Chandra observations of comets 8P/Tuttle and 17P/Holmes during solar minimum

We present results for Chandra X-ray Observatory observations of two comets made during the minimum of solar cycle 24. The two comets, 17P/Holmes (17P) and 8P/Tuttle (8P), were very different in their activity and geometry. 17P was observed, for 30 ks right after its major outburst, on 2007 October 31 (10:07 UT), and comet 8P/Tuttle was observed in 2008 January for 47 ks. During the two Chandra observations, 17P was producing at least 100 times more water than 8P but was 2.2 times further away from the Sun. Also, 17P was at a relatively high solar latitude (+19. ? 1) while 8P was observed at a lower solar latitude (3. ? 4). The X-ray spectrum of 17P is unusually soft with little significant emission at energies above 500 eV. Depending on our choice of background, we derive a 300�1000 eV flux of 0.5�4.5 � 10?13 erg cm?2 s?1, with over 90% of the emission in the 300�400 eV range. This corresponds to an X-ray luminosity between 0.4 and 3.3 � 1015 erg s?1. However, we cannot distinguish between this significant excess emission and possible instrumental effects, such as incomplete charge transfer across the CCD. 17P is the first comet observed at high latitude during solar minimum. Its lack of X-rays in the 400�1000 eV range, in a simple picture, may be attributed to the polar solar wind, which is depleted in highly charged ions. 8P/Tuttle was much brighter, with an average count rate of 0.20 counts s?1 in the 300�1000 eV range. We derive an average X-ray flux in this range of 9.4 � 10?13 erg cm?2 s?1 and an X-ray luminosity for the comet of 1.7 � 1014 erg s?1. The light curve showed a dramatic decrease in flux of over 60% between observations on January 1 and 4. When comparing outer regions of the coma to inner regions, its spectra showed a decrease in ratios of Cvi/C v, Oviii/Ovii, as predicted by recent solar wind charge exchange (SWCX) emission models. There are remarkable differences between the X-ray emission from these two comets, further demonstrating the qualities of cometary X-ray observations, and SWCX emission in general as a means of remote diagnostics of the interaction of astrophysical plasmas.