The detection of Interstellar Objects passing through the Solar System offers the promise of constraining the physical and chemical processes involved in planetary formation in other extrasolar systems .
While the effect of outgassing by 1I/2017 U1 ( ’ Oumuamua ) was dynamically observed , no direct detection of the ejected material was made .
The discovery of the active interstellar comet 2I/Borisov means spectroscopic investigations of the sublimated ices is possible for this object .
We report the first detection of gas emitted by an interstellar comet via the near-UV emission of CN from 2I/Borisov at a heliocentric distance of r = 2.7 au on 2019 September 20 .
The production rate was found to be Q ( CN ) = ( 3.7 \pm 0.4 ) \times 10 ^ { 24 } s ^ { -1 } , using a simple Haser model with an outflow velocity of 0.5 km s ^ { -1 } .
No other emission was detected , with an upper limit to the production rate of C _ { 2 } of 4 \times 10 ^ { 24 } s ^ { -1 } .
The spectral reflectance slope of the dust coma over 3900 Å < \lambda < 6000 Å is steeper than at longer wavelengths , as found for other comets .
Broad band R _ { c } photometry on 2019 September 19 gave a dust production rate of Af \rho = 143 \pm 10 cm .
Modelling of the observed gas and dust production rates constrains the nuclear radius to 0.7 - 3.3 km assuming reasonable nuclear properties .
Overall , we find the gas , dust and nuclear properties for the first active Interstellar Object are similar to normal Solar System comets .