540 research outputs found
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.Comment: accepted for publication in MNRAS additional supporting material can
be found at http://www.astro.uni-jena.de/~nina/supporting_info.pdf the
abstract has been adjusted to fit the length requirement (RXJ1856 =
RXJ1856.5-3754, RXJ0720 = RXJ0720.4-3125, U Sco = Upper Scorpius, Tr 10 =
Trumpler 10
Constraints on the Equation-of-State of neutron stars from nearby neutron star observations
We try to constrain the Equation-of-State (EoS) of supra-nuclear-density
matter in neutron stars (NSs) by observations of nearby NSs. There are seven
thermally emitting NSs known from X-ray and optical observations, the so-called
Magnificent Seven (M7), which are young (up to few Myrs), nearby (within a few
hundred pc), and radio-quiet with blackbody-like X-ray spectra, so that we can
observe their surfaces. As bright X-ray sources, we can determine their
rotational (pulse) period and their period derivative from X-ray timing. From
XMM and/or Chandra X-ray spectra, we can determine their temperature. With
precise astrometric observations using the Hubble Space Telescope, we can
determine their parallax (i.e. distance) and optical flux. From flux, distance,
and temperature, one can derive the emitting area - with assumptions about the
atmosphere and/or temperature distribution on the surface. This was recently
done by us for the two brightest M7 NSs RXJ1856 and RXJ0720. Then, from
identifying absorption lines in X-ray spectra, one can also try to determine
gravitational redshift. Also, from rotational phase-resolved spectroscopy, we
have for the first time determined the compactness (mass/radius) of the M7 NS
RBS1223. If also applied to RXJ1856, radius (from luminosity and temperature)
and compactness (from X-ray data) will yield the mass and radius - for the
first time for an isolated single neutron star. We will present our
observations and recent results.Comment: refereed NPA5 conference proceedings, in pres
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