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Radio Astronomy Sp.-V/AQuan/1999/10/07:17:14 Page 121 Chapter 6 Radio Astronomy Robert M. Hjellming 6.1 Introduction ......................... 121 6.2 Atmospheric Window and Sky Brightness ....... 123 6.3 Radio Wave Propagation ................. 125 6.4 Radio Telescopes and Arrays .............. 128 6.5 Radio Emission and Absorption Processes ...... 131 6.6 Radio Astronomy References .............. 140 6.1 INTRODUCTION Radio astronomy is defined by three things. First is the range of frequencies that constitute the radio windows of the Earth’s atmosphere, ranging roughly from 20 MHz to 1000 GHz. Second are the astronomical objects that emit radio waves with sufficient strength to be detectable at the Earth; some emit by thermal processes, but most are seen because of the emission from relativistic electrons (cosmic rays) interacting with local magnetic fields. Third, radio radiation behaves more like waves than particles (photons), allowing the measurement of both amplitude and phase of the radiation field. The capability to measure phase gives radio interferometry the capability to do the highest resolution imaging of astronomical objects currently possible in astronomy. The variety and number of radio sources is so large that in Table 6.1 we merely summarize a number of source catalogs devoted to these topics. 121 Sp.-V/AQuan/1999/10/07:17:14 Page 122 122 / 6 RADIO ASTRONOMY Table 6.1. List of radio source catalogs. Type of Source Contents ◦ ◦ 4C Radio Survey [1, 2] 4844 56 cm sources > 2Jy,−7 ≤ δ ≤ 80 ◦ ◦ Bologna B2 Sky Survey [3–5] 408 MHz, 21.7 ≤ δ ≤ 40 ◦ ◦ Bologna B3 Sky Survey [6, 7] 408 MHz, 37 ≤ δ ≤ 47 ◦ 20 cm N. Sky Catalog [8] 365, 1400, 4850 MHz, 0 ≤ δ ≤ 75 ◦ ◦ ◦ ◦ 6 cm Gal. Plane Survey [9] Parkes 64 m, |b|≤2 , l = 190 − 360 − 40 ◦ ◦ 6 cm South & Tropical Surveys [10, 11] Parkes 64 m, −78 ≤ δ ≤−10 11 cm All Sky Catalog [12] Bright sources ◦ ◦ ◦ 11 cm N. Sky Catalog [13] 6483 sources, |b|≤5 , 240 ≤ l ≤ 357 Bright Galaxies [14] All radio data pre-1975, normal galaxies 20 cm Gal. Plane Survey [15] VLA B Conf. Survey ◦ ◦ ◦ 21 cm Gal. Plane Survey [16] Eff. 100m, |b|≤4 ,95 ≤ l ≤ 357 Galaxy HI [17, 18] Galaxies, vradial ≤ 3000 km/s Atlas of Galactic HI [19] 21 cm H emission line profiles Molecular lines [20, 21] Frequencies, other molecular data Pulsars [22] Catalog of known pulsars Opt. Pos. Radio Stars [23] 221 radio stars ◦ 20 cm Radio Sources δ>−5 [24, 25] 1400 MHz sky atlas References 1. Pilkington, J.D.H., & Scott, P.F. 1965, Mem. R.A.S., 69, 183 2. Gower, J.F.R., Scott, P.F., & Willis D. 1967, Mem. R.A.S., 71, 144 3. Colla, G. et al. 1970, A&AS, 1, 281 4. Colla, G. et al. 1972, A&AS, 7,1 5. Colla, G. et al. 1973, A&AS, 11, 291 6. Fanti, C. et al. 1974, A&AS, 18, 147 7. Ficcara, A., Grueff, G., & Tomesetti, G. 1985, A&AS, 59, 255 8. White, R.L., & Becker, R.H. 1992, ApJS, 79, 331 9. Haynes, R.F., Caswell, J.L., & Simons, L.W.J. 1978, Aust. J. Phys. Suppl., 48,1 10. Griffith, M.R. et al. 1994, ApJS, 90, 179 11. Wright, A.E. et al. 1994, ApJS, 91, 111 12. Wall, J.V., & Peacock, J.A. 1985, MNRAS, 216, 173 13. Furst,¨ E. et al. 1990, A&AS, 85, 805 14. Haynes, R.F., Caswell, J.L., & Simons, L.W.J. 1978, Aust. J. Phys. Suppl., 45,1 15. Zoonematkermani, S. et al. 1990, ApJS, 74, 181 16. Reich, W., Reich, P., & Furst,¨ E. 1990, A&AS, 83, 539 17. Tully, R.B. 1988, Nearby Galaxies Catalog (Cambridge University Press, Cambridge) 18. Huchtmeier, W.K., & Richter, O.-G. 1989, A General Catalog of HI Observations of Galaxies: The Reference Catalog (Springer-Verlag, New York) 19. Hartman, D., & Burton, W.D. 1995, Atlas of Galactic HI (Cambridge University Press, Cambridge) 20. Lovas, F.J. 1992, J. Phys. Chem. Ref. Data, 21, 181 21. Lovas, F.J., Snyder, L.E., & Johnson, D.R. 1979, ApJS, 41, 451 22. Taylor, J.H., Manchester, R.N., & Lyne, A.G. 1993, ApJS, 66, 529 23. Requi´ eme,` Y. & Mazurier, J.M. 1991, A&AS, 89, 311 24. Condon, J.J., Broderick, J.J., & Seielstad, G.A. 1989, AJ, 97, 1064 25. Condon, J.J., Broderick, J.J., & Seielstad, G.A. 1991, AJ, 102, 2041 Other information and images from surveys can be obtained from Internet pages maintained by the National Center for Super Computing Applications (NCSA) Astronomy Digital Image Library (ADIL, http://imagelib.ncsa.uiuc.edu/imagelib.html) and the National Radio Astronomy Observatory (NRAO, http://info.aoc.nrao.edu). More extensive radio (and other) catalogs can be found from Internet pages for the NASA Astrophysics Data System (http://adswww.harvard.edu/index.html). In Figure 6.1 schematic radio spectra representative of a variety of continuum radio sources are plotted in the form of flux density (in units of Jy = Jansky = 10−23 ergs cm−2 s−1 Hz−1 = 10−26 Wm−2 Hz−1) as a function of frequency. The sources and spectra in Figure 6.1 are schematic Sp.-V/AQuan/1999/10/07:17:14 Page 123 6.2 ATMOSPHERIC WINDOW AND SKY BRIGHTNESS / 123 108 Active Sun 107 Sky Background 106 Jupiter (radiation belts) 105 Quiet Sun 104 Moon Crab Cas A Pulsar Cyg A (unpulsed) 103 Crab M31 Virgo A Nebula 3C295 102 Flux Density [Jy] 3C273 101 Orion Nebula Jupiter Mars 100 BL Lac SS433 10-1 Crab Pulsar (pulsed) Cyg X-3 (non-flaring) -2 10 Antares 10-3 10 100 1000 10000 Figure 6.1. Radio spectra of various types of sources in the form of flux density as a function of frequency ν. indicators of typical behavior for a wide variety of objects. Major solar-system radio sources are the active Sun, the quiet Sun, the Earth’s Moon, Mars, the surface of Jupiter, and Jupiter’s radiation belts. Stellar system sources are: pulsars, with the pulsed and unpulsed (dashed line) emission from the Crab pulsar; the partially ionized coronal emission of the red supergiant Antares; and the X-ray binaries SS433 and Cyg X-3 (quiescent). The optically thick and thin portions of the radio spectrum of the ionized gas from an HII region is indicated for the Orion Nebula. Cas A is a remnant of a supernova explosion in our Galaxy; and the Crab nebula spectrum is representative of the relatively rare plerion, i.e., nebulosities energized by pulsars. The radio spectrum for M31, the Andromeda nebula, is representative of spiral galaxy behavior. Strong radio galaxies are represented by Cyg A and Virgo A; BL Lac is a blazar, while 3C273 and 3C295 are strong quasars. Defining the convention that the spectral index α is the power law index for the flux density, α Sν ∝ ν , then positive spectral indices indicate optically thick emission while negative spectral indices indicate optically thin emission. Spectral indices near zero indicate optically thin emission for thermal sources, while similarly flat spectra for synchrotron radiation sources indicate optically thick emission from a wide range of optical depths. 6.2 ATMOSPHERIC WINDOW AND SKY BRIGHTNESS 6.2.1 Atmospheric Window The low-frequency end of the radio window is set by ionospheric absorption. Since the ionosphere has diurnal variations, solar cycle variations, variations depending upon the effect of particle storms impacting the atmosphere, and variations with Earth latitude and longitude, the lowest frequency where the atmosphere is transparent varies considerably over the range 10 to 30 MHz. From that low- frequency cut-off, up to about 22 GHz, the Earth’s atmosphere does not absorb radio waves, although variation in the index of refraction affects the phase of incoming radio waves at the higher frequencies. Sp.-V/AQuan/1999/10/07:17:14 Page 124 124 / 6 RADIO ASTRONOMY 1 1 1 mm PWV Atmospheric Transparency 8 mm PWV 0 0 0.01 0.1 1 10 100 1000 0 200 400 600 800 1000 Wavelength [cm] Frequency [GHz] Figure 6.2. The radio window for the Earth’s atmosphere shown in terms of plots of atmospheric transparency as a function of both wavelength and frequency. The solid line is for excellent conditions with 1 mm PWV and the dashed line is for normal conditions with 8 mm PWV. At the lowest frequencies plasma effects in the ionosphere induce variable Faraday rotation in incoming radio waves. In Figure 6.2 we plot models for the transparency of the Earth’s atmosphere for two values of precipitable water vapor (PWV), 1 and 8 mm, representing extremely low and normal levels, respectively. 6.2.2 Surface Brightness and Brightness Temperature All astronomy is based upon measurements of surface brightness on the celestial sphere: Iν(α, δ, t), where t is time, (α,δ) is position specified by right ascension and declination, ν is frequency, and I is the specific intensity which can be any of the four Stokes parameters. The surface brightness ν/ of a black body of temperature T is Bν = (2hν3/c2)(1/[eh kT − 1]), where h and k are the Planck and Boltzmann constants. For small values of hν/kT, the Rayleigh–Jeans approximation is ∼ valid, and Bν(T ) = 2kT/λ2; this is valid at longer radio wavelengths. For reasons related to the antenna measurement equation, radio astronomy often uses brightness temperature rather than surface brightness to describe measurements, where the brightness temperature is λ2 T ≡ Bν. (6.1) b 2k Equation (6.1) is often used even when black body or long wavelength approximations are not hν/kT applicable.
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