2024MNRAS.529.3521P

The discovery of interstellar comet 2I/Borisov offered the unique opportunity to obtain a detailed analysis of an object coming from another planetary system, and leaving behind material in our interplanetary space. We continuously observed 2I/Borisov between 2019 October 3 and December 13 using the 1.52-m Telescopio Carlos Sánchez equipped with Multicolour Simultaneous Camera for studying Atmospheres of Transiting exoplanets 2 instrument, and the 2.54-m Isaac Newton Telescope with Wide Field Camera. We characterize its morphology and spectro-photometric features using the data gathered during this extended campaign. Simultaneous imaging in four bands (g, r, i, and z_s) reveals a homogeneous composition and a reddish hue, resembling Solar system comets, and as well a diffuse profile exhibiting familiar cometary traits. We discern a stationary trend fluctuating around a constant activity level throughout 2019 October and November. Subsequently, a reduction in activity is observed in December. Dust production and mass-loss calculations indicate approximately an average of 4 kg s^–1 before perihelion, while after perihelion the net mass-loss is about 0.6 kg s^–1. Our simulations indicate the most probable size of coma dust particles should be in the range 200-250 nm, and the terminal speed around 300 m s^–1. The spectrum acquired with the 4.2-m William Herschel Telescope shows the presence of a strong CN line for which we find a gas production rate of 1.2 × 10²⁴ s^–1. We also detected NH₂ and OI bands. The ratio between NH₂ and CN productions is log (NH₂/CN) = -0.2. Overall, this observing campaign provides a new understanding of 2I/Borisov's unique characteristics and activity patterns.