2011MNRAS.417..617T

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 obtained. 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±0.80 mas for RX J1856.5-3754 by (Walter et al.), we find that it originated in the Upper Scorpius association 0.46±0.05 Myr ago. This kinematic age is slightly larger than the value we reported earlier (0.3 Myr) using the old parallax value of 5.6±0.6 mas by (Kaplan). Our result is strongly supported by its current radial velocity which we predict to be 6⁺¹⁹_- 20_ km/s. This implies an inclination angle to the line of sight of 88°± 6° consistent with estimates by (van Kerkwijk & Kulkarni) from the bow shock. No suitable runaway star was found to be a potential former companion of RX J1856.5-3754.

Making use of a recent parallax measurement for RX J0720.4-3125 of 3.6±1.6 mas by (Eisenbeiss), we find that this NS was possibly born in Trumpler 10 0.85±0.15 Myr ago. This kinematic age is somewhat larger than the one obtained using the old parallax value of 2.77±1.29 mas by (Kaplan et al.) (0.5 Myr). We suggest the B0 runaway supergiant HIP 43158 as a candidate for a former companion of the progenitor star. Then, the current distance of RX J0720.4-3125 to the Sun should be 286⁺²⁷_- 23_pc, in agreement with recent measurements. We then expect the radial velocity of RX J0720.4-3125 to be -76⁺³⁴_- 17_ km/s.