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The Rosat Hri X-Ray Survey of the Cygnus Loop

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The Rosat Hri X-Ray Survey of the Cygnus Loop Dartmouth College Dartmouth Digital Commons Open Dartmouth: Published works by Dartmouth faculty Faculty Work 1997 The Rosat Hri X-Ray Survey of the Cygnus Loop N. A. Levenson University of California, Berkeley J R. Graham University of California, Berkeley B. Aschenbach Max–Planck–Institut fur Extraterrestrische Physik W. P. Blair The Johns Hopkins University W. Brinkmann Max–Planck–Institut fur Extraterrestrische Physik See next page for additional authors Follow this and additional works at: https://digitalcommons.dartmouth.edu/facoa Part of the Astrophysics and Astronomy Commons Dartmouth Digital Commons Citation Levenson, N. A.; Graham, J R.; Aschenbach, B.; Blair, W. P.; Brinkmann, W.; Busser, J. U.; Egger, R.; Fesen, R. A.; Hester, J. J.; Kahn, S. M.; Klein, R. I.; McKee, C. F.; Petre, R.; Pisarski, R.; Raymond, J. C.; and Snowden, S. L., "The Rosat Hri X-Ray Survey of the Cygnus Loop" (1997). Open Dartmouth: Published works by Dartmouth faculty. 3450. https://digitalcommons.dartmouth.edu/facoa/3450 This Article is brought to you for free and open access by the Faculty Work at Dartmouth Digital Commons. It has been accepted for inclusion in Open Dartmouth: Published works by Dartmouth faculty by an authorized administrator of Dartmouth Digital Commons. For more information, please contact [email protected]. Authors N. A. Levenson, J R. Graham, B. Aschenbach, W. P. Blair, W. Brinkmann, J. U. Busser, R. Egger, R. A. Fesen, J. J. Hester, S. M. Kahn, R. I. Klein, C. F. McKee, R. Petre, R. Pisarski, J. C. Raymond, and S. L. Snowden This article is available at Dartmouth Digital Commons: https://digitalcommons.dartmouth.edu/facoa/3450 THE ASTROPHYSICAL JOURNAL, 484:304È312, 1997 July 20 ( 1997. The American Astronomical Society. All rights reserved. Printed in U.S.A. THE ROSAT HRI X-RAY SURVEY OF THE CYGNUS LOOP N. A.LEVENSON,1 J. R.GRAHAM,1 B.ASCHENBACH,2 W. P.BLAIR,3 W.BRINKMANN,2 J.-U. BUSSER,2 R.EGGER,2 R. A.FESEN,4 J. J.HESTER,5 S. M.KAHN,6 R. I.KLEIN,1,7 C. F. MCKEE,1,8 R.PETRE,9 R.PISARSKI,9 J. C. RAYMOND,10 AND S. L. SNOWDEN9 Received 1996 December 4; accepted 1997 February 11 ABSTRACT We describe and report progress on the joint U.S. and German campaign to map the X-ray emission from the entire Cygnus Loop with the ROSAT High Resolution Imager. The Cygnus Loop is the proto- type for a supernova remnant that is dominated by interactions with the interstellar medium and supplies fundamental physical information on this basic mechanism for shaping the interstellar medium. The global view that these high-resolution (FWHM D 10A) observations provide emphasizes the inhomo- geneity of the interstellar medium and the pivotal nature of cloudÈblast-wave interactions in determining the X-ray morphology of the supernova remnant. While investigating the details of the evolution of the blast wave, we also describe the interstellar medium in the vicinity of the Cygnus Loop, which the pro- genitor star has processed. Although we do not expect the X-ray observations to be complete until 1997 September, the incomplete data combined with deep Ha images provide deÐnitive evidence that the Cygnus Loop was formed by an explosion within a preexisting cavity. Subject headings: ISM: individual (Cygnus Loop) È supernova remnants È X-rays: ISM 1. INTRODUCTION wave interactions. The ROSAT HRI combines high 0.1È2.4 keV sensitivity (approximately three times better than the Supernova explosions are one of the most basic processes Einstein HRI;Zombeck et al. 1990), high spatial resolution that deÐne the physical and chemical state of the interstellar (FWHM B 6A on axis), and high contrast imaging (the half- medium (ISM). The interaction between supernova rem- energy radius is 3A on axis, with scattering less than a few nants (SNRs) and the ISM is complex and symbiotic. X-ray percent;Aschenbach 1988) to provide a powerful tool for observations are important in the study of SNRs because investigating cloudÈblast-wave interactions. thermal emission from hot material (kT D 0.1È1 keV) is the The region of the Milky Way toward Cygnus is viewed primary tracer of gas that the supernova remnant blast along the Carina-Cygnus arm and exhibits a complex array wave heats and accelerates. High-resolution X-ray observ- of bright and dark clouds on the Palomar Sky Survey. Most ations are important for studying SNR gas dynamics for remarkable are the arcs and loops of faint Ha nebulosity, three reasons. First, the length scale, , for cooling and l the smallest and brightest of which is the Cygnus Loop recombination behind a radiative shock is small: l B 2.5 (l 74.0, b 8.6). The Cygnus Loop is the prototype 10 v /n cm, wheren is the preshock number density \ \[ ] 17 s4 middle-aged SNR and is a particularly suitable candidate andv is7 the0 shock velocity0 in units of 100 km s (McKee s ~1 for a case study being nearby (770 pc;Minkowski 1958) and 1987).7Second, the preshock medium is not uniform, having bright with low foreground extinction [E(B V ) 0.1], clouds and substructure ranging over scales from tens of [ \ thus ensuring an unrivaled signal-to-noise ratio and spatial parsecs and downward. Third, gas-dynamical instabilities resolution. From an observational perspective, the Cygnus are important, and multidimensional hydrodynamic calcu- Loop is unique in terms of X-ray brightness, shock front lations predict that fully developed turbulence occurs when resolution (1A 4 1.1 10 cm), and interaction with a the blast wave collides with an interstellar cloud (Klein, ] 16 range of interstellar conditions. The Cygnus Loop is also at McKee, & Colella1994; Klein, McKee, & Woods 1995). sufficiently high galactic latitude that confusion with back- Consequently, structure is present down to the smallest ground emission from the plane of the galaxy is minimized. scales that can be resolved computationally. The improved Unlike other well-known younger SNRs such as Cas A, ROSAT HRI sensitivity and spatial resolution compared Tycho, and Kepler, the structure of the Cygnus Loop is with Einstein make it invaluable for exploring cloudÈblast- dominated by its interaction with its environment. The large diameter of the Cygnus Loop (3¡ 4 40 pc) assures that 1 Department of Astronomy, University of California, Berkeley, CA it is interacting with di†erent components of the ISM, from 94720. molecular gas on the northwest edge(Scoville et al. 1977), to 2 Max-Planck-Institut fu r Extraterrestrische Physik, Giessenbachs- trasse, D-85740 Garching, Germany. di†use atomic gas along the northeast and eastern limbs 3 Johns Hopkins University, Department of Physics and Astronomy, (DeNoyer 1975; Hester,Raymond, & Blair 1994), and low- Baltimore, MD 21218. density, hot, ionized gas to the south(Ku et al. 1984). Pre- 4 Department of Physics and Astronomy, Dartmouth College, vious studies of the Cygnus Loop have focused on Hanover, NH 03755. 5 Department of Physics and Astronomy, Arizona State University, individual regions selected for their brightness or morpho- Tempe, AZ 85287. logical peculiarity (e.g.,Hester & Cox 1986; Hester et al. 6 Departments of Astronomy and Physics, Columbia University, New 1994; Fesen,Kwitter, & Downes 1992; Graham et al. 1995; York, New York 10027. Levensonet al. 1996). The goal of this high-resolution inves- 7 Lawrence Livermore National Laboratory, Livermore, CA 94550. tigation(Graham & Aschenbach 1996) is to make an 8 Department of Physics, University of California, Berkeley, CA 94720. 9 Goddard Space Flight Center, Greenbelt, MD 20771. unbiased survey and thereby establish globally the state of 10 Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138. the medium into which the blast wave expands. A coherent 304 X-RAY SURVEY OF THE CYGNUS LOOP 305 picture emerges from our preliminary results(° 2). We therefore adopted the following observing strategy. We use present a uniÐed interpretation of the environment and the full 6A resolution to observe the brightest regions, which development of the Cygnus Loop(° 3), constrain the global restricts the e†ective Ðeld of view to 8@ radius. Each of these models that are relevant(° 4), and summarize the conclu- 24 pointings requires 15È20 ks to obtain 15 counts per pixel. sions in ° 5. In a comparable exposure time, the moderate surface brightness interior regions yield about 10 counts per 2. OBSERVATIONAL STRATEGY AND DATA REDUCTION 12A ] 12A pixel. At this resolution, the e†ective HRI Ðeld of Our primary goal is to map the entire Cygnus Loop view has a 12@ radius. We have observed these nine Ðelds for remnant at the highest spatial resolution that can be 20È25 ks. Observing the low surface brightness regions, we attained with the ROSAT HRI. Achieving this ambition is obtain 10 counts per cell only when using 15A or larger not simple, because the spatial resolution is limited by the pixels, for which the full HRI Ðeld of view is suitable. Each intrinsic instrumental properties and photon counting sta- of these 10 Ðelds requires a 35 ks exposure. Finally, one very tistics. The on-axis resolution is about 6A, but the o†-axis low surface brightness region along the western edge of the imaging performance is dominated by telescope aberra- ““ breakout ÏÏ requires a 50 ks exposure to obtain reliable tions, which increase with increasing Ðeld angle. At 20@, the counting statistics. The locations of the HRI pointings are edge of the useful Ðeld of view of the HRI, the resolution has shown on the Einstein IPC map (Fig. 1). degraded to about 30A. The large range of surface brightness A few Ðelds were observed during the initial operation of allows us the luxury of mapping the brightest regions at full ROSAT , and a concerted e†ort to make a complete map HRI resolution but forces us to seek a compromise between began during AO5, in 1994 October.
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