EDP Sciences
Free Access
Issue
A&A
Volume 545, September 2012
Article Number A128
Number of page(s) 11
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/201219758
Published online 18 September 2012

© ESO, 2012

1. Introduction

The study of X-ray source populations and diffuse X-ray emission in nearby galaxies is of major importance to improving our understanding the X-ray output of more distant galaxies as well as learning about processes that occur on interstellar scales within our own Galaxy. XMM-Newton and Chandra were used to perform deep X-ray surveys of the Local Group galaxies M 31 (Pietsch et al. 2005b; Stiele et al. 2011) and M 33 (Pietsch et al. 2004; Misanovic et al. 2006; Tüllmann et al. 2011). These deep observations of M 31 and M 33 allow for the first time the study of large samples of different source classes (with limiting point-source luminosities of  ~1035 erg s-1) in a galaxy other than the Milky Way. This includes, e.g., the study of about 80 supernova remnant (SNR) candidates in M 33, which is the largest sample of remnants detected at optical and X-ray wavelengths in any galaxy (Long et al. 2010), optical novae as the major class of supersoft X-ray sources (SSSs) in M 31 and M 33 (Pietsch et al. 2005a, 2007; Henze et al. 2010, 2011) and the discovery of type-I X-ray bursts from neutron star X-ray binaries in M 31 globular clusters (Pietsch & Haberl 2005).

The Large and Small Magellanic Clouds (LMC and SMC), nearby neighbours of the Milky Way, have different chemical compositions of low metallicity, are irregular in shape, and are strongly interacting both with the Milky Way and each other. These properties influence their star formation history and therefore, any study of stellar populations in the Magellanic Clouds (MCs) is particularly rewarding (see e.g. Antoniou et al. 2010, with respect to the Be/X-ray binary population of the SMC). Their proximity makes them ideal targets for X-ray studies. Limiting point-source luminosities of a few 1033 erg s-1 (a factor of  ~50 lower than for M 31 and M 33) are reached with XMM-Newton and extended objects like supernova remnants can easily be resolved (at the SMC distance of 60 kpc, Hilditch et al. 2005, the angular resolution of  ~10′′ provided by XMM-Newton corresponds to a linear size of 3 pc).

Previous X-ray surveys of the MCs performed with the imaging instruments of the Einstein (Long et al. 1981; Wang et al. 1991; Wang & Wu 1992), ASCA (only SMC; Yokogawa et al. 2003) and ROSAT (Kahabka et al. 1999; Haberl et al. 2000; Sasaki et al. 2000b; Haberl & Pietsch 1999; Sasaki et al. 2000a) satellites revealed discrete X-ray sources and large-scale diffuse emission. In particular, the high sensitivity and the large field of view of the ROSAT PSPC provided the most comprehensive catalogues of discrete X-ray sources ever compiled and revealed the existence of a hot thin plasma in the interstellar medium (ISM) of the MCs with temperatures between 106 and 107 K (Sasaki et al. 2002).

thumbnail Fig. 1

Combined EPIC exposure map of the SMC area. The pn and MOS exposure maps account for vignetting and for illustration are summed up with MOS1 and MOS2 each weighted with a factor of 0.4 to account for their relative sensitivity. The 30 numbered circles (radius 13′) mark the survey fields.

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However, owing to their relatively small distance, the MCs extend over large areas on the sky and high spatial resolution X-ray surveys with modern instrumentation need to complete a large number of raster observations. Chandra and XMM-Newton observed various targets in the MCs and only for the SMC do these “serendipitous” surveys cover a significant part of the galaxy. The disconnected fields of the Chandra Wing Survey (McGowan et al. 2008) are distributed around the eastern wing. The observations available in the XMM-Newton archive at the beginning of 2009 mainly covered parts of the SMC bar and wing with very different exposure times. To fill the gaps between the archival observations, we successfully applied for XMM-Newton observations of thirty fields in the SMC, which were performed between May 2009 and March 2010. Together with the archival data, the new observations cover the full extent of the SMC with the European Photon Imaging Camera (EPIC, Strüder et al. 2001; Turner et al. 2001) on board XMM-Newton.

First studies using the new XMM-Newton survey data mainly focussed on individual objects belonging to the SMC. Two known SSSs (Greiner 1996) were observed in bright state during our XMM-Newton survey. SMP SMC 22 (Sanduleak et al. 1978) is the most X-ray luminous central star of a planetary nebula in the SMC (Mereghetti et al. 2010) and the symbiotic binary SMC 3 is a highly variable X-ray source that was seen at its highest intensity state observed so far during our survey (Sturm et al. 2011b). A faint SSS identified with a Be star could be the first Be/white dwarf system discovered in the SMC (Sturm et al. 2012c). New Be/X-ray binaries were discovered in outburst during the XMM-Newton survey observations. For two of them, X-ray pulsations were discovered in the EPIC data (Sturm et al. 2011a; Coe et al. 2011), while two new transients show all the characteristics of Be/X-ray binaries, but no significant pulsations were detected (Coe et al. 2012). Another new Be/X-ray binary was discovered in a search for highly absorbed X-ray binaries in the EPIC data (Novara et al. 2011). Owen et al. (2011) combined all available XMM-Newton data of the SNR IKT 16 to study its morphology and X-ray spectrum.

Here, we present the first EPIC mosaic images from the survey and focus on extended X-ray sources such as SNRs in the SMC and galaxy clusters in the background. A detailed analysis of the images concerning the detection of point sources is presented inSturm et al. (2012b, hereafter SHP12, which resulted in 5236 detections of 3053 individual sources.

2. Observations and data analysis

The observations of the 30 SMC-survey fields were performed with all EPIC instruments in full-frame CCD readout mode with a 73 ms frame time for pn and 2.6 s for MOS. For the pn camera the thin and for the MOS cameras the medium optical blocking filters were used. The locations of the fields in the SMC are indicated in Fig. 1. We combined the survey data with all archival imaging data available in June 2010. We selected fields with pointing directions within 180′ around RA = 1h and Dec =  − 73.5°, but excluded five fields located very north and south that do not overlap with the other fields. We rejected observations severely affected by high background (see below). This finally resulted in 53 archival observations (including the calibration target 1E0102.2-7219), which we used for our image analysis. A short summary of these observations is provided in Table 1, as online material. We refer to Table 1 of SHP12 for more details on the instrumental set-ups and net exposure times of these observations and additional observations, which were used for the production of the point-source catalogue.

We used the XMM-Newton Science Analysis System (SAS), version 10.0.01 for the analysis of the EPIC data. To remove times of high background we inferred good time intervals (GTIs) from the background light curves created by epchain and emchain using the SAS task tabgtigen with thresholds of 8 and 2.5 cts ks-1 arcmin-2 for EPIC pn and EPIC MOS, respectively. We combined the GTIs using only common time intervals when data from all detectors were available, but included GTIs for a detector when others were not taking data. In this way, e.g., we ensure to include GTIs of MOS from the beginning of the observations before pn starts observing or observations when pn was partly in a non-imaging mode. Most of the survey observations were affected by relatively little background flaring activity as can be seen by the final net exposure times listed in the last column of Table 1 of SHP12. From the survey observations, only observations 0601210101 and 0601211701 received less than 60% of their requested exposure times. Both fields were observed again, together with field 0601212801, which lost exposure due to a ground station problem (resulting in observations 0601213201, 0601213301 and 0601213401). Together with the archival data, this resulted in a total number of 83 EPIC pn and 86 EPIC MOS observations covering the bar and wing of the SMC, which we used for our analysis (in timing mode, pn provides no imaging data, while for MOS, the outer CCDs are still in imaging mode, reducing the number of pn imaging exposures).

thumbnail Fig. 2

Distribution of the EPIC exposures in the SMC area covered by the XMM-Newton observations. Full, dashed, and dash-dotted lines indicate pn, MOS1, and MOS2, respectively.

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In a first run, we performed a source detection analysis on each observation using a maximum likelihood technique (SAS meta task edetect_chain) for 15 images from the three EPIC instruments in the five different energy bands 0.2 − 0.5 keV, 0.5 − 1.0 keV, 1.0 − 2.0 keV, 2.0 − 4.5 keV and 4.5 − 12 keV. The resulting source lists from each observation were correlated with the optical catalogue from the Magellanic Clouds Photometric Survey (MCPS) of Zaritsky et al. (2002). For astrometric boresight corrections, we selected secure identifications of optical counterparts to the X-ray sources (in most cases high mass X-ray binaries, HMXBs, but also foreground stars and background AGN). The inferred shifts in right ascension and declination were then applied to the attitude file of each observation and the processing repeated. In this way, astrometrically aligned images used for the full SMC mosaic and corrected source positions for a catalogue were produced. All images were corrected for exposure and the images from EPIC pn were corrected for out-of-time events to remove readout streaks. The MOS data were checked for noisy CCDs (Kuntz & Snowden 2008) using the SAS task emtaglenoise. The data of noisy CCDs were completely rejected for the creation of the mosaic images. A combined EPIC exposure map is presented in Fig. 1, which covers in total 5.5 square degrees. In most parts of the covered area, the exposure is around 25 ks (EPIC pn only) as demonstrated in Fig. 2. From the total covered area, 68% (60%, 59%) was observed with pn (M1, M2) exposures between 10 and 30 ks. An extremely long exposure was reached around the supernova remnant 1E0102.2-7219 in the north-east, which is used as a calibration target for XMM-Newton with regular observations every half a year. The maximum exposures are 427.7 ks, 585.4 ks, and 598.9 ks for EPIC pn, MOS1, and MOS2, respectively. For better orientation and comparison with the emission at other wavelengths, we mark in Fig. 3 the surveyed area on the H i map of the SMC obtained by Stanimirovic et al. (1999) and in Fig. 4 we show the corresponding area on an image obtained from the Magellanic Clouds Emission Line Survey2.

thumbnail Fig. 3

Location of the XMM-Newton survey with respect to the distribution of H i gas in the SMC (from Stanimirovic et al. 1999).

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thumbnail Fig. 4

Continuum-subtracted MCELS image where red, green, and blue correspond to the Hα, [S ii], and [O iii] emission line intensities. The contour surrounds the XMM-Newton survey region.

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In Fig. 5, a combined mosaic from three energy bands is presented as a colour (RGB) image. Sources with a soft X-ray spectrum appear as red sources while hard X-ray spectra are indicated by blue colours. Owing to their extremely soft X-ray spectrum with counts practically all below 700 eV, SSSs appear as very red unresolved sources in the RGB image. Two bright SSSs mentioned in the introduction, SMP SMC 22 and SMC 3, are located in the north and south west end of the SMC bar, respectively. Owing to the large off-axis angle in the high state observation the latter source appears blurred. Both SSSs are marked in Fig. 5 by their catalogue numbers (686 and 616, respectively). The HMXBs show the hardest spectra and appear blue in the image. The brightest is SMC X-1, which is south east in the eastern wing. Its location and those of the newly discovered Be/X-ray binaries from the introduction are also indicated in Fig. 5.

To investigate the faint large-scale diffuse emission (whose analysis is ongoing and whose results will be published elsewhere) and SNRs in the SMC, we subtracted the detector background using data from observations with the filter wheel closed3 (scaled to the intensity measured in the shaded detector corners, following the work of Bauer et al. 2008) and corrected for the telescope vignetting. Because no data in the corners are available in EPIC pn small- and large-window mode observations, these had to be excluded from our analysis.

2.1. Supernova remnants

In general, SNRs exhibit thermal X-ray spectra with emission mainly below 2 keV. Their colours in the RGB images range from red via orange and yellow to light green (see Fig. 6). Remarkable examples are the SNRs in the emission nebula N19 in the south west (Fig. 6, panel 2) where the colours allow us to confine the different remnants. Most of the bright SNRs were observed by XMM-Newton in the first years of the mission and investigated by van der Heyden et al. (2004). The spectral analyses in their study yielded typical temperatures between 0.3 keV and 2 keV. For three fainter remnants with low surface brightness, Filipović et al. (2008) found lower temperatures of around 0.18 keV, indicating older ages.

In Fig. 6, we mark with full circles the position and extent of the 23 SMC SNRs listed in the compilation ofBadenes et al. (2010, hereafter B10. Only three remnants are not seen in our X-ray images: DEM S130, N S21, and N83C, down to a surface brightness limit of  ~10-14 erg cm-2 s-1 arcmin-2. In particular, DEM S130 is located in the region with the highest cumulative exposure time near 1E0102.2-7219 without any indication of enhanced X-ray emission in our EPIC images (Fig. 6, panel 1). At the position of IKT 7 (Fig. 6, panel 7), a hard X-ray source is detected, which was identified with the 172 s Be/X-ray pulsar AX J0051.6-7311 (Yokogawa et al. 2000) by Haberl & Pietsch (2004). No significant extended emission is seen in our data around IKT 7. Since IKT 7 was proposed as an SNR candidate based only on hardness ratios from Einstein IPC data (Inoue et al. 1983) and it is not detected in the radio and optical bands4, we conclude that IKT 7 is not an SNR.

Several SNRs show a larger extent in the EPIC images than given in B10: N S19, IKT 5, B0050 − 728, DEM S128, and IKT 21 (Fig. 6, thin dashed lines). There is no obvious excess X-ray emission on top of N S19 (white circle in Fig. 6, panel 2), but the optical SNR might be part of a larger remnant with an elliptical shape (4′ × 2.8′, dashed ellipse in Fig. 6) or completely unrelated. IKT 5 (Fig. 6, panel 2) shows a more elliptical (peanut) shape extending further to the south. Similarly, B0050 − 728 (Fig. 6, panel 4), the largest SNR listed by B10, is either only part of an even larger SNR with a diameter of 7′, or one of a pair of close SNRs with similar temperatures. DEM S128 (Fig. 6, panel 1) covers an elliptical area (3.6′ × 2.8′) more than twice that given by B10. Finally, faint X-ray emission is seen around IKT 21 (Fig. 6, panel 1) within a circular region of diameter 5.2′, which is more than a factor of five larger than given by B10. In Table 2, we summarise the SNRs and new candidates with large extents and list their central coordinates and sizes as seen by XMM-Newton.

thumbnail Fig. 5

Combined EPIC pn and MOS mosaic image of the SMC region centred at RA = 01:00:30, Dec = –72:28:00 (J2000). The RGB colour image is composed of the three energy bands 0.2 − 1.0 keV (red), 1.0 − 2.0 keV (green), and 2.0 − 4.5 keV (blue) (exposure-corrected and for EPIC pn out-of-time event subtracted, but not detector-background subtracted and not vignetting-corrected). The boxes mark fields with SNRs (orange) and galaxy clusters (green), which are shown in more detail in Figs. 6 and 9. Several X-ray sources, which are mentioned in the text, are marked in white with their catalogue number from Sturm et al. (2012b): 686 the planetary nebula SMP SMC 22; 616 the symbiotic binary SMC 3; 1 the super-giant HMXB SMC X-1; 2519, 2563, 2732, and 3115 new Be/X-ray binaries.

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The EPIC images reveal three new candidate SNRs with low surface brightnesses and low temperatures (thick dashed lines in panels 2 and 3 in Fig. 6, marked with their XMM-names). The most compact of these is located east of B0045 − 733 and has a circular shape (1.5′ diameter). We designate this SNR candidate XMMU J0049.0 − 7306. Two larger SNR candidates, XMMU J0056.5 − 7208 and XMMU J0057.7 − 7213, are visible west of IKT 18, one circularly shaped (diameter 3.4′, but see discussion in Sect. 3) and the other more elliptical (4.8′ × 3′). At the northern end of XMMU J0057.7 − 7213, Filipović et al. (2005) list a radio SNR (ATCA source No. 345) that partially overlaps with the X-ray emission region. It is again unclear whether we see two remnants or one with different morphologies in different wave bands. In general, our findings increase the number of larger SNRs and, in particular, we extend the size distribution of SMC remnants to larger values. A more detailed analysis of the SMC SNRs is in progress and will be published elsewhere.

thumbnail Fig. 6

Supernova remnants in the SMC as seen by EPIC. Colours represent the same energy bands as in Fig. 5 and the panels are numbered according to Fig. 5. To more clearly reveal faint diffuse emission, all images were detector background-subtracted and both exposure- and vignetting-corrected and are shown with the scale indicated in the upper right. For a description of the markings see Sects. 2.1 (SNRs) and 2.2 (galaxy clusters). The bottom right panel shows the only known SMC SNR (Haberl et al. 2012) not covered by our survey.

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Table 2

Supernova remnants and candidates seen with large extents in the EPIC images.

thumbnail Fig. 7

EPIC pn (black) and MOS (red and green) spectra of the candidate SNR XMMU J0056.5−7208 extracted from observation 0601210601 (with net exposures of 29.6 ks and 34.1 ks, respectively). Top: data with best-fit Sedov model as histogram, bottom: residuals in units of sigma.

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The X-ray spectra of the new candidate SNRs are soft, indicating low temperatures. As an example, we show the EPIC spectra of XMMU J0056.5 − 7208 (Fig. 7). We selected single-pixel (PATTERN = 0) events for EPIC pn and all valid events (PATTERN 0 − 12) for MOS spectra (with FLAG = 0 for both instruments), resulting in 550, 230, and 220 net counts for the pn, MOS1, and MOS2 spectra, respectively. Owing to the large extraction area (circle with radius 96′′), the background (~70%) dominates the total spectrum and therefore, we binned to a signal-to-noise ratio of at least 3 per bin, which also ensured a minimum of 25 source counts per bin. To model the spectrum, we used the Sedov (vsedov in XSPEC) model (Borkowski et al. 2001) based on NEI-version 2.0. Metal abundances were fixed to 0.2 solar, as is typical in the SMC (Russell & Dopita 1992). To reduce the number of free model parameters in the fit, we forced the mean shock temperature and the electron temperature immediately behind the shock front to be the same. The absorption column density was divided into two parts, a fixed value of 6 × 1020 cm-2 with solar abundances for the Galactic foreground (Dickey & Lockman 1990), and a column density, which was allowed to vary in the fit, to account for the absorption in the SMC (where also the abundances where set to 0.2 and limited to a maximum of 3 × 1021 cm-2, the total SMC absorption in the direction of the SNR; Stanimirovic et al. 1999). Under these assumptions, we derived an upper limit to the SMC column of 3 × 1021 cm-2 and a shock temperature of 0.5 keV, which is also poorly constrained with an upper limit of 2.3 keV. Also for the ionisation time-scale, only an upper limit of 1.5 × 1013 s cm-3 could be derived. The formal best-fit model has a reduced χ2 of 1.28 for 16 degrees of freedom. The total observed flux in the 0.2 − 2.0 keV energy band was derived to be 5 × 10-14 erg cm-2 s-1, which translates into a surface brightness of 2.8 × 10-14 erg cm-2 s-1 arcmin-2. Using the radius-temperature relation ty = 3.8 × 102  (e.g. Xu et al. 2005), we estimate the dynamical age of the SNR to be  ~16 kyr. However, we stress that the low statistical quality of the X-ray spectra, which have a high background, does not allow us to differentiate between the various models generally used to explain SNR spectra and more X-ray data is required for all the faint SNRs in the SMC to derive more reliable parameters.

We examined the MCELS images in the regions around the new SNR candidates. Only for XMMU J0056.5 − 7208 (Fig. 8) is a shell structure visible, mainly in Hα and [S ii]. The structure is of elliptical shape, but could also be composed of two merging circular shells. The EPIC image with enhanced contrast in Fig. 8 appears to also reveal weak X-ray emission in the southern part where the optical emission is enhanced. In the brighter parts in the south, the shell has a ratio of the [S ii] to Hα intensity of around 0.4. This is at the lower limit of what is generally accepted for an SNR (typically  >0.4; Fesen et al. 1985), but given the low elemental abundance in the SMC, lower ratios are expected and also seen for other well-established SMC SNRs from the full MCELS image and optical spectroscopy (cf. Payne et al. 2007).

2.2. Clusters of galaxies

The EPIC images reveal other extended sources, which appear in darker green in the RGB images (Fig. 5) indicating that they have spectra harder than those of supernova remnants. These sources are not concentrated in the SMC bar where most SNRs are found, which suggests that they are most likely clusters of galaxies behind the SMC. In our galaxy cluster classification, we also include groups of galaxies that are characterised by X-ray emitting gas with somewhat lower temperatures than more massive clusters (Mulchaey 2000). In rare cases, giant H ii regions can also show relatively hard X-ray spectra (e.g. IC 131 in M 33, Tüllmann et al. 2009), but these can be identified as such by looking for emission at other wavelengths such as Hα. Five fields, including the most prominent clusters of galaxies, are marked with green boxes in Fig. 5 and zoomed images are shown in Fig. 9. Three more can be seen in the SNR images in Fig. 6, which are marked with arrows in panels 1 and 11. A summary of the candidate clusters of galaxies can be found in Table 3.

thumbnail Fig. 8

The region around XMMU J0056.5 − 7208. Left: zoom-in of the MCELS image from Fig. 4. Right: zoom-in of the X-ray image from Fig. 6, panel 3 with enhanced contrast. The contours mark the summed Hα, [S ii], and [O iii] intensity at 100 and 150 × 10-17 erg cm-2 s-1.

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thumbnail Fig. 9

Candidates for clusters of galaxies seen in the SMC field. Two sources from the catalogue identified with galaxies are marked with arrows. Presentation as in Fig. 6.

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The X-ray brightest and most extended candidate cluster is XMMU J011926.0-730134, which is located in the eastern wing of the SMC (north east of SMC X-1). The EPIC pn and MOS2 instruments (no MOS1 data is available as the source was located on MOS1 CCD 6, which was no longer operating at the time of observation 0601212301) collected  ~3100 counts within a circular area of radius 2′, which was sufficient for our spectral analysis. Spectra were extracted and binned in the same way as described in Sect. 2.1. We modelled the spectra with plasma emission attenuated by three absorption components (phabs*vphabs*vphabs*vmekal in XSPEC). The first accounts for the absorption in the Milky Way with a column density fixed at 4.5 × 1020 cm-2 (Dickey & Lockman 1990) and assuming solar abundances (Wilms et al. 2000), the second models the absorption through the SMC with column density fixed at 5.2 × 1021 cm-2 (Stanimirovic et al. 1999) and SMC abundances, and the third, with a column density that was allowed to vary in the fit, is responsible for the remaining absorption to the galaxy cluster (here we assume an abundance of 0.5 solar). This model results in an acceptable fit with a reduced χ2 of 1.01 for 79 degrees of freedom (Fig. 10). The best-fit value of the absorbing column density is 1.9 ± 0.9 × 1021 cm-2, of the temperature 1.5 ± 0.1 keV and of the redshift z = 0.052 ± 0.013 (errors denote 90% confidence ranges for one parameter of interest). Two known galaxies are listed in the 6dF galaxy survey (Jones et al. 2009) within an angular distance of 1′ from the cluster centre, g0119275-730130 with a redshift of 0.0656 and g0119251-730217 with z = 0.0677. Both redshifts are within the uncertainty range for the X-ray determined redshift, supporting the cluster nature of the X-ray emission.

A comparison with deep radio images of the SMC (Sturm et al., in prep., and references therein) revealed radio emission with jet-like structures at the position of XMMU J004203.0-730730. Their morphology suggests that they arise from two independent galaxies each possessing a single jet or from one galaxy moving through the intra-cluster medium and leaving the jets behind. A radio point source was also found in the centre of XMMU J010454.5-721022, which is most likely associated with an AGN. The host galaxy likely belongs to the cluster, although it cannot be excluded that it is located behind or in front of the cluster. Additional, more distant galaxy clusters that appear as sources with small extents are included in the point source catalogue of SHP12.

Table 3

Candidates for galaxy clusters.

A region with complex extended emission is shown in Fig. 9, panel 5. In the south-east, the source catalogue contains two sources, which are classified as galaxies (1711 and 1726, indicated by arrows). Source 1711 is identified with the galaxy 2MASX J01090126-7229058 with an extent of 0.6′ × 0.7′ (Skrutskie et al. 2006). Further to the north-west, weak extended emission might originate from hot cluster gas, but it remains unclear whether the galaxies belong to this cluster or they are unrelated.

thumbnail Fig. 10

EPIC pn (black) and MOS2 (red) spectra of the candidate galaxy cluster XMMU J011926.0-730134, fitted with an absorbed thin plasma emission (mekal) model. Net exposures are 27.4 ks and 32.9 ks, respectively. Top: data with best-fit model as histograms. Bottom: residuals in units of sigma.

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3. Discussion and conclusions

We have performed a deep X-ray survey of the SMC using the EPIC instruments on board XMM-Newton between May 2009 and March 2010, which, together with archival observations, covers the bar and eastern wing of the galaxy. While a detailed analysis of the population of discrete X-ray sources (unresolved and with small extents) is described in SHP12, we present here first mosaic images of the main body of the SMC. We study the morphology of more extended X-ray emission from supernova remnants and galaxy clusters. Several galaxy clusters can easily be identified in our EPIC images by their extended appearance and X-ray colours.

We have found that several of the known SNRs listed with their spatial extent in the catalogue of B10 appear larger in the X-ray images. This is particularly true for large SNRs with low temperatures (suggesting old remnants) and low surface brightnesses, which can be better mapped with the high sensitivity of XMM-Newton in soft X-rays. However, a larger extent could also be caused by the presence of more than one SNR merging into larger bubbles. In these cases, the consistent X-ray colours require the neighbouring remnants, which could have formed in star clusters, to have similar ages.

We propose three new SNRs with large extents, low surface brightnesses, and soft X-ray spectra (labelled with their XMM names in Fig. 6). The high sensitivity of the EPIC instruments at low energies allowed us to detect their faint and soft emission with a typical surface brightness of 10-14 erg cm-2 s-1 arcmin-1 (0.2−2.0 keV). Only one of the three remnants (XMMU J0056.5−7208) is seen in the MCELS images in the Hα, [S ii], and [O iii] emission lines. Shell-like optical filaments suggest that we see either one SNR extending further to the south-west of the bulk of the X-ray emission or two merging SNR shells with the southern one being fainter in X-rays but brighter in the optical than the northern shell. Our survey covers all other known SNRs in the SMC, except the recently discovered faint SNR far out in the eastern wing, which is located outside our survey area (Haberl et al. 2012) and also exhibits a low surface brightness and low temperature, indicative of older SNRs. For completeness, the image of this remnant (designated XMMU J0127.7 − 7332), which harbours the Be/X-ray binary pulsar SXP 1062 at its centre is included in the image gallery of Fig. 6. Overall, our studies of the X-ray morphology of the SNRs in the SMC extend their size distribution to larger values. In particular for B0050 − 728, the largest SNR known in the SMC, we have found an X-ray diameter as large as 7′, which corresponds to 122 pc at a distance of 60 kpc. In the MCs, this is only exceeded by the LMC SNR DEM L203 (B10).


4

Magellanic Cloud Supernova Remnant Database: http://www.mcsnr.org/Default.aspx

Acknowledgments

The XMM-Newton project is supported by the Bundesministerium für Wirtschaft und Technologie/Deutsches Zentrum für Luft- und Raumfahrt (BMWI/DLR, FKZ 50 OX 0001) and the Max-Planck Society. The Magellanic Clouds Emission Line Survey (MCELS) data were provided by R. C. Smith, P. F. Winkler, and S. D. Points. The MCELS project has been supported in part by NSF grants AST-9540747 and AST-0307613, and through the generous support of the Dean B. McLaughlin Fund at the University of Michigan, a bequest from the family of Dr. Dean B. McLaughlin in memory of his lasting impact on Astronomy. The National Optical Astronomy Observatory is operated by the Association of Universities for Research in Astronomy Inc. (AURA), under a cooperative agreement with the National Science Foundation. R. Sturm acknowledges support from the BMWI/DLR, FKZ 50 OR 0907. N. La Palombara, S.L. Mereghetti and A. Tiengo acknowledge financial contributions by the Italian Space Agency through ASI/INAF agreements I/009/10/0 and I/032/10/0 for data analysis and XMM-Newton operations, respectively.

References

Online material

Table 1

XMM-Newton observations of the large programme SMC survey.

Table 1

continued: archival and calibration (1E0102.2-7219) observations.

All Tables

Table 2

Supernova remnants and candidates seen with large extents in the EPIC images.

Table 3

Candidates for galaxy clusters.

Table 1

XMM-Newton observations of the large programme SMC survey.

Table 1

continued: archival and calibration (1E0102.2-7219) observations.

All Figures

thumbnail Fig. 1

Combined EPIC exposure map of the SMC area. The pn and MOS exposure maps account for vignetting and for illustration are summed up with MOS1 and MOS2 each weighted with a factor of 0.4 to account for their relative sensitivity. The 30 numbered circles (radius 13′) mark the survey fields.

Open with DEXTER
In the text
thumbnail Fig. 2

Distribution of the EPIC exposures in the SMC area covered by the XMM-Newton observations. Full, dashed, and dash-dotted lines indicate pn, MOS1, and MOS2, respectively.

Open with DEXTER
In the text
thumbnail Fig. 3

Location of the XMM-Newton survey with respect to the distribution of H i gas in the SMC (from Stanimirovic et al. 1999).

Open with DEXTER
In the text
thumbnail Fig. 4

Continuum-subtracted MCELS image where red, green, and blue correspond to the Hα, [S ii], and [O iii] emission line intensities. The contour surrounds the XMM-Newton survey region.

Open with DEXTER
In the text
thumbnail Fig. 5

Combined EPIC pn and MOS mosaic image of the SMC region centred at RA = 01:00:30, Dec = –72:28:00 (J2000). The RGB colour image is composed of the three energy bands 0.2 − 1.0 keV (red), 1.0 − 2.0 keV (green), and 2.0 − 4.5 keV (blue) (exposure-corrected and for EPIC pn out-of-time event subtracted, but not detector-background subtracted and not vignetting-corrected). The boxes mark fields with SNRs (orange) and galaxy clusters (green), which are shown in more detail in Figs. 6 and 9. Several X-ray sources, which are mentioned in the text, are marked in white with their catalogue number from Sturm et al. (2012b): 686 the planetary nebula SMP SMC 22; 616 the symbiotic binary SMC 3; 1 the super-giant HMXB SMC X-1; 2519, 2563, 2732, and 3115 new Be/X-ray binaries.

Open with DEXTER
In the text
thumbnail Fig. 6

Supernova remnants in the SMC as seen by EPIC. Colours represent the same energy bands as in Fig. 5 and the panels are numbered according to Fig. 5. To more clearly reveal faint diffuse emission, all images were detector background-subtracted and both exposure- and vignetting-corrected and are shown with the scale indicated in the upper right. For a description of the markings see Sects. 2.1 (SNRs) and 2.2 (galaxy clusters). The bottom right panel shows the only known SMC SNR (Haberl et al. 2012) not covered by our survey.

Open with DEXTER
In the text
thumbnail Fig. 7

EPIC pn (black) and MOS (red and green) spectra of the candidate SNR XMMU J0056.5−7208 extracted from observation 0601210601 (with net exposures of 29.6 ks and 34.1 ks, respectively). Top: data with best-fit Sedov model as histogram, bottom: residuals in units of sigma.

Open with DEXTER
In the text
thumbnail Fig. 8

The region around XMMU J0056.5 − 7208. Left: zoom-in of the MCELS image from Fig. 4. Right: zoom-in of the X-ray image from Fig. 6, panel 3 with enhanced contrast. The contours mark the summed Hα, [S ii], and [O iii] intensity at 100 and 150 × 10-17 erg cm-2 s-1.

Open with DEXTER
In the text
thumbnail Fig. 9

Candidates for clusters of galaxies seen in the SMC field. Two sources from the catalogue identified with galaxies are marked with arrows. Presentation as in Fig. 6.

Open with DEXTER
In the text
thumbnail Fig. 10

EPIC pn (black) and MOS2 (red) spectra of the candidate galaxy cluster XMMU J011926.0-730134, fitted with an absorbed thin plasma emission (mekal) model. Net exposures are 27.4 ks and 32.9 ks, respectively. Top: data with best-fit model as histograms. Bottom: residuals in units of sigma.

Open with DEXTER
In the text

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