Simultaneous Swift X-ray and UV views of comet C/2007 N3 (Lulin)

¹ Department of Physics and Astronomy, University of Leicester, Leicester, LE1 1RH, UK
e-mail: jac48@star.le.ac.uk; amr30@star.le.ac.uk
² Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
e-mail: dennis@astro.umd.edu
³ Astrophysics Science Division, Code 662, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
e-mail: stefan.m.immler@nasa.gov

Received: 25 August 2011
Accepted: 6 March 2012

Abstract

Aims. We present an analysis of simultaneous X-Ray and UV observations of comet C/2007 N3 (Lulin) taken on three days between January 2009 and March 2009 using the Swift observatory.

Methods. For our X-ray observations, we used basic transforms to account for the movement of the comet to allow the combination of all available data to produce an exposure-corrected image. We fit a simple model to the extracted spectrum and measured an X-ray flux of 4.3 ± 1.3  ×  10^-13 erg   cm^-2   s^-1 in the 0.3 to 1 keV band. In the UV, we acquired large-aperture photometry and used a coma model to derive water production rates given assumptions regarding the distribution of water and its dissociation into OH molecules about the comet’s nucleus.

Results. We compare and discuss the X-ray and UV morphology of the comet. We show that the peak of the cometary X-ray emission is offset sunward of the UV peak emission, assumed to be the nucleus, by approximately 35 000 km. The offset observed, the shape of X-ray emission and the decrease of the X-ray emission comet-side of the peak, suggested that the comet was indeed collisionally thick to charge exchange, as expected from our measurements of the comet’s water production rate (6–8  × 10²⁸   mol   s^-1). The X-ray spectrum is consistent with solar wind charge exchange emission, and the comet most likely interacted with a solar wind depleted of very highly ionised oxygen. We show that the measured X-ray lightcurve can be very well explained by variations in the comet’s gas production rates, the observing geometry and variations in the solar wind flux.