Astron. Astrophys. 341, 602 609 (1999) ASTRONOMY AND ASTROPHYSICS ROSAT all-sky survey map of the Cygnus Loop: Overall structure and comparison with radio map B. Aschenbach and D.A. Leahy Max-Planck-Institut für Extraterrestrische Physik, D-85740 Garching bei München, Germany Received 20 March 1998 / Accepted 27 August 1998 Abstract. The Cygnus Loop is one of the largest and nearest supernova remnants. It was observed by the PSPC detector of the Röntgen-Satellite ROSAT during the survey phase of the ROSAT mission. This has resulted in the highest sensitivity complete x-ray image of the Cygnus Loop. It also has the high spatial resolution of one arcmin and contains spectral information. It is thus the best map of the Cygnus Loop for studying the overall spatial and spectral structure of its x-ray emission. The Cygnus Loop shows x-ray emission from a more extensive region than previously appreciated, including the south blowout region and a newly detected west bubble. Significant fine scale spectral variations are detected. The structure of the Cygnus Loop in x-rays is compared to the radio structure recently observed at 1420 MHz with one arcmin resolution using the synthesis telescope of the Dominion Radio Astrophysical Observatory. Key words: shock waves ISM: Cygnus Loop ISM: supernova remnants X-rays: ISM 1. Introduction The Cygnus Loop is a well-studied large (3 diameter) supernova remnant (SNR) with a clear shell-type morphology in radio, infrared, optical, and x-ray bands. Previous radio observations are summarized by Green et al. (1990), and new radio observations are presented in Leahy et al. (1997). The optical nebula is clearly visible on the Palomar sky survey plates. An optical and x-ray study of an optical knot on the SE rim of the Cygnus Loop is given by Graham et al. (1995), which also includes references to previous optical studies of the Cygnus Loop. Ku et al. (1984) presented the Einstein IPC x-ray image. More recent x-ray observations of the Cygnus Loop are described by Leahy, Fink and Nousek (1990), Hatsukade and Tsunemi (1990), Ballet and Rothenflug (1989), and references therein. The newest data has come from the ASCA and ROSAT missions: an x-ray spectroscopic study of the NE rim of the Send offprint requests to: B. Aschenbach Permanent address: Department of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada T2N 1N4 Cygnus Loop with ASCA is given by Miyata et al. (1994); Levenson et al. (1996) present an HRI observation of the western edge of the Cygnus Loop; Levenson et al. (1997) give a progress report on the ROSAT HRI survey of the Cygnus Loop; Decourchelle et al. (1997) present ROSAT images and spectra of a region in the northern part of the Cygnus Loop. Recently, Miyata et al. (1998) report about the chemical composition of selected regions, in particular the centre, of the Cygnus Loop based on ASCA data. Here we present the ROSAT PSPC x-ray images of the entire Cygnus Loop, in soft and broad bands, the color map and the energy image. The comparison of radio and x-ray emission from supernova remnants is of great interest. The x-ray emission traces the hot gas behind the shock front, while the radio emission is from the relativistic electron population emitting synchrotron radiation in the ambient magnetic field. The synchrotron emission should have both upstream (relative to the shock position) and downstream components. The extent of these is governed by both the amplification of the magnetic field in the shock compression and the relative density of relativistic electrons upstream and downstream. The latter is determined by the scattering mechanisms for the electrons, which as yet are not well understood (e.g. see the review by Draine and McKee, 1993). The comparison of the ROSAT x-ray image is made with a new complete radio image (Leahy et al. 1997) taken with the synthesis telescope of the Dominion Radio Astrophysical Observatory, near Penticton, Canada. 2. Observations and image analysis The ROSAT data for the Cygnus Loop were extracted from the all-sky survey (also referred to as RASS, Voges (1993)). The ROSAT instruments are described in Aschenbach (1988), Pfeffermann et al. (1986) and Trümper (1983). EXSAS/MIDAS data processing software was used to produce 512 512 maps, with 40 pixels, for the 7 energy bands.08-.19,.20-.41,.42-.51,.52-.69,.70-.90,.91-1.31, and 1.32-2.01 kev. The average vignetting corrected exposure time was 608.7 s. The Cygnus Loop is progressively more limb brightened in the lower energy bands compared to the higher energy bands. Three broad-band energy maps were also computed: in.08-.41,.52-.90,.91-2.01 kev bands. The.08-.41 kev map has a total
B. Aschenbach & D.A. Leahy: ROSAT X-ray map of the Cygnus Loop 603 intensity of 861 c/s, the.52-.90 kev map has 574 c/s, and the.91-2.01 kev map has 224 c/s, for a total intensity of 1659 c/s from the Cygnus Loop. The.08-.41 kev band map is shown in Fig. 1. A higher spatial resolution x-ray map was made for the broad energy band.20-2.01 kev by restricting the photon event selection to less than 20 from the center of the optic axis on the PSPC detector. Since the ROSAT mirrors have poorer spatial resolution at energies below 0.2 kev, we did not include the.08-.2 kev map. This broad band, high spatial resolution map of the Cygnus Loop is shown in Fig. 2. A color coded map showing the 3-energy-band spectrum of the Cygnus Loop was constructed by assigning the.08-.41 kev map intensity to red intensity,.52-.90 map intensity to green intensity, and.90-2.01 map intensity to blue intensity. The resulting X-ray color map is shown in Fig. 3. The distribution of photon energies across the Cygnus Loop was also examined by constructing an energy image, where the content of each pixel corresponds to the mean energy per count in that pixel. The energy image is the most direct way to look for any spectral changes, although without intensity information, and is shown in Fig. 4. The radio image was produced (Leahy et al. 1997) from observations with the Dominion Radio Astrophysical Observatory s Synthesis Telescope at 1420 MHz. This radio image is the highest resolution complete radio image of the Cygnus Loop. Its spatial resolution of 1 arcminute is well matched to the ROSAT PSPC survey data. The radio map was put in the same projection, i.e. tangent plane, with the same projection center as the x-ray map. The conversion introduced an offset of the radio pixels with respect to the x-ray pixels of 1.75, small compared to the 40 pixel size. The radio map is shown in Fig. 5 with the contours of the.08-.41 kev x-ray map overlaid. Fig. 6 shows the color-coded ratio of x-ray to radio in units of counts per second per pixel divided by Kelvin (brightness temperature). 3. Discussion 3.1. Overall structure of the Cygnus Loop in x-rays The RASS maps presented here are a significant improvement over previous x-ray maps of the Cygnus Loop due to the increased spatial resolution and sensitivity of the ROSAT PSPC detectors compared to previous instruments. The RASS survey maps, due to the scanning mode of observation, also have the advantage of uniform coverage of the Cygnus Loop, essentially free from artifacts, which otherwise is very difficult to achieve for such a large source. A comparison of the current maps with e.g. the Einstein IPC image shows that all previous structure is detected, but with better sensitivity, primarily due to the PSPC s low instrument background, and better spatial resolution. The RASS maps of the Cygnus Loop further show a number of features of the x-ray emission not previously known. The high sensitivity.08-.41 kev map (Fig. 1) shows emission from the south blowout region. Only the northeast tip of this region had been detected previously, in the Einstein IPC map. A spur on the west side of the blowout is newly detected here in x-rays. A nearly circular bubble of about 0.4 radius on the west limb of the Cygnus Loop, just below center, is also detected for the first time. Structure in the carrot region (in the northwest, at about 0.6 of the distance from center to rim) is now clearly visible. But the most pervasive feature of the overall appearance is the filamentary structure throughout the Cygnus Loop. This can also be described as a set of overlapping limbbrightened shell segments, with radii of curvature ranging from about 5 to 50 arcminutes, with the smaller radii segments on the N and W rims, and the larger radii segments on the E to S rims and in the E and S interior regions. The higher resolution, but lower sensitivity, map is shown in Fig. 2. It does not show the faint filamentary and shell structure as well as Fig. 1, but shows the sharpness of the brighter features. The entire rim of the Cygnus Loop is now seen to be sharp. A double rim is clearly seen along the W to NW perimeter. This is most likely a projection effect, where we look at two edges of a corrugated outer shock along the line of sight. There is also a less obvious double rim along the NE perimeter. Other sharp features include the carrot, the prominent V adjacent to the SW rim, the east rim, and bright filaments in the NNW and near the center of the Cygnus Loop. The color maps show spectral variations of the x-ray emitting gas in the Cygnus Loop. Miyata (1996) has fit non equilibrium ionization models to the ASCA observations of the Cygnus Loop. The results of spectral fits to the SIS data (with similar results for the GIS data) indicate very significant spatial variations in temperature (0.24 to 0.80 kev) but less significant variations in element abundances and in ionization timescale. Although the spectral resolution of the PSPC is not enough to disentangle these effects and their impact on the spectra, we tentatively interpret the PSPC spectral variations in terms of temperature variations. The temperature is lower in the bright filaments (i.e. compare Fig. 1 with Figs. 3, 4): the anticorrelation of temperature with brightness is very strong. The spectral variations of the fainter emission, shown in Figs. 3, 4, show that the diffuse interior of the Cygnus Loop is significantly hotter than the bright filaments on the rim and in the interior, seen in Fig. 1. The higher brightness of the rim than the center, and the lower temperature of the rim than the center can be both accounted for by models of approximate pressure equilibrium, i.e. lower temperatures for higher density features and therefore higher brightness, enhanced by projection effects, which provide higher brightness close to the rim because we see predominantly along the filaments compressed by the shock wave, and lower brightness towards the central region because we see them face-on. Some of the notable x-ray structures are seen to have different temperatures than their nearby emission in the Cygnus Loop. The south blowout region in its northern part (Figs. 3, 4) has a hot interior, and appears even hotter than the hottest part of the rest of the Cygnus Loop. The hard spectrum could be due to the complete lack of cool filamentary emission in addition to the hot diffuse emission in that region. The west rim of the south blowout, and also the entire west bubble are of temperature similar to the cool bright filaments on the rim of the
604 B. Aschenbach & D.A. Leahy: ROSAT X-ray map of the Cygnus Loop Fig. 1. The ROSAT 0.08-0.41 kev map of the Cygnus Loop. Coordinates are right ascension, declination of epoch 2000.0. Colour scale is logarithmic, units are counts/s pixel. Fig. 3. Three band x-ray color image of the Cygnus Loop. The three basic colours red, green and blue refer to the x-ray count density in bands 80 410 ev, 520 900 ev and 910 2010 ev, respectively. Coordinates are right ascension, declination of epoch 2000.0. Fig. 2. The ROSAT high-resolution broad-band (0.2-2 kev) map of the Cygnus Loop. Coordinates are right ascension, declination of epoch 2000.0. Colour scale is logarithmic, units are counts/s pixel. Fig. 4. The ROSAT X-ray energy image of the Cygnus Loop. Coordinates are right ascension, declination of epoch 2000.0. Colour scale is linear, units are 10 ev/count. Cygnus Loop. Many of the interior filaments are also similarly cool: such as just interior to the NW rim and interior to the SW rim. Other fainter interior filaments appear to be warmer, such as the filaments at the center of the Cygnus Loop and the south part of the carrot feature. However this may be entirely due to observing a mixture of cool filament emission and hot interior emission along the same line of sight. Fig. 3 also shows that the rim clearly has cool (red in Fig. 3), intermediate (yellow/green) and hot (blue) regions. The SW and NW parts of the rim are cool; the N rim, the WNW rim and most of the SE rim are hot; and the NE to E rim and the bright spot
B. Aschenbach & D.A. Leahy: ROSAT X-ray map of the Cygnus Loop 605 Fig. 5. The DRAO Synthesis Telescope radio map of the Cygnus Loop (colour scale is logarithmic, units are K) with overlay of contours of the ROSAT.08-.41 x-ray map. The x-ray contours in units of counts/(s pixel) run from 0.001 to 0.128 stepped by a factor of 2. Coordinates are right ascension, declination of epoch 2000.0. Fig. 6. Ratio of x-ray brightness to radio brightness for the Cygnus Loop; colour scale is logarithmic, units are counts/(s pixel K). Coordinates are right ascension, declination of epoch 2000.0.
606 B. Aschenbach & D.A. Leahy: ROSAT X-ray map of the Cygnus Loop on the W rim are intermediate in temperature. But all parts of the rim are cooler than the interior, as noted above. The V feature on the SW rim has a cool right wing, and a hot left wing, so appears to be two linear structures seen in projection rather than a single physically connected feature. This temperature difference of the V has also been noted by Miyata (1996). The double rim in the NW has a slightly hotter inner rim than the outer rim, particularly at the south end where the two rims merge. The NE double rim (see Fig. 2) also has a cooler outer rim than the inner rim. The bright filaments in the entire east and north portions of the Cygnus Loop are hotter than the filaments in the northwest to west region. Some more details can be extracted from a comparison of the.08-.41,.52-.90 and.91-2.01 kev band maps. The comparison confirms that the brighter emission is cooler than the faint emission. The.91-2.01 kev map is relatively brighter in the center (more in the south center) and the.52-.90 kev map is brighter in a thick rim. The.08-.41 kev map compared to the.52-.90 kev map gives the following results for specific structures. The.08-.41 kev map is brighter and shows more structure all around the east, north and west rims, but the SE bright region looks nearly the same in both maps. The interior except for the central filaments is brighter in.52-.90 kev map than the.08-.41 kev map. For the south blowout: its west rim is brighter in the.08-.41 kev map; its east rim has the same structure in both maps; and a small section of the south rim is not visible in either. The interior of the blowout is brighter in the.52-.90 kev map. The spur on the west side of the south blowout is clearly visible in the.08-.41 kev map but also appears to have weak diffuse emission in the.52-.90 kev map. The west circular bubble feature is clearly defined in the.08-.41 kev map but also appears to have weak diffuse emission in the.52-.90 kev map. Band ratio maps (.52-.90 kev)/(.08-.41 kev), (.91-2.01 kev)/(.08-.41 kev) and (.91-2.01 kev)/(.52-.90 kev) were also created. A comparison of the (.91-2.01 kev)/(.08-.41 kev) map and the.08-.41 kev map shows almost perfect anticorrelation between brightness and energy ratio. This is consistent with the x-ray color map of Fig. 3. Similarly, the (.52-.90 kev)/(.08-.41 kev) map and the.08-.41 kev map show strong anticorrelation, but not as strongly. A slight change in energy ratio (cooler toward rim) is also seen in the (.91-2.01 kev)/(.52-.90 kev) map but the change is smaller than in the (.91-2.01 kev)/(.08-.41 kev) map or in the (.52-.90 kev)/(.08-.41 kev) map. The (.91-2.01 kev)/(.52-.90 kev) map shows that the interior of the south blowout is hotter than even the central region, confirming the conclusion from the color map Fig. 3. The west bubble and the rim of the south blowout are both found to be very cool though faint, in contrast to the interior of the south blowout which is hot. In summary, significant temperature variations which have not been detected previously are seen along the rim and in the bright filaments interior to the rim. Also distinctive temperatures are seen in the newly detected western bubble, south blowout interior and south blowout rim. An evident interpretation is that the Cygnus Loop was caused by a supernova exploding in a cavity (e.g. see Hester, Raymond and Blair 1994). The temperature variations of the bright emission near the rim are caused by variations in density in the dense cavity wall which has been just recently shocked. The interior hot diffuse emission is from the shock heated cavity gas. 3.2. Comparison of the X-ray and radio structure The x-ray map of the Cygnus Loop (Fig. 1) shows a strong filamentary structure as well as general diffuse emission, both of which are brightest in the north. This is in contrast to the radio emission (Fig. 5). The radio map also shows much filamentary structure, but has a relatively stronger diffuse component, and is brightest in the south. Next we compare the x-ray and radio maps in more detail. The ROSAT PSPC image shows that the south blowout region is detected, but faint, in x-rays: in contrast it is very bright in radio. The spur on the west side of the blowout has been clearly mapped in radio only recently (Leahy et al. 1997; Fig. 5 here), although it was detected by Moffat (1971) and Green (1984). The newly detected west bubble on the west limb of the Cygnus Loop, appears to have very weak diffuse radio emission associated with it in the map of Leahy et al. (1997). The carrot feature is seen as a somewhat isolated vertical group of filaments in the northwest, at about three-fifths of the radius from the center, in both the x-ray and radio maps. However the x-ray and radio emission are spatially separated: the x-ray emission of the east side of the carrot is east of the radio emission by nearly a constant 3 arcmin along the length of the east side, and there is no x-ray emission from the west side of the carrot. The serpent radio filament of the Cygnus Loop (Leahy et al. 1997) is the long bright radio filament which can be traced from near the NNW rim through the center of the Cygnus Loop and south into the south blowout region. It was first noted by Green (1984). By blinking the radio map and the high resolution x-ray map, one can trace the serpent radio filament in x-rays all the way along its length, except for a short piece 13 arcmin long which includes the radio source CL4. The serpent also lies along the east edge of the western hot region observed in the x-ray hardness ratio maps. This is also seen here in Fig. 3: the interior west of the serpent has a harder spectrum than the interior east of the serpent. I.e., the serpent divides two regions of different hardness ratio in x-rays. The bright radio emission in the south blowout region is similarly at the east edge of a hot region seen in x-rays. There are significant differences in the distribution of x- ray and radio brightness in the Cygnus Loop. However, many filamentary features are seen at both wavelengths, as can be seen from an examination of Fig. 1 and Fig. 5 (and discussed below). Fig. 6 shows the ratio of x-ray surface brightness Σ x to radio surface brightness Σ r in the Cygnus Loop in units of counts per second per pixel per Kelvin. The x-ray and radio surface brightnesses are given by: Σ x ɛ(t )n 2 e dl, Σ r n rel B (γ+1)/2 perp dl. Here n e is the thermal electron density, n rel is the relativistic electron density, and we have as-
B. Aschenbach & D.A. Leahy: ROSAT X-ray map of the Cygnus Loop 607 sumed a power law electron energy distribution, with index γ, and perpendicular component of magnetic field B perp. The x-ray emissivity is not a strong function of temperature, and since the temperature varies only between 0.24 kev to 0.8 kev within the Cygnus Loop (Miyata 1996) the expected ROSAT PSPC count rate would vary by just ±40%, assuming N H =4. 10 21 cm 2. We take the approximation that temperature is nearly constant along the line of sight, which is good since a single line of sight is likely to encompass a smaller temperature range than the entire variation in the Cygnus Loop. Then ɛ can be taken outside the integral. Further, if n 2 e and n rel B perp (γ+1)/2 are constant along the line-of-sight, the ratio reduces to n 2 e/n rel Bperp, 1.75 where we have used a value of 2.5 for γ. In the more likely physical situation of strong variations along the line-of-sight, the ratio will still simplify if the dominant contributions for both integrals come from the same region, and n 2 e and n rel B perp (γ+1)/2 are distributed similarly in this region. Then one obtains the ratio of n 2 e/n rel Bperp 1.75 in this dominant region. The observed surface brightness ratio shows strong variations: from 0.0005 to 0.2 counts per second per pixel per Kelvin. The central and west interior and south blowout have low ratio (dark blue in Fig. 6); most of the rim and the NE half of the interior have an intermediate ratio (green in Fig. 6); the N-NE rim, much of the east interior, and spots along the W and NW rim have high ratio (red or white in Fig. 6). The distribution of the ratio is highly structured (Fig. 6). The local structure is well correlated with structure in both the radio and x-ray maps. However, it is dominated by the image with the larger local relative variation. In many places, bright radio filaments show up as blue (e.g. along the east and northeast rims, and along the serpent), and bright x-ray filaments show up in red (e.g. north, west and southwest rims). But this is not generally true: the interior to the east rim is red due to the faintness in radio; the western interior is blue due to the faintness in x-rays. Good correlation but not exact correspondence is found between the brightest x-ray filaments and the radio filaments in the NE rim region, the southwest rim region which includes the V (very prominent in Fig. 3), the west side of the south blowout region, and the north rim region. This correlation is best seen in a comparison of the maps in Fig. 2 and Fig. 5; the x-ray contours in Fig. 5 do not show the fine structure of the x-ray filaments very well. But, for instance, for the north rim region, the top part of the large northern X filament in x-ray closely matches the radio X filament. The current radio and x-ray maps show that the positions of the brightest interior x-ray and radio filaments are the same to within about two arcminutes. Since the Cygnus Loop is a nearby supernova remnant, this already gives limits on the upstream diffusion length and thus a lower limit to the magnetic field fluctuations which scatter the relativistic electrons. They are comparable to those obtained already using more distant supernova remnants (Achterberg et al. 1994). Despite the positional coincidence, we see strong variations in the ratio of x-ray brightness to radio brightness. Traces of intensity in both the x-ray and radio images, along the same line, were made South to North (S-N) and East to West (E-W) across a diameter of the Cygnus Loop. These traces are shown in Leahy and Aschenbach (1996). The S-N trace through the center of the Cygnus Loop shows radio emission extending beyond the x-ray emission by about 2. Other S-N traces show variations on whether x-ray or radio extends further, but generally the x-ray and radio emission edges are within 3.5 of each other. The E-W trace shows x-ray emission 1.2 outside the radio emission on the east rim, but on the west rim the x- ray emission has two steps: one outside the radio rim by 2 ; the second inside the radio rim by 1. Both S-N and E-W traces show less interior emission in x-rays compared to radio. A more complete study has been made on the relative positions of x-ray and radio emission at the outer shock of the Cygnus Loop. A set of slits, each 5 pixels wide by 100 pixels long (each pixel is 40 in size), was positioned completely around the Cygnus Loop x-ray image, with the slit long dimension locally perpendicular to the shock, defined by the x-ray edge. A second set was positioned on the radio image at the identical sky coordinates. Brightness profiles in x-ray and radio were produced by extracting the slit images, then averaging across the width of each slit image. The errors in x-ray brightness profile can be calculated from counting statistics. They are a function of position but generally are about 0.05 count/s/pix. The errors are insignificant for the radio map, where the variation in galactic background is responsible for the background variation. The position errors for both the x-ray and radio maps are less than 5. To determine the edge a linear background was used in a 10 arcmin region outside the Cygnus Loop. The edge was determined by where the emission was first significantly above the background. Three sample profile pairs are shown in Figs. 7, 8 and 9. The profile pair in Fig. 7, from the Cygnus Loop southeast rim (at R.A. 20h 53m 50.5s, Dec. 29 51 35 ), has an x-ray to radio offset consistent with 0. The profile pair in Fig. 8, from the Cygnus Loop NW rim (at R.A. 20h 48m 1.6s, Dec. 32 16 53 ), has the radio edge outside of the x-ray edge by 4 arcmin. This is one of the largest offsets of all of the profiles. The profile pair in Fig. 9, from the Cygnus Loop NE rim (at R.A. 20h 57m 17.2s, Dec. 31 38 29 ), has the radio edge inside of the x-ray edge by 1.6 arcmin. A histogram of the differences in the positions of the edge of radio emission relative to the edge of the x-ray emission for all of the profiles along the entire limb of the Cygnus Loop is given in Fig. 10. Most of these radio x-ray edge offsets are in the range of ±1.8 arcmin, i.e. the FWHM of the distribution in Fig. 10 is 3.6. The mean value of the offset is 7. This is consistent with no offset, since the error in determination of any individual x-ray or radio edge is of order the spatial resolution of 1 arcmin for both wavelengths, and the error in determination of the mean of all of the offsets with no systematic error is 5. This yields a 3-σ upper limit on the offset of 15. From examination of the x-ray and radio images, we know that in some cases the difference in position is due to brightness variations, where one filament may be bright in x-rays but faint in radio and a nearby filament bright in radio but faint in x-rays. Thus the most plausible explanation of the offsets is that we are
608 B. Aschenbach & D.A. Leahy: ROSAT X-ray map of the Cygnus Loop Fig. 7. Brightness profiles in x-ray (solid line, units: 0.5 counts s 1 pixel 1 ) and radio (dashed line, units: brightness temperature K) from a slit crossing the southeast limb of the Cygnus Loop. Fig. 9. Brightness profiles in x-ray (solid line, units: 0.5 counts s 1 pixels 1 ) and radio (dashed line, units: brightness temperature K) from a slit crossing the northeast limb of the Cygnus Loop. Fig. 8. Brightness profiles in x-ray (solid line, units: 0.5 counts s 1 pixels 1 ) and radio (dashed line, units: brightness temperature K) from a slit crossing the northwest limb of the Cygnus Loop. seeing many filaments with greatly varying brightness ratios, due to greatly varying thermal particle and relativistic particle densities and magnetic fields. Thus we see radio outside x-ray if the outermost filament is radio bright and radio inside x-ray if the outermost filament is x-ray bright. 4. Conclusions With the ROSAT PSPC all-sky survey data, new high sensitivity high resolution x-ray maps of the Cygnus Loop have been made. We have detected x-ray emission clearly from the south blowout, the west spur on the south blowout, and the west bubble. The properties of the x-ray and radio emission change smoothly between the north circular part of the Cygnus Loop and the south blowout, which is evidence that the south blowout is an asymmetry in the Cygnus Loop and not a separate supernova remnant. The x-ray emission shows finer structures, and is more limb brightened in lower than in higher energy bands. The Cygnus Fig. 10. Histogram of offsets of the radio edge relative to the x-ray edge for the full set of brightness profiles. Loop is hotter in its center; it is brighter and cooler at its rim and in the filaments. This is consistent with the observed brightness temperature anticorrelation, which in turn can be explained by brighter cooler emission resulting from recently shocked clouds. This would be expected, for example, if the Cygnus Loop were the result of an explosion in a wind-blown cavity and only recently has the shock encountered the higher density of the cavity wall. The cavity wall needs to be very irregular in shape and density to produce the numerous arc like and cloud like features all along the perimeter of the Cygnus Loop as observed in x-rays (Aschenbach 1996). Strong variations in the ratio of x-ray surface brightness to radio surface brightness demonstrate strong inhomogeneities in the quantity n 2 e/n rel Bperp. 1.75 The magnetic field and relativistic particle variations globally are not correlated with the density inhomogeneities in the x-ray emitting plasma. However, in a
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