We observed the interstellar comet 2I/Borisov using the Neil Gehrels- Swift Observatory ’ s Ultraviolet/Optical Telescope .
We obtained images of the OH gas and dust surrounding the nucleus at four epochs spaced before and just after perihelion ( -2.56 AU to 2.03 AU ) .
Water production rates increased steadily before perihelion from ( 7.0 \pm 1.5 ) \times 10 ^ { 26 } molecules s ^ { -1 } on Nov. 1 , 2019 to ( 10.7 \pm 1.2 ) \times 10 ^ { 26 } molecules s ^ { -1 } on Dec. 1 .
This rate of increase in water production rate is slower than that of most Jupiter-family comets and quicker than most dynamically new comets .
After perihelion , the water production rate decreased rapidly to ( 4.9 \pm 0.9 ) \times 10 ^ { 26 } molecules s ^ { -1 } on Dec. 21 .
Our sublimation model constrains the minimum radius of the nucleus to 0.37 km , and indicates an active fraction of at least 55 % of the surface . A ( 0 ) f \rho calculations show variation between 90 and 106 cm with a slight trend peaking before the perihelion , lower than previous and concurrent published values .
The observations confirm that 2I/Borisov is carbon-chain depleted and enriched in NH _ { 2 } relative to water .