Discovery of an [O iii] Emission Shell around the X-Ray Binary CI Cam

Abstract We report the serendipitous discovery of a 8 ′ × 12 ′ emission shell brightest in [O iii] centered on the X-ray binary B[e] star, CI Cameleopardalis (CI Cam). This shell, detected during a survey of optical emission associated with the Galactic supernova remnant G150.3+4.5, is seen outside but immediately adjacent to the remnant’s optical filaments along its northwestern edge. Assuming this shell is related to CI Cam’s strong winds and transient outbursts, the adoption of CI Cam’s Gaia DR3 statistically corrected distance of 4.1 − 0.2 + 0.3 kpc yields shell dimensions ≃9.5 pc × 14.3 pc. While the distance to the G150.3+4.5 remnant is uncertain, the appearance of CI Cam’s emission shell so close to the SNR’s optical filaments appears likely to be a chance coincidence.