We present an analysis of simultaneous X-Ray and UV observations ofcomet
C/2007 N3 (Lulin) taken on three days between January 2009 and March 2009 using
the Swift observatory. 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 ergs cm-2
s-1 in the 0.3 to 1.0 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.
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 10^28 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.Comment: Paper accepted for publication in Astronomy and Astrophysics, 6 March
2012, 12 pages, 8 colour figures, one tabl
