Multi-frequency study of Local Group supernova remnants

Abstract

<i>Context. <i/>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.<i>Aims. <i/>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 H, [S ii] and [O iii] 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.<i>Methods. <i/>We analysed the X-ray and Radio-Continuum spectra and present multi-wavelength morphological studies of this SNR.<i>Results. <i/>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 <i>α<i/> = -0.36 <i>±<i/> 0.09 and a diameter of <i>D<i/> = 52.4 <i>±<i/> 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 <i>kT<i/> 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.<i>Conclusions. <i/>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.