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X-Ray and Extreme Ultraviolet Emission from Comets Lisse et al.: X-Ray and Extreme UV Emission from Comets 631 X-Ray and Extreme Ultraviolet Emission from Comets C. M. Lisse University of Maryland T. E. Cravens University of Kansas K. Dennerl Max-Planck-Institut für Extraterrestrische Physik The discovery of high energy X-ray emission in 1996 from C/1996 B2 (Hyakutake) has created a surprising new class of X-ray emitting objects. The original discovery (Lisse et al., 1996) and subsequent detection of X-rays from 17 other comets (Table 1) have shown that the very soft (E < 1 keV) emission is due to an interaction between the solar wind and the comet’s atmosphere, and that X-ray emission is a fundamental property of comets. Theoretical and ob- servational work has demonstrated that charge exchange collisions of highly charged solar wind ions with cometary neutral species is the best explanation for the emission. Now a rapidly chang- ing and expanding field, the study of cometary X-ray emission appears to be able to lead us to a better understanding of a number of physical phenomena: the nature of the cometary coma, other sources of X-ray emission in the solar system, the structure of the solar wind in the helio- sphere, and the source of the local soft X-ray background. 1. INTRODUCTION total X-ray power, or luminosity, of C/Hyakutake was mea- sured to be approximately 109 W. The emission was also Astrophysical X-ray emission is generally found to origi- extremely “soft”, or of low characteristic photon energy — nate from hot collisional plasmas, such as the million-degree only X-rays of energies less than ~1 keV (or wavelengths gas found in the solar corona (e.g., Foukal, 1990), the 100- longer than ~1.2 nm) were detected from C/Hyakutake. The million-degree gas observed in supernova remnants (e.g., total amount of energy emitted in X-rays from a comet is Cioffi, 1990), or the accretion disks around neutron stars and approximately 10–4 the energy delivered to a comet from black holes. As electromagnetic radiation with wavelength λ the Sun due to photon insolation and solar wind impact between about 0.01 nm and 100 nm (1 nm = 10–9 m), ex- (Lisse et al., 2001). treme ultraviolet (EUV) and X-ray radiation are important for solar system and astrophysical applications because the 2. OBSERVED CHARACTERISTICS OF photons are sufficiently energetic and penetrating to ionize COMETARY X-RAY EMISSION neutral atoms and molecules, and can thus drive chemical reactions. In fact, current estimates of the X-ray burden per Shortly after the initial C/Hyakutake detection, soft X-ray atom in the young solar system are some 103–104 photons emission from four other comets was found in the ROSAT per atom, even as far out as the proto-Kuiper belt, as the archival database, confirming the discovery (Dennerl et al., young Sun was much more X-ray active than now (Feigel- 1997). X-ray emission has now been detected from 18 com- son, 1982; Dorren et al., 1995). ets to date (Table 1) using a variety of X-ray sensitive space- The Sun is not the only source of X-rays in the solar sys- craft — BeppoSAX, ROSAT, the Extreme Ultraviolet Ex- tem (Cravens, 2000a, 2002a). Prior to 1996, X-rays were plorer (EUVE), and more recently, Chandra and XMM- found in scattering of solar X-rays from the terrestrial atmos- Newton. All comets within 2 AU of the Sun and brighter phere and in the terrestrial aurora, as scattered solar X-rays than V = 12 have been detected when observed. We now off the illuminated surface of the Moon, and from the jovian recognize that X-ray emission is a characteristic of all ac- aurora. Nonetheless, the 1996 discovery, using the Röntgen tive comets. Satellite (ROSAT), by Lisse and co-workers (Lisse et al., The observed characteristics of the emission can orga- 1996) (Fig. 1) of strong X-ray emission from Comet C/ nized into the following four categories: (1) spatial mor- 1996 B2 (Hyakutake) was very surprising because cometary phology, (2) total X-ray luminosity, (3) temporal variation, atmospheres are known to be cold and tenuous, with charac- and (4) energy spectrum. Any physical mechanism that pur- teristic temperatures between 10 and 1000 K. Compared to ports to explain cometary X-ray emission must account for other X-ray sources, comets are moderately weak — the all these characteristics. 631 632 Comets II TABLE 1. Observation times, instruments, and energies* for comets detected† through December 2002 in the X-ray or extreme ultraviolet. Comet Time Instrument Energy (keV) Detection Reference 45P/Honda-Mrkos-Pajdušáková Jul 1990 ROSAT PSPC/WFC 0.09–2.0 Yes [1] C/1990 K1 (Levy) Sep 1990 ROSAT PSPC /WFC 0.09–2.0 Yes [1] Jan 1991 0.09–2.0 Yes [1] C/1991 A2 Arai Nov 1990 ROSAT PSPC/WFC 0.09–2.0 Yes [1] C/1990 N1 (Tsuchiya-Kiuchi ) Nov 1990 ROSAT PSPC/WFC 0.09–2.0 Yes [1] Jan 1991 0.09–2.0 Yes [1] 2P/Encke Nov 1993 EUVE DS 0.02–0.10 No [8] Jul 1997 EUVE Scanners 0.02–0.18 Yes [9] Jul 1997 ROSAT HRI/WFC 0.09–2.0 Yes [9] 19P/Borrelly Nov 1994 EUVE DS 0.02–0.10 Yes [8] 6P/d’Arrest Sep 1995 EUVE DS 0.02–0.10 Yes [8] C/1996 B2 (Hyakutake) Mar 1996 ROSAT HRI /WFC 0.09–2.0 Yes [2] Mar 1996 EUVE DS 0.02–0.10 Yes [3] Mar 1996 ALEXIS 0.06–0.10 No Apr 1996 XTE PCA 2.0–10.0 No Jun 1996 ASCA 0.20–6.0 No Jun 1996 ROSAT HRI /WFC 0.09–2.0 Yes [4] Jul 1996 ROSAT HRI /WFC 0.09–2.0 Yes Aug 1996 ROSAT HRI /WFC 0.09–2.0 Yes Sep 1996 ROSAT HRI /WFC 0.09–2.0 Yes C/1995 O1 (Hale-Bopp) Apr 1996 ROSAT HRI/WFC 0.09–2.0 No Sep 1996 EUVE Scanners 0.02–0.10 Yes [5] Sep 1996 BeppoSAX 0.1–200 Yes [6] Sep 1996 ROSAT HRI/WFC 0.09–2.0 Yes? [7] Sep 1996 ASCA 0.20–6.0 No [5] Mar 1997 XTE PCA 2.0–10.0 No Oct 1997 ROSAT HRI /WFC 0.09–2.0 No Nov 1997 ROSAT HRI /WFC 0.09–2.0 No Nov 1997 EUVE DS 0.02–0.10 Yes [8] Feb 1998 ROSAT PSPC/WFC 0.09–2.0 No C/1996 Q1 (Tabur) Sep 1996 ASCA 0.20–6.0 No Sep 1996 ROSAT HRI /WFC 0.09–2.0 Yes [1] Oct 1996 ROSAT HRI /WFC 0.09–2.0 Yes [1] 55P/Temple-Tuttle Jan 1998 EUVE Scanners 0.02–0.18 Yes Jan 1998 ASCA SIS 0.20–6.0 No Jan 1998 ROSAT HRI /WFC 0.09–2.0 Yes Feb 1998 ROSAT HRI /WFC 0.09–2.0 Yes 103P/Hartley 2 Feb 1998 ROSAT PSPC/WFC 0.09–2.0 Yes C/1998 U5 (LINEAR) Dec 1998 ROSAT PSPC/WFC 0.09–2.0 Yes C/2000 S4 (LINEAR) Jul 2000 Chandra ACIS-S 0.2–10.0 Yes [10] Aug 2000 Chandra ACIS-S 0.2–10.0 Yes [10] C/1999 T1 McNaught-Hartley Jan 2001 Chandra ACIS-S 0.2–10.0 Yes [11] Jan 2001 XMM-Newton 0.2–12.0 Yes Feb 2001 FUSE 0.113–0.117 No [12] Lisse et al.: X-Ray and Extreme UV Emission from Comets 633 TABLE 1. (continued). Comet Time Instrument Energy (keV) Detection Reference C/2001 A2 (LINEAR) Jun 2001 Chandra HRC/LETG 0.2–2.0 No Jun 2001 XMM-Newton 0.2–12.0 Yes? Jul 2001 FUSE 0.113–0.117 No [12] C/2000 WM1 (LINEAR) Dec 2001 Chandra ACIS/LETG 0.2–2.0 Yes Dec 2001 FUSE 0.113–0.117 Yes [12] Jan 2002 XMM-Newton 0.2–12.0 Yes [13] C/2002 C1 (Ikeya-Zhang) Apr 2002 Chandra ACIS-S 0.2–10.0 Yes [13] May 2002 XMM-Newton 0.2–12.0 Yes? [13] *The full energy range of the observing instrument is given. † This table summarizes published and unpublished (1) dedicated observations, whether successful or not; and (2) successful serendipitous observations. The table is sorted according to time of first observation of the comet. References: [1] Dennerl et al. (1997); [2] Lisse et al. (1996); [3] Mumma et al. (1997); [4] Lisse et al. (1997a); [5] Krasnopolsky et al. (1997); [6] Owens et al. (1998); [7] Lisse et al. (1997b); [8] Krasnopolsky et al. (2000); [9] Lisse et al. (1999); [10] Lisse et al. (2001); [11] Krasnopolsky et al. (2002); [12] Weaver et al. (2002); [13] K. Dennerl et al. (personal communication, 2003). 2.1. Spatial Morphology al., 2002) at distances that exceed 104 km for weakly ac- tive comets, and can exceed 106 km for the most luminous X-ray and EUV images of C/1996 B2 (Hyakutake) made (Dennerl et al., 1997) (Fig. 2). The spatial extent for the by the ROSAT and EUVE satellites look very similar (Lisse most extended comets is independent of the rate of gas emis- et al., 1996; Mumma et al., 1997) (Fig. 1). Except for im- sion from the comet. The region of peak emission is cres- ages of C/1990 N1 (Dennerl et al., 1997) and C/Hale-Bopp cent-shaped with a brightness peak displaced toward the 1995 O1 (Krasnopolsky et al., 1997), all EUV and X-ray Sun from the nucleus (Lisse et al., 1996, 1999).
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