arXiv:astro-ph/0104212v1 11 Apr 2001
X-Rays from Superbubbles in the Large Magellanic Cloud. VI. A
Sample of Thirteen Superbubbles
Bryan C. Dunne, Sean D. Points, and You-Hua Chu
Department of Astronomy, University of Illinois, 1002 West Green Street, Urbana, IL
email@example.com, firstname.lastname@example.org, email@example.com
We present ROSAT observations and analysis of thirteen superbubbles in the
Large Magellanic Cloud. Eleven of these observations have not been previously
reported. We have studied the X-ray morphology of the superbubbles, and have
extracted and analyzed their X-ray spectra. Diffuse X-ray emission is detected
from each of these superbubbles, and X-ray emission is brighter than is theo-
retically expected for a wind-blown bubble, suggesting that the X-ray emission
from the superbubbles has been enhanced by interactions between the superbub-
ble shell and interior SNRs. We have also found significant positive correlations
between the X-ray luminosity of a superbubble and its Hα luminosity, expansion
velocity, and OB star count. Further, we have found that a large fraction of
the superbubbles in the sample show evidence of “breakout” regions, where hot
X-ray emitting gas extends beyond the Hα shell.
Subject headings: ISM: bubbles, galaxies: individual (LMC)
Superbubbles are large (∼100 pc across) shells in the interstellar medium (ISM) created
by the combined action of stellar winds and supernova explosions of massive stars in an
OB association. The hot (?106K) shock-heated gas interior to superbubbles emits X-ray
radiation. X-ray observations of superbubbles can reveal a wealth of information on the
structure and interior of superbubbles. An excess of diffuse X-ray emission in superbubbles
can indicate the presence of interior supernova remnants (SNRs) shocking the inner walls of
the superbubble shell (Chu & Mac Low 1990, hereafter Paper I; Wang & Helfand 1991).
The diffuse X-ray emission can also be used to find “breakout” regions where the hot gas
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from the superbubble interior may be leaking out into the ISM. Unresolved peaks superposed
on the diffuse X-ray emission may also indicate the presence of stellar X-ray sources interior
to the superbubble.
The Large Magellanic Cloud (LMC) provides an ideal laboratory for observing super-
bubbles in the X-ray spectrum. The coverage of Galactic superbubbles is far from complete
because of extinction from the disk of the Milky Way. Non-Magellanic Cloud extragalactic
superbubbles are too far away to be angularly resolved by X-ray instruments such as ROSAT.
The LMC, however, provides a sample in excess of 20 superbubbles, at a common distance
(∼50 kpc, Feast 1999), that are resolvable by modern X-ray detectors. Observations of
the superbubbles in the LMC can provide us with great insight into the interaction among
superbubbles, SNRs, and the ISM.
We have been studying X-ray emission from superbubbles in the LMC. In Paper I, Ein-
stein observations were used to show that seven LMC superbubbles are diffuse X-ray sources
with luminosities much higher than those expected by the wind-blown, pressure-driven bub-
ble models of Weaver et al. (1977). Off-center SNRs are proposed to be responsible for
the excess X-ray emission. ROSAT observations of the superbubble N44 confirmed its dif-
fuse X-ray emission and provided the first useful X-ray spectra of N44 for determinations of
plasma temperatures (Chu et al. 1993, hereafter Paper II). To illustrate that excess X-ray
emission from superbubbles is caused by an intermittent process, Chu et al. (1995, hereafter
Paper III) analyzed ROSAT observations of four X-ray-dim superbubbles, and showed that
these superbubbles do not have excess X-ray emission. For high-resolution spectral analy-
sis, ASCA observations of N44 were made; the ASCA data showed that the hot gas in the
breakout region is slightly cooler than that in the superbubble interior (Magnier et al. 1996,
hereafter Paper IV). ROSAT observations of the H II complex N11 were analyzed to study
the interaction between OB associations, H II regions, and superbubbles (Mac Low et al.
1998, hereafter Paper V).
In this latest study, we have analyzed ROSAT observations of eleven Hα-indentified
superbubbles in the LMC whose observations had not been reported previously. Diffuse
X-ray emission was detected in every one of these eleven superbubbles. We have re-analyzed
N44 and N11 in order to have a homogeneous set of results for comparisons. We have modeled
these superbubbles using the pressure-driven bubble models of Weaver et al. (1977). In this
paper, we report the ROSAT observations of the thirteen superbubbles studied, and discuss
the X-ray luminosities and other properties of the superbubbles and their relationship with
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2. X-Ray Dataset and Analysis
ROSAT Archival Dataset
Our dataset is based on a selection of known LMC superbubbles around OB associations
with a well-defined Hα morphology. We have further constrained the sample to include
only those superbubbles with previously unreported ROSAT observations with at least 5 ks
exposure. The superbubbles studied are in N51, N57, N103, N105, N144, N154, N158, N160,
N206 (nomenclature of Henize 1956), and 30 Dor C. Two superbubbles are present in N51,
making the total number of superbubbles eleven. For comparisons with previous results, we
have also included N11 and N44 in the dataset. The coordinates, sizes, Hα luminosities,
expansion velocities, and local OB associations of this sample of thirteen superbubbles, as
well as alternative designations, are summarized in Table 1.
Two detectors are available on board the ROSAT satellite: the Position-Sensitive Pro-
portional Counter (PSPC) and the High-Resolution Imager (HRI). We have used PSPC
observations to investigate physical conditions and distribution of the 106K gas interior to
most of the LMC superbubbles in our dataset. The PSPC is sensitive to X-ray photons
with energies in the range of 0.1–2.4 keV and has an energy spectral resolution of ∼40% at
1 keV, with a field of view of ∼2◦. As the HRI is better suited to revealing points sources
rather than diffuse emission, we have used HRI observations to investigate the distribution
of X-ray emitting gas only in the superbubble N206, which fell close to the outer edge of the
field-of-view in the PSPC observations. The HRI is sensitive in the energy range of 0.1–2.0
keV, with a field of view of ∼40′. Further information on the PSPC and HRI can be found
in Pfefferman et al. (1987) and the ROSAT Mission Description (1991). A summary of the
individual observations can be found in Table 2.
2.2.X-Ray Data Analysis
We have studied both the X-ray morphology and X-ray spectra of the superbubbles in
the dataset. All of the data were reduced using standard routines in IRAF1and the PROS2
1Image Analysis and Reduction Facility – IRAF is distributed by the National Optical Astronomy Ob-
servatories, which are operated by the Association of Universities for Research in Astronomy, Inc., under
cooperative agreement with the National Science Foundation.
2PROS/XRAY Data Analysis System – http://hea-www.harvard.edu/PROS/pros.html
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Analysis of the X-ray morphology of each superbubble was conducted using smoothed
PSPC and HRI images. The images were binned to 4′′pixels and then smoothed with
Gaussian function of σ = 4 pixels. We have compared the X-ray morphologies with the
Hα morphologies observed in the PDS scans of the Curtis Schmidt plates of Kennicutt &
Hodge (1986). In Figure 1, we present the Hα images overlaid with X-ray contours. We also
present the X-ray images overlaid with the same contours to ensure the clarity of the contour
levels. The contours are at levels of 50%, 70%, 85%, and 95% of the peak level within the
superbubble. For bright X-ray objects in the field not actually part of the superbubble (such
as SNRs), we have plotted additional contours at 2, 4, 8, and 16 times the superbubble peak
level. These contours are plotted as dashed lines.
2.2.2. Spectral Fits
The X-ray spectra of the superbubbles were extracted from the PSPC data. We defined
source regions for each superbubble. Then, possible stellar sources (i.e., X-ray binaries, Wolf-
Rayet stars) were excised from the data before the spectra were extracted. Additionally, we
selected several background regions around each superbubble (This is especially important
for superbubbles superposed on large extended LMC sources of diffuse X-ray emission, such
as the 30 Doradus complex and the supergiant shells LMC2 and LMC3). The background-
subtracted spectra were then extracted from the PSPC event files.
The observed X-ray spectrum of each superbubble is a convolution of several factors:
the intrinsic X-ray spectrum of the superbubble, the intervening interstellar absorption, and
the PSPC response function. Because the interstellar absorption and the PSPC response
function are dependent on photon energy, we must assume models of the intrinsic X-ray
spectrum and the interstellar absorption to make the problem tractable.
emission from the superbubble interiors appears largely diffuse, we have used the Raymond &
Smith (1977) thin-plasma emission model and the Morrison & McCammon (1983) effective
absorption cross-section per hydrogen atom to describe the intrinsic X-ray spectra of the
superbubbles and the foreground absorption, respectively. We then simulate the observed
spectrum, combining the assumed models for the intrinsic spectrum and the interstellar
absorption with the response function of the PSPC. The observed spectrum is fitted with
the simulated spectra; the χ2of the fits determines the best-fit.
As the X-ray
We performed a χ2grid search of simulated spectral fits to determine the best-fit levels
for the temperature, kT, and absorption column density, NH. From these model fits, we can
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calculate the un-absorbed X-ray flux, and therefore the X-ray luminosity, LX, of the diffuse
X-ray emission from the superbubbles. The normalization factor of the best fit can be
used to determine the volume emission measure, adopting a distance of 50 kpc for the LMC
(Feast 1999). If we assume a uniform density in the X-ray-emitting gas, the volume emission
measure can be expressed as N2
factor, and V is the volume of the superbubble interior. Assuming the superbubbles have an
ellipsoidal shape, we can use the observed diameters of the superbubbles to determine Ne
The net exposure time, background-subtracted source counts, scaled background counts,
and best-fit values of kT, NH, LXand Ne
(1977) thermal plasma model are given in Table 3. Plots of the fits to the superbubbles’
X-ray spectra are shown in Figure 2.
efV , where Neis the electron density, f is the volume filling
√f for a 30% solar abundance Raymond & Smith
We have also sought to further constrain the models by using observations of the H I
column density to independently determine the total absorption column density, NH. We
have divided the absorption column into Galactic and LMC components and determined
each separately. Arabadjis & Bregman (1999) demonstrated that in the X-ray spectrum, at
Galactic latitudes |b| > 25◦, the contribution of molecular gas to the total absorption column
density is comparable to the contribution of the neutral hydrogen gas. We have therefore
approximated the Galactic absorption column density by NH≃ 2 × NHI. Continuing this
approximation to the LMC absorption column density is more complicated. Measurements
of LMC H I column density can sample material both in front of and behind a feature such
as a superbubble. We have used the simplifying assumption that half of the LMC H I gas is
foreground to the superbubbles and half is background. Thus, the LMC component of the
total absorption column density is1
is therefore given by NH= 2 × (NHI)Galactic+ (NHI)LMC.
We have used the observations of Galactic and LMC H I column densities by Dickey
& Lockman (1990) and Rohlfs et al.(1984) to determine the total absorption column
density. These calculated values of NHare, on average, nearly an order of magnitude larger
than values of NH determined by the best-fit models to the X-ray spectra (see Table 4).
Indeed, several of the Galactic NHvalues are alone larger than those derived from the best-
fit models. The calculated values of NH also show a much narrower range of absorption
column densities to the LMC superbubbles (log NH = 21.3–21.7) than the values of NH
derived from the best-fit models to the X-ray spectra (log NH= 20.4–22.0).
2×2 ×NHI= NHI. The total absorption column density
We have re-determined best-fit values for kT, LX, and Nebased again on a 30% solar
abundance Raymond & Smith (1977) thermal plasma model, but with NH fixed at the
calculated values determined above. A summary of the results are given in Table 4. Plots
of these “fixed NH” fits to the superbubbles’ X-ray spectra are shown in Figure 3.
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3.Individual Superbubble Properties
We will now discuss each of the superbubbles studied individually. For each super-
bubble, although we primarily use the nomenclature of Henize (1956, e.g., N11), we give
alternative designations as cataloged by Davies, Elliott, & Meaburn (1976, e.g., DEM L
192). We also give the OB associations and star clusters encompassed by the superbubble
as reported in Lucke & Hodge (1970, e.g., LH63). General descriptions of the Hα morphol-
ogy, as seen in the PDS scans of the Curtis Schmidt plates of Kennicutt & Hodge (1986),
and comparisons with X-ray morphology, from the smoothed PSPC images, are given. We
also discuss breakout regions, possible identifications of X-ray hotspots with known stellar
sources from Breysacher (1981, e.g., Br81) and Sanduleak (1969, e.g., Sk−66◦28), and other
interesting X-ray features. The linear sizes of the superbubbles are calculated assuming 1′
= 15 pc.
N11 (DEM L 34) is the second largest H II complex in the LMC (Kennicutt & Hodge
1986). N11 contains a superbubble 150 pc × 100 pc in size, surrounded by several bright
Hα knots and smaller Hα shells. The superbubble has been labeled Shell 1 in Paper V. Shell
1 encompasses the OB association LH 9. Diffuse X-ray emission is detected toward Shell 1
(See Figure 1a).
This diffuse X-ray is centrally bright and appears to be confined by the observed Hα
shell. The X-ray emission peaks at a location coincident with HD 32228, a known Wolf-
Rayet star also cataloged as Br81 and Sk−66◦28. Several smaller peaks are also evident in
the diffuse X-ray emission. A more thorough interpretation of the X-ray emission can be
found in Paper V.
3.2. N44 (DEM L 152)
N44 is a bright H II complex, similar to N11. N44 contains a superbubble, cataloged as
DEM L 152, around the OB association LH 47. The 100 pc × 75 pc superbubble is well-
detected in Hα emission with well-defined shell walls. Diffuse X-ray emission is detected
toward DEM L 152 (See Figure 1b). Additionally, diffuse X-ray emission is detected ∼6′to
the northeast of DEM L 152; this emission has been identified by Paper II as originating
from a supernova remnant.
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The diffuse X-ray emission correlates well with the Hα shell of DEM L 152. The X-ray
emission is limb-brightened, forming an X-ray shell just interior to the Hα shell. A breakout
region on the southern edge of the superbubble and an X-ray blister on the eastern edge of
the shell are detected in the PSPC image as well. A more thorough interpretation of the
X-ray emission and the breakout regions can be found in Paper II and Paper IV.
3.3. N51 (DEM L 192, DEM L 205)
N51 is a nebular complex encompassing five OB associations, LH 51, LH 54, LH 55,
LH 60, and LH 63 (Paper I). Two ionized gas shells are visible in the Hα image (see Figure 1c)
and are cataloged as DEM L 192 and DEM L 205. DEM L 192 is the larger shell with a size
of 135 pc × 120 pc and contains the OB associations LH 51 and LH 54. DEM L 205 is the
smaller shell with a size of 65 pc × 50 pc. The morphology of DEM L 205 is that of a blister
with the OB association LH 63 at the base. Diffuse X-ray emission is detected toward both
of these optical shells.
The diffuse X-ray emission toward DEM L 192 is limb-brightened and confined within
the optical Hα shell. Therefore, it is reasonable to conclude that the X-ray emission is
produced by hot gas interior to the superbubble. Additionally, an X-ray hotspot is detected
within the superbubble and coincides with the Wolf-Rayet star Br31, also cataloged as
Sk−67◦104. Emission from this hotspot was excluded in the thermal plasma model fit of
DEM L 192.
It is not clear whether the X-ray hotspot is a peak in the diffuse emission or a stellar
source. Only 42±11 PSPC counts were detected from the hotspot. This count level is
inadequate to constrain the three parameters upon which a thermal plasma emission model
depends. Further high resolution X-ray observations are needed to explore the nature of this
DEM L 205 also shows limb-brightening in the PSPC images. Unlike DEM L 192, no
X-ray hotspots are detected within DEM L 205. On the southwestern side of this shell, where
the Hα surface brightness is lowest, the X-ray emission appears more extended than the main
shell. X-ray emission of similar morphology is also detected in the Einstein observations of
N51 (Paper I). This may indicate a breakout of the hot gas interior to DEM L 205 into a
lower density region; however, the Hα image hints at the presence of very faint outer Hα
filaments confining this emission.
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3.4.N57 (DEM L 229)
N57 is a nebular complex encompassing the OB association LH 76. Two ionized gas
shells are visible in the Hα image (See Figure 1d) and are cataloged as DEM L 229 and
DEM L 231. DEM L 229 is the larger shell; it is 200 pc × 100 pc in size and contains LH 76.
The smaller shell, DEM L 231, 30 pc × 30 pc in size, is a ring nebula around the Wolf-Rayet
star Br48 (Chu & Lasker 1980; Chu, Weis, & Garnet 1999). Of these two shells, diffuse
X-ray emission is only detected toward DEM L 229. Additionally, a bright X-ray source is
visible to the north of DEM L 229, the X-ray source is not associated with DEM L 229 and
has been identified as the SNR candidate, SNR 0532-675 (Mathewson et al. 1985; Williams
et al. 1999).
The diffuse X-ray emission toward DEM L 229 is coincident with the interior of the
superbubble. The emission also appears to be largely confined by the optical Hα shell. This
indicates that the X-ray emission originates from the hot gas in the superbubble interior.
The X-ray emission appears brightest towards the southern part of DEM L 229. Details of
the X-ray morphology cannot be confidently determined, however, as N57 is detected on the
outer edge of the PSPC (See Table 2) where the point-spread function becomes quite poor.
3.5. N103 (DEM L 84)
N103 is a nebular complex encompassing the star cluster NGC1850. It has two main
components, N103A (DEM L 85), the 20 pc × 15 pc Hα knot on the east, and N103B3
(DEM L 84), a 120 pc × 120 pc superbubble on the west. Supernova remnant 0509−68.7
is present just exterior to the eastern edge of the superbubble (Mathewson et al.
Williams et al. 1999), and the superbubble is brightest in Hα on the eastern side closest to
the SNR. Diffuse X-ray emission is detected toward both the superbubble and the supernova
remnant (See Figure 1e).
The X-ray emission toward N103B appears to have several distinct structures. The most
obvious is the very strong extended source that is coincident with the supernova remnant
0509−68.7. Another prominent structure is a large limb-brightened X-ray ring with a point
X-ray source at the center. Chu et al.(2000) found that this X-ray ring is centered on
the nearby star cluster HS122 (Hodge & Sexton 1966), which is projected at the southwest
3It should be noted here that the label “N103B” is often erroneously used to refer to the supernova
remnant 0509−68.7. We are using “N103B” to refer to the Hα structure as originally identified by Henize
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rim of the superbubble. We therefore cannot assume that the majority of the diffuse X-ray
emission detected toward N103B is caused by hot gas inside the superbubble of N103B.
Further comparison of the X-ray and Hα images presented in Figure 1e reveal diffuse X-
ray emission in the region between the supernova remnant of N103B and the X-ray ring
and coincident with the superbubble. Because of the PSPC’s low scattering levels, it is
reasonable to assume that this X-ray emission arises from hot gas within the superbubble.
We have used this region as a sample of the X-ray emission from the superbubble of N103B.
3.6. N105 (DEM L 86)
N105 is a nebular region encompassing the OB association LH 31 (also cataloged as
NGC1858) and the star cluster NGC1854. Two bright Hα knots are visible as well as Hα
emission from the filaments of the 90 pc × 60 pc superbubble shell. The larger Hα knot is
coincident with LH 31; the smaller knot is coincident with NGC1854. Diffuse X-ray emission
is detected toward the larger Hα knot (See Figure 1f).
The strongest X-ray emission from N105 is coincident with LH 31, indicating that the
X-ray emission is likely emitted by hot gas produced by the OB association. The diffuse
X-ray is not confined by the Hα emission of N105; instead, the emission extends eastward
from N105 to a nearby patch of Hα emission, DEM L 87. The diffuse X-ray emission appears
to lose intensity approximately at the edge of DEM L 87. This suggests that the hot, X-ray
emitting gas created by LH 31 is expanding eastward through a lower density medium to
the denser gas in DEM L 87. At the interface with DEM L 87, the X-ray emission drops off,
further suggesting interaction between the X-ray emitting gas of N105 and DEM L 87.
3.7. N144 (DEM L 199)
N144, encompassing the OB association LH 58, is a nebular region near the western rim
of the supergiant shell LMC 3. A 120 pc × 75 pc ionized gas shell is visible in the Hα image
(See Figure 1g). The morphology of N144 is a roughly circular complex made up of many
blister-like bubbles surrounding a central shell structure. The central shell has the brightest
Hα emission of the nebula on its northern side. Diffuse X-ray emission is weakly detected
toward several of the bubble regions, including the central bubble.
The X-ray emission is generally coincident with N144, and the strongest X-ray emission
is toward the central shell seen in the Hα image. This indicates that the X-ray emission is
likely to originate from the hot gas in the superbubble interior. There are two peaks in the X-
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ray emission from the central shell. These peaks are generally coincident with the Wolf-Rayet
stars Br34 and Br32, also cataloged as Sk−68◦82 and Sk−68◦80, indicating probable sources
for the peaks in the diffuse X-ray emission (Hail, Dunne, & Chu 2001). The X-ray emission
extends beyond the Hα emitting shell of the central bubble to the northeast and southwest.
Each of these “wings” from central concentration has its own weaker concentration of X-ray
emission. This suggests that multiple-bubble structures in the N144 region have interior hot
diffuse gas. The southwest “wing” of the X-ray emission shows further extension beyond the
Hα shell of N144. This may indicate a breakout of the hot gas interior to N144 into a lower
3.8. N154 (DEM L 246)
N154 is a nebular complex to the south of the 30 Doradus region. The superbubble
encompasses the OB associations LH 81 and LH 87. The 180 pc × 120 pc Hα shell of the
superbubble is an angular, almost-rhomboid shape, with the strongest Hα emission coming
from the northeast and southwest sides. Diffuse X-ray emission is detected toward N154
(see Figure 1h). A bright X-ray source to the southwest of N154 has been identified as SNR
0534-699 (Mathewson et al. 1983; Williams et al. 1999).
The diffuse X-ray toward N154 is centrally bright, and the morphology of the emission
is that of an ellipsoidal running southwest to northeast - similar to the structure and size of
the Hα emission. It is reasonable to therefore assume that the diffuse X-ray emission arises
from hot gas in the interior of the superbubble N154. There is significant X-ray emission
trailing from N154 toward the northeast. Due to the strong X-ray background emission and
number of X-ray sources near the 30 Doradus complex, it is difficult to determine if this
emission trail is a breakout region of hot X-ray emitting gas from N154 or a blending of
X-ray emission from N154 with emission from neighboring regions.
3.9. N158 (DEM L 269)
N158 is a complex nebular structure to the south of the 30 Doradus. The complex
encompasses the OB associations LH 101 and LH 104. The superbubble is located on the
northern part of the N158 complex and has a well-defined, 100 pc × 90 pc shell in the Hα.
Diffuse X-ray emission is detected toward N158 (See Figure 1i). Two strong X-ray sources
are detected to the north and west, respectively, of N158 as well. The source north of N158
appears to be coincident with the known SNR and pulsar PSR B0540−69.3 (Mathewson et
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al. 1983; Bica et al. 1998), and the position of the source west of N158 is consistent with
the Einstein X-ray source 0538.5−6925, a foreground Galactic star (Cowley et al. 1997).
The diffuse X-ray emission toward N158 appears to be coincident with the superbubble.
Although the X-ray emission is not confined by the observed Hα shell, the morphology of the
X-ray emission suggests that the emission is associated with N158, but that several breakout
regions have formed along the shell where hot, X-ray emitting gas is escaping the interior of
the superbubble. It must be cautioned, however, that the X-ray morphology of this region
is exceedingly complex.
3.10. N160 (DEM L 284)
The nebular complex N160 is located on the southern side of the 30 Doradus region.
The superbubble in N160 dominates the morphology of the complex. The superbubble is
180 pc × 150 pc in size, and encompasses the OB association LH 103. The Hα morphology
is roughly circular, with a possible “blowout” region apparent on the northeast edge (Points
et al. 1999). The Hα emission is strongest on the southern edge of the superbubble, closest
to LH 103. Diffuse X-ray emission is detected toward the superbubble (See Figure 1j).
Additionally, there is a pair of strong X-ray sources south of N160. These sources have
been identified: the brighter source is the X-ray binary LMC X-1 and the dimmer source is
SNR 0540-697 (Chu et al. 1997; Williams et al. 2000).
The diffuse X-ray emission detected toward the superbubble in N160 is concentrated near
LH 103. Little significant emission is detected from the remainder of the the superbubble.
This suggests that the stars and/or supernovae in LH 103 are producing hot X-ray emitting
gas. The gas interior to the superbubble may be too hot to be detected in the PSPC energy
bandpass, or the majority of the hot gas may simply have already escaped.
3.11. N206 (DEM L 221)
N206 (also cataloged as DEM L 221) is a nebular complex encompassing the OB as-
sociations LH 66 and LH 69. N206 contains both a superbubble and a smaller supernova
remnant, SNR 532-710 (Mathewson et al. 1983; Williams et al. 1999). The 30 pc × 30 pc
remnant is located on the eastern side of the nebular complex, and has a faint circular Hα
shell. The superbubble has a larger circular shell, 110 pc × 110 pc, with the brightest Hα
emission coming from the eastern and southern sides of the bubble. Diffuse X-ray emission is
detected toward both the remnant and the superbubble (See Figure 1k). The diffuse X-ray
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emission detected toward the supernova remnant has been previously explored by Williams
et al. (1999).
The diffuse X-ray emission toward the superbubble of N206 appears to have a several
enhancements, possibly in a limb-brightened ring. The emission is coincident with a region
of the superbubble that is not bright in Hα, but the diffuse X-ray emission appears confined
by faint Hα structures on the western side of the superbubble. Therefore, it is reasonable to
conclude that the X-ray emission is produced by hot gas interior to the superbubble.
One of the enhancements of the diffuse X-ray emission is coincident with the Wolf-Rayet
star Br44. The X-ray emission from this enhancement in the superbubble may therefore have
a stellar source rather than a diffuse one. The emission from this enhancement has therefore
been excluded from the thermal plasma model fit of the superbubble in N206.
3.12. 30 Dor C (DEM L 263)
30 Dor C is a superbubble located in the southwestern region of the 30 Doradus complex.
The superbubble encompasses the OB association LH 90. The Hα emission shows a strong
shell structure, 100 pc × 90 pc in size.
superbubble (See Figure 1l).
Diffuse X-ray emission is detected toward the
The X-ray emission detected toward 30 Dor C is limb-brightened and appears confined
within the Hα shell. It is therefore reasonable to conclude that the X-ray emission arises from
within the superbubble. The limb-brightened shell is well-defined all around the superbubble
except on the southwest, where the X-ray emission is near the background levels.
The absorption column density towards 30 Dor C is known to change dramatically across
the face of the superbubble (Osterberg 1997). We have therefore divided 30 Dor C into two
parts (east and west) to account for the change in absorption column density in our spectral
fits. Unfortunately, the LMC NHImaps from Rohlfs et al. (1984) do not have the spatial
resolution to detect this change, so we must still use a single value in our “fixed NH” fits of
30 Dor C.
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4.1.Distribution of X-ray Luminosities
Based on the spectral fits performed for each superbubble, we have computed X-ray
luminosities. We have used these luminosities to investigate the superbubble luminosity
function. In Figure 4, we present the X-ray luminosity function for the superbubbles in our
dataset. We have plotted the luminosities as determined by both the “best-fit NH” model
fits and the “fixed NH” model fits. The “best-fit NH” luminosity function illustrates that
most of the superbubbles in our sample have an X-ray luminosity around 1035−36ergs sec−1.
A single high-end outlier is also shown at over 1037ergs sec−1; this outlier is N11–Shell 1.
The “fixed NH” luminosity distribution favors higher X-ray luminosities than the “best-fit
NH” luminosity distribution as well as a broader range in luminosities (excepting the outlier
on the “best-fit NH” luminosity function).
4.2. Pressure Driven Models
To compare the X-ray luminosities of the superbubbles derived from observation with
the pressure-driven bubble models of Weaver et al.
have followed the procedure described in Paper I and corrected in Paper III. Assuming that
the shell thickness is small compared to the radius of the bubble, an electron temperature in
the shell of Te≃ 104K, and a mean atomic mass of the ambient medium µa= (14/11)mH,
we can use equations (7)–(10) from Paper III to derive the X-ray luminosity:
(1977) for the LMC superbubbles, we
LX≃ (6.7 × 1029erg · sec−1) · ξ · I · EM5/7· R12/7
where ξ is the metallicity, assumed to be 0.3, I is a dimensionless function of the temperatures
interior to the superbubble, which has a value ∼2, EM is the emission measure of the 104
K shell gas in cm−6pc, Rpc is the radius of the superbubble in parsecs, and vexp is the
superbubble expansion velocity in km sec−1. The dimensions and expansion velocities of the
superbubbles are given in Table 1. Unfortunately, we do not have expansion velocities for all
of the superbubbles. The emission measure was determined from the continuum-subtracted
Hα image derived from the PDS scans of the Curtis-Schmidt plates of Kennicutt & Hodge
(1986). The emission measures and theoretical X-ray luminosities are presented in Table 5.
Of course, the emission measure can vary greatly around Hα shell, so we have taken an
rough mean for each superbubble. The predicted X-ray luminosities range from 1034.3–1035.1
erg sec−1. These luminosities range from ∼3 to ∼50 times lower than the X-ray luminosities
– 14 –
determined from the PSPC data with the “fixed NH” fits. This suggests that the majority of
the X-rays are produced by different mechanisms than those described in the pressure-driven
bubble model, confirming that the superbubbles in our sample are X-ray bright.
4.3. X-Ray Luminosity Correlations
We have also compared the superbubble volume, Hα luminosity, expansion velocity, and
bright star count with the X-ray luminosity, for both “best-fit NH” and “fixed NH” model
fits (See Figures 5, 6, 7, & 8). The superbubble volumes were determined from the sizes
of the Hα shell (See Table 1), assuming an ellipsoidal shape. The expansion velocities and
Hα luminosities are also given in Table 1. The bright star counts are based on the OB
association star counts in Lucke & Hodge (1970); again, Table 1 lists the OB associations
encompassed by each superbubble. Although the scatter level of these plots is obviously
high, we have attempted to fit each plot with a linear trend line to test for correlations
between X-ray luminosity and other superbubble properties. The correlation coefficients of
the trends are detailed in Table 6. Positive correlations are found between X-ray luminosity
and each of the other properties. The correlations are moderate for the X-ray luminosities as
determined by the “best-fit NH” model fits and generally stronger for the X-ray luminosities
as determined by the “fixed NH” model fits. The strongest correlation is between “fixed
NH” X-ray luminosity and bright star count. It must be considered, however, that the
correlation between X-ray luminosity and superbubble volume may be due to a surface
brightness selection effect.
The correlations demonstrate that the X-ray luminosity of a superbubble is affected by
the richness and age of the OB associations within its shell walls. The bright star count
of a superbubble will obviously be directly related to the richness of its OB associations.
Also, OB association richness will provide stellar winds to power the expansion, and thereby
increase the expansion velocity, of the superbubble. The Hα luminosity of a superbubble will
be positively affected by the richness of OB association, as more stars provide more ionizing
flux, and negatively affected by age, as the powerfully-ionizing, early-type stars exhaust
themselves. The X-ray luminosity–OB association richness relationship has already been
demonstrated; however, the X-ray luminosity can increase with the age of a superbubble. As
demonstrated by Paper I and Wang & Helfand (1991), the X-ray luminosity of a superbubble
can be enhanced by SNRs. Thus, a superbubble that has already had several bright stars go
supernova can be brighter in X-rays than a superbubble with much younger OB associations.
We would therefore expect the correlation between X-ray luminosity and Hα luminosity to
be weaker than the correlation between X-ray luminosity and bright star count, which it is
– 15 –
for the “fixed NH” model fits.
4.4. Stellar Sources and Breakout Regions
We have described the X-ray morphology for each superbubble and compared those
morphologies to the Hα morphologies and known stellar sources of each superbubble. We
have found that in a significant fraction of the superbubbles, peaks in the X-ray emission are
coincident with known stellar sources, such as Wolf-Rayet stars. High-resolution observations
are need to determine whether the X-ray peaks are caused by stellar emission or stellar wind
interactions with the superbubble interior gas. In addition, nearly half of the superbubbles
show some evidence of breakout regions in their X-ray morphologies, where hot gas appears
to be leaking from the superbubble interior into the surrounding regions. Again, further
studies of the diffuse X-ray gas will be needed to confirm whether these regions are true
We have presented ROSAT observation of thirteen LMC superbubbles. Eleven of these
observations had not been reported previously. In each of these superbubbles, diffuse X-
ray emission brighter than is theoretically expected for a wind-blown bubble was detected.
Based on the previous findings in Paper I and Wang & Helfand (1991), it is reasonable to
conclude that the X-ray emission from the superbubbles has been enhanced by interactions
between the superbubble shell and interior SNRs. We have also found significant positive
correlations between the X-ray luminosity of a superbubble and its Hα luminosity, expansion
velocity and OB star count. Further, we have found that a large fraction of the superbubbles
in the sample show evidence of breakout regions. In Paper IV it was demonstrated that
breakout regions can significantly affect the evolution of a superbubble, draining energy
and pressure that would otherwise go into expansion. We also suggest that because these
breakout regions appear so frequently, the superbubbles may be a significant source of hot
gas for the interstellar medium.
We would like to thank Robert Gruendl for his useful communications in preparing this
paper. This research has made use of data obtained through the High Energy Astrophysics
Science Archive Research Center Online Service, provided by the NASA/Goddard Space
Flight Center. This research was made possible by ADP grants NAG 5-7003 and NAG