JAXA  Special  Publication     JAXA-SP-09-008E190
Detecting X-ray emission from Cometary Atmospheres using the SuzakuX-ray Imaging Spectrometer
G. V. Brown1, P. Beiersdorfer1, D. Bodewits2, F. S. Porter2, Y. Ezoe3, K. Hamaguchi2, M. Hanya4, M. Itoh5,C. A. Kilbourne2, T. Kohmura6, Y. Maeda7, H. Negoro9, Y. Tsuboi4, H. Tsunemi9, Y. Urata10
1 Lawrence Livermore National Laboratory, 2 NASA/Goddard Space Flight Center,3 Tokyo Metropolitan University, 4 Chuo University,5 Kobe University, 6 Kogakuin University,7 Institute of Space and Astronautical Science, 8 Nihon University,9 Osaka University, 10 Saitama UniversityE-mail: gregbrown@llnl.gov
Abstract
The Suzaku X-ray imaging spectrometer has been used to observe the X-ray emission from comets73P/Schwassmann-Wachmann 3C and 8P/Tuttle. Comet 73P/Schwassmann-Wachmann 3C was observedduring May and June of 2006, while it was near perihelion and passed within 0.1 AU of the Earth. Comet8P/Tuttle was observed during January of 2008 when it was at its closest approach to the Earth at 0.25AU, and again near perihelion at a distance of 0.5 Au from Earth. In the case of comet 73P/SchwassmannWachmann 3C, the XIS spectra show line emission from highly charged oxygen and carbon ions as wellas emission from what is most likely L-shell transitions from Mg, Si, and S ions. This line emission iscaused by charge exchange recombination between solar wind ions and cometary neutrals, and can be usedas a diagnostic of the solar wind. Here we present some of the results of the observation of the comet73P/Schwassmann-Wachmann 3C.Key words: Comets, Suzaku, X-ray, X-ray Imaging Spectrometer, Charge Exchange
X-ray emission from cometary atmospheres has beenobserved now for over a decade, and the dominant x-rayproduction mechanism is widely accepted to be chargeexchange recombination (CEX) between solar wind ionsand cometary neutrals. In this process a bound electronfrom the cometary neutral undergoes radiationless trans-fer to an excited state of the solar wind ion. The excitedstate then decays radiatively, emitting one or more pho-tons in the process. The spectrum produced by CEXdepends on the ion species, the neutral species, and thecollision velocity between the two particles (Beiersdorfer2001,Beiersdorfer 2003,Bodewits 2004, Bodewits 2006,Wargelin 2008). Thus, a CEX x-ray spectrum emittedfrom a comet is a powerful diagnostic of the solar windion abundance, the relative speed between the solar windion and the cometary neutral and the comet’s neutralgas density and species distribution. It is thus possibleto use the x-ray spectrum produced by a comet to re-motely diagnose the composition and speed of the solarwind.Comet 73P Schwassmann-Wachmann 3 was discoveredin 1930 by F. C. A. Schwassmann and A. A. Wachmann,and has an orbital period of5.5 years. Owing to thefact that it was faint and small, it was not identiﬁed
again until 1979, when its optical magnitude was 12.5at its closest approach to the Earth. It was not observedin 1985, but was observed in 1990 with an optical mag-nitude of 9. During its 1995 apparition, the brightnesssurprisingly went up several orders of magnitude as aresult of the fact that the comet had broken into sev-eral smaller fragments. For its apparition in 2006, over60 fragments were being tracked.The X-ray ImagingSpectrometer (XIS) on Suzaku observed fragment C onMay 13 at its closest approach to Earth at a distance of0.074 AU, and again at perihelion on June 8. During theJune 8 observation, a solar ﬂare occurred and the X-rayemission from the comet brightened as a result.Figure 1 shows the X-ray spectrum measured dur-ing the June 8th observation, and also the ﬁt to thespectrum. The data were ﬁt using laboratory-measuredcharge exchange data from C V, C VI, N VI, N VII, andO VII. In addition, lines at 0.21,0.24,0.27, and 0.4 keVwere also included to achieve a good ﬁt. Many of theselines are likely produced by charge exchange onto L-shellions of silicon, sulfur, and magnesium (see for exampleFrankel et al 2009 and Koutroupma et al 2010). Oneuseful diagnostic of the speed of the solar wind ions isthe hardness ratio. The hardness ratio is deﬁned as the
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The Energetic Cosmos : from Suzaku to ASTRO-H191
 counts/seconds/keV/cm2
0.900.800.700.600.500.400.300.20 energy (keV)
 73P/SW3C XIS Fit
Fig. 1. X-ray spectrum measured by the back illuminated CCD of Suzaku’s X-ray Imaging Spectrometer. This spectrum was measured onJune 8th, 2006 when the comet was at perihelion. The data were ﬁt using laboratory measured charge exchange spectra from C V & VI,N VI & N VII, and O VII. Additional lines at 0.21, 0.24, 0.27 and 0.4 keV are also included. The emission below 300 eV is most likelydominated by L-shell emission from S, Si, and Mg.
relative intensity of the n > 3 to 1 transitions relativeto the n = 2 to 1 transitions. The larger the hardnessratio, the slower the speed of the solar wind ion rela-tive to the cometary neutral (Beiersdorfer et al. 2001).Based on the hardness ratio measured from the hydro-genic C5+ Rydberg series, the velocity of the C6+ ions is∼ 300 km/s, consistent with a slow solar wind. We alsonote that no hydrogenic O7+ was included in our ﬁt,and thus no fully ionized O8+ was present in the solarwind during these observations. This result is consistentwith measurements using the Solar Wind Ion Composi-ton Spectrometer (SWICS) on the Advanced Composi-tion Explorer (ACE), which also showed that no O8+
was present in the solar wind during this observation.
In 2008, comet 8P/Tuttle was observed twice by theXIS. The ﬁrst observation during the ﬁrst week of Jan-uary when the comet was at a distance of 0.26 AU fromEarth, 1.1 AU from the Sun, and at a solar latitude of ∼1 degrees south, i.e., nearly in the ecliptic. During thesecond observation later in January when the the cometwas 0.48 AU from the Earth, 1.0 AU from the Sun, andhad a solar latitude of ∼ -20 degrees. The ACE sampledthe solar wind during both observations. As a result,for the second observation, the solar wind was sampledsimultaneously in the ecliptic (by ACE), and out of theecliptic (by the X-ray emission from 8P/Tuttle).
In 2013, AstroH is planned to launch carrying a highresolution X-ray microcalorimeter array, the Soft X-raySpectrometer (SXS). This instrument is expected to havea resolution of 7 eV across the 0.4 to 10 keV energy bandand an eﬀective area between 10 and 100 cm2 between0.4 and 1 keV. This instrument will make possible forthe ﬁrst time to resolve the X-ray spectral features fromcometary atmospheres.
AcknowledgementsPart of this work was performed under the auspices of theU. S. Department of Energy by Lawrence Livermore Na-tional Laboratory under Contract DE-AC52-07NA27344and is also supported by NASA grant number 06-SUZ2-0058 to LLNL.
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