The Optical Spectrum of 3c 273.

Abstract

Photoelectric spectrophotometric observations of both the continuum and emission lines in the spectrum of 3C 273 have been obtained using spectrum-scanning instruments. Observations cover the wavelength range from 3200 to X 10800. For laboratory wavelengths to the blue of X 5500 the entire continuous spectrum and the hydrogen emission lines can be explained in terms of a hot hydrogen nebula at an electron temperature of 160000 K. In this model the total thermal emission is 2.3 X 10 erg , and most of the oxygen is required to be in the form of O+++. The continuous spectrum can also be explained in terms of a hydrogen nebula at an electron temperature of about 14000 K, but in this case it is necessary to add arbitrarily a flat spectrum, such as synchrotron radiation, which must account for more than 90 per cent of the continuous spectrum. In this latter model the observed hydrogen lines are somewhat too strong. To the red of the laboratory wavelength X 5500 there is in all models a strong excess of radiation which should probably be associated with the observed radio4requency synchrotron emission. Assuming the high electron-temperature model to be correct, one is led on the basis of the observed light fluctuations to a nebula with a radius of only a few parsecs and a mass of 2 X 10 solar masses. At the density implied by such a model, electron scattering is very important and makes it even more difficult to explain the light variations. Kinetic energy of expansion is sufficient for at most 10 years so that 3C 273 is either younger than 10 years, or there is a continuous supply of energy being provided. If one could disregard the light variations, the best model from the standpoint of the observations would have an electron temperature of 160000 K, a diameter of 2000 pc, and a mass of 1011 solar masses.