Magnesium-rich ejecta in the SNR G284.3-1.8 around the high-mass gamma-ray binary 1FGL J1018.6-5856

We present results from two Chandra observations of the 16.6 day X-ray/γ-ray high-mass binary 1FGL J1018.6-5856 located at the center of the supernova remnant (SNR) G284.3-1.8. The binary spectra, separated by 0.25 in binary phase, are fit with an absorbed power-law model with for both observations (the flux during the second observation is a factor of 1.7 smaller). In the high-resolution ACIS-I image we found a hint of extended emission -3″ southeast of the binary, significant at the level. Binary evolution codes reproduce the system's observed properties with two massive stars with an initial 18 day period, undergoing mass transfer and leaving behind a heavy neutron star. The initial mass of the progenitor star in this scenario is 27 ± 4 . Chandra and XMM-Newton images of the remnant show it has a relatively low X-ray surface brightness. The two brightest regions of extended X-ray emission, with luminosities erg s^-1 for d = 5 kpc, lie in the northern and western portions and show significantly different spectra. The northern patch is consistent with shocked ISM, with a low temperature and long ionization timescale. However, the western patch is dominated by ejecta, and shows significantly enhanced Mg content relative to other ejecta products. The abundance ratios inferred resemble those from the Large Magellanic Cloud remnant N49B. To our knowledge, this is only the second case of such Mg-rich ejecta found in an SNR. Nucleosynthesis models for core-collapse supernovae predict Mg-rich ejecta from very massive progenitors of .