RX J050736−6847.8: A Large Supernova Remnant around an X-Ray Binary in the Large Magellanic Cloud

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Abstract
RX J050736-6847.8 in the Large Magellanic Cloud (LMC) is an intriguing X-ray source consisting of a large ring (diameter ~ 150 pc) of diffuse emission and a central compact source. It is projected in the vicinity of the superbubble N103 around the star cluster NGC 1850. RX J050736-6847.8's ring of diffuse X-ray emission, offset from the superbubble N103, is not bounded by any optical shell structure, while RX J050736-6847.8's central compact X-ray source is projected within the cluster HS122. We have analyzed archival ROSAT observations of RX J050736-6847.8 to determine the physical properties of its X-ray–emitting gas. The X-ray luminosity of the diffuse X-ray emission is 5–6 × 1035 ergs s-1 in the 0.1–2.4 keV band, within the range for supernova remnants (SNRs). Assuming a shell geometry with a fractional shell thickness (ΔR/R) of 0.05–0.2, we find the density of the hot gas to be 0.05–0.09 cm-3 and a hot gas mass of ~820 M⊙. The physical properties of this shell of hot gas are consistent with Sedov's solution for a ~5 × 104 yr old SNR in a low-density (~0.015 cm-3) medium formed by a supernova of an explosion energy of 3 × 1051 ergs. The density is so low that no detectable optical emission is expected. Therefore, we suggest that this ring of X-ray emission originates in a SNR, the largest known in the LMC. The large size, low density, and the regular X-ray morphology suggest that this SNR is located in the halo of the LMC; the small absorption column density further suggests that this SNR is on the near side of the LMC halo. The central compact source of RX J050736-6847.8 is probably an X-ray binary in the cluster HS122. The relationship between the compact source and the diffuse ring is unknown.