arXiv subjects

Jacob Askew

Publications and source records attributed to Jacob Askew.

6 recordsLinked to original sources

arXiv2026

Dense Molecular Clumps with Large Blue Asymmetries: Evidence for Collapse

An analysis of the Millimetre Astronomy Legacy Team 90 GHz (MALT90) survey has produced a sample of 27 candidate dense molecular clumps with large collapse motions, as revealed by large ``blue'' asymmetrical line profiles of the optically thick \hcop\, line. %with respect to the optically thin \nthp\, line. New, more sensitive molecular line observations of this sample, conducted with the Mopra 22-m telescope, confirm the blue asymmetries in the \hcop\, line profiles, with large, positive values of the asymmetry parameter $A$ ($\bar{A}_{HCO^+} = 0.69\pm0.01$), and positive, but smaller asymmetries in the \hcn\, and \hnc\, lines: ($\bar{A}_{HCN} = 0.35\pm0.01$ and $\bar{A}_{HNC} = 0.28\pm0.01$), as expected for a less optically thick tracer in collapsing clumps. The small, positive mean asymmetry parameters for \cch\, and \htcop, $\bar{A}_{C_2H} = 0.15\pm0.02$ and $\bar{A}_{H^{13}CO^+} = 0.18\pm0.03$, likely indicate slightly optically thick emission for at least some clumps. The hyperfine ratios for \nthp\, are in their optically thin, LTE, values, but for \hcn\ they are not; the $F=1 \to 1$ hyperfine line shows abnormally weak intensities. A simple two-component model shows that self-absorption of the background $F = 1 \to 1$ hyperfine line by the main $F = 2 \to 1$ hyperfine line of a cold, foreground, redshifted cloud can reproduce the observed \hcn\, hyperfine intensities and match the \hcn\, and \hcop\, line profiles. All of these results are consistent with self-absorption of the optically thick lines on the red side of the profile, as expected for collapsing clumps. A simple two-cloud model suggests that this sample represents dense clumps with extreme collapse velocities, $V_{inf} \sim 2.4$ \kms.

astro-ph.GA↗

arXiv2024

The neutron star mass, distance, and inclination from precision timing of the brilliant millisecond pulsar J0437$-$4715

The observation of neutron stars enables the otherwise impossible study of fundamental physical processes. The timing of binary radio pulsars is particularly powerful, as it enables precise characterization of their (three-dimensional) positions and orbits. PSR~J0437$-$4715 is an important millisecond pulsar for timing array experiments and is also a primary target for the Neutron Star Interior Composition Explorer (NICER). The main aim of the NICER mission is to constrain the neutron star equation of state by inferring the compactness ($M_p/R$) of the star. Direct measurements of the mass $M_p$ from pulsar timing therefore substantially improve constraints on the radius $R$ and the equation of state. Here we use observations spanning 26 years from Murriyang, the 64-m Parkes radio telescope, to improve the timing model for this pulsar. Among the new precise measurements are the pulsar mass $M_p=1.418\pm 0.044$ $M_{\odot}$, distance $D=156.96 \pm 0.11$ pc, and orbital inclination angle $i=137.506 \pm 0.016^\circ$, which can be used to inform the X-ray pulse profile models inferred from NICER observations. We demonstrate that these results are consistent between multiple data sets from the Parkes Pulsar Timing Array (PPTA), each modeled with different noise assumptions. Using the longest available PPTA data set, we measure an apparent second derivative of the pulsar spin frequency and discuss how this can be explained either by kinematic effects due to the proper motion and radial velocity of the pulsar or excess low-frequency noise such as a gravitational-wave background.

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arXiv2023

The Parkes Pulsar Timing Array Third Data Release

We present the third data release from the Parkes Pulsar Timing Array (PPTA) project. The release contains observations of 32 pulsars obtained using the 64-m Parkes "Murriyang" radio telescope. The data span is up to 18 years with a typical cadence of 3 weeks. This data release is formed by combining an updated version of our second data release with $\sim 3$ years of more recent data primarily obtained using an ultra-wide-bandwidth receiver system that operates between 704 and 4032 MHz. We provide calibrated pulse profiles, flux-density dynamic spectra, pulse times of arrival, and initial pulsar timing models. We describe methods for processing such wide-bandwidth observations, and compare this data release with our previous release.

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arXiv2023

Search for an isotropic gravitational-wave background with the Parkes Pulsar Timing Array

Pulsar timing arrays aim to detect nanohertz-frequency gravitational waves (GWs). A background of GWs modulates pulsar arrival times and manifests as a stochastic process, common to all pulsars, with a signature spatial correlation. Here we describe a search for an isotropic stochastic gravitational-wave background (GWB) using observations of 30 millisecond pulsars from the third data release of the Parkes Pulsar Timing Array (PPTA), which spans 18 years. Using current Bayesian inference techniques we recover and characterize a common-spectrum noise process. Represented as a strain spectrum $h_c = A(f/1 {\rm yr}^{-1})^{\alpha}$, we measure $A=3.1^{+1.3}_{-0.9} \times 10^{-15}$ and $\alpha=-0.45 \pm 0.20$ respectively (median and 68% credible interval). For a spectral index of $\alpha=-2/3$, corresponding to an isotropic background of GWs radiated by inspiraling supermassive black hole binaries, we recover an amplitude of $A=2.04^{+0.25}_{-0.22} \times 10^{-15}$. However, we demonstrate that the apparent signal strength is time-dependent, as the first half of our data set can be used to place an upper limit on $A$ that is in tension with the inferred common-spectrum amplitude using the complete data set. We search for spatial correlations in the observations by hierarchically analyzing individual pulsar pairs, which also allows for significance validation through randomizing pulsar positions on the sky. For a process with $\alpha=-2/3$, we measure spatial correlations consistent with a GWB, with an estimated false-alarm probability of $p \lesssim 0.02$ (approx. $2\sigma$). The long timing baselines of the PPTA and the access to southern pulsars will continue to play an important role in the International Pulsar Timing Array.

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arXiv2023

The gravitational-wave background null hypothesis: Characterizing noise in millisecond pulsar arrival times with the Parkes Pulsar Timing Array

The noise in millisecond pulsar (MSP) timing data can include contributions from observing instruments, the interstellar medium, the solar wind, solar system ephemeris errors, and the pulsars themselves. The noise environment must be accurately characterized in order to form the null hypothesis from which signal models can be compared, including the signature induced by nanohertz-frequency gravitational waves (GWs). Here we describe the noise models developed for each of the MSPs in the Parkes Pulsar Timing Array (PPTA) third data release, which have been used as the basis of a search for the isotropic stochastic GW background. We model pulsar spin noise, dispersion measure variations, scattering variations, events in the pulsar magnetospheres, solar wind variability, and instrumental effects. We also search for new timing model parameters and detected Shapiro delays in PSR~J0614$-$3329 and PSR~J1902$-$5105. The noise and timing models are validated by testing the normalized and whitened timing residuals for Gaussianity and residual correlations with time. We demonstrate that the choice of noise models significantly affects the inferred properties of a common-spectrum process. Using our detailed models, the recovered common-spectrum noise in the PPTA is consistent with a power law with a spectral index of $\gamma=13/3$, the value predicted for a stochastic GW background from a population of supermassive black hole binaries driven solely by GW emission.

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arXiv2022

Analysis of the ionized interstellar medium and orbital dynamics of PSR~J1909-3744 using scintillation arcs

Long-term studies of binary millisecond pulsars (MSPs) provide precise tests of strong-field gravity and can be used to measure neutron-star masses. PSR~J1909$-$3744, a binary MSP has been the subject of several pulsar timing analyses. The edge-on orbit enables measurement of its mass using the Shapiro delay; however, there is degeneracy in the sense of the inclination angle, $i$, and multiple solutions for the longitude of ascending node, $Ω$. Radio pulsars scintillate due to inhomogeneities in the ionized interstellar medium (IISM). This can result in scintillation arcs in the power spectrum of the dynamic spectrum that can use these to study the interstellar medium and constrain binary pulsar orbits. Here, we study the scintillation of PSR~J1909-3744 using observations from the 64-m Parkes Radio Telescope (Murriyang) over $\approx$13\, years, using techniques to study scintillation in a lower signal-to-noise regime. By monitoring annual and orbital variations of the arc-curvature measurements we are able to characterise the velocity of the IISM. We find that the statistics of the IISM remained stationary over this time and a slightly anisotropic model (axial ratio $\gtrsim1.2$) is preferred. We measure the relative distance to a single dominant thin scattering screen at $s=0.49\pm0.04$, or $D_s=590\pm50$\,pc, with an angle of anisotropy $ζ=85\pm6^\circ$ (East of North) and velocity in the direction of anisotropy $V_{\textrm{IISM}, ζ}=14\pm10$\,km\,s$^{-1}$. By combining a physical model of the IISM and current pulsar timing results, we also constrain $Ω=225\pm3^\circ$ and $i=86.46\pm0.05^\circ$.

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