Pre-perihelion observations of long-period comet C/2013 X1 (PANSTARRS)

We present and analyse the results of quasi-simultaneous spectroscopic, photometric, and polarimetric observations of hyperbolic comet C/2013 X1 (PANSTARRS) obtained at the 6-m Big Telescope Alt-azimuth (BTA) telescope Special Astrophysical Observatory (SAO) and 2.6-m Shajn telescope Crimean Astrophysical Observatory (CrAO). A wide fan-shaped structure and a weak tail were detected in the comet. The mean V - R colour of the coma was estimated to be neutral compared to the solar value. The Af rho parameter, a proxy to the dust production in the comet, was about 1672 +/- 21 cm in the R band. Emissions of the CN, C-2, C-3, and NH2 molecules were identified in the cometary spectrum, which covers the wavelength range 3800 - 7100 & Aring;. When the comet was at a distance of 2.66 au from the Sun, the minimum degree of polarization of about -1.5 per cent was detected in the near-nucleus region of the coma, in the range up to about 10 000 km from the optocentre. Further, polarization gradually increased (in absolute value) with distance from the nucleus, reaching -6.5 per cent at about 50 000 km. To reproduce the observed values of linear polarization and the phase-angle dependence of polarization for long-period comets, we used the Sh-matrix method with conjugated Gaussian random particles as light scatters, and the chemical composition of dust particles in the coma of 74 per cent amorphous carbon, 25 per cent of Mg-rich silicates, and 1 per cent of water ice.