A. Y. De Horta

New XMM-Newton observations of supernova remnants in the Small Magellanic Cloud

Context. A complete overview of the supernova remnant (SNR) populati on is required to investigate their evolution and interacti on with the surrounding interstellar medium in the Small Magel lanic Cloud (SMC). Aims. Recent XMM-Newton observations of the SMC cover three known SNRs (DEM S5, SNR B0050 −72.8, and SNR B0058 −71.8), which are poorly studied and are X-ray faint. We used new mult i-frequency radio-continuum surveys and new optical obser vations at Hα, [S ii], and [Oiii] wavelengths, in combination with the X-ray data, to invest igate their properties and to search for new SNRs in the SMC. Methods. We used X-ray source selection criteria and found one SMC obj ect with typical SNR characteristics (HFPK 334), that was initially detected by ROSAT. We analysed the X-ray spectra a nd present multi-wavelength morphological studies of the t hr e SNRs and the new candidate. Results. Using a non-equilibrium ionisation collisional plasma mod el, we find temperatures kT around 0.18 keV for the three known remnants and 0.69 keV for the candidate. The low temperature , low surface brightness, and large extent of the three remna nts indicates relatively large ages. The emission from the new candidate ( HFPK 334) is more centrally peaked and the higher temperatur e s ggests a younger remnant. Our new radio images indicate that a pulsa r wind nebulae (PWN) is possibly associated with this object . Conclusions. The SNRs known in the SMC show a variety of morphological stru ctures that are relatively uncorrelated in the di fferent wavelength bands, probably caused by the di fferent conditions in the surrounding medium with which the re mnant interacts.Context. A complete overview of the supernova remnant (SNR) population is required to investigate their evolution and interaction with the surrounding interstellar medium in the Small Magellanic Cloud (SMC). Aims. Recent XMM-Newton observations of the SMC cover three known SNRs (DEM S5, SNR B0050−72.8, and SNR B0058−71.8), which are poorly studied and are X-ray faint. We used new multi-frequency radio-continuum surveys and new optical observations at Hα ,[ Sii], and [O iii] wavelengths, in combination with the X-ray data, to investigate their properties and to search for new SNRs in the SMC. Methods. We used X-ray source selection criteria and found one SMC object with typical SNR characteristics (HFPK 334), that was initially detected by ROSAT. We analysed the X-ray spectra and present multi-wavelength morphological studies of the three SNRs and the new candidate. Results. Using a non-equilibrium ionisation collisional plasma model, we find temperatures kT around 0.18 keV for the three known remnants and 0.69 keV for the candidate. The low temperature, low surface brightness, and large extent of the three remnants indicates relatively large ages. The emission from the new candidate (HFPK 334) is more centrally peaked and the higher temperature suggests a younger remnant. Our new radio images indicate that a pulsar wind nebulae (PWN) is possibly associated with this object. Conclusions. The SNRs known in the SMC show a variety of morphological structures that are relatively uncorrelated in the different wavelength bands, probably caused by the different conditions in the surrounding medium with which the remnant interacts.

Multifrequency study of SNR J0533-7202, a new supernova remnant in the LMC

We present a detailed study of Australia Telescope Compact Array (ATCA) observations of a newly discovered Large Magellanic Cloud (LMC) supernova remnant (SNR), SNR J0533-7202. This object follows a horseshoe morphology, with a size 37 pc x 28 pc (1-pc uncertainty in each direction). It exhibits a radio spectrum with the intrinsic synchrotron spectral index of alpha= -0.47+-0.06 between 73 and 6 cm. We report detections of regions showing moderately high fractional polarisation at 6 cm, with a peak value of 36+-6% and a mean fractional polarisation of 12+-7%. We also estimate an average rotation measure across the remnant of -591 rad m^-2. The current lack of deep X-ray observation precludes any conclusion about high-energy emission from the remnant. The association with an old stellar population favours a thermonuclear supernova origin of the remnant.

Radio planetary nebulae in the Magellanic Clouds

We report the extragalactic radio-continuum detection of 15 planetary nebulae (PNe) in the Magellanic Clouds (MCs) from recent Australia Telescope Compact Array+Parkes mosaic surveys. These detections were supplemented by new and high-resolution radio, optical and infrared observations which helped to resolve the true nature of the objects. Four of the PNe are located in the Small Magellanic Cloud (SMC) and 11 are located in the Large Magellanic Cloud (LMC). Based on Galactic PNe the expected radio flux densities at the distance of the LMC/SMC are up to ∼2.5 and ∼2.0 mJy at 1.4 GHz, respectively. We find that one of our new radio PNe in the SMC has a flux density of 5.1 mJy at 1.4 GHz, several times higher than expected. We suggest that the most luminous radio PN in the SMC (N S68) may represent the upper limit to radio-peak luminosity because it is approximately three times more luminous than NGC 7027, the most luminous known Galactic PN. We note that the optical diameters of these 15 Magellanic Clouds (MCs) PNe vary from very small (∼0.08 pc or 0.32 arcsec; SMP L47) to very large (∼1 pc or 4 arcsec; SMP L83). Their flux densities peak at different frequencies, suggesting that they may be in different stages of evolution. We briefly discuss mechanisms that may explain their unusually high radio-continuum flux densities. We argue that these detections may help solve the ‘missing mass problem’ in PNe whose central stars were originally 1–8 M� . We explore the possible link between ionized haloes ejected by the central stars in their late evolution and extended radio emission. Because of their higher than expected flux densities, we tentatively call this PNe (sub)sample – ‘Super PNe’.

Multi-frequency study of supernova remnants in the Large Magellanic Cloud - confirmation of the supernova remnant status of DEM L205

Context. The Large Magellanic Cloud (LMC) is an ideal target for the study of an unbiased and complete sample of supernova remnants (SNRs). We started an X-ray survey of the LMC with XMM-Newton, which, in combination with observations at other wavelengths, will allow us to discover and study remnants that are either even fainter or more evolved (or both) than previously known. Aims. We present new X-ray and radio data of the LMC SNR candidate DEM L205, obtained by XMM-Newton and ATCA, along with archival optical and infrared observations. Methods. We use data at various wavelengths to study this object and its complex neighbourhood, in particular in the context of the star formation activity, past and present, around the source. We analyse the X-ray spectrum to derive some remnant’s properties, such as age and explosion energy. Results. Supernova remnant features are detected at all observed wavelengths : soft and extended X-ray emission is observed, arising from a thermal plasma with a temperature kT between 0.2 keV and 0.3 keV. Optical line emission is characterised by an enhanced [S ii]-to-Hα ratio and a shell-like morphology, correlating with the X-ray emission. The source is not or only tentatively detected at near-infrared wavelengths (shorter than 10 μm), but there is a detection of arc-like emission at mid and far-infrared wavelengths (24 and 70 μm) that can be unambiguously associated with the remnant. We suggest that thermal emission from dust heated by stellar radiation and shock waves is the main contributor to the infrared emission. Finally, an extended and faint non-thermal radio emission correlates with the remnant at other wavelengths and we find a radio spectral index between −0.7 and −0.9, within the range for SNRs. The size of the remnant is∼79 × 64 pc and we estimate a dynamical age of about 35 000 years. Conclusions. We definitely confirm DEM L205 as a new SNR. This object ranks amongst the largest remnants known in the LMC. The numerous massive stars and the recent outburst in star formation around the source strongly suggest that a core-collapse supernova is the progenitor of this remnant.

Highly absorbed X-ray binaries in the Small Magellanic Cloud

Many of the high mass X-ray binaries (HMXRBs) discovered in recent years in our Galaxy are characterized by a high absorption, most likely intrinsic to the system, that can impede their detection at the softest X-ray energies. Exploiting the good coverage obtained with sensitive XMM-Newtonobservations, we have undertaken a search for highly absorbed X-ray sources in the Small Magellanic Cloud (SMC), which is known to contain a large number of HMXRBs. After a systematic analysis of 62 XMM-NewtonSMC observations, we obtained a sample of 30 sources with evidence of an equivalent hydrogen column density larger than 3×10 23 cm 2 . Five of these sources are clearly identified as HMXRBs, four being previously known (including three X-ray pulsars) and one, XMMU J005605.8-720012, being reported here for the first time. For the latter, we present optical spectroscopy confirming the asso ciation with a Be star in the SMC. The other sources in our sample have optical counterparts fainter than magnitude ∼16 in the V band, and many have possible NIR counterparts consistent with highly reddened early-type stars in the SMC. While their number is broadly consistent with the expected population of background highly absorbed active galactic nuclei, a few of them could be HMXRBs in which an early-type companion is severely reddened by local material.

Multi-frequency study of Local Group supernova remnants - The curious case of the Large Magellanic Cloud SNR J0528–6714

Context. The SNRs known in the Local Group show a variety of morphological structures that are relatively uncorrelated in the different wavelength bands. This variety is probably caused by the different conditions in the surrounding medium with which the remnant interacts. Aims. Recent ATCA, XMM-Newton and MCELS observations of the Magellanic Clouds (MCs) cover a number of new and known SNRs which are poorly studied, such as SNR J0528–6714. This particular SNR exhibits luminous radio-continuum emission, but is one of the unusual and rare cases without detectable optical and very faint X-ray emission (initially detected by ROSAT and listed as object [HP99] 498). We used new multi-frequency radio-continuum surveys and new optical observations at Hα ,[ Sii ]a nd [Oiii] wavelengths, in combination with XMM-Newton X-ray data, to investigate the SNR properties and to search for a physical explanation for the unusual appearance of this SNR. Methods. We analysed the X-ray and Radio-Continuum spectra and present multi-wavelength morphological studies of this SNR. Results. We present the results of new moderate resolution ATCA observations of SNR J0528–6714. We found that this object is a typical older SNR with a radio spectral index of α = −0.36 ± 0.09 and a diameter of D = 52.4 ± 1.0 pc. Regions of moderate and somewhat irregular polarisation were detected which are also indicative of an older SNR. Using a non-equilibrium ionisation collisional plasma model to describe the X-ray spectrum, we find temperatures kT of 0.26 keV for the remnant. The low temperature, low surface brightness, and large extent of the remnant all indicate a relatively advanced age. The near circular morphology indicates a type Ia event. Conclusions. Our study revealed one of the most unusual cases of SNRs in the Local Group of galaxies – a luminous radio SNR without optical counterpart and, at the same time, very faint X-ray emission. While it is not unusual to not detect an SNR in the optical, the combination of faint X-ray and no optical detection makes this SNR very unique.

Multi-frequency observations of SNR J0453–6829 in the LMC - A composite supernova remnant with a pulsar wind nebula

Context. The Large Magellanic Cloud (LMC) is rich in supernova remnants (SNRs), which can be investigated in detail with radio, optical, and X-ray observations. SNR J0453–6829 is an X-ray and radio-bright remnant in the LMC, within which previous studies revealed the presence of a pulsar wind nebula (PWN), making it one of the most interesting SNRs in the Local Group of galaxies. Aims. We study the emission of SNR J0453–6829 to improve our understanding of its morphology, spectrum, and thus the emission mechanisms in the shell and the PWN of the remnant. Methods. We obtained new radio data with the Australia Telescope Compact Array and analysed archival XMM-Newton observations of SNR J0453–6829. We studied the morphology of SNR J0453–6829 from radio, optical, and X-ray images and investigated the energy spectra in the different parts of the remnant. Results. Our radio results confirm that this LMC SNR hosts a typical PWN. The prominent central core of the PWN exhibits a radio spectral index αCore of −0.04 ± 0.04, while in the rest of the SNR shell the spectral slope is somewhat steeper with αShell = −0.43 ± 0.01. We detect regions with a mean polarisation of P (12 ± 4)% at 6 cm and (9 ± 2)% at 3 cm. The full remnant is of roughly circular shape with dimensions of (31 ± 1) pc × (29 ± 1) pc. The spectral analysis of the XMM-Newton EPIC and RGS spectra allowed us to derive physical parameters for the SNR. Somewhat depending on the spectral model, we obtain for the remnant a shock temperature of around 0.2 keV and estimate the dynamical age to 12 000–15 000 years. Using a Sedov model we further derive an electron density in the X-ray emitting material of 1.56 cm −3 , typical for LMC remnants, a large swept-up mass of 830 M� ,a nd an explosion energy of 7.6 × 10 50 erg. These parameters indicate a well evolved SNR with an X-ray spectrum dominated by emission

Multi-frequency Observations of a Superbubble in the LMC: The Case of LHA 120-N 70

We present a detailed study of new Australia Telescope Compact Array and XMM-Newton observations of LHA 120-N 70 (hereafter N 70), a spherically shaped object in the Large Magellanic Cloud, classified as a superbubble. Both archival and new observations were used to produce high quality radio continuum, X-ray, and optical images. The radio spectral index of N 70 is estimated to be α = –0.12 ± 0.06, indicating that while a supernova (SN) or supernovae have occurred in the region at some time in the distant past, N 70 is not the remnant of a single specific SN. N 70 exhibits limited polarization with a maximum fractional polarization of 9% in a small area of the northwest limb. We estimate the size of N 70 to have a diameter of 104 pc (±1 pc). The morphology of N 70 in X-rays closely follows that in radio and optical, with most X-ray emission confined within the bright shell seen at longer wavelengths. Purely thermal models adequately fit the soft X-ray spectrum which lacks harder emission (above 1 keV). We also examine the pressure output of N 70 where the values for the hot (P X) and warm () phases are consistent with other studied H II regions. However, the dust-processed radiation pressure (P IR) is significantly smaller than in any other object studied in Lopez et al. N 70 is a very complex region that is likely to have had multiple factors contributing to both the origin and evolution of the entire region.

Radio confirmation of Galactic supernova remnant G308.3-1.4

We present radio-continuum observations of the Galactic supernova remnant (SNR) candidate, G308.3‐1.4, made with the Australia Telescope Compact Array, Molonglo Observatory Synthesis Telescope and the Parkes radio-telescope. Our results combined with Chandra Xray images confirm that G308.3‐1.4 is a bona fide SNR with a shell morphology. The SNR has average diameter of D = 34± 19 pc, radio spectral index of α = −0.68± 0.16 and linear polarisation of 10 ± 1%; We estimate the SNR magnetic field B ≈ 29 μG. Employing a Σ− D relation, we estimate a distance to G308.3‐1.4 of d = 19± 11 kpc. The radio morphology, although complex, suggests a smaller size for the SNR than previously implied in an X-Ray study. These results imply that G308.3‐1.4 is a young to middle-aged SNR in the early adiabatic phase of evolution.

Active galactic nuclei behind the SMC selected from radio and X-ray surveys

Context. The XMM-Newton survey of the Small Magellanic Cloud (SMC) revealed 3053 X-ray sources with the majority expected to be active galactic nuclei (AGN) behind the SMC. However, the high stellar density in this field often does not allow assigning unique optical counterparts and hinders source classification. On the other hand, the association of X-ray point sources with radio emission can be used to select background AGN with high confidence, and to constrain other object classes like pulsar wind nebula. Aims. To classify X-ray and radio sources, we use clear correlations of X-ray sources found in the XMM-Newton survey with radio-continuum sources detected with ATCA and MOST. Methods. Deep radio-continuum images were searched for correlations with X-ray sources of the XMM-Newton SMC-survey point-source catalogue as well as galaxy clusters seen with extended X-ray emission. Results. Eighty eight discrete radio sources were found in common with the X-ray point-source catalogue in addition to six correlations with extended X-ray sources. One source is identified as a Galactic star and eight as galaxies. Eight radio sources likely originate in AGN that are associated with clusters of galaxies seen in X-rays. One source is a pulsar wind nebula candidate. We obtain 43 new candidates for background sources located behind the SMC. A total of 24 X-ray sources show jet-like radio structures.