The origin of RXJ1856.5-3754 and RXJ0720.4-3125 – updated using new parallax measurements

RX J1856.5-3754 and RX J0720.4-3125 are the only young isolated radio-quiet neutron stars (NSs) for which trigonometric parallaxes were measured. Due to detection of their thermal emission in X-rays they are important to study NS cooling and to probe theoretical cooling models. Hence, a precise determination of their age is essential. Recently, new parallax measurements of RX J1856.5-3754 and RX J0720.4-3125 were ob-tained. Considering that NSs may originate from binary systems that got disrupted due to an asymmetric supernova, we attempt to identify runaway stars which may have been former companions to the NS progenitors. Such an identification would strongly support a particular birth scenario with time and place. We trace back each NS, runaway star and the centres of possible birth associations (assuming that most NSs are ejected directly from their parent association) to find close encounters. The kinematic age is then given by the time since the encounter. We use Monte Carlo simulations to account for observational uncertainties and evaluate the outcome statistically. Using the most recent parallax measurement of 8.16±0.80mas for RX J1856.5-3754 by Wal