Rybicki And Lightman: “Radiation Processes In Astrophysics .

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Bibliography Rybicki and Lightman: “Radiation Processes in Astrophysics”Longair: “High Energy Astrophysics”Rohlfs and Wilson: “Tools of Radio Astronomy”Dyson and Williams: “The Physics of the Interstellar Medium”Shu: “The Physics of Astrophysics I: Radiation”

Radiation ProcessesWe can measure the following quantities: The energy in the radiation as a function ofa) position on the skyb) frequency The radiation’s polarisation.From these measurements we can hope to determine Physical parameters of the source (e.g. temperature, composition, size) The radiation mechanism. The physical state of the matter.Need to understand the difference between often used terms: luminosity,flux, flux density, specific intensity and specific energy density.

Luminosity (L) A source’s luminosity is the power thatit emits in radiation.SI units are W.Also used are erg s-1 10-7 W Have to define a frequency rangee.g. the total (or bolometric) luminosity ofa lightbulb is 50 Wthe luminosity in the visible range isonly 1 W For a spherical blackbody sourceL 4πR2σT4E ò L dt

Flux (φφ) The flux of a source is the powerreceived per unit area.The flux from a uniform sphericalsource at distance d is given byφ L4πd 2SI units are Wm-2. e.g. Flux from a light bulb at adistance of 10 m is 0.04 Wm-2E òò φ dt dA

Flux Density (Fν) Flux density is the power received perunit area per unit frequency. Flux isthe integral of flux density with respectto frequency.ν2φ ò Fν dνν1 ν ν 2 ν 1 is the bandwidth Also called specific flux - ‘specific’refers to Hz-1. NB Astronomers often say ‘flux’ whenthey mean ‘flux density’filter SI units are Wm-2Hz-1 Also used are Jy 10-26 Wm-2Hz-1E òòòFν dν dA dt

Specific Intensity, Surface Brightness (Iν) Specific intensity (or surface brightness) isthe power received (or emitted) per unitarea per unit frequency per unit solidangle.Fν ò Iν cosθ dΩÞ Fν at surface of a spherical source π Iν Specific intensity is independent ofdistance.Also define the mean intensity Jν Jν 14πòπ4θIν dΩ An isotropic radiation field is one whereIν is independent of angle, so Jν Iν . ΩfilterUnits are Wm-2Hz-1sr-1E ò òòò Iν cos θ dν dΩ dA dt

Calculating Fν and Iν for a blackbody The specific intensity of a black-body is given by the Planck function, Bν2hν 31æ FνIν Bν 2ç c (exp hkTν 1) è Ωöfor a uniform brightness source øA derivation of Bν is given in the appendix. In the Rayleigh-Jeans region where hν kTIν Bν 2kTλ2e.g. A black-body source of angular radius 1 arcsec and temperature 2700 K isobserved by a telescope with a beam FWHM 1 arcmin at 1GHz.I ν (source) 3.02 x 10-17 Wm-2Hz-1sr-1. (We are in the Rayleigh-Jeans region)Fν (source) Iν (source) x Ω(source) 2.2 x 10-27 Wm-2Hz-1 0.22 Jy.(Ω(source) π(1/3600 x π/180)2 )

Determining Ω In general the integration for calculating the received flux density from thespecific intensity usingFν ò Iν cosθ dΩ is not trivial since it must take into account the telescope’s response function,also called the beam.If the source is much smaller than the telescope’s beam then this makes anegligible difference and Fν Iν Ω(source) (as in the previous example).Another simple case is if the source has constant specific intensity inside thebeam; the integration then gives Iν Ω(beam) where Ω(beam) is the effective beamsolid angle. The effective beam solid angle for a Gaussian beam of FWHM2πθFWHM is Ω (beam) θ FWHM4 ln 2Extending the previous example: the telescope also picks up radiation from the2.7 K Cosmic Microwave Background .Iν (CMB) 3.02 x 10-20 Wm-2Hz-1sr-1. (We are in the Rayleigh-Jeans region)Fν(CMB) Iν (CMB) x Ω(beam) 2.9 x 10-27 Wm-2Hz-1 0.29 Jy.(Ω(beam) (1/60 x π/180)2 x 1.13)

Brightness Temperature (TB) Radio astronomers often use the concept of brightness temperaturefor sources that do not have a black-body spectrum. It is the temperaturethat a black-body source would have in order to be the same brightness.In general the brightness temperature is a function of observingfrequency, and does not correspond to an actual physical temperature.Iν λ2TB (ν ) 2ke.g. At 350 GHz the atmosphere emits with a surface brightness of 2x10-15 Wm-2Hz-1sr-1.Therefore it has a brightness temperature of 52 K.At 30 GHz the surface brightness of the atmosphere is 2x10-18 Wm-2Hz-1sr-1. Thiscorresponds to a brightness temperature of 7 K.

Radiation Energy Density (U) uν(Ω) is the specific energy density i.e. the energy in the radiationfield per unit frequency per unit volume per unit solid angle. Thevolume of field incident on the target is c cosθ dA dt. ThereforedE ò òòò uν (Ω)c cos θ dν dΩ dA dtÞ uν (Ω) Iνcuν is uν(Ω) integrated over all angles; it has units Jm-3Hz-1 U is uν integrated over all frequencies i.e. the total energydensity in EM fields; it has units Jm-3. So for an isotropic fielduν ò uν (Ω) dΩ U ò uν dν 4πc4πJνcò J ν dν

Radiation Pressure (P) For photons of mtm p, E pc. Since p p cosθ, then themomentum per unit area per unit time per unit frequency, pν ofa radiation field ispν 1c2Icosθ dΩvò For isotropic radiation (i.e. Jν Iν) perfectly reflected by awall (so the total momentum change is twice the incidentmomentum), and remembering P dF/dA, F dp/dt thenP òòν πI cos2 θ dΩ dν2c ν2P 13 U(dΩ 2π sinθ dθ )

Longair: “High Energy Astrophysics” Rohlfs and Wilson: “Tools of Radio Astronomy” Dyson and Williams: “The Physics of the Interstellar Medium” Shu: “The Physics of Astrophysics I: Radiation” Radiation Processes We can measure the following quantities: The energy in the radiation as a function of a) position on the sky b) frequency The radiation’s .

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