Origin of asymmetries in X-ray emission lines from the blast wave of the 2014 outburst of nova V745 Sco

Abstract

The symbiotic nova V745 Sco was observed in outburst on 2014 February 6. Its observations by the Chandra X-ray Observatory at days 16 and 17 have revealed a spectrum characterized by asymmetric and blue-shifted emission lines. Here we investigate the origin of these asymmetries through three-dimensional hydrodynamic simulations describing the outburst during the first 20 days of evolution. The model takes into account thermal conduction and radiative cooling and assumes a blast wave propagates through an equatorial density enhancement. From the simulations, we synthesize the X-ray emission and derive the spectra as they would be observed with Chandra. We find that both the blast wave and the ejecta distribution are efficiently collimated in polar directions due to the presence of the equatorial density enhancement. The majority of the X-ray emission originates from the interaction of the blast with the equatorial density enhancement and is concentrated on the equatorial plane as a ring-like structure. Our "best-fit" model requires a mass of ejecta in the outburst $M_{ej} \approx 3\times 10^{-7}\,M_{\odot}$ and an explosion energy $E_b \approx 3 \times 10^{43}$ erg and reproduces the distribution of emission measure vs temperature and the evolution of shock velocity and temperature inferred from the observations. The model predicts asymmetric and blue-shifted line profiles similar to those observed and explains their origin as due to substantial X-ray absorption of red-shifted emission by ejecta material. The comparison of predicted and observed Ne and O spectral line ratios reveals no signs of strong Ne enhancement and suggests the progenitor is a CO white dwarf.