source dynamics, e.g. strengths of shock High Energy Astrophysics: Synchrotron Radiation I 7/06

8 waves; velocities of jets Background theory. Electric field We have the expression for the Fourier component of electric field of the pulse from a moving electron derived from the Lienard- Weichert potentials: re iq e ir c 4c 0 n n exp it n Xt dt c () High Energy Astrophysics: Synchrotron Radiation I 8/06

9 The Fourier components of a transverse radiation field can be expressed in terms of unit vectors e and e 2 perpendicular to the wave direction such that E E e + E 2 e 2 (2) In any particular application we choose e and e 2 so as to simplify the expression for the resultant electric field and Stokes parameters, defined by: High Energy Astrophysics: Synchrotron Radiation I 9/06

10 c 0 I E T E * + E 2 E * 2 c 0 Q E T E * E 2 c 0 U E T * E 2 + E E * 2 E * 2 () V -- c E i T * E 2 E E * 2 High Energy Astrophysics: Synchrotron Radiation I 0/06

11 Interpretation of T Another way to interpret the parameter T which is appropriate when considering radiation from a distribution of particles is: or In the latter case each pulse originates from a different electron in the distribution. Nomenclature T time between pulses T No of pulses per unit time We refer to the e and e 2 components of the electric field as the two modes of polarisation. High Energy Astrophysics: Synchrotron Radiation I /06 (4) (5)

12 .2 Emissivities The basic relation is: dw dddt c r T 2 E E * (6) High Energy Astrophysics: Synchrotron Radiation I 2/06

13 In terms of IQUV j I dw dddt dw dddt j Q j U dw dddt dw dddt dw 2 j V i dddt dw dddt dw* dddt dw* dddt (7) High Energy Astrophysics: Synchrotron Radiation I /06

14 . Helical motion of a relativistic particle For synchrotron emission, we evaluate the above integrals using the expression for helical motion: B v B Pitch angle High Energy Astrophysics: Synchrotron Radiation I 4/06

15 sin cos B t + 0 v c sinsin B t+ 0 (8) cos B 0 qb Gyrofrequency m qb m Non-relativistic gyrofrequency q sign of charge q pitch angle 0 The derivation of the equations for the helical trajectory of a charged particle in a magnetic field are given in the background notes. High Energy Astrophysics: Synchrotron Radiation I 5/06 (9)

16 Usually we are concerned with electrons ( ) so that: v c sincos B t+ 0 sinsin B t + 0 cos (0) Note that the motion of the velocity vector is anti-clockwise for an electron. High Energy Astrophysics: Synchrotron Radiation I 6/06

17 Coordinates of particle Integrating the velocity gives for the position vector: sin sin B t+ 0 B Xt c sin cos B t+ 0 B + X 0 () tcos High Energy Astrophysics: Synchrotron Radiation I 7/06

18 This consists of a linear motion in the z-direction superimposed upon a circular motion in the x y plane. The radius of the circular motion is the gyroradius c r sin G B mcsin qb (2) High Energy Astrophysics: Synchrotron Radiation I 8/06

19 .4 Approximations for ultrarelativistic particles» Sometimes is appropriate. 2 2 Most of the radiation beamed into a cone of half-angle. This means that we can expand functions in terms of small powers of the angle between the particle velocity and the direction of emission. We set up the calculation in the following way (following Rybicki & Lightman): High Energy Astrophysics: Synchrotron Radiation I 9/06

20 B e z To observer n vt a e 2 Normal to trajectory v y e is defined as the instant where the partix Tangent to particle motion at t 0. The origin of time t 0 High Energy Astrophysics: Synchrotron Radiation I 20/06

21 cle s velocity comes closest to the direction of the observer. The x axis represents the instantaneous direction of motion at t 0. The y axis represents the direction of the centre of curvature. The z axis completes the system of coordinates (i.e. e e e 2 ). n is the direction of the observer and makes an angle to the direction of the electron at t 0. We take small. Since t 0 represents the instant at which the angle between the velocity and the observer s direction is a minimum, the component of n in the direction of the normal, m is zero. The parameter a is the instantaneous radius of curvature of the particle High Energy Astrophysics: Synchrotron Radiation I 2/06

22 Formally, the pulse of radiation reaching the observer originates from the entire trajectory of the particle. However, most of this radiation originates from a very small region of the particle s orbit near the origin of the above coordinate system..5 Radius of curvature Summary of the theory of dimensional curves: x x t Velocity vector v Unit tangent vector Arc length: t dx dt v - v ds ---- v dt () High Energy Astrophysics: Synchrotron Radiation I 22/06

23 The curvature ( ), radius of curvature ( a) and curvature normal ( m) are given by: dt ---- m ds --m a dt ---- ds dt dt dtds Using the previous expression for velocity: -- dt ---- v dt (4) v c sincos B t sinsin B t cos v t - sincos v B t sinsin B t cos (5) High Energy Astrophysics: Synchrotron Radiation I 2/06

24 Since the particle passes through the origin at vector is: t 0 the position x csin sin B t B csin cos B t ctcos B (6) r G sin B t r G cos B t ctcos Remember that the gyroradius c r sin G B (7) High Energy Astrophysics: Synchrotron Radiation I 24/06

25 The curvature is determined from: -- dt ---- m v dt This implies B sin sin c B t cos B t 0 (8) m sin B t cos B t 0 B sin a c c sin (9) NB. The radius of curvature is not the same as the radius of the projection of the orbit onto the plane perpendicular to B. As one expects, a when 2. r G B r G sin 2 High Energy Astrophysics: Synchrotron Radiation I 25/06

26 .6 The magnetic field in the particle-based system of coordinates Since the normal vector m sin B t cos B t 0 is perpendicular to the magnetic field, then the magnetic field lies in the plane of e and e. i.e. in the same plane as n. Also since the velocity makes an angle of with the magnetic field, the direction of B is as shown in the diagram. High Energy Astrophysics: Synchrotron Radiation I 26/06

27 4 Semi-quantitative treatment of synchrotron emission 4. Small angle approximation Recall the expression for the electric field of a radiating charge: E rad q c 0 r n n n n (20) For a relativistically moving particle, the electric field is highly peaked in the direction of motion, i.e. when High Energy Astrophysics: Synchrotron Radiation I 27/06

28 n is close to zero. Since is the angle between and n n cos High Energy Astrophysics: Synchrotron Radiation I 28/06 (2) So n is a minimum at 0 and increases rapidly for, i.e. for. Since», then the angles we are dealing with are very small. The expression for n introduces the variable / This variable occurs frequently in the following theory. (22)

29 4.2 Estimate of critical frequency 2 Centre of curvature P P 2 Observer High Energy Astrophysics: Synchrotron Radiation I 29/06

30 The angle turned through and the arc length of the particle orbit are related by: where a is the previously determined radius of curvature. From the above geometry d ds -- a (2) 2 -- s a 2 a -- Time taken for trajectory to sweep through relevant angle t 2 t s ---- c 2a c (24) (25) High Energy Astrophysics: Synchrotron Radiation I 0/06

31 This is not the period for which the pulse is observed P P 2 s D distance to observer Front of pulse emitted from here at t Back of pulse emitted from here at t 2 Partticle moves distance s in time t 2 t. High Energy Astrophysics: Synchrotron Radiation I /06

32 Times of arrival of front and back of pulse: D + s D t t t c 2 t c s v t 2 t t 2 t ---- t c 2 t - t c 2 t v - c t2 t (26) i.e. the time between pulses is reduced because the trailing part of the pulse has less distance to travel. This is one example of the effect of time retardation. High Energy Astrophysics: Synchrotron Radiation I 2/06

33 The factor Since then v - can be expanded in powers of as follows: c (27) (28) (29) High Energy Astrophysics: Synchrotron Radiation I /06

34 The observed duration of the pulse is therefore t 2 t t 2 t a c c sin B B sin c (0) Fourier theory tells us that if the length of a pulse is t then the typical frequencies present in such a pulse are t. Hence the typical frequencies expected in a synchrotron pulse are given by: B sin qb m sin qb m 2 sin () High Energy Astrophysics: Synchrotron Radiation I 4/06

35 The critical frequency resulting from the exact synchrotron theory (to follow) is close to this estimate: c -- 2 B sin qb 2 m sin qb 2 m 2 sin (2) The critical frequency which characterises synchrotron emission is a factor of 2 qb higher than the cyclotron frequency and a factor of m higher than the gyrofrequency at which particles gyrate around the magnetic field lines. High Energy Astrophysics: Synchrotron Radiation I 5/06

36 4. Typical example - relativistic electron in a typical interstellar magnetic field B 0 5 G nt 0 4 eb 2 B Hz m e c -- eb m 2 0 sin.0 sinhz csin 0 r G sin m B () Note that the critical frequency in this example is about microwave radio frequency. 0 GHz a High Energy Astrophysics: Synchrotron Radiation I 6/06

37 4.4 Relation between frequency and Lorentz factor 2 -- eb m 2 sin m ē -- 2 / Bsin 2 / 2 / Bsin 2 / / nt GHz Bsin 2 / / 0G GHz (4) High Energy Astrophysics: Synchrotron Radiation I 7/06

38 5 Detailed development of synchrotron emission Angle turned through in time t: -- d v dt d ds -- a -- a vt a (5) High Energy Astrophysics: Synchrotron Radiation I 8/06

39 In the new set of axes, the motion is circular with no component in the z direction. The velocity in these axes is therefore: v c vt vt cos----- sin a a i.e. vt vt cos----- sin a a (6) High Energy Astrophysics: Synchrotron Radiation I 9/06

40 5. Axes based upon instantaneous orbital plane B e z e To observer n e x v vt a y Centre of motion Normal to trajectory e 2 e Tangent to particle motion at t 0. High Energy Astrophysics: Synchrotron Radiation I 40/06

41 Rotate axes by angle about e 2 - gives new unit vectors e e n 5.2 Significance of and e e The axes and are respectively parallel and perpendicular to the e e projection of the magnetic field on the plane perpendicular to the line of sight given by the vector n.this is the plane of propagation of the radiation. High Energy Astrophysics: Synchrotron Radiation I 4/06

42 5. Integral to evaluate We have our expression for the Fourier component of the electric field of a pulse: re iq e ir c 4c 0 n n exp it n Xt dt c (7) 5.4 Determination of n n We first calculate a simple expression for the quantity n n. High Energy Astrophysics: Synchrotron Radiation I 42/06

43 Use: n cose + sine vt vt cos e a + sin----- e a 2 (8) This gives: n vt cos----- n e a + vt sin----- n e2 a vt cos----- sine a + vt sin a e (9) vt n n cos a vt sin e sin a e High Energy Astrophysics: Synchrotron Radiation I 4/06

44 We make the following approximations: vt cos----- a vt sin a vt a (40) so that n n e vt ct e e a e a (4) Note: In order to determine terms that are relevant to the small amount of circular polarisation that emerges from a synchrotron source, we would have to expand to the next order in. High Energy Astrophysics: Synchrotron Radiation I 44/06

45 The term i t n Xt c This term in the argument of the exponential involves Xt. Since dx v c vt dt cos----- a and X0 0 sin vt a (42) High Energy Astrophysics: Synchrotron Radiation I 45/06

46 then Xt ca vt sin v vt cos a a (4) a sin vt vt cos a a giving t n Xt t c cos 0 sin t a -- cos vt sin----- c a a -- sin vt c a vt cos a (44) High Energy Astrophysics: Synchrotron Radiation I 46/06

47 n Xt 5.5 Expansion of t in small powers of and t: c then t cos vt sin a vt a -- v t a a vt -- cos c sin a t v - -- v c 2c - t2 -- v t a 2 c (45) (46) High Energy Astrophysics: Synchrotron Radiation I 47/06

48 v In the first term, we use and put c 2 2 v c in the remainder, giving: t a -- cos c t The characteristic angle vt sin a 2 2 t n Xt c t t c2 t a 2 -- c2 2 t a 2 enters into the expression. (47) High Energy Astrophysics: Synchrotron Radiation I 48/06

49 Summary so far i t n n e vt ct e e a -----e a n Xt c i t c t 2 a 2 (48) New variables We put these equations into a dimensionless form by defining new variables High Energy Astrophysics: Synchrotron Radiation I 49/06

50 y c t a a c The purpose of introducing these variables is to make quantities in the integrand of the expression for the Fourier transform of order unity. In so doing it helps to represent the new dimensionless variables in a way that makes their physical significance apparent. (49) High Energy Astrophysics: Synchrotron Radiation I 50/06

51 Significance of y The quantity a c is the time taken for the particle s velocity to swing through an angle. Hence y Note: t Characteristic time (50) y is a dimensionless variable One expects the major contribution to come from the region of the integrand wherein y. Integration over t will be replaced by integration over y. High Energy Astrophysics: Synchrotron Radiation I 5/06

52 Significance of a c c a a -- c -- c c sin B sin B (5) B sin c Since the radiation is concentrated at the critical frequency, the dominant contribution to the result will be around. High Energy Astrophysics: Synchrotron Radiation I 52/06

53 Rearrangement of integrand For n n : ct c n n e -----e a e -- a ye a c e ---- ye (52) High Energy Astrophysics: Synchrotron Radiation I 5/06

54 For i t n Xt c i t n Xt c i t c t 2 a 2 i a y c + c a 2 a c y (5) i a y c + -- a y c High Energy Astrophysics: Synchrotron Radiation I 54/06

55 i t n Xt c ia c y + --y i 2 -- a c y + --y (54) --i y y For t variable of integration: t a y dt c a dy c (55) High Energy Astrophysics: Synchrotron Radiation I 55/06

56 6 Evaluation of Fourier components of electric field re iq e ir c 4c 0 n n n Xt exp it dt c nn e ---- ye i t n Xt c --i y y dt a dy c High Energy Astrophysics: Synchrotron Radiation I 56/06

57 Therefore we have a parallel component and a perpendicular component of the electric field: re re iq a e ir c 4c 0 c iq a c 0 c 2 e ir c exp 2 yexp --i y 2 --i y + --y + --y dy dy (56) High Energy Astrophysics: Synchrotron Radiation I 57/06

58 The integrals over y have real and imaginary parts: exp Parallel component In the integral for 2 --i y (57) the real part only contributes since the imaginary part is odd and the different contributions over 0 cancel. Therefore, we evaluate: + --y re + cos isin 2 -- y 2 -- y + --y + --y 0 and High Energy Astrophysics: Synchrotron Radiation I 58/06

59 exp 2 --i y + --y dy cos 2 -- y + --y dy Perpendicular component In the integral for re the imaginary part only contributes since the exponential term is multiplied by y. We therefore evaluate: y 2 --i y + --y exp y d i ysin 2 -- y + --y dy (58) High Energy Astrophysics: Synchrotron Radiation I 59/06

60 These integrals are expressed in terms of Bessel functions: cos ysin 2 -- y 2 -- y + --y + --y (59) where K and K 2 are modified Bessel functions of order and 2 respectively. These integrals are evaluated in the Appendix. dy K K 2 High Energy Astrophysics: Synchrotron Radiation I 60/06

61 Final expressions re re iq a ir e c 2 c c 0 q a 2 ir c 0 c 2 e c K K2 (60) High Energy Astrophysics: Synchrotron Radiation I 6/06

62 7 Ellipticity of single electron emission The waves corresponding to the above Fourier components are, for the parallel component: E r t E E 2 + E e it + E * e it e it e it (using the reality condition of the Fourier transform). (6) High Energy Astrophysics: Synchrotron Radiation I 62/06

63 and for the perpendicular component E r t E E 2 + E e it + E * e it e it e it (62) High Energy Astrophysics: Synchrotron Radiation I 6/06

64 Using the expressions for the Fourier components: E r t q a K c 0 r c ie i r c - t + ie i r c - t (6) q a K c 0 r c sin t r - c High Energy Astrophysics: Synchrotron Radiation I 64/06

65 E rt q a c 0 rc 2 K2 e i r c - t + e i r c - t (64) q a c 0 rc 2 K2 cos t r - c High Energy Astrophysics: Synchrotron Radiation I 65/06

66 Now consider the path traced out with time by the fixed r. Write E r t a sin t E r t a cos t r - c r - c E vector at a (65) This describes an ellipse in the e e coordinate system. We shall see that a 0 but that a can be positive or negative. High Energy Astrophysics: Synchrotron Radiation I 66/06

67 E E a 0 a 0 E E r t - c r E rt a sin t - E c rt a cos t r - c High Energy Astrophysics: Synchrotron Radiation I 67/06

68 a q a K c 0 r c a q a c 0 r c 2 K q a c 0 rc 2 K2 (66) q a c 0 r c 2 2K 2 The semi-major and semi-minor axes of the electric vector depend upon (through the variable ) and we have to bear in mind the High Energy Astrophysics: Synchrotron Radiation I 68/06

69 fact that / c (67) The rotation of the electric vector and the shape of the polarisation ellipse depends upon The angular dependence of the radiation field can be understood from the plots of the functions f x K f x 2 K 2 (68) High Energy Astrophysics: Synchrotron Radiation I 69/06

70 where x c (69) These are plotted below for x. The variation is similar for all x. High Energy Astrophysics: Synchrotron Radiation I 70/06

71 f f f f High Energy Astrophysics: Synchrotron Radiation I 7/06

72 As can be seen both the parallel and perpendicular components of the electric field vanish rapidly for. Axes of ellipse This plot shows the variation of the ratio of a a f f for x. Since this ratio is always less than for all the parallel axis is always the minor axis and the perpendicular axis is always the major axis. High Energy Astrophysics: Synchrotron Radiation I 72/06

73 Summary of polarisation properties of single electron emission From the above: a is positive for 0 and negative for 0 Therefore the radiation is always left polarised for 0 and right polarised for 0 The ratio a a so that the major axis is always e. That is the major axis of the polarisation ellipse is perpendicular to the projection of the magnetic field on the plane of propagation. The following figure summarises the above. Note that 0 corresponds to looking exactly along the direction of the electron velocity. High Energy Astrophysics: Synchrotron Radiation I 7/06

74 Right polarised V 0 e 0 n v e B Left polarised V 0 0 v n 8 Integrated single electron emissivity This is a good example of how the integrated emission from a source can exhibit polarisation properties that are different from the individual components. A qualitative feature that one can note from the above is that the High Energy Astrophysics: Synchrotron Radiation I 74/06

75 integrated emission from a single electron will have a low circular polarisation because of the positive and negative contributions to V from negative and positive Note that U 0 because the axis of the ellipse is in the direction. e Recall our expression for the energy of the pulse corresponding to the various components of the polarisation tensor: dw dd c r 2 E E * (70) High Energy Astrophysics: Synchrotron Radiation I 75/06

76 B v n d d Illustrating the calculation of total emission from a single electron. We integrate over all directions n. High Energy Astrophysics: Synchrotron Radiation I 76/06

77 The total spectral energy of the pulse, per unit solid angle is: dw dd dw dd dw dd dw dd (7) The two terms correspond to emission into the two modes. We can use this expression to determine the total energy emitted by the electron in one gyroperiod and hence the total spectral power. We first consider the total energy per unit solid angle per unit circular frequency emitted over one entire gyration of the particle. Begin with the energy radiated per unit solid angle, at a particular point on the orbit, that we have just calculated: High Energy Astrophysics: Synchrotron Radiation I 77/06

78 dw dd c r 2 E E * dw dd 2 q a c 0 c 2 2 K2 c r 2 E E * (72) 2 q a c 0 c 2 4 K2 2 High Energy Astrophysics: Synchrotron Radiation I 78/06

79 These expressions are integrated over all solid angles associated with the directions n. Let be the polar angles for the direction n. The solid angle for emission around this direction is: (7) We can use the solid angle corresponding to a complete orbit since the spectrum of a pulse is independent of. For a complete orbit: We have d sindd d 2sind + (74) (75) High Energy Astrophysics: Synchrotron Radiation I 79/06

80 where is the angle we have used in the previous calculations. Since the contribution to the integral over solid angle mainly comes from, then we can put d d d 2sind (76) High Energy Astrophysics: Synchrotron Radiation I 80/06

81 Hence, the total energy emitted over one gyrational period into the parallel and perpendicular modes is: (77) The integral over 02 is replaced by an integral over because of the concentration of dw, dd around 0». dw dw, sin d dd 2sin d dw, d dd and the rapid vanishing of the Bessel functions for High Energy Astrophysics: Synchrotron Radiation I 8/06

82 These expressions give the amount of energy radiated per orbit. The energy radiated per unit time is given by dividing by the period of the orbit T B 2m qb -- T qb m (78) Hence, the power (i.e. energy per unit time per unit circular frequency) emitted in the parallel mode is P 2 q 2 a c 0 c sin K d qb m (79) High Energy Astrophysics: Synchrotron Radiation I 82/06

83 We change the variable of integration to, thereby introducing another factor of into the denominator. We also use, for the local radius of curvature, a c sin B a c B sin 2 (80) Various factors in the numerical factor in from of the expression for P combine to give 2 c in the denominator, where 2 9 c B 6 sin 2 (8) High Energy Astrophysics: Synchrotron Radiation I 8/06

84 All of this combines to give: P qb sin c m 0 c K d (82) The numerical factor in front of the expression for the other power P is identical and P q Bsin c m 4 K2 2 d 0 c (8) High Energy Astrophysics: Synchrotron Radiation I 84/06

85 remembering that / / c (84) and in- The two integrals involve integration over the variable volve as parameter x c (85) High Energy Astrophysics: Synchrotron Radiation I 85/06

86 These integrals were evaluated by Westfold (959) in one of the fundamental papers on synchrotron emission. The results are: K d 4 K2 2 d where the functions are defined by x x 2 Fx x 2 Fx + Gx Gx Fx x K 5 zdz Gx xk 2 x (86) (87) High Energy Astrophysics: Synchrotron Radiation I 86/06

87 Hence the components of the single electron emissivities per unit frequency are: P P qb sin Fx 0 c m qb sin Fx 0 c m Gx + Gx (88) High Energy Astrophysics: Synchrotron Radiation I 87/06

88 The total synchrotron power per unit circular frequency from a single electron is: P tot P + P x qb sin F x 0 c m c (89) High Energy Astrophysics: Synchrotron Radiation I 88/06

89 The functions Fx and Gx are plotted on linear and logarithmic scales in the following figures. The asymptotic forms are as follows: Small x Large x Fx Gx Fx Gx x / 2 / x / 2 / x -----e 2 x x -----e 2 x (90) High Energy Astrophysics: Synchrotron Radiation I 89/06

90 .0 Features F(x), G(x) F(x) G(x) Peak of Fx at x c 0.29 Emission is fairly broadband with x c High Energy Astrophysics: Synchrotron Radiation I 90/06

91 F(x), G(x) e+00 e-0 e-02 F(x) G(x) Logarithmic version of the plot of Fx and Gx, showing the power law dependence of Fx and Gx for small x. e-0 e-04 9 Synchrotron cooling e-04 e-0 e-02 e-0 e+00 e+0 x c 9. Integration of synchrotron power A fairly immediate implication of the power radiated by a single electron is the loss of energy by the electron. High Energy Astrophysics: Synchrotron Radiation I 9/06

92 Since the power per unit frequency is: P qb sin F x 0 c m (9) then the total power radiated over all frequencies is P 0 Pd qb sin c m c qb sin Fx d 0 c m 0 0 Fx dx (92) High Energy Astrophysics: Synchrotron Radiation I 92/06

93 The integral and Therefore, Fx dx x K 5 t dt dx 0 P 0 x c -- qb sin m q4 B 2 sin c 2 m (9) (94) (95) High Energy Astrophysics: Synchrotron Radiation I 9/06

94 In terms of energy ( E mc 2 ): P q4 B 2 sin E m 4 c 5 (96) The power of appearing in this expression is why synchrotron emission from protons is not usually considered. m Other expressions for total power emitted by electrons Take q e so that the total power emitted by an electron is P e4 B 2 sin c 2 m 2 (97) High Energy Astrophysics: Synchrotron Radiation I 94/06

95 This is often expressed in terms of the Thomson cross-section for an electron. The classical radius of an electron is defined by Electrostatic potential energy e m 4 0 r e c 2 0 r 0 Rest mass energy e m e c 2 (98) m High Energy Astrophysics: Synchrotron Radiation I 95/06

96 The Thomson cross-section is T r2 e m 2 e c m 2 (99) This cross-section is important, inter alia, when considering the scattering of photons by electrons. In terms of T the power emitted by an electron is: P e4 B 2 sin c 2 m 2 c T B o sin 2 2 (00) High Energy Astrophysics: Synchrotron Radiation I 96/06

97 9. Synchrotron cooling of electrons By conservation of energy, the rate of energy lost by the electron is de P c dt T B o sin 2 2 (0) Hence, the rate of change of Lorentz factor is: d dt de m e c dt T B m e c o sin 2 2 (02) High Energy Astrophysics: Synchrotron Radiation I 97/06

98 This equation can be put in the form ---- d dt 2 T B m e c o sin 2 (0) Let 0 be the value of at t 0, then assuming that B and remain constant during the time t, then T B sin m e c 2 t o (04) High Energy Astrophysics: Synchrotron Radiation I 98/06

99 This defines a synchrotron cooling time: t syn m e c B2 sin T 0 0 (05) Features decreases with increasing the higher energy electrons t syn cool the fastest decreases with increasing magnetic field. t syn High Energy Astrophysics: Synchrotron Radiation I 99/06

100 Example Take typical values for the lobes of a radio galaxy: B 0G nt (06) t syn yrs (07) Estimates of the ages of radio galaxies often exceed 0 8 years. Hence there is a need to re-energise particles in the lobes of radio galaxies. This introduces the need for particle acceleration. High Energy Astrophysics: Synchrotron Radiation I 00/06 sin

101 0 Appendix: Evaluation of synchrotron integrals The evaluation of the synchrotron integrals follows from the result (Abramowitz and Stegun, eqn ) Iax cosay + xy where 0 dy Aiz is the Airy function. In our case, x -- a 2 a / Ai a / -- 2 x (08) (09) High Energy Astrophysics: Synchrotron Radiation I 0/06

102 Differentiating, Iax with respect to x gives: so that 0 Iax y sin ay + xydy x ysinay + xydy 0 a 2 / Ai a / x a 2 / Ai a / x (0) () High Energy Astrophysics: Synchrotron Radiation I 02/06

103 Furthermore, the Airy function and its derivative are related to the modified Bessel functions of order and 2 via: K -- 2 / Aiz K z Aiz z z -- 2 / 2 See equations (0.4.26) and (0.4.) of Abramowitz and Stegun. (2) For x 2 and a 2, the argument of the Airy function and its derivative is z a / x / / 2 /. () High Energy Astrophysics: Synchrotron Radiation I 0/06

104 Hence, the argument,, of the corresponding Bessel functions is. Hence, 0 cosay + xydy a / Ai a / / Ai / K x / 2 (4) High Energy Astrophysics: Synchrotron Radiation I 04/06 / 2 / K

105 and 0 ysinay + xydy a 2 / Ai a / / Ai / K 2 / 2 x / / K 2 (5) High Energy Astrophysics: Synchrotron Radiation I 05/06

106 Finally, the integrals from to are twice the integrals from 0 to : 0 cos --y y 2 y --y 2 sin + --y 2 dy dy K K 2 (6) High Energy Astrophysics: Synchrotron Radiation I 06/06