The morphology of cometary X-ray emission

¹ Max-Planck-Institut für Astrophysik, 85748 Garching, Germany e-mail: ruw@mpa-garching.mpg.de
² Max-Planck-Institut für extraterrestrische Physik, 85748 Garching, Germany e-mail: kod@mpe.mpg.de
³ University of Maryland, Department of Astronomy, College Park, MD 20742, USA e-mail: lisse@astro.umd.edu

Received: 2 April 2004
Accepted: 10 August 2004

Abstract

X-ray emission from comets is generated by charge exchange of heavy solar wind ions in a high ionization state with the neutral particles of the cometary coma. We use model calculations and theoretical considerations to study the morphology of the emission. The emission is located in a bowl-shaped region on the subsolar side of the nucleus. We derive several length scales. By comparison with hydrodynamic scales we find that the maximum emission comes from inside the bow shock. The size of the emission region depends on the size of the cometary coma, i.e., mainly on the gas production, while the intensity depends mainly on the energy content of the heavy ions in the solar wind flow. This dependence allows a determination of the gas production of the comet and the heavy ion energy flux from a statistical study of the observed spatial distribution of X-ray photons. Here we apply this procedure to X-ray observations from comets C/1996 B2 (Hyakutake), C/1990 K1 (Levy), 2P/Encke and C/2000 WM1 (LINEAR). The gas production rates determined by our method agree very well with those obtained by other methods. The variability of the emission documented in several recorded outbursts correlates with the solar wind proton density.