Presentation on theme: "1 射电天文基础 姜碧沩北京师范大学天文系 2009/08/24-28 日，贵州大学. 2009/08/24-28 日射电天文暑期学校 2 Emission Mechanisms of Continuous Radiation The Nature of Radio Sources Radiation."— Presentation transcript:
2009/08/24-28 日射电天文暑期学校 2 Emission Mechanisms of Continuous Radiation The Nature of Radio Sources Radiation from an Accelerated Electron The Frequency Distribution of Bremsstrahlung for an Individual Encounter The Radiation of an Ionized Gas Cloud Nonthermal Radiation Mechanisms Review of the Lorentz Transformation The Synchrotron Radiation of a Single Electron The Spectrum and Polarization of Synchrotron Radiation The Spectral Distribution of Synchrotron Radiation Energy Requirements of Synchrotron Sources Low-Energy Cutoffs in Nonthermal Sources Inverse Compton Scattering
2009/08/24-28 日射电天文暑期学校 3 The Nature of Radio Sources Two large families –Locations: galactic and extragalactic –SED: The nature of discrete sources was investigated by measurements at different frequencies to determine the spectral characteristics Roughly constant flux density with increasing frequency More intense at lower frequency –Emission mechanisms Thermal Nonthermal
2009/08/24-28 日射电天文暑期学校 13 Nonthermal Radiation Mechanisms Relativistic electrons moving in intricately “tangled” magnetic fields of extended coronas believed to surround certain kinds of stars Radiation from relativistic cosmic ray electrons that move in the general interstellar magnetic field
2009/08/24-28 日射电天文暑期学校 14 Review of the Lorentz Transformation
2009/08/24-28 日射电天文暑期学校 18 The Synchrotron Radiation of a Single Electron
2009/08/24-28 日射电天文暑期学校 19 The Total Power Radiated
2009/08/24-28 日射电天文暑期学校 20 The Angular Distribution of Radiation
2009/08/24-28 日射电天文暑期学校 21 The Frequency Distribution of the Emission
2009/08/24-28 日射电天文暑期学校 22 The Spectrum and Polarization of Synchrotron Radiation The instantaneous radiation is in general elliptically polarized, but since the position angle of the polarization ellipse is rotating with the electron, the time averaged polarization is linear. This is true also for the radiation emitted by an ensemble of monoenergetic electrons moving in parallel orbits.
2009/08/24-28 日射电天文暑期学校 24 The Spectral Distribution of Synchrotron Radiation from an Ensemble of Electrons
2009/08/24-28 日射电天文暑期学校 25 Homogeneous Magnetic Field
2009/08/24-28 日射电天文暑期学校 26 Random Magnetic Field
2009/08/24-28 日射电天文暑期学校 27 Energy Requirements of Synchrotron Sources
2009/08/24-28 日射电天文暑期学校 28 Low-Energy Cut-offs in Nonthermal Sources Synchrotron radiation at frequencies below the low-frequency cutoff ν 1 should have a spectral index of n=1/3 In synchrotron radiation fields spontaneous photon emission will be accompanies by absorption and stimulated emission as in any other radiation fields. This absorption can become important in compact, high-intensity radio sources at low frequencies when the optical depth becomes large. The Razin effect Foreground thermal plasma may absorb may synchrotron emission at lower frequencies
2009/08/24-28 日射电天文暑期学校 29 Inverse Compton Scattering Compton Scattering –An X-ray or gamma-ray photon collides with a particle, usually an electron. Some of the photon’s energy is transferred to the particle and the photon is reradiated at a longer wavelength Inverse Compton Scattering –A low-energy photon collides with a fast-moving electron. The electron passes on a small proportion of its energy to the photon, the photon’s wavelength decreases. The electron has to suffer a large number of collisions before it loses an appreciable fraction of its energy
2009/08/24-28 日射电天文暑期学校 30 The Sunyaev-Zeldovich Effect Photons from a cold source, the 2.7K background, interact with a hot foreground source, a cluster of galaxies. Such clusters have free electrons with T k >10 7 K, so the bremsstrahlung radiation peaks in the X-ray range. The net effect of an interaction of the photons and electrons is to shift longer wavelength photons to shorter wavelength
2009/08/24-28 日射电天文暑期学校 31 Energy Loss from High-Brightness Sources
2009/08/24-28 日射电天文暑期学校 32 Exercise The Orion hot core is a molecular source with an average temperature of 160K, angular size 10", located 500pc from the Sun. The average local density of H 2 is 10 7 cm -3. –Calculate the line-of-sight depth of this region in pc, if this is taken to be the diameter –Calculate the column density N(H 2 ) which is the integral of density along the line-of-sight. Assume that the region is uniform –Obtain the flux density at 1.3mm using T dust =160K, the parameter b=1.9 and solar metallicity in equation (9.7) –Use the Rayleigh-Jeans relation to obtain the dust continuum main beam brightness temperature from this flux density in a 10" beam. Show that this is much smaller than T dust. –At long millimeter wavelengths, a number of observations have shown that the optical depth of such radiation is small. Then the observed temperature is T=T dust τ dust, where the quantities on the right hand side of this equation are the dust temperature and dust optical depth. From this relation determine τ dust. –At what wavelength is τ dust =1 if τ dust ~λ -4 ?
2009/08/24-28 日射电天文暑期学校 33 Exercise From Fig. 9.1, determine the ‘turnover’ frequency of the Orion A HII region, that is the frequency at which the flux density stops rising and starts to decrease. This can be obtained by noting the frequency at which the linear extrapolation of the high and low frequency parts of the plot of flux density versus frequency meet. At this point, the optical depth τ ff of free-free emission through the center of Orion A is unity, that is τ ff =1, call this frequency ν 0. From equation (9.36) in ‘Tools’, the relation of turnover frequency, electron temperature T e and emission measure EM=N e 2 is ν 0 =0.3045(T e ) -0.643 (EM) 0.476. This relation applies to a uniform density, uniform temperature region, actual HII regions have gradients in both quantities, so this relation is at best only a first approximation. Determine EM for an electron temperature T e =8300K The FWHP size of Orion A is 2.5’, and Orion A is 500pc from the Sun. What is the linear diameter for the FWHP size? Combine the FWHP size and emission measure to obtain the RMS electron density.
2009/08/24-28 日射电天文暑期学校 34 Exercise The source Cas A is a cloud of ionized gas associated with the remnant of a star which exploded about 330 years ago. The radio emission has the relation of flux density as a function of frequency shown in Fig. 9.1 in ‘Tools’. For the sake of simplicity, assume that the source has a constant temperature and density, in the shape of a ring, which thickness 1’ and outer radius of angular size 5.5’. What is the actual brightness temperature at 100MHz, 1GHz, 10GHz, 100GHz?