LS 5039 is a high-mass binary with a period of 4 days , containing a compact object and an O star , one of the few high-mass binaries detected in \gamma -rays .
Our Chandra ACIS observation of LS 5039 provided a high-significance ( \approx 10 \sigma ) detection of extended emission clearly visible for up to 1′ from the point source .
The spectrum of this emission can be described by an absorbed power-law model with photon index \Gamma = 1.9 \pm 0.3 , somewhat softer than the point source spectrum \Gamma = 1.44 \pm 0.07 , with the same absorption , N _ { H } = ( 6.4 \pm 0.6 ) \times 10 ^ { 21 } cm ^ { -2 } .
The observed 0.5–8 keV flux of the extended emission is \simeq 8.8 \times 10 ^ { -14 } erg s ^ { -1 } cm ^ { -2 } , or 5 % of the point source flux ; the latter is a factor of \sim 2 lower than the lowest flux detected so far .
Fainter extended emission with comparable flux and a softer ( \Gamma \approx 3 ) spectrum is detected at even greater radii ( up to 2′ ) .
Two possible interpretations of the extended emission are a dust scattering halo and a synchrotron nebula powered by energetic particles escaping the binary .
We discuss both of these scenarios and favor the nebula interpretation , although some dust contribution is possible .
We have also found transient sources located within a narrow stripe south of LS 5039 .
We discuss the likelihood of these sources to be related to LS 5039 .