shell and Ðlled-center morphologies. One leading explanation for the center-ðlled X-ray

Transcription

1 THE ASTROPHYSICAL JOURNAL, 488:307È316, 1997 October 10 ( The American Astronomical Society. All rights reserved. Printed in U.S.A. ROSAT OBSERVATIONS OF THE SUPERNOVA REMNANT CTB 1 WILLIAM W. CRAIG Columbia Astrophysics Laboratory, 538 West 120th Street, New York, NY 10027; bill=astro.columbia.edu CHARLES J. HAILEY Columbia Astrophysics Laboratory, 538 West 120th Street, New York, NY 10027; chuckh=astro.columbia.edu AND RYSZARD L. PISARSKI Astrophysics Data Facility, Goddard Space Flight Center, Code 631, Greenbelt, MD 20771; rlp=ssvs.gsfc.nasa.gov Received 1996 September 11; accepted 1997 May 13 ABSTRACT We present ROSAT PSPC observations of the supernova remnant CTB 1 (G116.9 ]0.2). The remnant is detected for the Ðrst time in the X-ray region. The X-ray morphology places the remnant into the class of center-ðlled remnants with a centrally peaked X-ray emission proðle and accompanying optical and radio shells. The emission is consistent with a thermal plasma with T \ 3 ] 106 K and absorption N \ 7 ] 1021 cm~2. A breakout ÏÏ along the northeast rim of the remnant is coincident with the position H of an X-ray pulsar. A much larger feature appears to be a partial shell which may be associated with a previous supernova remnant that swept the area around CTB 1 clean. The appearance of the remnant is most easily explained by evaporating clouds. Projection e ects caused by an asymmetric remnant are discussed as another, less plausible, explanation. Subject headings: ISM: individual (CTB 1, G117.7]0.6) È supernova remnants È X-rays: ISM 1. INTRODUCTION Intermediate-age supernova remnants (SNR) can be a valuable probe of the interstellar medium (ISM), as well as of blast-wave physics and particle acceleration. As the number of these objects studied has increased, particularly in the X-ray band, it has become clear, however, that these objects are a heterogeneous population that must be understood in their own right before their usefulness as a probe of ISM structure is realized. Excepting plerionic sources such as the Crab, two broad classes of intermediate-age (adiabatic phase expansion) SNRs are known. The Ðrst, with a shell-like appearance in the radio, optical, and X-ray passbands, includes wellknown objects such as the Cygnus Loop (see, e.g., Fesen, Blair, & Kirshner 1982; Leahy & Aschenbach 1996) and Vela (see, e.g., Kahn et al. 1985). The second, the center- Ðlled ÏÏ remnant class, has a number of members, among them G299.2[2.9 (Busser, Egger, & Aschenbach 1996), 3C400.2 (Saken et al. 1995), W44 (Rho et al. 1994), and HB9 (Leahy & Aschenbach 1995). These objects retains a shelllike appearance in the radio and optical but have a center- Ðlled appearance in the X-ray (0.1È2.0 kev) passband. There are also objects (see, e.g., W51C, Koo, Kim, & Seward 1995; W28, Long et al. 1991), which show characteristics of both shell and Ðlled-center morphologies. One leading explanation for the center-ðlled X-ray appearance is o ered by White & Long (1991; hereafter WL). In the WL model, density enhancements (clouds) in the ambient ISM are overtaken by the blast wave from the SN and are engulfed in a hot, thin plasma. If the cloud sizes and evaporation times are in the right range, the clouds are evaporated on an appropriate timescale, signiðcantly enhancing the X-ray emission in the center of the remnant. The WL model has been shown to be consistent with the X-ray emission from a number of center-ðlled SNRs (Long et al. 1991; Rho et al. 1994). There remain, however, a number of questions about the applicability of the WL 307 model. Among them are the possible deðcit of Ha emission expected from the boundary of the evaporating clouds, and the tendency for center-ðlled remnants to appear only at high (N [ 5 ] 1021 cm~2) column densities. Expanding H the survey of galactic SNRs in both the optical and X-ray passbands is the best way to address these questions. CTB 1 (G116.9]0.2) is a galactic supernova remnant that had been unstudied in the X-ray until the ROSAT observations reported on here. The remnant has an almost complete shell in both the optical (van den Bergh, Marscher, & Terzian 1973; Fesen et al. 1996) and in the radio (Willis 1973; Dickel & Willis 1980; Landecker, Roger, & Dewdney 1982). The distance to CTB 1 as derived by bulk optical velocity and a galactic rotation curve (Hailey & Craig 1994) is 3.1 kpc. The radius of the optical shell, as well as that of the X-ray emission, is D14 pc, assuming this distance. The radio images suggest a breakout region in the northeast region of the remnant. An X-ray pulsar (Hailey & Craig 1995) has been discovered in this region. Optical spectra (Hailey & Craig 1994) indicate a smooth density gradient along the rim, with the western portion of the blast wave encountering a signiðcantly higher ambient density than in the east. The remnant was not detected by the Einstein X-Ray Observatory, in part because the nearest pointing was not centered on the remnant itself. Thus the question of whether CTB 1 has a center-ðlled or shell-like X-ray appearance was not answered until these ROSAT observations were made. 2. OBSERVATIONS Since the X-ray emission that might be expected from CTB 1 was larger than the central 40@ diameter Ðeld of view of the ROSAT PSPC, two pointings were performed. The Ðrst, which we refer to as the central pointing, placed the center of CTB 1 within the central 40@ region of the PSPC. The second, referred to as the northeast pointing, was centered on the northeast breakout region. The pointings over-

2 308 CRAIG, HAILEY, & PISARSKI Vol. 488 TABLE 1 ROSAT OBSERVATION LOG Exposure Field Date of Observations a(2000) d(2000) (s) Central Aug 15 23:58: :24: Northeast Aug 16È17 00:03: :33: FIG. 1.ÈProcessed X-ray image for the northeast pointing. Data have been separated into seven energy bands, corrected in the manner described by Snowden et al. (1994) and then combined to form this image. The radio and optical shell of CTB 1 is marked as feature A. Feature B, the limblike extension to the north, is suggestive of a partial shell and is marked by the larger circle (see text). Heavy absorption to the south obscures the shell in the lower part of the image. An X-ray pulsar, described in Hailey & Craig (1995), is marked as feature C. lapped slightly although in most cases they were analyzed separately to avoid complications due to a varying energy and spatial response (particularly outside the central 40@ region) in the ROSAT PSPC. Pointing parameters and exposure times are shown in Table 1. TABLE 2 ENERGY BANDS Beginning End Band Channel Channel Band Band Band Band Band Band Band The raw ROSAT PSPC images as produced by the standard processing clearly show X-ray emission associated with CTB 1. As expected from the high column density toward the remnant, the X-ray Ñux is relatively low. The low surface brightness necessitated a careful treatment of the dataset to fully remove instrumental e ects that could complicate the analysis. We followed the complete prescription for processing of extended sources in the PSPC as given in Snowden et al. (1994). The count rate was Ðrst examined as a function of orbital position of the spacecraft, and periods with obviously elevated count rates were trimmed from the data set. The raw photon list was binned in the seven discrete bandpasses, as shown in Table 2. An energy-dependent exposure correction was then performed, after Ðrst modeling and removing scattered solar X-rays and the long term enhancement ÏÏ of the PSPC gain. Afterpulse events were also rejected. These corrections are relatively minor (less than 5% of the data were rejected) in the

3 No. 1, 1997 ROSAT OBSERVATIONS OF CTB FIG. 2.ÈProcessed image for the central pointing, processed as in Fig. 1 case of these observations. The full process is important to perform, however, particularly when interpreting faint structure outside the central 40@ region of the PSPC. 3. ANALYSIS AND RESULTS 3.1. Spatial Structure The images in each of the individual energy bands were smoothed with an adaptive Ðlter which adjusted in size to include at least 50 photons in the smoothing box. The images for energies from 0.5È2.0 kev were then combined to produce Figures 1 and 2. Because of the heavy absorption toward CTB 1, no photons from the remnant were seen at lower energies. The northeast pointing is marked with the features we discuss in this paper. The CTB 1 remnant is TABLE 3 SPECTRAL FIT RESULTS kt Location (kev) N ] 1022 H s2/d.o.f. Central UR ^ ^ Central UL ^ ^ Central LR ^ ^ Central LL ^ ^ Central all ^ ^ NE lower ^ ^ NE upper ^ ^ NE all ^ ^ shown with its optical/radio shell outlined as feature A in Figure 1. Clearly this SNR is not a shell-like remnant in the X-ray band. The optical/radio shell, as marked on the Ðgure, appears to contain the remnant completely except in the northeast region where the emission has broken out, perhaps into a lower density region (see 4.2). The large, faint limb of emission, marked as feature B, is suggestive of a circular, shell-like SNR. This feature and its implications are discussed in 3.3. The X-ray pulsar found in the northeast region (Hailey & Craig 1995) is marked as feature C. The comparison of the X-ray morphology with the radio maps further conðrms the center-ðlled appearance. The 21 cm map contours from Landecker, Roger, & Dewdney (1982) are shown in Figure 3 overlaid on the X-ray gray scale. The crisp 21 cm contours clearly outline the relatively bright X-ray emission in the central region of CTB 1. The tightly spaced contours along the western rim of the remnant coincide with the higher density seen in the earlier optical work (Hailey & Craig 1994). The northeast region, which is incompletely covered in this map, shows evidence for some less structured radio emission coincident with the X-ray emission. The 1.4 GHz map of Dickel & Willis (1980), as shown overlaid on the X-ray gray scale in Figure 4, covers a wider region and shows that the radio emission extends out to the northeast breakout region, with some suggestion that the radio contours in the northeast also bound the X-ray emission. The shell of optical Ðlaments, as seen on the digitized

4 310 CRAIG, HAILEY, & PISARSKI Vol. 488 FIG. 3.ÈThe 21 cm contours from Landecker, Roger, & Dewdney (1982) overlaid onto the total counts from a mosaic of the two pointings. The shell seen in the radio contours is coincident with the optical emission. POSS O plates overlaid with the X-ray contours in Figure 5, also bound the X-ray emission from the CTB 1 remnant. There is no apparent optical emission from the northeast region. The gap in the shell-like optical emission toward the northeast is coincident with the X-ray breakout. More recent work by Fesen et al. (1996) gives a complete look at the optical emission from the remnant and conðrms the breakout in the northeast and the generally symmetric shape of the remnant Spectral Analysis The spectral properties of the X-ray emission were analyzed using the XSPEC package. After background subtraction and elimination of point sources no emission was seen below 0.5 kev. Since the Ñux from the remnant is rather low, large regions were summed and then Ðtted to determine the physical properties of the emission. The central cavity, i.e., the interior of CTB 1 itself, was well Ðtted by a thermal plasma model with a temperature of 3 ] 106 K absorbed by an equivalent column density of N \ 7 ] 1021 cm~2 as shown in Figure 6a. ConÐdence contours H for the Ðt parameters in this region are shown in Figure 7a. Fits to smaller regions within the central cavity gave results consistent with these conðdence contours with no statistically signiðcant variations in either temperature or column density in the central cavity. The results, for both the central and northeast pointings, are shown in Table 3. For completeness both a bremstrahlung and a power-law model were Ðtted to the data as well. The bremstrahlung model gave a slightly worse Ðt (with a reduced s2 of 1.4 as compared to 1.2 from the thermal model). The power-law models gave a statistically good Ðt only for unphysical values of the power law index (a D 7È9). Given the low number of counts, Ðts to a nonequilibrium ionization model do not well constrain the additional Ðtting parameter and are not presented. The X-ray emission from the northeast breakout region appears to be spatially related to the central region and was Ðtted in the same way. Results as shown in Figures 6b and 7b are similar to the emission from the central cavity with a slightly lower temperature of 2 ] 106 K and a column density of N \ 8 ] 1021 cm~2. Results from Ðts of two smaller sections H of the northeast emission were consistent with the numbers derived from the Ðt to the entire region (Table 3). The good agreement between the intervening column density derived for the central pointing and the northeast pointing, along with the morphological evidence discussed above, gives us conðdence that the X-ray emission from the two pointings is indeed related T he Northern L imb The limblike feature, marked as C ÏÏ in Figure 1, is real. The small number of counts in the feature prevent us from constraining Ðt parameters well. However, the emission is

5 No. 1, 1997 ROSAT OBSERVATIONS OF CTB FIG. 4.ÈThe contours from the 1.4 GHz map of Dickel & Willis (1980) overlaid onto the total counts from the central pointing. The extension of radio emission to the northeast is spatially correlated with the X-ray emission. consistent with the highly absorbed thermal emission from both the central and northeast pointings. There are no photons from this feature below 0.5 ke, indicating that the feature, if not coincident with CTB 1, is at least located in the Perseus arm near the CTB 1 region. In addition, the emission, if circular, is centered quite close to the X-ray pulsar RSJ 0002]6246 (Hailey & Craig 1995). Finally, the X-ray emission appears to be associated with a previously known H I shell (Lozinskaya 1992). The H I shell has a bulk velocity of D [30 km s~1, implying a distance of D3 kpc from galactic rotation curve models. The consistency of these various distance measures with CTB 1 permits us to tentatively interpret the emission as coming from a previously unknown SNR, G117.7]0.6, centered at l \ and b \ 0.6. The shell radius at that distance is 37 pc. The deðcit of radio emission from this object is perhaps not too surprising. Pfe ermann, Aschenbach, & Predehl (1991) discovered a similarly large SNR during the ROSAT all-sky survey that had not previously been seen in radio surveys. Follow-up observations found that the radio surface brightness of the Pfe ermann, Aschenbach, & Predehl object is a factor of 4 lower than that of any remnant known to date Blast Wave Properties The distance to CTB 1 as derived in the earlier optical work (Hailey & Craig 1994) is 3.1 kpc. The radius of the optical shell, as well as that of the X-ray emission, is D14 pc. The X-ray luminosity, then, is L D 7 ] 1034 ergs, and X the derived preshock density, n, is 0.08 cm~3, assuming a 0 spherical cavity for CTB 1. This rather low density was suggested by the optical work where the density-sensitive [S II] j6717/j6731 ratio was shown to be in the low-density limit throughout the remnant. The low density is also implied by the radio results (Landecker, Roger, & Dewdney 1982). This density yields a mass swept up by the blast wave of only 22 solar masses. Then, assuming a Sedov expansion, the temperature of the X-rayÈemitting gas allows us to derive a velocity of V \ 505 km s~1 and an age for the remnant of about 9000 shock yr. The X-ray shock velocity as derived from the optical shock properties (Hailey & Craig 1994) is reasonably consistent at D400 km s~1, and the age, again as derived from the optical analysis, is D 7500È11,000 yr. The derived initial explosion energy is also rather low, at v D 1050 ergs DISCUSSION The uniform optical and radio shells which deðne CTB 1 are indicative of a blast wave expanding into a relatively uniform ISM. The optically derived density gradient that runs from the higher density western rim toward the northeast breakout is rather smooth and is not consistent with, for example, an expansion near the edge of a molecular cloud such as that seen in 3C 391 (Rho & Petre 1996). The larger partial shell and northeast breakout are indications

6 312 CRAIG, HAILEY, & PISARSKI Vol. 488 FIG. 5.ÈDigitized POSS O plate, with the total X-ray contours from the central pointing superimposed. The very faint optical shell is located inside the heavy dotted circle which marks its extent (see Fesen et al for much deeper optical imagery). The lack of optical emission in the northeast is clearly related to the X-ray breakout.ïï that the ISM in the region may have been strongly shaped by previous SN explosions or pre-sn stellar winds. The spatial distribution of the X-ray emission provides a new look at the structure of the region. We look Ðrst at the emission within the remnant itself and then its relation with the environs X-Ray Emission ProÐle As demonstrated by Long et al. (1991) (see their Fig. 9), a standard Sedov model cannot produce a centrally peaked proðle, regardless of the amount of absorption toward a remnant. Their analytical calculations, assumed equilibrium ionization and a Raymond & Smith (1977) thermal model. Since it was not clear from their text whether the emission had been folded through an instrument response function, we constructed our own model of a remnant. We also assumed ionization equilibrium and a thermal plasma model but took the additional step of folding the emission through the response function of the ROSAT PSPC. In cubes 0.2 pc on a side we calculate the emission given a Sedov density proðle and an initial explosion energy of 1050 ergs (appropriate for CTB 1). For each cube the Sedovdetermined temperature is used to determine the spectrum of emission from that point in the remnant. The resultant spectrum is then absorbed by the appropriate column and folded through the PSPC response matrix. After all cubes are Ðlled in this way, the emission is totaled along the line of sight and projected into two dimensions to generate a simulated X-ray image. To fully investigate the possible emission proðles expected from such a remnant, we model not only a standard sphere but also allow the radius of the object along the projection direction to vary. This produces an ellipsoidal cavity while still yielding a circular X-ray image. The results, as shown in Figures 8a and 8b, are relatively consistent with those of Long et al. (1991) for the spherical model. Additional structure in the emission proðle results from the folding of the emission through the PSPC response matrix as well as from the Ðnite cube size in the model. These e ects are relatively minor. It is indeed unlikely that a standard spherical Sedov remnant can have a centrally peaked appearance in the X-ray. More interesting are the nonspherical models. Shown in Figures 8cÈ8f are models for two nonsymmetric geometries. The deep ÏÏ remnant (Figs. 8c and 8d) has an extent in the projection direction equal to twice that in the other two axes. The shallow ÏÏ remnant (Figs. 8e and 8f ) is foreshortened in the projection direction with a radius equal to half that in the other two directions. For the deep ÏÏ model the emission as seen in the ROSAT PSPC would be more limb brightened for the two densities shown. For the shallow ÏÏ model with a density of 1.0 cm~3and an absorp-

7 No. 1, 1997 ROSAT OBSERVATIONS OF CTB FIG. 6.È(a) Fitted spectrum from the central pointing. The counts are summed over 10 arcmin~2 bins and clearly show the heavy absorption toward CTB 1. (b) Fitted spectrum from the northeast pointing. The column derived from Ðtting these spectra indicates that the central and northeast regions are physically related. tion N of D3 ] 1021 cm~2 or greater, the appearance is H far di erent with a proðle that is distinctly centrally peaked. The proðles generated can be compared to the radially averaged X-ray emission from our observations of CTB 1. The proðle, as shown in Figure 9, was generated by taking 10 radial cuts through the emission, out to a distance just short of the edge of the X-ray emission. Because of the breakout in the northeast, the proðle is not accurate toward FIG. 7.È(a) s2 conðdence contours from the central pointing. (b) Northeast region (the breakout). Contour levels are at the 69%, 91%, 95%, and 99% levels.

8 314 CRAIG, HAILEY, & PISARSKI Vol. 488 FIG. 8.ÈThe radial proðle of X-ray emission from a Sedov remnant, as folded through the ROSAT PSPC, is modeled. Curves are for di erent values of the interstellar absorption. (a) Values of N for the various curves. (a)è(b) The emission proðle expected from a spherically symmetric Sedov remnant for densities of 0.1 and 1.0 cm~3.(c)è(d) The same H calculation for a deep ÏÏ model with twice the extent along the line-of-sight direction as in the spherical model. (e)è( f ) A shallow ÏÏ model with only half the extent along line of sight as the spherical. For higher column densities of absorbing material it is possible to produce centrally peaked brightness proðles for nonsymmetric cavities. the edge of the remnant but should be rather accurate toward the middle. The centrally peaked proðle is quite clear; therefore, this emission cannot result from a spherical Sedov model, and some other mechanism must be invoked. It is an intriguing result, although perhaps not unexpected, that the geometry of a remnant in the projection direction can dramatically modify its appearance. In fact, it has been clear for some time that projection e ects can completely dominate the appearance of a remnant in all passbands (see e.g., Hester 1987; Graham et al. 1994). The particular modiðcation of the geometry that we discuss above, however, is not likely to apply to CTB 1. The centrally Ðlled proðle is only produced by the shallow ÏÏ nonspherical model, and a foreshortening by a factor of 2 is required. The circular appearance of the remnant, as can be seen clearly in the beautifully deep optical images by Fesen et al. (1997), is a strong argument against such a strong asymmetry. Although certainly not impossible, it would seem unlikely that the symmetry of the remnant would be broken in such an extreme way only along the projection direction. The emission proðle from CTB 1 is consistent with the WL model. Centrally peaked emission in that model requires two conditions. The lifetime of the evaporating clouds which enhance the emission in the central region of the remnant should have a ratio to the remnantïs age, q, on order 10È30. Also, the ratio of the mass in the clouds to that in the intercloud medium, C, should be in a range such that the ratio of C/q is D2È5. The appearance of the remnant in the WL model is determined primarily by the ratio of C/q. Attempts to constrain values of C and q independently from either the overall emission measure or its proðle are not deðnitive since the parameter space over which C and q can vary independently is large T he Northeast Breakout: T he Northern L imb and Pulsar We have seen from the X-ray emission that the remnant is centrally Ðlled. What can we learn about the surrounding area? The northeast region, where the X-ray emission appears to have broken out from the CTB 1 cavity, the candidate SNR G117.9]0.6 (the northern limb) and the

9 No. 1, 1997 ROSAT OBSERVATIONS OF CTB FIG. 9.ÈThe radial proðle of the X-ray emission from within CTB 1 (solid line). Ten radial cuts across the X-ray image, equally spaced in angle, were made. Each cut passed through the center of the remnant and terminated just short of the optical and radio shell radius. The cuts were averaged to produce the proðle shown. The x-axis scale assumes a 3.0 kpc distance to the remnant. The clearly centrally peaked proðle may be explained by either evaporative enhancement due to clouds in the interior of the remnant or, perhaps, by projection e ects resulting from a nonsymmetric SNR. A proðle for a shallow ÏÏ (see Fig. 8) nonspherical model with N \ 1.0 ] 1022 (dotted line) is overplotted for comparison. Although nonsphericity can explain a centrally Ðlled geometry, such asymmetry is not H likely in the case of CTB 1. X-ray pulsar could result from a number of plausible scenarios. The region of the Perseus arm of the Galaxy where CTB 1 is located is surrounded by OB associations (Roberts 1972), and, although no associations coincident with the region have been cataloged, the entire area has likely been strongly impacted by relatively recent massive star formation and subsequent supernova explosions. Several scenarios, which Ðt the data, follow from such an environment. The Ðrst, and perhaps most appealing picture, posits an initial supernova explosion in the northeast region. The blast wave from this object, expanding into a uniform medium (and thus shell-like in the X-ray) would now be seen as G117.9]0.6, some 37 pc in radius, and on order 3 ] 104 yr old. The neutron star left behind by this initial explosion would have a transverse velocity of D100 km s~1, not unreasonably low given the velocity distribution of young neutron stars as determined by Lyne & Lorimer (1994). The progenitor to CTB 1, then, would have exploded soon (within a few thousand years) after the blast wave from the Ðrst supernova passed through. To the west the blast wave from the second SNR would ride up the density gradient left behind by the Ðrst, accounting for the density gradient observed in that remnant. As can be seen in Figure 1, the two blast waves would now be coincident at the western rim of CTB 1, o ering a possible explanation for the young, hot shock seen there. To the east the CTB 1 blast wave would encounter a falling density gradient, accounting for the northeast breakout. The anomalously low average ambient density around CTB 1 would also be entirely consistent with an area swept clean by a previous SNR blast wave. The existence of evaporating clouds, required by the White & Long (1991) model, is difficult to understand given this interpretation of the region. In a variation on this scenario the progenitor to CTB 1 could have exploded outside the shell of the Ðrst SNR. The two shells would interact then in the northeast region with the blast wave from CTB 1, reheating the cavity of the Ðrst supernova remnant. Although this picture does not explain the low ambient density in the CTB 1 cavity, it does provide a natural explanation for the presence of the evaporating clouds which are the favored explanation for the center- Ðlled appearance of CTB 1. Although appealing in their ability to explain both regions, these scenarios appear to be indistinguishable from a third picture. Here we assume Ðrst that the northern limb (G117.9]0.6) is a chance superposition of emission. Then CTB 1 would be responsible for all the emission in the region. The northeast breakout would be plausibly caused by a cavity in the ambient ISM. A cloudy medium and evaporation or an asymmetric geometry could then naturally explain the center-ðlled proðle. In this picture, however, the neutron star becomes a problem. If it is indeed associated with CTB 1 (the age determined by Hailey & Craig 1995 is D104 yr with a distance of D3 kpc), then it would need a transverse velocity approaching 3000 km s~1 to have moved so far out into the northeast in 10,000 yr. The velocity in this scenario would be very much on the high side of the neutron star velocity distribution as reported by Lyne & Lorimer (1994). The density gradient within CTB 1, as seen in the optical, would result from a gradual change in shock velocity (see Hailey & Craig 1994; Fesen et al. 1996) Implications for Center-Ðlled X-Ray Models Given the sparse data set provided by the rather faint X-ray emission in the region, it is difficult to draw Ðrm conclusions about the the CTB 1 region. Both a preexisting distribution of cloudlets and a peculiar expansion geometry provide plausible explanations for the X-ray emission seen. This situation is not unusual in the study of intermediateage remnants. For SNR 3C 391 (Rho & Petre 1996) the progenitor is posited to have exploded near the edge of a molecular cloud which provided a natural density gradient into which the blast wave expands. This, in combination with a distribution of small clouds in the medium toward the outside of the Large Molecular Cloud, appears to provide the likeliest explanation of emission seen from 3C 391. The WL model appears to be consistent with observations of a number of remnants. Good examples are 3C (Saken et al. 1995), W44 (Rho et al. 1994), and W28 (Long et al. 1991). Any mechanism which enhances the density within the cavity, be it a previous blast wave, other large density gradient preextant in the ISM, or perhaps simply a projection e ect could also account for the observed emission. The sensitivity of ROSAT PSPC observations will continue to provide insights into a wide range of supernova remnants as the large data set continues to be analyzed. 5. SUMMARY We have presented the Ðrst X-ray observations of the supernova remnant CTB 1 and the surrounding region. In addition to detecting the remnant for the Ðrst time in this passband we have also discovered a large X-ray breakout region to the northeast of the remnant in the region where an X-ray pulsar has previously been detected. In addition, we see some evidence for the existence of an older, larger SNR G117.9]0.6, which may be responsible for much of

10 316 CRAIG, HAILEY, & PISARSKI the observed structure in the CTB 1 region. The center-ðlled X-ray structure we see is consistent with the WL evaporating cloud model. In a less likely scenario the present structure of CTB 1 is the result of an asymmetric expansion seen in projection. Continued analysis of the observations of similar objects with the sensitive ROSAT PSPC will be a great help in determining the physics behind the broad continuum of intermediate-age supernova remnants. We would like to thank the anonymous referee for his or her detailed comments on the paper which greatly improved both content and presentation. REFERENCES Busser, J.-U., Egger, R., & Aschenbach, B. 1996, A&A, 310, 1 Long, K. S., Blair, W. P., White, R. L., & Matsui, Y. 1991, ApJ, 373, 567 Dickel, J. R., & Willis, A. G. 1980, A&A, 85, 55 Lozinskaya, T. A. 1992, Supernovae and Stellar Wind in the Interstellar Fesen, R. A., Blair, W. P., & Kirshner, R. P. 1982, ApJ, 262, 171 Medium (New York: AIP) Fesen, R. A., Winkler, P. F., Rathore, Y., Downes, R. A., Wallace, D., & Lyne, A. G., & Lorimer, D. R. 1994, Nature, 369, 127 Tweedy, R. W. 1997, AJ, 113, 767 Pfe ermann, E., Aschenbach, B., & Predehl, P. 1991, A&A, 246, 28. Graham, J. R., Levenson, N. A., Hester, J. J., Raymond, J. C., & Petre, R. Raymond, J. C., & Smith, B. W. 1977, ApJ, 35, , ApJ, 444, 787 Rho, J. C., & Petre, R. 1996, 467, 698 Hailey, C. J., & Craig, W. W. 1994, ApJ, 434, 635 Rho, J. C., Petre, R., Schlegel, E. M., & Hester, J. J. 1994, ApJ, 430, 757 ÈÈÈ. 1995, ApJ, 455, L151 Roberts, W. W. 1972, ApJ, 173, 259 Hester, J. J. 1987, ApJ, 314, 187 Saken, J. M., Long, K. S., Blair, W. P., & Winkler, P. F. 1995, ApJ, 443, 231 Kahn, S. M., Gorenstein, P., Harnden, F. R., Jr., & Seward, F. D. 1985, Snowden, S. L., McCammon, D., Burrows, D. N., & Mendenhall, J. A. ApJ, 299, , ApJ, 424, 714 Koo, B., Kim, K., & Seward, F. 1995, ApJ, 447, 211 van den Bergh, S., Marscher, A. P., & Terzian, Y. 1973, ApJS, 26, 19 Landecker, T. L., Roger, R. S., & Dewdney, P. E. 1982, AJ, 87, 1379 White, R. L., & Long, K. S. 1991, ApJ, 373, 543 Leahy, D. A., & Aschenbach, B. 1995, A&A, Willis, A. G. 1973, A&A, 26, 237 ÈÈÈ. 1996, in Proc. Roentgenstrahlung from the Universe, ed. H. U. Zimmermann, J. Trumper, & H. Yorke, MPE Rept. 263, 261