1988Apj. . .335. .215P the Astrophysical Journal, 335:215-238
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.215P The Astrophysical Journal, 335:215-238,1988 December 1 © 1988. The American Astronomical Society. AU rights reserved. Printed in U.S.A. .335. 1988ApJ. A COMPREHENSIVE STUDY OF THE X-RAY STRUCTURE AND SPECTRUM OF IC 443 R. Petre and A. E. Szymkowiak Laboratory for High Energy Astrophysics, NASA/Goddard Space Flight Center F. D. Seward Harvard/Smithsonian Center for Astrophysics AND R. WlLLINGALE Leicester University Received 1986 August 26; accepted 1988 May 31 ABSTRACT We present the results of a comprehensive study of the X-ray emission from the supernova remnant IC 443, using the SSS, IPC, and HRI of the Einstein Observatory and the MED of the HEAO 1 A-2 experiment. We observe a soft X-ray appearance highly atypical of a supernova remnant in the adiabatic phase, with little correlation between X-ray and optical or radio features. The best-fit models of the low-energy X-ray spectrum of the brightest area of the remnant suggest either that the remnant has not yet attained ionization equi- librium or that the X-rays arise in a multiphase medium. Pronounced soft X-ray spectral differences across the remnant are accounted for by variations in absorption by an intervening molecular cloud. Our analysis sug- gests that despite the atypical X-ray appearance, the X-ray emission in IC 443 is probably confined to a thin (0.1 pc) shell. Subject headings: nebulae: individual (IC 443) — nebulae: supernovae — X-rays: sources I. INTRODUCTION compression of shocked gas and the consequent compression Recent detailed studies of individual supernova remnants of ambient magnetic fields and relativistic particles transverse have revealed that few, if any, closely resemble the elegant to the direction of the shock. mathematical abstractions constructed to describe them. The Spectrophotometry of the bright filaments has indicated that wealth of images and spectra available from the last decade’s typical temperatures and densities inside the optically emitting series of X-ray observatories, combined with advances in gas are on the order of 15,000 K and 500 cm-3 (Fesen and imaging and spectral resolution in wavebands accessible from Kirshner 1980). Ha interferometry has yielded a mean velocity ground-based observations, show that physical remnants do for the filaments of 65 km s-1 (Lozinskaya 1969). However, not faithfully reproduce the theorist’s spherically symmetric radial velocities as high as 220 km s-1 have been observed in Sedov (1959) hydrodynamics (see, e.g., Gorenstein and Danzi- some filaments (Lozinskaya 1979). The presence in the north- ger 1983). The concrete circumstellar and interstellar media are east of Ha emission from gas unassociated with the filaments, neither homogeneous nor smoothly varying, but are filled with having radial velocities of 400-500 km s_1 (Lozinskaya 1979), clumps of diverse density and composition. This variety tends suggests that the shock front is propagating into the surround- to give each supernova remnant a distinctive character shaped ing medium much faster than the optical filaments are moving. by its immediate environment, while at the same time tends to The detection of [Fe x] 26374 coronal emission near the bright preclude simple modeling of any but the smallest regions of a optical filaments confirms the presence of some gas in or remnant. Nevertheless, it is still possible to synthesize overall around these filaments with T æ 106 K, corresponding to a pictures of a supernova’s interaction with the inhomogeneous shock velocity of ~400 km s_1 (Woodgate, Lucke, and Socker interstellar medium. 1979). In the case of the supernova remnant IC 443, indications of a The interaction between the shock front and large inter- distinctive character arising as a consequence of its interaction stellar features is not isolated to the locale of the bright optical with structures in the interstellar medium are present through- filaments. Observations of CO line radiation at 2.6 cm have 4 out the reported observations in the X-ray, radio, and optical revealed the presence of a large (M % 10 M0), thin (3 ± 1 pc) bands. Although the remnant possesses a nearly complete molecular cloud covering virtually the entire southern and optical shell, its appearance is dominated by a complex western parts of IC 443 (Cornett, Chin, and Knapp 1977; Sco- network of bright filaments, filling the northeastern quadrant, ville et al 1977). This cloud is responsible for significant visual which has arisen from the encounter between the shock front extinction in the central regions of the remnant. That it is in and a large-scale density enhancement in the local interstellar front of IC 443 and interacting with it is demonstrated by the medium. This enhancement is probably an H i cloud (DeNoyer detection of 10 different shocked molecular species including 1978), which is probably associated with the H n region S249 -1 H2, CO, and OH, all with a typical velocity of —30 km s located 2° to the northeast of IC 443 (Fesen 1984). The contin- (DeNoyer 1979a, b; DeNoyer and Frerking 1981; Treffers uum radio surface brightness at 6 cm and 20 cm (Duin and van 1979). IC 443 has thus become a rich laboratory for the study der Laan 1975; Green 1986; Mufson et al 1986) bears a strik- of shock chemistry. A map of shocked H i by Giovanelli and ing resemblance on all scales to the optical appearance. This Haynes (1979) indicates that virtually the entire front face of IC similarity is probably due to unstable radiative cooling and 443 has encountered either the molecular cloud or the H i 215 © American Astronomical Society • Provided by the NASA Astrophysics Data System .215P 216 PETRE ET AL. Vol. 335 .335. cloud and that approximately 1000 M0 has been swept up by rocket observation. Though this image had ~ 6' resolution and the shock front. The presence of this H i shell strongly suggests consisted of only ~ 30 events, it located the brightest region of that at least some portions of IC 443 have entered the radi- soft X-ray emission not behind northeastern optical filaments ative, momentum-conserving expansion phase (Cox 1972; but along the northern segment of the shell, overlapping the 1988ApJ. Straka 1974; Falle 1975). With the shock velocities of 400 km filaments, with no strong spatial correlation. Levine et al sug- s_1 or higher elsewhere in the remnant suggesting adiabatic gested that the shocked regions of low preshock density expansion, it seems as though interaction with its dense neigh- responsible for the X-radiation and the regions of higher pre- bor has forced IC 443 into a sort of premature “ midlife crisis.” shock density emitting optically are spatially distinct, lying Recent discoveries about IC 443 have revealed that until adjacent to each other along the shock. All of the above experi- now even the most detailed work on the remnant has not taken ments lacked the sensitivity to spatially resolve X-ray emission into account two important aspects. First, observations using from the interior or the southern portions of IC 443. the IRAS satellite have revealed that IC 443 is an extremely Previous X-ray spectral observations have also lacked the bright source of far-infrared emission in the 12, 25, 60, and 100 sensitivity and the spatial and spectral resolution needed to fim bands (Braun and Strom 1986; Mufson et al 1986). The perform detailed spectral measurements of IC 443. Attempts IRAS band luminosity is by far the dominant source of radi- have been made to fit proportional counter spectra of ative cooling; Dwek et al (1987) estimate the IRAS band restricted portions of IC 443 (i.e., the northeast) with thermal luminosity to be 360 times the X-ray luminosity. The brightest bremsstrahlung models (e.g., Malina, Lampton, and Bowyer infrared emission regions are best spatially correlated with the 1976; Parkes et al 1977) or to fit spectra of the entire remnant shocked H i emission from the northeast and the molecular with multicomponent thermal spectra, including line emission cloud. Approximately half of the infrared emission arises as (Galas, Venkatesan and Garmire 1981; Charles et al 1981). thermal emission from shock-heated dust, assuming a normal The typical best-fit temperature of these fits is ~1 keV gas-to-dust ratio (Dwek et a/. 1987). The origin of the remain- ( = 1.2 x 107 K), with a column density of 3-4 x 1021 cm-2. As der is currently undetermined, although approximately 20% is supernova remnants are usually relatively complicated entities, probably from atomic line emission from shocked clouds with highly spatially dependent spectra dominated by line (Mufson ei a/. 1986). rather than continuum emission, simple models of an entire Second, optical spectroscopy by Fesen (1984) has revealed SNR are limited in the information they can provide to esti- that IC 443 as demarcated by the extent of the optical/radio mates of some average temperature and column density. shell does not represent the entire supernova remnant. His The first X-ray instrument sufficiently sensitive in the soft work demonstrates the existence of a “ breakout ” of the shock X-ray band to perform spatial mapping and detailed spectral into a large, low-density interstellar cavity to the southeast of measurements of IC 443 comparable in quality to measure- the optical/radio nebulosity. Fesen also found strong evidence ments in the optical and radio bands was the Einstein Observa- of a physical connection between IC 443 and the H n region tory (Giacconi et al 1979). In this paper we present a S249, thereby establishing a firm distance estimate for IC 443 compilation of the observations using three of the four focal of 1.5 kpc. Sensitive H i mapping by Braun and Strom (1986) plane instruments on Einstein, the Imaging Proportional confirms the existence of this breakout.