arXiv subjects

J. Askew

Publications and source records attributed to J. Askew.

3 recordsLinked to original sources

arXiv2025

Scintillating insights into PSR~J0737$-$3039A and the interstellar plasma of the Gum Nebula from MeerKAT

The double pulsar system PSR~J0737$-$3039A/B has enabled some of the most precise tests of strong-field gravity to date. Here, we present a scintillation analysis of the system based on an 18-month observation campaign with the MeerKAT radio telescope. We characterise this interference pattern to infer properties of scattering plasma and the orbital geometry of the system. Our preferred model supports a scattering screen located at a distance of $D_s = 360^{+30}_{-40}$ pc. This moderately anisotropic screen of ionized gas (axial ratio $A_R = 2.4 \pm 0.2$) lies near the edge of the Gum Nebula, which is believed to be a supernova remnant (SNR) or an \HII\, region. We estimate the expansion velocity of the nebula to be $V_{\textrm{s}} = 35 \pm 5$ km s$^{-1}$, implying a SNR age of $t \approx 1$ Myr. We also constrain the orbital orientation and inclination sense of the double pulsar to be $\Omega = 40^{\circ} \pm 3^{\circ}$ and $i > 90^{\circ}$, respectively. Assuming standard scattering geometry, our model yields a distance estimate consistent with the parallax-derived value of $D = 770 \pm 70$ pc from very long baseline interferometry. We conclude by discussing how future models of pulsar scintillation can enhance our understanding of the IISM and the properties of pulsars embedded within or lying behind such intervening structures.

astro-ph.HE↗

arXiv2024

Absorption and Self-Absorption of [C II] and [O I] Far Infrared Lines Towards a Bright Bubble in the Nessie Infrared Dark Cloud

Using the upGREAT instrument on SOFIA, we have imaged [C II] 157.74 and [O I] 63.18 micron line emission from a bright photodissociation region (PDR) associated with an ionized ``bubble'' located in the Nessie Nebula, a filamentary infrared dark cloud. A comparison with ATCA data reveals a classic PDR structure, with a uniform progression from ionized gas, to photodissociated gas, and on to molecular gas from the bubble's interior to its exterior. [O I] line emission from the bubble's PDR reveals self-absorption features. Toward a FIR-bright protostar, both [O I] and [C II] show an absorption feature at a velocity of $-18$ km/s, the same velocity as an unrelated foreground molecular cloud. Since the gas density in typical molecular clouds is well below the [O I] and [C II] critical densities, the excitation temperatures for both lines are low (~20 K). The Meudon models demonstrate that the surface of a molecular cloud, externally illuminated by a standard G_0 = 1 interstellar radiation field, can produce absorption features in both transitions. Thus, the commonly observed [O I] and [C II] self-absorption and absorption features plausibly arise from the subthermally excited, externally illuminated, photodissociated envelopes of molecular clouds. The luminous young stellar object AGAL337.916-00.477, located precisely where the expanding bubble strikes the Nessie filament, is associated with two shock tracers: NH3 (3,3) maser emission and SiO 2-1 emission, indicating interaction between the bubble and the filament. The interaction of the expanding bubble with its parental dense filament has triggered star formation.

astro-ph.GA↗

arXiv2023

Comparing recent PTA results on the nanohertz stochastic gravitational wave background

The Australian, Chinese, European, Indian, and North American pulsar timing array (PTA) collaborations recently reported, at varying levels, evidence for the presence of a nanohertz gravitational wave background (GWB). Given that each PTA made different choices in modeling their data, we perform a comparison of the GWB and individual pulsar noise parameters across the results reported from the PTAs that constitute the International Pulsar Timing Array (IPTA). We show that despite making different modeling choices, there is no significant difference in the GWB parameters that are measured by the different PTAs, agreeing within $1σ$. The pulsar noise parameters are also consistent between different PTAs for the majority of the pulsars included in these analyses. We bridge the differences in modeling choices by adopting a standardized noise model for all pulsars and PTAs, finding that under this model there is a reduction in the tension in the pulsar noise parameters. As part of this reanalysis, we "extended" each PTA's data set by adding extra pulsars that were not timed by that PTA. Under these extensions, we find better constraints on the GWB amplitude and a higher signal-to-noise ratio for the Hellings and Downs correlations. These extensions serve as a prelude to the benefits offered by a full combination of data across all pulsars in the IPTA, i.e., the IPTA's Data Release 3, which will involve not just adding in additional pulsars, but also including data from all three PTAs where any given pulsar is timed by more than as single PTA.

astro-ph.HE↗