1,046 research outputs found
Model-independent reconstruction of the primordial curvature power spectrum from PTA data
Recently released data from pulsar timing array (PTA) collaborations provide
strong evidence for a stochastic signal consistent with a gravitational-wave
background, potentially originating from scalar-induced gravitational waves
(SIGWs). However, in order to determine whether the SIGWs with a specific power
spectrum of curvature perturbations can account for the PTA signal, one needs
to estimate the energy density of the SIGWs, which can be computationally
expensive. In this paper, we use a model-independent approach to reconstruct
the primordial curvature power spectrum using a free spectrum cross over from
to with NANOGrav
15-yrs data set. Our results can simplify the task of assessing whether a given
primordial curvature power spectrum can adequately explain the observed PTA
signal without calculating the energy density of SIGWs.Comment: 17 pages, 1 figur
Constraints on primordial curvature power spectrum with pulsar timing arrays
The stochastic signal detected by NANOGrav, PPTA, EPTA, and CPTA can be
explained by the scalar-induced gravitational waves. In order to determine the
scalar-induced gravitational waves model that best fits the stochastic signal,
we employ both single- and double-peak parameterizations for the power spectrum
of the primordial curvature perturbations, where the single-peak scenarios
include the -function, box, lognormal, and broken power law model, and
the double-peak scenario is described by the double lognormal form. Using
Bayesian inference, we find that there is no significant evidence for or
against the single-peak scenario over the double-peak model, with (Bayes
factors) among these models . Therefore, we are not able
to distinguish the different shapes of the power spectrum of the primordial
curvature perturbation with the current sensitivity of pulsar timing arrays.Comment: 19 pages, 1 table, 7 figure
Observational evidence for a spin-up line in the P-Pdot diagram of millisecond pulsars
It is believed that millisecond pulsars attain their fast spins by accreting
matter and angular momentum from companion stars. Theoretical modelling of the
accretion process suggests a spin-up line in the period-period derivative
(-) diagram of millisecond pulsars, which plays an important role
in population studies of radio millisecond pulsars and accreting neutron stars
in X-ray binaries. Here we present observational evidence for such a spin-up
line using a sample of 143 radio pulsars with < 30 ms. We also find that
PSRs~J18233021A and J18242452A, located near the classic spin-up line,
are consistent with the broad population of millisecond pulsars. Finally, we
show that our approach of Bayesian inference can probe accretion physics,
allowing constraints to be placed on the accretion rate and the
disk-magnetosphere interaction.Comment: 10 pages, 4 figures, 2 tables. Accepted for publication by ApJ
Old Story New Tell: The Graphite to Diamond Transition Revisited
Graphite and diamond are two well-known allotropes of carbon with distinct
physical properties due to different atomic connectivity. Graphite has a
layered structure in which the honeycomb carbon sheets can easily glide, while
atoms in diamond are strongly bonded in all three dimensions. The transition
from graphite to diamond has been a central subject in physical science. One
way to turn graphite into diamond is to apply the high pressure and high
temperature (HPHT) conditions. However, atomistic mechanism of this transition
is still under debate. From a series of large-scale molecular dynamics (MD)
simulations, we report a mechanism that the diamond nuclei originate at the
graphite grain boundaries and propagate in two preferred directions. In
addition to the widely accepted [001] direction, we found that the growth along
[120] direction of graphite is even faster. In this scenario, cubic diamond
(CD) is the kinetically favorable product, while hexagonal diamond (HD) would
appear as minor amounts of twinning structures in two main directions.
Following the crystallographic orientation relationship, the coherent interface
t-(100)gr//(11-1)cd + [010]gr//[1-10]cd was also confirmed by high-resolution
transmission electron microscopy (HR-TEM) experiment. The proposed phase
transition mechanism does not only reconcile the longstanding debate regarding
the role of HD in graphite-diamond transition, but also yields the atomistic
insight into microstructure engineering via controlled solid phase transition.Comment: 35 pages, 5 figure
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