Out of the 71 sources common to the Parkes radio and RASS surveys of the LMC, 38 have been previously classified (see Table 2 (click here), Col. 8). Most are SNRs (30, including four SNR candidates) and background sources (six). Only two X-ray sources from the radio surveys are listed as HII regions. One of them is a chance coincidence with the X-ray binary LMC X-1 (Sect. 4.6) and the other is 30 Doradus.
Out of the 27 sources common to the Parkes radio and the ROSAT PSPC surveys of the SMC, 23 have been previously classified (see Table 3 (click here), Col. 8). There are 12 SNRs, seven background sources and three HII regions. One source (SMCB0035-7228) appears to be a chance coincidence with the SMC X-ray super-soft source (Sect. 4.3).
Of 52 confirmed radio HII regions in the LMC (Filipovic et al. 1997b, hereafter Paper VII), seven sources appear on the Einstein X-ray lists of Helfand et al. (1991), Wang & Helfand (1991b) and Wang et al. (1991). The small number of X-ray emitting HII regions in the RASS LMC point-source list relative to the number seen with the Einstein survey may result from several causes. Some of the Einstein sources are not found as they are below the RASS detection threshold, and others are too extended to be identified as point sources or are located in confused areas. Another possibility is that the improved RASS source positions may exclude the proposed HII regions as the origin of the X-ray source.
HII regions are excellent SN birth places and we expect a number of SNR X-ray sources to be associated with HII regions (Chu & Low 1990). Another process that could explain the appearance of HII regions in X-ray surveys is shock heating by stellar winds inside the HII regions, but Chu et al. (1995b) argue that stellar winds alone could not produce enough X-ray emission. The most extreme example of this is 30 Doradus which is a bright HII region at both radio and X-ray frequencies with no confirmed SNRs (Dickel et al. 1994). We believe, however, that stellar winds, together with embedded SNRs, could be sufficient for X-ray emission from large HII regions. Arthur & Henney (1996) proposed a model in which an SNR evolves inside an extremely diffuse stellar-wind bubble (formed by the OB association stars) but the density in the SNR is augmented through hydrodynamic ablation of cool, dense clumps by the post-blast SNR flow.
Most SNRs in the MCs are embedded in HII regions; 16
such objects have been found. Chu & Kennicutt (1988b) predicted that more
embedded SNRs will be identified in future radio surveys of higher angular
resolution and sensitivity. These SNRs are rather weak emitters of very
small size (<5
) and are obscured within much stronger
HII regions such that their detection is difficult with
the present radio survey data.
By comparison, the discovery of further young and luminous SNRs in the MCs
isolated from HII regions in significant numbers is not
likely (Clarke 1976). This conjecture is supported by our
study (Paper VII) where we predict that only a small
fraction of our unclassified sources can be SNRs.
To establish a criterion for the classification of the 33 "unknown'' LMC and five "unknown'' SMC sources and to check the "known'' sources, we plot in Figs. 3 (click here)a and 3b the source radio spectral index against the X-ray HR2. These colour-colour diagrams show several important trends.
Figure 3: The distribution of radio spectral index (
)
and X-ray hardness ratio 2 (HR2) for different classes of
sources towards a) the LMC and b) the SMC. Asterisks
represents SNRs; filled square - HII regions; open
circles - background sources and triangles - unclassified
sources. All background sources have 
First, all known background sources have positive HR2, with mean value
for the ones towards the LMC of 
and 
(see Fig. 4 (click here)b),
compared to the HR2 for the LMC SNRs (see Fig. 4a), which have a much wider
distribution (
. All sources with negative HR2 are SNRs or
SNR candidates.
Figure 4: Distribution of HR2 for a) the LMC SNRs, and b) background
sources towards the LMC
Figure 5: Distribution of radio spectral index for a) the LMC SNRs,
and b) background sources towards the LMC common to the RASS
Second, the LMC SNRs have a narrower range of radio spectral index
; see Fig. 5 (click here)a) than background sources
in the field of the LMC 
; see Fig. 5b).
Third, the background sources have two peaks of spectral index: one with
very steep radio spectra consisting of 12 sources with 
and 
, and another with flat and inverted spectra (10 sources) with
and 
.
Because of the overlap in radio spectral index (Paper VII), spectral index
alone is not sufficient to distinguish SNRs, HII regions and background
sources. As an additional help in source classification, we will treat
all sources outside the region defined by 
to
and 
to
as background sources. Also, all sources outside the area of the SMC
defined by 
to 
and
to 
will
be treated as background. No radio sources in these surrounding
areas are known to belong to either the LMC or the SMC.
Because of the small number of HII regions observed in previous X-ray surveys (4 out of 174, Paper VII), we do not believe that any of the previously unclassified (both radio and X-ray) sources in the field of the MCs are compact HII regions.
A list of background sources towards the LMC is presented in Paper VII. From
this list, six known background sources have been confirmed as X-ray emitters:
, 
,
LMC B0547 - 6746, LMC B0602 - 6443 and LMC B0611 - 6623
(see in Table 2 (click here), Col. 8 marked with "BG'').
There are 16 sources outside the LMC area defined in Sect. 4.1.
All of the sources (
,
, 
, 
,
, 
, 
,
, 
, 
,
, 
, 
,
LMC B0606 - 7041, LMC B0608 - 6510 and LMC B0611 - 6734)
have a positive HR2 and radio spectrum
typical of background sources. Here we suggest that all of these sources are
background objects (see Table 2 (click here), Col. 8 marked with "bg'').
Another five sources within the LMC area (LMC B0531-6518, LMC B0537-6506, LMC B0547-6729, LMC B0552-6948 and LMC B0553-6704) are classified as probable background sources because of their positive hardness ratios and/or radio spectra. This classification is strongly based on the HR2 because, as the radio spectra could not be estimated for some of these sources, they were detected at only one radio frequency. These sources are marked in Table 2 (click here), Col. 8, with "bg?''.
To conclude, 27 out of 71 sources in common towards the LMC appear
to be background (or candidates for background) sources. This is consistent
with the number expected from 
studies of such objects (Paper VII).
A catalogue of background radio sources towards the SMC can be found in Paper VII. From this list we found six radio sources (SMC B0037-7327; SMC B0040-7323; SMC B0047-7343; SMC B0049-7356; SMC B0053-7227 and SMC B0110-7318) that are common to the ROSAT PSPC list of sources (Kahabka et al. in preparation). These sources were marked with "BG'' in Table 3 (click here), Col. 8.
Another radio source 
listed in Paper VII as a background
candidate (see in Table 3 (click here), Col. 8 marked with "BG?'') was found in the
ROSAT PSPC X-ray surveys 
. This source, with positive
HR2, will be treated as a definite background object in future studies.
The X-ray super-soft source 
lies 
from the
radio source 
and therefore they are most probably
a chance coincidence. We believe that the radio source is likely to be
a background object.
In total, 8 out of 27 sources in common towards the SMC appear to be background objects. Seven of them have an X-ray counterpart and one is probably a chance coincidence.
Previous studies investigated 56 SNRs in the LMC, from which 37 are confirmed and 18 are candidates (Mathewson et al. 1983, 1984, 1985; Mills et al. 1984; Chu & Kennicutt 1988a, b, 1994; Chu et al. 1993, 1995a, b, 1997;
Smith et al. 1994;
Dickel et al. 1993, 1994;
Dickel & Milne 1995). All are detected at radio frequencies.
Three confirmed SNRs (
, 
and 
) listed in
Mathewson et al. (1983a),
Mills et al. (1984) and
Tuohy et al. (1982) could
not be detected in any of the six Parkes radio surveys (their emission is
below our detection limits). They were, however, detected at 843 MHz
with the Molonglo Observatory Synthesis Telescope (MOST)
(Mills et al. 1984). Positive X-ray detection of these three SNRs has
been reported by Wang et al. (1991) and
Pietsch et al. (in preparation). Another SNR
discovered by Chu et al. (1997)
is associated with HII region N159 and we
discuss this source in Sect. 4.6.
We have detected 26 SNRs and four SNR candidates in both the X-ray and our
Parkes radio surveys. All 30 sources: namely 
, 
,
, 
, 
,
, 
, 
,
, 
, 
,
, 
, 
,
, 
, 
,
, 
, 
,
, 
, 
,
, 
, 
,
, 
, 
and 
, show strong evidence for
being SNRs based on radio spectral index or X-ray HR2, or both. These sources
are marked in Table 2 (click here), Col. 8 with "SNR'' or "SNR?''.
Two previously unclassified radio sources (
and 
)
are strong candidates for being SNRs because of a negative HR2 and steep radio
spectral index.
There are another four previously unclassified radio sources (
,
, 
, 
)
which have a negative
HR2, but we have no radio spectral index data, since they are observed
only at one radio frequency. These are more likely to be SNRs than background
sources and therefore we consider them as SNR candidates (see in Table 2 (click here),
Col. 8 marked with "snr?'').
Thirty SNRs and six new SNR candidates from this study have been classified out of the total number of 71 sources. This increases the total number of SNR candidates known in the LMC from 18 to 24.
Wang et al. (1991) detected 28 X-ray SNRs from the Einstein survey of the LMC and Smith et al. (1994) and Chu et al. (1995a, 1997) found another seven SNRs from the ROSAT PSPC survey. Here we confirm the existence at X-ray frequencies of an additional five previously known radio SNRs and suggest a further six new SNR candidates. This brings the total number of X-ray SNRs and SNR candidates in the LMC to 46. As the total number of known SNRs and SNR candidates in the LMC is 62, 74% are now confirmed as X-ray sources.
There are four known SNRs and 12 SNR candidates in our Parkes radio surveys for which there are no X-ray counterparts (Table 4 (click here)). Table 4 (click here) contains SNRs drawn from different age populations. There are both strong and weak radio SNRs in Table 4 (click here) and so the non-detections in X-rays are not simply because the SNRs are radio weak. The speculation by Aschenbach (1995) for the existence of X-ray quiet SNRs is supported.
| (1) | (2) | (3) | (4) | (5) | (6) |
| Radio | S4.75 |  | Type | Reference | Comments |
| Source Name | (Jy) | ||||
| LMCB0450-6927 | 0.2670 |  | SNR? | 8; 31 | HeIII region |
| LMCB0450-7055 | 0.4960 |  | SNR1 | 23; 25 | PopII? |
| LMCB0454-7005 | 0.1660 |  | SNR? | 8; 20 | |
| LMCB0459-6612 | SNR? | 8 | Seen only at 2.45 GHz | ||
| LMCB0505-6548 | 0.0460 |  | SNR? | 8 | |
| LMCB0520-6531 | 0.5050 |  | SNR? | 8; 49 | X-ray - Superbubble |
| LMCB0521-6545 | 0.1410 |  | SNR? | 8; 20 | |
| LMCB0523-7138 | 0.1760 |  | SNR? | 8; 20 | Superbubble |
| LMCB0524-6627 | 0.050f |  | SNR1 | 20; 23 | Pop I; SNR embedded in HII region |
| LMCB0524-7121 | 0.1040 |  | SNR? | 8 | |
| LMCB0528-6716 | 0.1490 |  | SNR0 | 23 | |
| LMCB0528-7038 | 0.2440 |  | SNR? | 8 | |
| LMCB0529-6702 | 0.2080 |  | SNR? | 34 | Vicinity of Pulsar PSR B0529-66 |
| LMCB0537-6641 | 0.4420 |  | SNR? | 8 | |
| LMCB0538-6922 | 1.0120 |  | SNR0 | 27; 42 | Pop I; SNR embedded in HII region |
| LMCB0544-6621 | 0.0920 |  | SNR? | 8 | |
It is interesting to note that 33% (60 out of 182), or 48% (89 out of 182 if 29 SNR candidates are included) of the radio SNRs in our Galaxy have been seen in X-ray surveys (Aschenbach 1995). These percentages for our Galaxy are far smaller than for the LMC. The reason may be that soft X-ray emission from some of the Galactic SNRs may be absorbed by HI in the Galactic Plane, whereas X-ray emission from the LMC is less absorbed because of lower column-depths towards the MCs.
An estimate of the rate of supernova formation can be obtained if the age of the individual SNRs can be determined. To estimate the age of individual SNRs in the MCs, we follow the method of Van Buren & Greenhouse (1994) and adopt the relationship between age and radio flux density at 4.75 GHz ( S4.75). For calibration, we have scaled the flux density of Cas A ( S4.75=650 Jy) at its distance of 3 kpc, and age of 340 years (Whithfield 1957), to the distance of the LMC (50 kpc; Westerlund 1993) to give the relationship:
where T is the SNR age in years and S4.75 is in Jansky (Jy).
Assuming the S4.75 flux densities to be complete down to
Jy (which corresponds to a SNR age of 3400 yr), we have
computed the age of 38 SNRs and SNR candidates. It has been assumed that the
flux densities are not seriously affected by any confusing HII regions.
A comparison of our estimates of the individual SNR ages with diameters
taken from various optical and high-resolution radio images shows little
correlation and, therefore, we assign little confidence to the individual ages.
The mean period between successive SNR occurrences is 100 (
) yr.
This figure does not agree with the estimate of Chu & Kennicutt (1988b)
who give the birth-rate in the LMC of one SNR per 500 yr. However,
Chu & Kennicutt (1988b) predict that their estimate of the rate will
change with the discovery of new SNRs hidden in
HII regions and superbubbles.
Using the radio supernova rate and the relation between star-formation rate
(SFR) and supernova rate (Condon 1992; Eq. (20)) it
is possible to determine the SFR in the LMC. We obtain an SFR
of 
) 
. This result is
consistant, albeit slightly higher than the upper
limit of 
suggested by Kennicutt (1991).
Our estimate of supernova birth-rate in the LMC
seems large in comparison to the rate in our Galaxy (one every 30-50 yr).
Probably, the problem lies in the Van Buren & Greenhouse (1994) relationship which is
too simplistic given the poor correlation between ages and flux for well-known SNRs.
Also, Condon's (1992) relation between SFR (in ) and SN rate
assumes some kind of universal initial mass function (IMF), while we have strong
indications for bimodal mass function in the MCs, with the large star masses
(hence SNs) strongly favoured in clusters and associations
(Massey et al. 1995).
From the list of 20 SMC SNRs and SNR candidates (Ye 1988), 12 were found in our Parkes radio surveys (Paper VII). Five well-known SNRs and three SNRs candidates could not be detected in any of our radio surveys but they can all be detected with the MOST radio telescope and are also in the ROSAT PSPC surveys.
All 12 radio SMC SNRs from the Parkes surveys have counterparts in the ROSAT
PSPC survey. These 12 sources are: 
, 
,
, 
,
,
, 
,
,
, 
,
and 
. They show typical SNR
characteristics and here we confirm their SNR nature.
Radio sources SMC B0043-7330 and SMC B0054-7235 have not
been classified before and we found counterparts in X-ray sources
and 
. These sources were also
detected in H and IR surveys and therefore we believe that
they could be good SNR candidates. However, neither of
these sources has a conclusive radio spectral index or HR2.
Using the same method as for the LMC, we estimate the birth-rate of SNRs and
the SFR in the SMC. From 12 SMC SNRs, 10 have radio flux at 4.85 GHz
greater than 0.1 Jy, which is our completeness level. Using an
estimated age of these 10 SMC SNRs, we find that the birth-rate of the
SNRs in the SMC is one every 350 (
) yr. As for the LMC,
this figure does not agree with the previously published estimate of
Mathewson et al. (1983) who give the birth-rate in the SMC of
one SNR per 800 yr.
Using this birth-rate we obtain an SFR for the SMC of
. This result is also
consistent with the upper limit of 
suggested by Kennicutt (1991).
Only five radio HII regions (
, 
,
; 
and 
) are correlated
with X-ray sources in this study. The X-ray emission from the well-known radio
HII region(s) (N 159;
Hunt & Whiteoak 1994) is caused by the
X-ray binary 
and is therefore not associated with the
HII region. Chu et al. (1997) found X-ray emission from
N159A (2
east of LMCX-1) in the ROSAT HRI image, which they interpret as being
coused by an SNR. However, Hunt & Whiteoak (1994) did not find any evidence of such an SNR in the
high-resoulution (
10
) ATCA radio observations.
There are six sources towards the LMC
(LMC B0456-6803,
LMC B0454-6806, LMC B0513-6729,
LMC B0523 - 6623,
LMC B0528 - 6542 and LMC B0557 - 6854)
and two towards the
SMC (SMC B0034-7155 and
SMC B0058-7228)
that could be either SNR candidates or background objects with flat radio spectra
and positive HR2 close to zero. The classification of these sources,
however, remains ambiguous.
There are four foreground stars in the field of the RASS (Pietsch et al. in
preparation)
that coincide with our radio sources (
,
, and ).
We believe that source is a background source
(Sect. 4.2). We classified the source as an
SNR candidate (Sect. 4.4) and the classification of the remaining
two sources ( and ) is ambiguous.
All of these sources belong to the group of expected random coincidences (see Sect. 3).
So far we have discussed sources detected in the Parkes radio surveys (Papers IV, IVa and V) and the ROSAT X-ray surveys (Pietsch et al. in preparation; Kahabka et al. in preparation). However, there are other sources towards the LMC detected in both radio and X-ray from other surveys which are not listed in Table 2 (click here). There are no such sources in the field of the SMC.
In Table 5 (click here) we list an additional 14 sources towards the LMC which have
been catalogued in Papers IV and IVa but which have not been detected in
the RASS. These sources have been detected at X-ray wavelengths by the
Einstein surveys by Long et al. (1981) and Wang et al. (1991), and with the
ROSAT PSPC by
Trümper et al. (1991). Also, we add three confirmed radio SNRs
(, and detected
with the MOST and discussed in Sect. 4.4)
which are listed in the RASS but not seen in our radio surveys (Table 5 (click here)).
All abbreviations and lists of references in Table5 (click here) are based on the nomenclature used in Table 2 (click here), with the exception of the X-ray information (count rate and HR2).
| (1) | (2) | (3) | (4) | (5) | (6) |
| Radio | X-ray |  | Type | Reference | Comments |
| Source Name | Source Name | ||||
| LMCB0453-6700 | RXJ04531-6655 |  | SNR | 4 | |
| LMCB0500-7014 | LHG 7; W 7 |  | SNR1 | 15; 20; 27; | PopI; SNR embedded |
| 28; 40; 41 | in HII region | ||||
| LMCB0501-6629 | RXJ0502-6624 |  | SNR? | 28 | Vicinity of PSR B0502-66? |
| B0505-679 | LMCRASS 135; | -0.50 | SNR3 | 22; 27; 35 | |
| LHG 10; W 9 | |||||
| B0509-675 | LMCRASS 152; | -0.48 | SNR3 | 27; 35 | |
| LHG 14; W 12 | |||||
| LMCB0509-6720 | RXJ0509-6717 |  | SNR? | 7; 17 | |
| LMCB0517-7151 | W 18 |  | BG | 1; 12 | |
| LMCB0522-6800 | W 30 |  | HII | 2; 3; 6; 13; | Near a non-thermal source; |
| 26; 28; 31 | HeIII region | ||||
| LMCB0523-6806 | W 31 |  | HII | 2; 3; 6; 13; 26; 28 | |
| LMCB0526-6731 | W 40 |  | HII | 2; 3 | SB(X) |
| LMCB0532-6743 | W 51 |  | HII | 2; 3; 28; 43; 44; 54 | Curved radio spectrum; Diffuse emission |
| LMCB0535-6948 | LHG 56; W 63 |  | HII | 3; 37; 54 | |
| LMCB0536-6914 | LHG 62; W 71; |  | SNR | 5; 27 | PopI; SB(X); SNR embedded |
| RXJ05362-6911 | in HII region | ||||
| LMCB0536-6920 | W 66 | SNR | 45 | Seen only at 8.55 GHz | |
| LMCB0539-6606 | LHG 75 |  | |||
| LMCB0540-6927 | RXJ05402-6928 | Seen only at 8.55 GHz | |||
| B0543-689 | LMCRASS 278; | -0.29 | SNR | 27 | |
| LHG 82; W 91 | |||||
The numbers in this column refer to references given at the end of
Table 2 (click here).
Of these 17 sources, nine are known SNRs, five are HII regions and one is a
known background source (see Table 5 (click here), Col. 4). The classification of
sources 
and 
is ambiguous but
they are likely to be SNR candidates.