The origin of RXJ1856.5-3754 and RXJ0720.4-3125 -- updated using new
  parallax measurements
  RXJ1856 and RXJ0720 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 RXJ1856 and RXJ0720 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 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. Using the most recent parallax measurement of
8.16+/-0.80 mas for RXJ1856, we find that it originated in the U Sco
association 0.46+/-0.05 Myr ago. This is slightly larger than the value we
reported earlier (0.3 Myr). Our result is strongly supported by its current
radial velocity that we predict to be 6+19-20 km/s. This implies an inclination
angle of 88+/-6 deg consistent with the bow shock. No suitable runaway star was
found to be a potential former companion of RXJ1856. Making use of a recent
parallax measurement for RXJ0720 of 3.6+/-1.6 mas, we find that this NS was
possibly born in Tr 10 0.85+/-0.15 Myr ago. This is somewhat larger than the
one obtained using the old parallax value (0.5 Myr). We suggest the B0 runaway
supergiant HIP 43158 as a candidate for a former companion. Then, the current
distance of RXJ0720 to the Sun should be 286+27-23 pc, in agreement with recent
measurements. We then expect the radial velocity of RXJ0720 to be -76+34-17
km/s.
