14 research outputs found
X-PSI Parameter Recovery for Temperature Map Configurations Inspired by PSR J0030+0451
In the last few years, the NICER collaboration has provided mass and radius
inferences, via pulse profile modeling, for two pulsars: PSR J0030+0451 and PSR
J0740+6620. Given the importance of these results for constraining the equation
of state of dense nuclear matter, it is crucial to validate them and test their
robustness. We therefore explore the reliability of these results and their
sensitivity to analysis settings and random processes, including noise,
focusing on the specific case of PSR J0030+0451. We use X-PSI, one of the two
main analysis pipelines currently employed by the NICER collaboration for mass
and radius inferences. With synthetic data that mimic the PSR J0030+0451 NICER
data set, we evaluate the recovery performances of X-PSI under conditions never
tested before, including complex modeling of the thermally emitting neutron
star surface. For the test cases explored, our results suggest that X-PSI is
capable of recovering the true mass and radius within reasonable credible
intervals. This work also reveals the main vulnerabilities of the analysis: a
significant dependence on noise and the presence of multi-modal structure in
the posterior surface. Noise particularly impacts our sensitivity to the
analysis settings and widths of the posterior distributions. The multi-modal
structure in the posterior suggests that biases could be present if the
analysis is unable to exhaustively explore the parameter space. Convergence
testing, to ensure an adequate coverage of the parameter space and a suitable
representation of the posterior distribution, is one possible solution to these
challenges.Comment: 27 pages, 13 figure
Pulse Profile Modeling of Thermonuclear Burst Oscillations I: The Effect of Neglecting Variability
We study the effects of the time-variable properties of thermonuclear X-ray
bursts on modeling their millisecond-period burst oscillations. We apply the
pulse profile modeling technique that is being used in the analysis of
rotation-powered millisecond pulsars by the Neutron Star Interior Composition
Explorer (NICER) to infer masses, radii, and geometric parameters of neutron
stars. By simulating and analyzing a large set of models, we show that
overlooking burst time-scale variability in temperatures and sizes of the hot
emitting regions can result in substantial bias in the inferred mass and
radius. To adequately infer neutron star properties, it is essential to develop
a model for the time variable properties or invest a substantial amount of
computational time in segmenting the data into non-varying pieces. We discuss
prospects for constraints from proposed future X-ray telescopes.Comment: Accepted for publication in MNRA
Pulse Profile Modelling of Thermonuclear Burst Oscillations II: Handling variability
Pulse profile modelling is a relativistic ray-tracing technique that can be
used to infer masses, radii and geometric parameters of neutron stars. In a
previous study, we looked at the performance of this technique when applied to
thermonuclear burst oscillations from accreting neutron stars. That study
showed that ignoring the variability associated with burst oscillation sources
resulted in significant biases in the inferred mass and radius, particularly
for the high count rates that are nominally required to obtain meaningful
constraints. In this follow-on study, we show that the bias can be mitigated by
slicing the bursts into shorter segments where variability can be neglected,
and jointly fitting the segments. Using this approach, the systematic
uncertainties on the mass and radius are brought within the range of the
statistical uncertainty. With about 10 source counts, this yields
uncertainties of approximately 10% for both the mass and radius. However, this
modelling strategy requires substantial computational resources. We also
confirm that the posterior distributions of the mass and radius obtained from
multiple bursts of the same source can be merged to produce outcomes comparable
to that of a single burst with an equivalent total number of counts.Comment: submitted to MNRAS. The Zenodo link will go public after peer review.
Comments are welcom
Atmospheric Effects on Neutron Star Parameter Constraints with NICER
We present an analysis of the effects of uncertainties in the atmosphere
models on the radius, mass, and other neutron star parameter constraints for
the NICER observations of rotation-powered millisecond pulsars. To date, NICER
has applied the X-ray pulse profile modeling technique to two
millisecond-period pulsars: PSR J0030+0451 and the high-mass pulsar PSR
J0740+6620. These studies have commonly assumed a deep-heated fully-ionized
hydrogen atmosphere model, although they have explored the effects of
partial-ionization and helium composition in some cases. Here we extend that
exploration and also include new models with partially ionized carbon
composition, externally heated hydrogen, and an empirical atmospheric beaming
parametrization to explore deviations in the expected anisotropy of the emitted
radiation. None of the studied atmosphere cases have any significant influence
on the inferred radius of PSR J0740+6620, possibly due to its X-ray faintness,
tighter external constraints, and/or viewing geometry. In the case of PSR
J0030+0451 both the composition and ionization state could significantly alter
the inferred radius. However, based on the evidence (prior predictive
probability of the data), partially ionized hydrogen and carbon atmospheres are
disfavored. The difference in the evidence for ionized hydrogen and helium
atmospheres is too small to be decisive for most cases, but the inferred radius
for helium models trends to larger sizes around or above 14-15 km. External
heating or deviations in the beaming that are less than at emission
angles smaller than 60 degrees, on the other hand, have no significant effect
on the inferred radius.Comment: 26 pages, 12 figures (2 of which are figure sets), 3 tables. Accepted
for publication in Ap
Recruitment of Glycosyl Hydrolase Proteins in a Cone Snail Venomous Arsenal: Further Insights into Biomolecular Features of Conus Venoms
Cone snail venoms are considered an untapped reservoir of extremely diverse peptides, named conopeptides, displaying a wide array of pharmacological activities. We report here for the first time, the presence of high molecular weight compounds that participate in the envenomation cocktail used by these marine snails. Using a combination of proteomic and transcriptomic approaches, we identified glycosyl hydrolase proteins, of the hyaluronidase type (Hyal), from the dissected and injectable venoms (“injectable venom” stands for the venom variety obtained by milking of the snails. This is in contrast to the “dissected venom”, which was obtained from dissected snails by extraction of the venom glands) of a fish-hunting cone snail, Conus consors (Pionoconus clade). The major Hyal isoform, Conohyal-Cn1, is expressed as a mixture of numerous glycosylated proteins in the 50 kDa molecular mass range, as observed in 2D gel and mass spectrometry analyses. Further proteomic analysis and venom duct mRNA sequencing allowed full sequence determination. Additionally, unambiguous segment location of at least three glycosylation sites could be determined, with glycans corresponding to multiple hexose (Hex) and N-acetylhexosamine (HexNAc) moieties. With respect to other known Hyals, Conohyal-Cn1 clearly belongs to the hydrolase-type of Hyals, with strictly conserved consensus catalytic donor and positioning residues. Potent biological activity of the native Conohyals could be confirmed in degrading hyaluronic acid. A similar Hyal sequence was also found in the venom duct transcriptome of C. adamsonii (Textilia clade), implying a possible widespread recruitment of this enzyme family in fish-hunting cone snail venoms. These results provide the first detailed Hyal sequence characterized from a cone snail venom, and to a larger extent in the Mollusca phylum, thus extending our knowledge on this protein family and its evolutionary selection in marine snail venoms
X-PSI Parameter Recovery for Temperature Map Configurations Inspired by PSR J0030+0451
In the last few years, the NICER collaboration has provided mass and radius inferences, via pulse profile modeling, for two pulsars: PSR J0030+0451 and PSR J0740+6620. Given the importance of these results for constraining the equation of state of dense nuclear matter, it is crucial to validate them and test their robustness. We therefore explore the reliability of these results and their sensitivity to analysis settings and random processes, including noise, focusing on the specific case of PSR J0030+0451. We use X-ray Pulse Simulation and Inference (X-PSI), one of the two main analysis pipelines currently employed by the NICER collaboration for mass and radius inferences. With synthetic data that mimic the PSR J0030+0451 NICER data set, we evaluate the recovery performances of X-PSI under conditions not previously tested, including complex modeling of the thermally emitting neutron star surface. For the test cases explored, our results suggest that X-PSI is capable of recovering the true mass and radius within reasonable credible intervals. This work also reveals the main vulnerabilities of the analysis: a significant dependence on noise and the presence of multimodal structure in the posterior surface. Noise particularly impacts our sensitivity to the analysis settings and widths of the posterior distributions. The multimodal structure in the posterior suggests that biases could be present if the analysis is unable to exhaustively explore the parameter space. Convergence testing, to ensure an adequate coverage of the parameter space and a suitable representation of the posterior distribution, is one possible solution to these challenges
X-PSI: A Python package for neutron star X-ray pulse simulation and inference
International audienc
Atmospheric Effects on Neutron Star Parameter Constraints with NICER
International audienceWe present an analysis of the effects of uncertainties in the atmosphere models on the radius, mass, and other neutron star parameter constraints for the NICER observations of rotation-powered millisecond pulsars. To date, NICER has applied the X-ray pulse profile modeling technique to two millisecond-period pulsars: PSR J0030+0451 and the high-mass pulsar PSR J0740+6620. These studies have commonly assumed a deep-heated fully-ionized hydrogen atmosphere model, although they have explored the effects of partial-ionization and helium composition in some cases. Here we extend that exploration and also include new models with partially ionized carbon composition, externally heated hydrogen, and an empirical atmospheric beaming parametrization to explore deviations in the expected anisotropy of the emitted radiation. None of the studied atmosphere cases have any significant influence on the inferred radius of PSR J0740+6620, possibly due to its X-ray faintness, tighter external constraints, and/or viewing geometry. In the case of PSR J0030+0451 both the composition and ionization state could significantly alter the inferred radius. However, based on the evidence (prior predictive probability of the data), partially ionized hydrogen and carbon atmospheres are disfavored. The difference in the evidence for ionized hydrogen and helium atmospheres is too small to be decisive for most cases, but the inferred radius for helium models trends to larger sizes around or above 14-15 km. External heating or deviations in the beaming that are less than at emission angles smaller than 60 degrees, on the other hand, have no significant effect on the inferred radius