Long-term activity and outburst of comet C/2013 A1 (Siding Spring) from narrow-band photometry and long-slit spectroscopy

We present a unique data set of more than one year's worth of regular observations of comet C/2013 A1(Siding Spring) taken with TRAPPIST, along with low-resolution spectra obtained with the ESO/VLT FORS 2 instrument. The comet made a close approach to Mars on October 19, 2014, and was then observed by many space-borne and ground-based telescopes. We followed the evolution of the OH, NH, CN, C-3, and C-2 production rates as well as the Af rho parameter, a proxy for the dust production. We detected an outburst two weeks after perihelion, with gas and dust production rates increased by a factor of five within a few days. By modelling the shape of the CN and C-2 radial profiles, we determined that the outburst happened on November 10 around 15:30 UT (+/- 5 h) and measured a gas expansion velocity of 1.1 +/- 0.2 kms(-1). We used a thermal evolution model to reproduce the activity pattern and outburst. Our results are consistent with the progressive formation of a dust mantle explaining the shallow dependence of gas production rates, which may be partially blown off during the outburst. We studied the evolution of gas composition, using various ratios such as CN/OH, C-2/OH, or C-3/OH, which showed little or no variation with heliocentric distance, including at the time of the outburst. This indicates a relative level of homogeneity of the nucleus composition.