12,536 research outputs found
An analysis of the timing irregularities for 366 pulsars
We provide an analysis of timing irregularities observed for 366 pulsars.
Observations were obtained using the 76-m Lovell radio telescope at the Jodrell
Bank Observatory over the past 36 years. These data sets have allowed us to
carry out the first large-scale analysis of pulsar timing noise over time
scales of > 10yr, with multiple observing frequencies and for a large sample of
pulsars. Our sample includes both normal and recycled pulsars. The timing
residuals for the pulsars with the smallest characteristic ages are shown to be
dominated by the recovery from glitch events, whereas the timing irregularities
seen for older pulsars are quasi-periodic. We emphasise that previous models
that explained timing residuals as a low-frequency noise process are not
consistent with observation.Comment: Accepted by MNRAS. High resolution images available from the article
on AD
Gaia astrometry for stars with too few observations - a Bayesian approach
Gaia's astrometric solution aims to determine at least five parameters for
each star, together with appropriate estimates of their uncertainties and
correlations. This requires at least five distinct observations per star. In
the early data reductions the number of observations may be insufficient for a
five-parameter solution, and even after the full mission many stars will remain
under-observed, including faint stars at the detection limit and transient
objects. In such cases it is reasonable to determine only the two position
parameters. Their formal uncertainties would however grossly underestimate the
actual errors, due to the neglected parallax and proper motion. We aim to
develop a recipe to calculate sensible formal uncertainties that can be used in
all cases of under-observed stars. Prior information about the typical ranges
of stellar parallaxes and proper motions is incorporated in the astrometric
solution by means of Bayes' rule. Numerical simulations based on the Gaia
Universe Model Snapshot (GUMS) are used to investigate how the prior influences
the actual errors and formal uncertainties when different amounts of Gaia
observations are available. We develop a criterion for the optimum choice of
priors, apply it to a wide range of cases, and derive a global approximation of
the optimum prior as a function of magnitude and galactic coordinates. The
feasibility of the Bayesian approach is demonstrated through global astrometric
solutions of simulated Gaia observations. With an appropriate prior it is
possible to derive sensible positions with realistic error estimates for any
number of available observations. Even though this recipe works also for
well-observed stars it should not be used where a good five-parameter
astrometric solution can be obtained without a prior. Parallaxes and proper
motions from a solution using priors are always biased and should not be used.Comment: Revised version, accepted 21st of August 2015 for publication in A&
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