We present results from two Chandra observations of the 16.6 d X-ray/ \gamma -ray high-mass binary 1FGL J1018.6 - 5856 located at the center of the supernova remnant G284.3 - 1.8 .
The binary spectra , separated by 0.25 in binary phase , are fit with an absorbed power-law model with \Gamma \approx 1.7 - 1.8 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 \approx 2 ^ { \prime \prime } – 3 ^ { \prime \prime } southeast of the binary , significant at the 3 \sigma 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 \approx 2 M _ { \odot } neutron star .
The initial mass of the progenitor star in this scenario is 27 \pm 4 M _ { \odot } . 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 \sim 10 ^ { 33 } 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 SNe predict Mg-rich ejecta from very massive progenitors of > 25 M _ { \odot } .