---
abstract: 'We report the progress to date from an ongoing unbiased ultraviolet survey of supernova remnants in the Magellanic Clouds using the Far Ultraviolet Spectroscopic Explorer ($\fuse$) satellite. Earlier work with $\fuse$ and other instruments has indicated that optical and/or X-ray characteristics of supernova remnants are not always good predictors of their brightness in the ultraviolet. This survey is obtaining spectra of a random large sample of Magellanic Cloud supernova remnants with a broad range of radio, optical, and X-ray properties. We proposed 39 objects in the Large Magellanic Cloud and 11 objects from the Small Magellanic Cloud, with a standard request of 10 ks per object using the $\fuse$ 30 square aperture. To date, 39 objects have been observed in the survey (38 in the LMC and one in the SMC) and 15 have been detected, a detection rate of nearly 40%. Our survey has nearly tripled the number of UV-detected SNRs in the Magellanic Clouds (from 8 to 22). Because of the diffuse source sensitivity of $\fuse$, upper limits on non-detected objects are quite sensitive in many cases, dependent upon night observing fraction and whether stellar light contamination plays a role for a given object. Estimated total luminosities in O VI, based simply on scaling the flux at the observed positions to an entire object, span a broad range from considerably brighter to many times fainter than the inferred soft X-ray luminosities, indicating that O VI can be an important and largely unrecognized coolant in certain objects. We compare the optical and X-ray properties of the detected and non-detected objects but do not find a simple indicator for ultraviolet detectability. Non-detections may be due to clumpiness of the emission, high foreground extinction, slow shocks whose emission gets attenuated by the Magellanic interstellar medium, or a combination of these effects. The characteristics of individual detected supernova remnants are summarized in an Appendix.'
author:
- 'William P. Blair, Parviz Ghavamian, Ravi Sankrit, & Charles W. Danforth'
title: ' [*Far Ultraviolet Spectroscopic Explorer*]{} Survey of Magellanic Cloud Supernova Remnants'
---

Introduction
============

Supernova remnants (SNRs) as a class are important constituents of the interstellar medium (ISM). They are responsible for much of the enrichment of the ISM of a galaxy, and they are a significant energy source for stirring and mixing the ISM. They may also be important in some regions for triggering star formation in molecular clouds. SNRs typically emit over nearly the entire electromagnetic spectrum, and multiwavelength observations have become an important source of physical information about the ejecta (in young objects), and the circumstellar medium and ISM into which the shock wave propagates.

Ultraviolet spectroscopy of SNRs has proven to be an important tool for providing physical information and insight into this important class of objects (Blair 2001; Raymond 2001; and references therein). Emission lines in the UV tend to come from material at temperatures that are intermediate between the X-ray emitting gas and the optical filaments. Information on shock velocities, preshock densities, and abundances of important elements such as carbon that have no bright optical lines can all be derived from UV observations of shocks. $\fuse$ observations have the added bonus of high spectral resolution, which provides kinematic and line profile information. Thus the importance of overlying absorption and in some cases the shock geometry can be assessed directly (Blair, Sankrit, & Tulin 2002; Sankrit, Blair, & Raymond 2003).

Primarily because of interstellar extinction, however, there are remarkably few objects available for study in the UV. In our Galaxy, the Cygnus Loop and Vela SNRs are the objects that have been studied extensively; both are nearby and have low foreground extinction. Notable but difficult observations of the Crab Nebula (Blair et al. 1992), SN 1006 (Raymond, Blair, & Long 1995; Korreck et al. 2004) and Puppis A (Blair et al. 1995) have also been performed.

The foreground galactic extinction toward the Magellanic Clouds is relatively low, but the number of SNRs observed in the FUV is still very modest (see Table 1). The young ‘core collapse’ SNRs N132D (LMC) and 1E 0102-7219 (SMC) are very bright in soft X-rays and have been studied in the UV as well (Morse et al. 1996; Blair et al. 2000a). The X-ray and optically-bright SNR N49 has received considerable attention (Vancura et al. 1992; Blair et al. 2000b; Sankrit, Blair, & Raymond 2004). However, the next brightest optical SNR in the LMC, N63A, has not been observed in the ultraviolet since the early days of the International Ultraviolet Explorer (IUE) satellite (Benvenuti, Dopita, & D’Odorico 1980) even though it should be readily detectable by modern instrumentation. Three out of four of the Balmer-dominated SNRs identified by Tuohy et al. (1982) have only recently been detected with $\fuse$ (Ghavamian et al. 2006). Not only is this a small sample, but the objects observed have tended to have specific characteristics that attracted attention to them, and with very bright optical and/or X-ray emission in particular. They are not ‘typical’ SNRs, and so the picture we derive for the global impact of SNRs on their host galaxy is seriously biased.

Positions for FUV spectral observations have often been selected based on the optical appearances or characteristics of the region or object being observed, including not only the surface brightness but also particular line ratios or signatures (e.g., Raymond et al. 1988; Blair et al. 2002; and references therein). More recently, a few FUV observation positions have been selected based on the appearance in soft X-rays rather than optical (Raymond et al. 1997; Sankrit et al. 2001). With very few FUV images available for guidance, observers have had little else to go by.

However, there have been a growing number of indications that selection criteria for FUV observations of SNRs have been too narrow. FUV spectro-imaging of the Cygnus Loop and Vela SNRs from the Voyager Ultraviolet Spectrometers (Blair et al. 1991, 1995) and of Vela recently by the SPEAR experiment (Nishikida et al. 2006) show patchy, variable spatial distributions in key FUV emission lines, albeit at modest spatial resolutions. Higher spatial resolution FUV images of selected regions in the Cygnus Loop are available from the Ultraviolet Imaging Telescope (UIT). Danforth et al. (2000) show comparisons of UIT images to both X-ray and optical images, indicating all wavelengths are patchy and show relative spatial variations. Even though the UIT bandpass did not isolate individual emission lines, spatial variations of the FUV emission are clearly present. Some of the spectroscopic positions observed in the Cygnus Loop and Vela SNRs also correspond to very faint optical filaments and yet the UV emission is bright (Sankrit et al. 2001; Blair et al. 2002; Sankrit & Blair 2002; Raymond et al. 2003).

A serendipitous $\fuse$ observation of a SNR in the SMC H II region NGC 346 (N66) has provided another important example of potential observational bias (Danforth et al. 2003). With the $\fuse$ MDRS aperture placed on a star in the NGC 346 star cluster, strong O VI and C III $\lambda$977 emission lines were detected in the LWRS aperture a couple of arcminutes away. Reconstructing the LWRS aperture location showed that it was projected onto the limb of a known radio SNR 0057-7226 (Ye, Turtle, & Kennicutt 1991). The SNR had been observed previously in FUV absorption with $\fuse$, using the Luminous Blue Variable HD 5980 as a background source (Koenigsberger et al. 2001; Hoopes et al. 2001), but no FUV emission had been detected. Indeed, [*no optical shell or emission from the SNR is evident in images of the region*]{}, and only extremely faint optical emission from the SNR can be detected in H$\alpha$ echelle data (Chu & Kennicutt 1988; Danforth et al. 2003). The SNR is an X-ray source (Wang & Wu 1992; Yokogawa et al. 2000; Nazé et al. 2002), but with a luminosity of only $\rm L_{x}(0.3 - 10 ~ keV) \simeq 1.4 \times 10^{35} ~ \LUM$ (Nazé et al. 2002) is certainly not in the class that would make it a priority FUV target. It is unlikely that an FUV observation of this SNR would have ever been attempted had the serendipitous spectrum not been obtained, and yet this is a bright FUV emission source.

To understand the systematics of ultraviolet emission from a wider sample of SNRs, and in particular to investigate the properties of more typical SNRs, we are conducting an FUV survey of Magellanic Cloud SNRs that is not biased by optical, radio, or X-ray characteristics of the selected objects. While the survey is ongoing, we have a significant subset of the proposed observations in hand that allows us to address some of the primary questions about FUV detectability. Section 2 addresses the target selection criteria and describes the survey strategy and observational details. Section 3 describes a general analysis of the detected objects and compares to optical and X-ray data. Details about the individual objects detected are described in the Appendix.

Target Selection and Observations
=================================

The $\fuse$ Survey program category allows one to propose a set of targets for potential observation, with no guarantees that any particular target will be observed. Targets are selected and inserted into observation timelines in ways that provide scheduling flexibility to the observatory, while accomplishing the scientific objectives of the proposed program. We thus compiled a list of potential targets from the optical SNR catalogs of Mathewson et al. (1983; 1984, 1985) and from the X-ray SNR catalog of Williams et al. (1999a). We supplemented this list with a number of individual objects reported in the literature (Smith et al. 1994; Chu et al. 1995; 1997; 2000; Dickel et al. 2001; Smith et al. 2004), and a handful of objects where optical longslit echelle data from CTIO (Danforth 2003) indicated high velocity emission in H$\alpha$ but for which no previous SNR identification had been suggested. Such objects may represent SNRs ‘buried’ in H II regions that have not heretofore called attention to themselves, similar to the SMC SNR 0057-7226 mentioned earlier. We culled this list by removing objects previously observed with $\fuse$ (see Table 1) and a small subset for which optical extinction measurements or X-ray derived neutral hydrogen column densities indicated severe attenuation of any potential FUV emission. Finally, a selection of six SNRs newly identified in the Magellanic Cloud Emission Line Survey (MCELS) team (Smith et al. 2004) were added in the second year of the survey. Our final list contained 39 objects in the LMC and 11 objects in the SMC. The observations reported below took place over two observing cycles, using $\fuse$ program identifiers D904 (44 LMC and SMC remnants) and E900 (six new MCELS objects in the LMC).

The $\fuse$ instrument covers the wavelength range 905 – 1187 Å, with a nominal point source resolution R = $\lambda/\Delta\lambda ~ \ge$ 20,000. For diffuse sources of interest to this program, the spectral resolution is driven instead by the spectrograph aperture size and filling factor. We are primarily interested in the LWRS (30 square) apertures for this paper. A filled LWRS aperture produces a roughly square-topped instrumental profile of width 106 $\VEL$ near 1035 Å. $\fuse$ contains four optical channels, each with its own focal plane and spectrograph aperture plate.

Small, thermally-induced distortions in the $\fuse$ optical bench discovered after launch do not allow rigid co-alignment of the apertures from each channel over time. Typical misalignments correspond to a few to as much as 10, and can vary even during the course of a given orbit or integration. For reference, the LiF1 LWRS aperture positions are shown in this paper. The LiF1 channel was used for guiding, and so its position on the sky was held fixed while the other channels drifted relative to it. In cases where the object is large in comparison to the $\fuse$ apertures, all of the apertures should be sampling similar SNR emission. However, on the smaller objects, channel misalignments can cause different sampling and impact the assessments of relative O VI and C III line strengths. Further details about the $\fuse$ instrument and on-orbit performance information are provided by Moos et al. (2000; 2002) and Sahnow et al. (2000).

The $\fuse$ survey strategy involved a standard 10 ks request on each object. These faint emission line sources could all be observed in time-tagged mode. The size of the LWRS apertures plus the sensitivity of the $\fuse$ detectors combine to provide exceptional diffuse source sensitivity compared with other FUV spectrographs that have been flown. For instance, a nominal 10 ks observation would detect emission in O VI $\lambda$1032 that is 100 times fainter than the bright LMC SNR N49 (Blair et al. 2000; Sankrit et al. 2004), with limiting flux levels near $\rm 5 ~\times ~ 10^{-15} ~ ergs~cm^{-2}~s^{-1}$. Actual observation times ranged above and below the nominal 10 ks request, at the convenience of the $\fuse$ schedulers. (See discussion below and Table 2.)

The intent of the survey was to place the LiF1A LWRS aperture at or near the projected center of each SNR. This strategy had two primary motivations. First of all, to do anything other than this would apply a bias to our survey that was not desirable. We did not want to point at features selected from optical or X-ray images of the objects. Secondly, it may be advantageous to point through the centers, where typical Doppler shifting of $\pm$30 – 150 $\VEL$ would move any SNR emissions out from under potential overlying absorptions by the host galaxy. This is an important factor for strong resonance lines such as C III $\lambda$977 and O VI, the strongest lines seen in most SNR spectra.

In practice, since published catalog coordinates were used for many of the objects, the $\fuse$ apertures did not always lie on the exact central positions. In objects with spotty optical emission, for instance, some of the Mathewson et al. (1983, 1984, 1985) coordinates are off center with respect to newer X-ray data, which tends to show the full extent of more objects. Also, in retrospect, some of the MCELS-supplied coordinates did not correspond to the object centers. In all of the SNR figures that follow, we show the actual LiF1A LWRS aperture location observed (i.e., coordinates shown in Table 2), so any miscenterings will be obvious. Since most of the objects in our survey were from 1  to 5 in extent, the LWRS apertures were usually filled with potential emission, even if modest channel misalignments or miscenterings were present. A number of the spectra are impacted to some degree by stellar emission, which affects our ability to sense the presence of potential faint emission features. Strong emission, if present, is still visible even if modest stellar contamination is present.

To date, 39 objects out of the 50 potential targets have been observed, all but one (SNR 0104-723) in the LMC. The distribution of these objects across the face of the LMC in shown in Figure 1, where detections and non-detections are indicated with separate symbols. Table 2 summarizes the targets observed and the results in terms of detections and non-detections. The details of the spectral analysis and search for SNR emission lines are described in the next section.

Analysis and Discussion
=======================

Each $\fuse$ data set has been reprocessed with the most current version of the CalFUSE pipeline publicly available at the time of our analysis, CalFUSE 3.0.7. The output from this version of CalFUSE makes it trivial to inspect both the total data sets and the orbital-night only time periods. The total data sets were inspected first. However, scattered light and backscattered solar line emission can cause confusion in the total data sets, especially for the SiC channels (which include the C III $\lambda$977 line). The redshift of each Magellanic Cloud is enough to separate the SNR C III emission (when present) from the back-scattered solar emission, but potential faint emission can still be lost in the higher background. For those objects with detected C III and/or O VI line emission, we have chosen to extract, plot, and measure the night-only fractions to avoid contamination problems when possible. (Two detected objects had no orbital night integration, however. See Table 2.)

The search for emission centered on the brightest transitions normally seen in the FUV – O VI $\lambda\lambda$1031.9138,1037.6154 and C III $\lambda$977.020. The O VI $\lambda$1032 line in the LiF1 channel was the primary feature searched for because the LiF1 channel has the highest effective area and the LWRS aperture position was known accurately. For most detected objects, the LiF2B channel also showed O VI emission, but at lower signal levels (due to the smaller effective area of this channel). O VI $\lambda$1038 was an important secondary indicator, although it is nearly always impacted severely by overlying absorption. Comparison with published grids of shock models (e.g., Hartigan, Raymond, & Hartmann 1987) indicate that strong C III emission occurs for shocks with velocities above about 100 $\VEL$ while O VI emission ‘turns on’ above $\sim$160 $\VEL$, when the shock becomes effective at ionizing $\rm O^{+4}$ to $\rm O^{+5}$. Hence, we also searched for C III, which in principle could be seen even if no O VI was detectable. The SiC channels both cover the C III line region, but the SiC2A channel has higher effective area, so this channel was searched for SNR emission. Of course, channel misalignment makes the exact location of the SiC2 LWRS aperture uncertain relative to the LiF1 LWRS aperture, as described in Section 2.

Out of the 39 objects observed to date, 15 detections have been obtained, as indicated in Table 2, a detection rate of 38.5%. Summary figures showing images of each of these objects and $\fuse$ data sections are shown in Figures 2-16. Optical H$\alpha$ and \[S II\] CCD images in these figures were provided to us by the MCELS team (Smith et al. 1999), headed by R. Chris Smith of NOAO. The X-ray images have been obtained from the public HEASARC data archive for Chandra, ROSAT, XMM-Newton, and Einstein. Squares in the image panels show the nominal LiF1A LWRS aperture position and orientation for each observation. Since the LWRS aperture is 30 in size, this also provides the spatial scale for each image. $\fuse$ data segments near the O VI and C III regions are shown in separate panels of each Figure. Because we are searching for faint emission lines, we found it beneficial to bin most data sets over 8 CalFUSE 3.0.7 pixels ($\sim$30 $\VEL$) for display in the Figures. A brief discussion of each detected SNR is presented in the Appendix.

As can be seen in Figures 2 - 16, the line profiles exhibit a wide range of shapes. Some are broad (several hundred $\VEL$), some narrow (only slightly above the filled aperture resolution), and some are multi-peaked. The O VI $\lambda$1032 profile is most instructive, as it is often the strongest line seen and has relatively good signal-to-noise ratio. These figures each contain horizontal bars with tick marks that indicate the positions of [*potential*]{} ISM absorption features, primarily $\rm H_2$ and O I lines. The bar marked “MW" (for Milky Way) shows the Galactic rest frame, and the other bar is shifted to the rest frame of the appropriate Magellanic Cloud (+275 $\VEL$ for LMC; +165 $\VEL$ for SMC). The longest tick marks indicate the expected positions of C III and O VI lines. Of course, the same transitions that we expect from the SNRs in emission are strong resonance lines that can also be in absorption from the intervening ISM at both galactic and MC velocities.

Many objects detected in our survey show both O VI and C III emission at detectable levels. Interestingly, some objects show O VI but no C III (e.g., SNR 0509-687), but there is only one detection in C III-only (SNR 0527-658), and this detection is marginal. The presence of O VI emission in most of the detected objects indicates that each of them has some shocks with velocities in excess of 160 $\VEL$. In SNR 0527-658, at least at the observed location, such shocks are apparently absent. Additional observations at other locations within SNR 0527-658 would be needed to understand whether the absence of O VI is intrinsic to the object or due to the clumpiness of the O VI spatial distribution.

We have measured the observed line widths, approximate centroid velocities, and integrated total line fluxes for all SNRs with detections, and list these values in Table 3. These measurements have been made with IDL tools that permit user interaction in the measurements. Each spectrum is displayed and a cursor is used to set limits around each line for integrating the flux above the background for each feature. Since most of the line profiles are distinctly non-Gaussian, the fluxes derived in this way are simply integrated flux above background, not the equivalent flux for a Gaussian fit. These measurements are only lower limits to the actual fluxes since unknown but potentially significant amounts of emission are apparently absorbed by foreground ISM. (See discussion below.) The full-width-zero-intensity (FWZI) values listed in Table 3 indicate the full range over which the integration was made. The values listed in Table 3 are all significantly above the value expected for a filled LWRS aperture, indicating that intrinsic line broadening is detected in all cases. The central velocities listed are not the centroid of measured flux, but rather the center of the FWZI velocity range used. Two things are immediately obvious from inspection of Table 3. For all objects with both O VI and C III lines, the central velocities for C III are more redshifted than for O VI, and the FWZI line widths are significantly higher for O VI than for C III. Both of these facts are consistent with significant impacts from overlying absorption. Inspection of any of Figures 3 - 59 from the Danforth et al. (2002) $\fuse$ ISM atlas of sight lines in the Magellanic Clouds shows that the C III ISM absorption at LMC velocities is typically very saturated and broader than the ISM absorption from O VI. Hence, the portion of the C III SNR emission that is visible from under this ISM absorption is narrower and appears more redshifted. While O VI ISM absorption is also usually present, it is not usually saturated and varies in strength between sight lines. In the SNRs, often a broad base of emission is seen in O VI $\lambda$1032 (for example, SNR 0454-665, Figure 5), and thus a larger FWZI is measured and the central velocity is less shifted than for C III.

Overlying absorption is also obviously present in many cases from inspection of the line profiles themselves. There are some cases where an apparently broad O VI emission line has been partially but not totally absorbed on the blue side by LMC and/or galactic ISM (e.g., SNR 0506-680, Figure 6). In general, the line widths for C III are equal to or narrower than widths for O VI, consistent with a saturated C III ISM absorption (even stronger than in O VI) impacting the observed profile. The line profiles often give the impression of being cut off on the blue side, as if they are peeking out from under the overlying LMC absorption. We see only one example where substantial emission is seen blueward of the rest LMC (or SMC) wavelength, that being SNR 0536-706 (Figure 15), and in this case the blue wing of the line appears to be truncated by Milky Way O VI absorption. Since we are looking through the middle of each SNR, we are preferentially seeing redshifted emission from the back side of each SNR, while the front-side shell emission is typically absorbed or partially absorbed by MC and Milky Way ISM absorptions.

We now address the one object with detected C III and no O VI emission, SNR 0527-658 (Figure 9). It is conceivable that O VI emission is present in this object but just not at the location of the LWRS aperture, or that this object is dominated by shocks slower than $\sim 160 \VEL$. We have no independent way of checking which of these possibilities may apply in this case. If the former suggestion is correct, it would imply a rather clumpy distribution of O VI, at least in some objects. If the latter situation is true, it may provide a reason why few (only one to date) C III-only objects are observed. Assuming we are systematically seeing the redshifted sides of the shells, objects with slower shocks (and hence lower bulk motions) will tend to be buried (or mostly buried) underneath the strongly saturated MC C III ISM absorption line. We note that the observed C III profile in the spectrum of SNR 0527-658 appears to be cut off sharply on the blue side. Apparently in this case, the relative motions are such that the C III line is partially seen in spite of the overlying absorption.

In general, however, a combination of effects tend to complicate the situation with C III. The uncertainty in position of the SiC channels with respect to LiF1, the lower effective area, and the propensity for significant spectral comtamination by solar-backscattered C III all work against C III detection at various levels. (See more detailed discussion below for Figure 17.)

Non-detections come in two basic varieties, those for which no significant evidence for O VI or C III emission lines is seen, and those observations for which significant stellar light contaminates the $\fuse$ data and makes detection of faint emission lines difficult to impossible. Figure 17 shows the C III and O VI regions of $\fuse$ data for SNR 0507-685, one of the non-detections without significant stellar contamination. This example shows both total and night-only data segments for comparison. No O VI lines are detected, and while some spectral structure appears at nearly the correct wavelength for LMC C III emission, the large change between the total and night-only data sets implies the observed feature is not intrinsic SNR emission. In the case of the SiC2A channel, this is an instrumental feature caused by the miscentering of low pulse-height photons from the strong airglow feature to the left. Still, the presence of such structure in the spectrum limits the ability to set a significant upper limit at C III unless a significant amount of orbital-night exposure time is available.

Figure 18 shows the same spectral regions for SNR 0519-697 (N120), one of the objects with stellar contamination. This example shows that a peak near the expected O VI position is present at a low level, but its interpretation is uncertain. Many hot stars contain P-Cygni wind features of varying strengths and shapes that could produce some O VI emission. On the other hand, some SNR emission could be present at an undetermined level and be masked by the variable stellar continuum. In cases such as this (indicated in Table 2), our ability to place limits on O VI are made inherently more difficult.

Thus, the quality of the upper limits varies based on the achieved integration times for each object and whether stellar contamination is present. Also, the unknown velocity widths for the undetected objects makes placing a specific upper limit rather arbitrary. We choose to adopt the method of Dixon, Sankrit, & Otte (2006), who have determined upper limits for a wide range of $\fuse$ data empirically, assuming a width appropriate for a filled LWRS aperture and the LiF1 channel, viz.

$$\rm U(1032) ~=~ \frac{273.1}{\sqrt{t_{exp}}}$$

where $\rm t_{exp}$ is the night exposure time in s, and U(1032) is a 3$\sigma$ upper limit at $\lambda$1032 in units of 1000 Line Units (KLU), where 1 KLU = $\rm 4.5 \times 10^{-19} ~ ergs ~cm^{-2} ~ s^{-1}~arcsec^{-2}$. Thus, a night integration of 5 ks corresponds to U(1032) = 3.9 KLU or a 3$\sigma$ LWRS upper limit of $\rm 1.56 \times 10^{-15} ~ ergs ~cm^{-2} 
~ s^{-1}$, assuming a filled aperture and an unresolved line width (106 $\rm km ~ s^{-1}$). This is more than two orders of magnitude below the measured F(1032) in the bright LMC remnant N49 (Blair et al. 2000b), although the line width in that case was over 400 $\VEL$.

Few of the non-detections can be attributed to short integration times (see Table 2), and the inferred upper limits on emission in O VI, at least at the observed locations, are quite small. Of course in many objects the region sampled by the $\fuse$ LWRS aperture is a tiny part of the projected surface area. A clumpy distribution of O VI emission could be present, so the absence of detected O VI does not necessarily indicate a complete absence of O VI emission from an object. For completeness, we indicate the $\fuse$ aperture positions on optical (MCELS) and X-ray images (when available) of all non-detections in Figures 19-24. Inspection of these Figures relative to the detections (Figures 2-16) demonstrates that, judging from optical and X-ray morphology alone. one would be hard pressed to determine [*a priori*]{} which objects will be detectable in the FUV and which will not. For instance, some large, old optical shell remnants are seen (SNR 0450-709, Figure 2) and some are not (SNR 0453-672, Figure 19, panel 3). Perhaps more telling, several of the most solid detections with $\fuse$ are unimpressive optical and/or X-ray SNRs (e.g., SNR 0454.5-6713, Figure 4; SNR 0506-680, Figure 6) while some non-detections are relatively strong optical and/or X-ray sources (e.g., SNR 0455-687, Figure 20 top panel; SNR 0519-697, Figure 21 second panel). There are several SNRs with apparent central X-ray emission consistent with ejecta in both the detected and non-detected samples, but it is not clear we are seeing FUV emission associated with the ejecta in any case, so this does not appear to be a significant factor in FUV detectability.

In Figures 25 and 26, we study some systematics of the sample. In Figure 25, we show histograms of the detections and non-detections as a function of angular size. Since many objects are non-circular, we have simply adopted the average of the ranges shown in Table 2 as being representative for this purpose. There is some indication that detections tend to be smaller diameter than non-detections, but the effect is not large. In particular, in the 1 - 4 range, there are 11 detections and 15 non-detections. Angular size does not appear to be a primary driver for FUV detectability.

However, there may be some effect at the smallest diameters. In Figure 26, we show the observed F($\lambda$1032) from Table 3 against angular size for the detected SNRs. SNR 0535-660 (N63A) is exceptional, being due to an encounter of a fast shock with a dense cloud (e.g. Warren et al. 2003), and would be located at 45 on the vertical scale of this plot. However, the Figure shows the next three brightest objects all below 1.5 in angular size. Between 1.5 and 4, there is significant dispersion in values, but all objects are as bright or brighter than the single detected larger object. A single upper limit is shown at the position of the one SNR detected only in C III. However, by referring back to Figure 25, there are numerous other SNRs in the mid-range of angular diameter that would provide upper limits below $\rm 1.0 \times 10^{-14} ~ \FLUX$, indicating an even larger dispersion than shown in Figure 26.

There are at least two objects in the non-detection list with very faint optical emission but well-formed X-ray shells (SNR 0453-685, Figure 19 fourth panel; SNR 0534-699, Figure 23, top panel). These two objects are modest in size, and may be dominated by non-radiative shocks (e.g., Ghavamian et al. 2006 and references therein). Since most of our detections show both O VI and C III lines at relative strengths consistent with radiative shocks (Hartigan et al. 1987), it may be systematically easier to detect such objects over non-radiative cases.

Another factor in non-detections could simply be foreground extinction. Accurate optical color excesses or columns densities are not available for many of the objects we have observed. While the foreground galactic extinction toward the MCs is generally low, some objects could be impacted by local extinction. MC extinction curves tend to show sharper upturns at short UV wavelengths than the galactic curve (e.g. Prévot et al. 1984; Fitzpatrick 1985) so this effect cannot be ignored. Since many of the detections are quite faint, some non-detections could just be due to excessive but unknown attenuation. With the line flux information in Table 3, we can in principle calculate O VI luminosities for detected objects. However, a number of assumptions must be made, with the quality of those assumptions varying considerably from object to object. For instance, to scale an O VI flux measured through the LWRS aperture to a total O VI flux, one must assume a uniform flux across the object and scale to its projected area, an assumption that may be particularly inaccurate for the larger SNRs. Also, since many of the O VI line profiles show evidence of overlying absorption, a correction for the missing flux must be included. A moderately good assumption involves assuming an optically thin 2:1 ratio for $\lambda$1032:$\lambda$1038 to get a total O VI flux. Finally a correction for overlying extinction is required to estimate an intrinsic flux from the observed (corrected) value. Since extinction values are not known accurately for the individual objects, we choose below to assume a standard value of E(B - V) = 0.1 and a galactic extinction curve (e.g., Cardelli, Clayton, & Mathis 1989), implying a factor of 3.5 correction at O VI. (Note: this ignores any intrinsic LMC absorption, so larger corrections may be appropriate in some cases.) Proper scaling by distance then provides an estimate of the O VI luminosity of an object. Rather than attempt this calculation for all detected objects, we choose to perform it for a few selected objects where it appears reasonable combinations of assumptions can be made to provide some insight into the importance of O VI as a source of cooling relative to, for instance, the soft X-ray emission.

SNR 0454-665 (N11L; Figure 5) is a relatively small diameter SNR with a well-detected O VI line. Using the angular size listed in Table 2, a modest geometric correction factor (3.1) is needed to scale up to the whole object. If we assume from the line profile that roughly half of the actual O VI $\lambda$1032 line is seen through the overlying absorption, a factor of 1.5 to include $\lambda$1038, the factor of 3.5 extinction correction factor discussed above, and a distance of 50 kpc, we derive $\rm L(O~VI) ~=~ 4.2 \times 10^{35}~ergs ~ s^{-1}$. Williams et al. (1999b) used ROSAT data to estimate $\rm L_{x}(0.5 - 2~ keV) ~=~ 8.8 \times 10^{34}~ergs ~ s^{-1}$, or nearly a factor of 5 less than our estimated L(O VI).

As a slightly larger object, SNR 0532-710 (N206, Figure 12) has a geometric scaling factor from LWRS of $\sim$27. However, the sampled region appears to be representative of the shell interior, and if anything the O VI emission might be expected to limb-brightened. From inspection of the line profile, we again estimate $\sim$50% attenuation from overlying MC and MW absorption and assume the same $\lambda$1038 and extinction correction factors and distance as above, yielding $\rm L(O~VI) ~=~ 2.4 \times 10^{35}~ergs ~ s^{-1}$. Williams et al. (2005) report a Chandra/XMM-Newton estimate of $\rm L_{x}(0.3 - 8~ keV) ~=~ 8(\pm4) \times 10^{35}~ergs ~ s^{-1}$, dependent on model spectrum assumptions, or roughly several times the estimated O VI luminosity in this case.

By way of contrast, we select an FUV non-detected object, SNR 0453-685 (D90403; Fig. 19, bottom panel) with a night observing time of 3.8 ks. Interestingly, this object is quite bright in X-rays but shows no obvious optical emission in the MCELS data. We determine a 3$\sigma$ upper limit of $\rm 2.5 \times 10^{-13} ~\FLUX$ in $\lambda$1032. (This assumes an unresolved line and a filled LWRS aperture.) With a geometric correction factor of $\sim$16 and other parameters as for N206 above, we find $\rm L(O~VI) ~\le~ 1.9 \times 10^{34}~ergs ~ s^{-1}$. We are unaware of a detailed model of the X-ray emission from this SNR, but if we simply use the relative X-ray surface brightnesses for this object and N206 from Williams et al. (1999a, Table 2), scale by the projected areas, and assume the same spectral modelling as done for N206 (Williams et al. 2005), we derive an approximate X-ray luminosity of $\rm L_{x}(0.3 - 8~ keV) ~\simeq~ 6 \times 10^{36}~ergs ~ s^{-1}$, or at least two orders of magnitude brighter than the O VI upper limit. It is unlikely that extinction alone could account for such a large depression of the L(O VI), and substantial real variations of the relative luminosities are likely present. While none of these estimates is high fidelity, clearly there is a broad range of L(O VI):$\rm L_x$ in the sampled objects, and the luminosity in just the O VI lines can rival the entire soft X-ray luminosity in some cases.

Summary and Conclusions
=======================

It is clear from the $\fuse$ survey that there are numerous SNRs in the Magellanic Clouds that are observable in the far ultraviolet. We have detected 15 SNRs (14 previously undetected in the ultraviolet plus 0535-660 \[N63A\]) out of a total of 39 observed, bringing the total number of FUV-detected SNRs in the Magellanic Clouds to 22. The detected objects span a wide range of parameter space, from relatively small, bright objects to large (old) shells, and from both X-ray shells and filled-center morphologies. Almost as interesting are the non-detections, because the sensitivity of $\fuse$ allows significant upper limits to be placed in many cases. These objects must have substantially higher foreground columns and/or much slower shock velocities at the observed locations than the detected objects in order to escape detection. Given the likelihood of non-uniform spatial distributions of the FUV emission, non-detections at the observed positions do not necessarily indicate these objects have no FUV emission. Potential clumpiness, especially for the largest angular size SNRs, may be a significant reason for non-detection. A significant subset of our spectra were contaminated by stellar light, making the upper limits somewhat less conclusive in these cases.

The line profiles for detected objects generally show evidence of significant overlying self-absorption by the ISM of the host galaxy (if not also the Milky Way, depending on the SNR intrinsic line widths). Although we typically looked on a sight line through the center of each SNR, we tend to see emission at wavelengths just redward of the host galaxy rest frame, as if we are systematically seeing the receding side of each detected SNR. This is also indicative of overlying absorption, which would tend to impact emission from the approaching side of each SNR more severely. In one case, SNR 0536-706, we may be seeing front-side/back-side shell emission, although the observed profile in O VI could also represent a broader emission line with the center removed by O VI self-absorption (see sec. 5.14).

All but one of the detected SNRs were seen in O VI $\lambda$1032. Many but not all were detected in C III $\lambda$977 as well. A single SNR was seen marginally in C III but [*not*]{} in O VI. This may be due to a selection effect arising from SNRs with slower shocks (hence, no O VI) not being visible through the more substantial ISM absorption at C III. However, without additional observations, we cannot rule out the idea that O VI emission is present but patchy in this object, and hence simply not seen.

Although many assumptions are involved, estimates of the luminosities of these objects in O VI show a broad range relative to the soft X-ray luminosities (which also involve a number of assumptions). We have examples where the O VI luminosity is considerably brighter than the inferred soft X-ray luminosity, to some of the FUV non-detected objects where the inferred upper limit on O VI is perhaps two orders of magnitude or more below the X-ray luminosity. The reason or reasons for such diversity are not entirely clear, although many important parameters, such as line of sight extinction and clumpiness of the FUV emission, are not currently well established for many of these objects.

The $\fuse$ data reported in this paper were obtained with the NASA-CNES-CSA $\fuse$ satellite, which is operated by Johns Hopkins University with financial support through NASA contract NAS 5-32985. It is a pleasure to thank the $\fuse$ operations team for their efforts in obtaining these data. We also thank Chris Smith, Roger Leiton Thompson, and Claudio Aguilera of the MCELS team for providing us with the optical images used in this paper. This work has been supported by NASA guest investigator grants NAG5-12423 and NNG04GJ25G, both to the Johns Hopkins University.

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Appendix
========

In this Appendix, we provide brief explanatory comments on each of the detected Magellanic Cloud SNRs.

SNR 0448-669 (Figure 3)
-----------------------

This SNR was only recently identified, as part of ongoing analysis of the MCELS imaging survey (Smith et al. 2004). The position provided was apparently poorly centered and instead sits on the northern limb of the optical shell. No X-ray image is available. The $\fuse$ detection of O VI is modest, although both lines of the doublet are present and some indication of self-absorption by LMC halo O VI is evident, especially for $\lambda$1032. The C III line is not detected with certainty although only 3.8 ks of the 15.7 ks integration was in orbital night. With a line width of nearly 700 $\VEL$, the blue wing of $\lambda$1032 may be getting clipped by Milky Way halo absorption.

SNR 0450-709 (Figure 2)
-----------------------

This low surface brightness, non-descript shell SNR is one of the largest Magellanic SNRs in angular size, first reported by Mathewson et al. (1985) in the optical and radio. Williams et al. (2004) have recently provided a detailed investigation. The XMM-Newton data show only a faint enhancement above background levels with no distinct shape seen. The X-rays appear to be brighter interior to the shell, making the SNR similar to a handful of large old galactic SNRs such as W28 and 3C400.2 (Long et al. 1991). Despite its apparent late stage of evolution, the SNR was detected in both C III and O VI with $\fuse$, although as a faint source. The detection has been aided by the relatively long (9.5 ks) orbital night integration for this object. The central velocity of the detected SNR emission appears to be just slightly redward of the LMC mean velocity. The C III emission appears truncated on the blue side by LMC absorption and is artificially narrow. Both O VI lines appear to be broader lines impacted by overlying (mainly LMC) absorption. Intrinsic fluxes are correspondingly uncertain.

SNR 0454.5-6713 (Figure 4)
--------------------------

This SNR was discovered as a diffuse X-ray source, and was found optically by Smith et al. (1994). It is located within the LMC H II region N9. It’s optical morphology is nondescript, with just a few radiative filaments embedded within faint, diffuse emission. Faint, diffuse X-ray emission fills the region of optical emission and extends somewhat farther to the south and southwest. The Chandra X-ray morphology gives the impression that this SNR is a member of the composite or “mixed morphology" class, with a faint outer shell and bright, filled center. The bright central X-ray emission in the Chandra image is probably due to ejecta, and Seward et al. (2004) point to a likely Type Ia supernova origin. The $\fuse$ aperture position, based on the estimated optical centroid, is clearly off-center based on the X-ray image and samples primarily the diffuse regions in X-ray and optical. The strong, relatively narrow C III and O VI emission lines lie just longward of the LMC rest frame velocity and appear to be due to radiative shocks. The lines could be impacted by blue wing absorption, but the lack of an extended red wing on the strong, narrow lines argues for relatively low velocity dispersion within the aperture.

SNR 0454-665 (N11L; Figure 5)
-----------------------------

This SNR is a member of the original group of SNRs identified in the LMC by Mathewson & Clarke (1973) using the optical \[S II\] to H$\alpha$ criterion, and has been studied in detail by Williams et al. (1999b). Although it is part of the large N11 H II region complex, it forms a well-defined and relatively isolated shell with an apparent “jet" or breakout to the NE, reminiscent of the optical jet in Cas A (Fesen & Gunderson 1996). With a relatively small angular size, the ROSAT PSPC image shows little structure and leaves the impression of filled-center emission. The $\fuse$ spectrum with only 2.2 ks of orbital night nonetheless shows very bright and well-detected O VI $\lambda\lambda$1032,1038 and C III lines with evidence for absorption on the blue sides of the profiles. In particular, the $\lambda$1032 line provides evidence of a much broader and stronger line than is actually detected. Radiative shock emission apparently dominates the line emission from material in the $\fuse$ aperture.

SNR 0506-680 (N23; Figure 6)
----------------------------

Once again, the faintness of the diffuse, patchy optical emission from this SNR has caused the Mathewson et al. (1983) coordinate to be off-center in comparison with the Chandra X-ray data, which extends considerably to the north and west from the bulk of the optical emission. In this case, the $\fuse$ aperture lies on the SE bright X-ray limb. Even with only 2.7 ks of orbital night integration, both O VI and C III are detected, although the C III line is quite faint. The O VI emission is quite strong and very broad relative to C III in this object. The line shape for O VI $\lambda$1032 makes it clear that both galactic and LMC overlying absorption are impacting the observed fluxes. The brightness of the X-ray and O VI intensity and the faintness of the optical emission and C III line make it likely that non-radiative shocks contribute significantly to the observed emission.

SNR 0509-687 (N103B; Figure 7)
------------------------------

This young, small diameter SNR nearly fits within the nominal $\fuse$ LiF1A LWRS aperture. The bright X-ray emission corresponds to the region of knotty optical emission, implicating a shock-cloud interaction on the west side, but fainter X-ray emission extends toward the east. No discernable C III emission is present, although the small angular size of the SNR makes it possible in principle that the $\fuse$ SiC apertures could have been misaligned enough to miss or only partially cover the SNR. (We note that the LiF2B channel, which also covers the O VI spectral region, was apparently misaligned–see main text.) The O VI $\lambda$1032 emission is quite strong and broad, and the $\lambda$1038 line is obviously impacted more severely by overlying absorption. A small amount of scattered starlight contaminates the spectrum near O VI, causing the apparent pedestal of emission.

SNR 0520-694 (Figure 8)
-----------------------

This relatively faint and moderately sized optical shell SNR lies in a rich star field within the bar of the LMC. Significant stellar emission contaminates the $\fuse$ aperture position, but we feel confident in claiming detection by virtue of the strong, relatively narrow C III emission just longward of the LMC rest velocity. Although an O VI P-Cygni feature in the stellar spectrum cannot be ruled out without more information, an interpretation of the dip at $\lambda$1032 as ISM absorption would make it likely that the apparent emission just longward of the LMC rest velocity is faint, broad O VI emission from the SNR. This is the feature measured and reported in Table 3.

SNR 0527-658 (DEML 204; Figure 9)
---------------------------------

This is one of the larger SNRs in the survey, and is an apparent member of the composite class, with a large outer optical shell and filled-center X-ray emission. This is the only SNR in the survey that shows C III emission but no detectable emission in O VI, and even the C III detection is somewhat marginal. However, the appearance of the nominal C III line is consistent with the hypothesis we have put forward in the main text for why there are almost no such detections: the intrinsic C III SNR line is largely absorbed by intervening ISM. For this object, the kinematics are such that the C III line peeks out on the red side of the LMC absorption and is just visible. The intrinsic C III line is likely quite strong in this large (older) radiative shell-type SNR, and the absence of detectable O VI indicates few shocks remaining at velocities in excess of 160 $\VEL$. There is a possible emission line at the position of C II $\lambda$1037.02, the excited state component of the more well known C II $\lambda$1036.34 ISM absorption line, although significant overlying absorption may be present. The possible presence of this line is also consistent with the emission from this object being dominated by slow radiative shocks.

SNR 0528-692 (LHG40; Figure 10)
-------------------------------

This SNR was first reported in X-rays by Long, Helfand, & Grabelsky (1981) and optical and radio detections were provided by Mathewson et al. (1984). A faint optical radiative shell is visible, but the X-ray emission is ill-defined in the ROSAT PSPC data. This is another object in the LMC bar, and stellar contamination is nearly impossible to avoid. The $\fuse$ spectrum of this object includes only orbital day data, and so additional lines are due to residual dayglow, including the feature near Milky Way C III rest velocity. Despite the stellar contamination, the SNR O VI emission is clearly seen in both lines at the LMC velocity. Interestingly, no C III at LMC velocities is detected, which is surprising given the radiative shell nature of the optical emission. It may be that the receding shell is too faint to be sween and the near-side emission is lost in the ISM absorption from the LMC.

SNR 0530-701 (Figure 11)
------------------------

This SNR was only recently identified, as part of ongoing analysis of the MCELS imaging survey (Smith et al. 2004), and appears to be a large, old, radiative shell-type SNR. No X-ray data are available. The supplied coordinate for this object was on the southern rim instead of the center. The $\fuse$ detection of O VI is quite weak but moderately broad. Absorption may affect the blue side of the $\lambda$1032 line, and $\lambda$1038 is only marginally present. The C III line is not detected with certainty, although the small fraction of orbital night data for this observation (and the use of the total data set for this object) both complicate the interpretation of C III.

SNR 0532-710 (N206; Figure 12)
------------------------------

This composite or “mixed morphology" SNR shows a classic symmetrical shell in the optical (with the shocked radiative emission showing particularly well in \[S II\]), but a center-filled X-ray morphology. A detailed study has been published by Williams et al. (2005), with the addition of a possible pulsar wind nebula discovered in X-ray and radio data. In this moderate sized SNR, the central X-ray emission is likely due to ejecta. The $\fuse$ spectrum shows faint, broad O VI emission and a very narrow C III line just longward of the LMC rest velocity. Clearly overlying absorption is impacting these profiles, and the blue side of C III could just be missing. It is likely that the observed emission is due to the radiative shell.

SNR 0534-705 (DEM 238; Figure 13)
---------------------------------

This moderately sized faint shell-type optical SNR was reported by Mathewson et al. (1983). The Chandra data show extended faint X-ray emission filling the optical shell, but a brighter central concentration (somewhat offset toward the southwest) that is likely due to ejecta. Even though the $\fuse$ aperture is filled partially with ejecta, the narrowness of the observed line profiles makes it likely that the observed lines are fron the readiative shell. Both O VI lines are seen but $\lambda$1038 is weak, implying overlying absorption. The C III line is comparable to O VI $\lambda$1032 in peak intensity but is narrower, largely attributable to ISM absorption on the blue side of the profile.

SNR 0535-660 (N63A; Figure 14)
------------------------------

This SNR was detected with IUE (Benvenuti et al. 1980) but has not been observed with more modern instrumentation. The IUE spectra only showed C IV $\lambda$1550, He II $\lambda$1640, and C III $\lambda$1909 emission lines superimposed on a continuum of stellar contamination and/or dust-scattered starlight. The SNR is buried within an extended H II region and OB association. The bizarre optical morphology is dominated by a shocked cloud located on the near side of the SNR from our sight line. The true size and shape of the SNR is apparent from the soft X-rays (e.g., Warren et al. 2003). The $\fuse$ aperture covered this shocked cloud and the spectrum is contaminated by significant continuum. Nonetheless, very strong O VI emission lines are detected just shortward of the LMC rest velocity. There is no clear evidence for C III. Since IUE observed C III $\lambda$1909, it is likely that the SiC channels were misaligned by enough that they missed the bright radiative shocked cloud. The strength of the O VI emission implies a radiative shock origin, even though C III and other cooler ions are missing. Given that It is probable that the FUV emission is dominated by radiative shocks. This is consistent with a dense, shocked cloud being hit relatively recently by the blast wave.

SNR 0536-706 (DEML 249; Figure 15)
----------------------------------

This SNR is a large, faint optical shell SNR that is brighter on the eastern side of the shell. The X-ray emission from ROSAT PSPC is low resolution, but rather diffuse and strongest at the center, possibly indicating it is a member of the mixed-morphology class (Williams et al. 1999a). The $\fuse$ spectrum shows a narrow C III line centered at the LMC rest velocity and clearly double-peaked O VI lines that bracket the LMC rest velocity. The presence of C III emission at the rest velocity argues that this object must be on the near side of the LMC, and thus suffers little overlying absorption from LMC ISM C III. The O VI line morphology could then arise from one of two mechanisms: either O VI emission arises from approaching and receding sides of the shell and we are resolving this structure, or a broader O VI emission line is undergoing significant self-absorption from within the SNR itself. Whatever the cause, this spectral character is unique in the objects we have observed in this survey.

SNR 0104-723; Figure 16
-----------------------

This is the only SMC remnant observed to date in the $\fuse$ survey, and it was first reported by Mathewson et al. (1984). It is detected in O VI. Both O VI lines appear to be impacted by overlying absorption from both our Galaxy and the SMC. C III may be marginally detected. The strong emission at $\lambda$977 in the rest frame is backscattered solar emission in the $\fuse$ SiC channel and is not real.

**Figure Captions**

Note: Due to size restrictions, all figures included as separate ‘JPG’ files.

[**Fig. 1:**]{} Overview image of the Large Magellanic Cloud showing the distribution of detected and non-detected SNRs. Detected objects are indicated with stars symbols, with filled stars being previous detections (from Table 1) and white stars being new detections from Table 2. Non-detections are indicated with crosses. H$\alpha$ image from Gaustad et al. (2001).

[**Fig. 2:**]{} Summary figure for SNR 0450-709. The top panel shows optical and X-ray images of the SNR with the $\fuse$ LWRS aperture (30 square) overlaid for the location and position angle at the time of the observation. Top left: H$\alpha$; Top middle: \[S II\]; Top right: soft X-ray (from the source indicated by the label). The $\fuse$ aperture (30 square) provides the scale. The middle and bottom panels show $\fuse$ data sections centered on C III $\lambda$977, and O VI $\lambda\lambda$1032,1038, respectively. Each spectral panel contains two horizontal bars with tick marks, indicating the positions of potential overlying absorption features, one at zero velocity for Galactic absorption (labelled MW), and one shifted to the mean velocity of the host Magellanic Cloud (+275 $\VEL$ for LMC; +165 $\VEL$ for SMC). The long, bold tick marks are at the expected positions of the C III and O VI resonance lines, which are in emission from the SNR, but potentially in absorption by overlying gas. Medium tick marks indicate positions of $\rm H_2$ transitions, and short tick marks indicate positions of interstellar O I lines. Very short ticks in the bottom panel indicate the position of C II $\lambda$1036.3, another very strong ISM absorption line. The earth symbol indicates terrestrial H airglow emissions from Ly $\gamma$ (C III panel) and Ly $\beta$ (O VI panel).

[**Fig. 3:**]{} Same as Figure 2, but for SNR 0448-669. In this case, no X-ray data are available, and the coordinate provided was apparently off-center and on the northern limb of the shell.

[**Fig. 4:**]{} Same as Figure 2, but for SNR 0454.5-6713.

[**Fig. 5:**]{} Same as Figure 2, but for SNR 0454-665 (N11L).

[**Fig. 6:**]{} Same as Figure 2, but for SNR 0506-680.

[**Fig. 7:**]{} Same as Figure 2, but for SNR 0509-687 (N103B).

[**Fig. 8:**]{} Same as Figure 2, but for SNR 0520-694. In this case, no X-ray data are available.

[**Fig. 9:**]{} Same as Figure 2, but for SNR 0527-658.

[**Fig. 10:**]{} Same as Figure 2, but for SNR 0528-692.

[**Fig. 11:**]{} Same as Figure 2, but for SNR 0530-701. In this case, no X-ray data are available, and the coordinate provided was apparently off-center and on the southern limb of the shell. Due to the small fraction of orbital night data, the total dat set is used for this figure. The additional lines visible are due to dayglow emissions, including especially the strong line at Milky Way C III rest velocity.

[**Fig. 12:**]{} Same as Figure 2, but for SNR 0532-710.

[**Fig. 13:**]{} Same as Figure 2, but for SNR 0534-705.

[**Fig. 14:**]{} Same as Figure 2, but for SNR 0535-660 (N63A). The aperture includes the bright optical shocked cloud, which is known to be on the near-side (blue-shifted) of the SNR. Bright O VI emission is seen (despite considerable stellar contamination) but there is no indication of C III.

[**Fig. 15:**]{} Same as Figure 2, but for SNR 0536-706.

[**Fig. 16:**]{} Same as Figure 2, but for SNR 0104-723. This is the only SMC SNR surveyed to date. The velocity scale of the top bar has been adjusted to that of the SMC.

[**Fig. 17:**]{} C III and O VI spectral regions for the non-detection of SNR 0507-685 in the LMC. The upper line in each panel shows the total data set (10.9 ks CHECK), while the lower line shows the orbital-night data section for comparison (4.9 ks CHECK). The dashed lines show the zero point of the total data sets, which have been arbitrarily offset upwards for display purposes. The apparent weak emission at the LMC C III position in the total data is absent in the night-only segment, indicating it is not intrinsic SNR emission.

[**Fig. 18:**]{} Same as Figure 18, but for SNR 0519-697, an example where stellar light contaminated the $\fuse$ data. It is conceivable that LMC ISM absorption is present but being filled in with emission from the SNR. However, without a separate template spectrum of the star, there is no way to tell. Hence, the ability to place an upper limit on emission is compromised in cases such as this.

[**Fig. 19:**]{} Optical and X-ray images of SNRs declared non-detections, in the same format as the top panels in Figures 2-16. The nominal $\fuse$ LiF1 LWRS aperture positions are superimposed and provide the scale for each panel. Shown are SNRs 0449-693, 0453.1-6655, 0453-672, and 0453-685.

[**Fig. 20:**]{} Same as Figure 19, but for SNRs 0455-687 (N86), 0500-702, 0506-657, and RXJ0507-685 (see also Figure 17).

[**Fig. 21:**]{} Same as Figure 19, but for SNRs 0513-692, 0519-697 (see also Fig. 18), 0521-657, and 0523-679.

[**Fig. 22:**]{} Same as Figure 19, but for SNRs 0524-664, 0528-672, 0532-673, and 0532-675.

[**Fig. 23:**]{} Same as Figure 19, but for SNRs 0534-699, 0536-676, 0536-692, and 0536-693.

[**Fig. 24:**]{} Same as Figure 19, but for SNRs 0538-693, 0543-689, and 0547-697, where both “a" and “b" components (DEML316A and B) were observed separately. (See Williams et al. 1997.)

[**Fig. 25:**]{} Histograms of detected SNRs (solid line) and non-detected SNRs (dashed line) as a function of angular size (see text). The bin centered at 5.5 represents all objects larger than 5.

[**Fig. 26:**]{} A plot of measured O VI $\lambda$1032 flux versus angular size for detected SNRs. There is some tendancy for the smallest objects to have the highest O VI fluxes, but a large dispersion of values is indicated for objects above 1.5 in size. SNR 0535-660 (N63A) would be at 45 units on the vertical scale. The single detection in C III-only, SNR 0527-658, is shown as an upper limit, but other non-detections would also produce upper limits at or below the limit on this object.

[lccccc]{} \[tbl:table1\] SNR0057-7226 & 00:59:27 & $-$72:10:05 & P103,P203 & Yes & 1\
SNR0102-7219 & 01:04:04 & $-$72:01:50 & A075,C083 & Yes & 2\
SNR0505-679 (DEM L71) & 05:05:43 & $-$67:52:38 & P214,C072 & Yes & 3\
SNR0509-675 & 05:09:32 & $-$67:31:17 & P214 & Yes & 3\
SNR0519-690 & 05:19:34 & $-$69:02:10 & P214 & Yes & 3\
SNR0525-696 (N132D) & 05:25:01 & $-$69:38:34 & A075,B095 & Yes & 4\
SNR0525-661 (N49) & 05:26:04 & $-$66:05:18 & X005,C055 & Yes & 5,6\
SNR0548-704 & 05:47:50 & $-$70:24:52 & P214 & No & 3\

References: (1) Danforth et al. (2003); (2) Sasaki et al. (2006); (3) Ghavamian et al. (2006); (4) Beasley et al. (2006); (5) Blair et al. (2000b); (6) Sankrit et al. (2004).

[lccc]{} \[tbl:table3\] SNR0448-669 (E90001) & & &\
   $\lambda$(cent) \[Å\] & 1033.18 & 1038.87 & ...\
   V(cent) \[km/s\] & +369 & +361 & ...\
   $\Delta$V(FWZI) \[km/s\] & 676 & ... & ...\
   F($\lambda$) \[$\FLUX$\] & 2.8E-14 & 9.4E-15 & ...\
SNR0450-709 (D90401) & & &\
   $\lambda$(cent) \[Å\] & 1032.62 & 1038.32 & 978.13\
   V(cent) \[km/s\] & +206 & +201 & +341\
   $\Delta$V(FWZI) \[km/s\] & 627 & ... & 219\
   F($\lambda$) \[$\FLUX$\] & 1.2E-14 & 3.6E-15 & 1.2E-14\
SNR0454.5-6713 (D90405) & & &\
   $\lambda$(cent) \[Å\] & 1032.94 & 1038.64 & 978.25\
   V(cent) \[km/s\] & +299 & +284 & +377\
   $\Delta$V(FWZI) \[km/s\] & 336 & ... & 299\
   F($\lambda$) \[$\FLUX$\] & 3.2E-14 & 1.6E-15 & 7.8E-14\
SNR0454-665 (D90406) & & &\
   $\lambda$(cent) \[Å\] & 1032.67 & 1038.40 & 978.28\
   V(cent) \[km/s\] & +220 & +225 & +387\
   $\Delta$V(FWZI) \[km/s\] & 563 & ... & 291\
   F($\lambda$) \[$\FLUX$\] & 1.7E-13 & 6.5E-14 & 1.3E-13\
SNR0506-680 (D90409) & & &\
   $\lambda$(cent) \[Å\] & 1032.94 & 1038.64 & 978.34\
   V(cent) \[km/s\] & +294 & +292 & +405\
   $\Delta$V(FWZI) \[km/s\] & 684 & ... & 251\
   F($\lambda$) \[$\FLUX$\] & 1.4E-13 & 3.1E-14 & 2.3E-14\
SNR0509-687 (D90410) & & &\
   $\lambda$(cent) \[Å\] & 1033.01 & 1039.11 & ...\
   V(cent) \[km/s\] & +320 & +431 & ...\
   $\Delta$V(FWZI) \[km/s\] & 533 & ... & ...\
   F($\lambda$) \[$\FLUX$\] & 1.6E-13 & 6.9E-14 & $<$1.0E-14\
& & &\
& & &\
SNR0520-694 (D90448) & & &\
   $\lambda$(cent) \[Å\] & 1033.41: & ... & 978.45\
   V(cent) \[km/s\] & +436: & ... & +439\
   $\Delta$V(FWZI) \[km/s\] & 490: & ... & 393\
   F($\lambda$) \[$\FLUX$\] & 6.7E-14: & ... & 2.0E-13\
SNR0527-658 (D90417) & & &\
   $\lambda$(cent) \[Å\] & ... & ... & 978.38\
   V(cent) \[km/s\] & ... & ... & +418\
   $\Delta$V(FWZI) \[km/s\] & ... & ... & 315\
   F($\lambda$) \[$\FLUX$\] & $<$1.0E-14 & ... & 2.2E-14\
SNR0528-692 (D90418) & & &\
   $\lambda$(cent) \[Å\] & 1033.00 & 1038.76 & ...\
   V(cent) \[km/s\] & +320 & +330 & ...\
   $\Delta$V(FWZI) \[km/s\] & 453 & ... & ...\
   F($\lambda$) \[$\FLUX$\] & 4.8E-14 & 3.5E-14 & ...\
SNR0530-701 (E90005) & & &\
   $\lambda$(cent) \[Å\] & 1032.97 & 1038.69 & ...\
   V(cent) \[km/s\] & +308 & +309 & ...\
   $\Delta$V(FWZI) \[km/s\] & 604 & ... & ...\
   F($\lambda$) \[$\FLUX$\] & 1.4E-14 & 3.0E-15 & ...\
SNR0532-710 (D90419) & & &\
   $\lambda$(cent) \[Å\] & 1033.13 & 1038.90 & 978.38\
   V(cent) \[km/s\] & +355 & +370 & +418\
   $\Delta$V(FWZI) \[km/s\] & 650 & ... & 315\
   F($\lambda$) \[$\FLUX$\] & 1.5E-14 & 1.8E-15 & 2.2E-14\
SNR0534-705 (D90422) & & &\
   $\lambda$(cent) \[Å\] & 1033.34 & 1039.04 & 978.42\
   V(cent) \[km/s\] & +416 & +410 & +430\
   $\Delta$V(FWZI) \[km/s\] & 370 & ... & 231\
   F($\lambda$) \[$\FLUX$\] & 6.1E-14 & 2.0E-14 & 2.7E-14\
& & &\
& & &\
& & &\
SNR0535-660 (D90423) & & &\
   $\lambda$(cent) \[Å\] & 1032.67 & 1038.42 & ...\
   V(cent) \[km/s\] & +221 & +231 & ...\
   $\Delta$V(FWZI) \[km/s\] & 480 & ... & ...\
   F($\lambda$) \[$\FLUX$\] & 4.5E-13 & 3.5E-14 & ...\
SNR0536-706 (D90426) & & &\
   $\lambda$(cent) \[Å\] & 1033.05 & 1038.88 & 978.15\
   V(cent) \[km/s\] & +331 & +364 & +347\
   $\Delta$V(FWZI) \[km/s\] & 608 & ... & 200\
   F($\lambda$) \[$\FLUX$\] & 3.4E-14 & 8.5E-15 & 2.7E-14\
SNR0104-723 (D90444) & & &\
   $\lambda$(cent) \[Å\] & 1032.66 & 1038.35 & 978.34\
   V(cent) \[km/s\] & +218 & +211 & +405\
   $\Delta$V(FWZI) \[km/s\] & 604 & ... & 290\
   F($\lambda$) \[$\FLUX$\] & 2.6E-14 & 1.2E-14 & 1.6E-14\
