We present H \alpha and [ O III ] ( 5007 Å ) images of the nebula powered by the jet of the black hole candidate and microquasar Cygnus X–1 , observed with the 2.5m Isaac Newton Telescope ( INT ) .
The ring-like structure is luminous in [ O III ] and there exists a thin outer shell with a high [ O III ] / H \alpha flux ratio .
This outer shell probably originates in the collisionally excited atoms close to the front of the bow shock .
Its presence indicates that the gas is shock excited as opposed to photoionised , supporting the jet-powered scenario .
The shock velocity was previously constrained at 20 < v _ { s } < 360 km s ^ { -1 } ; here we show that v _ { s } \geq 100 km s ^ { -1 } ( 1 \sigma confidence ) based on a comparison of the observed [ O III ] / H \alpha ratio in the bow shock with a number of radiative shock models .
From this we further constrain the time-averaged power of the jet : P _ { Jet } = ( 4 – 14 ) \times 10 ^ { 36 } erg s ^ { -1 } .
The H \alpha flux behind the shock front is typically 4 \times 10 ^ { -15 } erg s ^ { -1 } cm ^ { -2 } arcsec ^ { -2 } , and we estimate an upper limit of \sim 8 \times 10 ^ { -15 } erg s ^ { -1 } cm ^ { -2 } arcsec ^ { -2 } ( 3 \sigma ) to the optical ( R -band ) continuum flux of the nebula .
The inferred age of the structure is similar to the time Cyg X–1 has been close to a bright H II region ( due to the proper motion of the binary ) , indicating a dense local medium is required to form the shock wave .
In addition , we search a > 1 degree ^ { 2 } field of view to the south of Cyg X–1 in H \alpha ( provided by the INT Photometric H \alpha Survey of the Northern Galactic Plane ; IPHAS ) for evidence of the counter jet interacting with the surrounding medium .
Two candidate regions are identified , whose possible association with the jet could be confirmed with follow-up observations in [ S II ] and deeper observations in [ O III ] and radio .