arXiv subjects

Alexander W. Raymond

Publications and source records attributed to Alexander W. Raymond.

7 recordsLinked to original sources

arXiv2025

Modeling Low-Temperature Plasmas Simulating Titan's Atmosphere

In the study presented here, we model the gas phase chemistry induced by plasma discharge at low temperature (150 K) in the NASA Ames COSmIC Simulation Chamber (COSmIC) using a 1-dimensional multi-fluid plasma model named CO-PRISM (COSmIC Plasma Reactivity and Ionization Simulation Model). Our model incorporates an extensive chemical reaction network to simulate the neutral-neutral and ion-neutral reactions occurring in the COSmIC experiments when using N2-CH4-based gas mixtures relevant to Titan's atmosphere. Our reaction network now includes crucial reactions involving the first electronically-excited state of atomic nitrogen, recent electron collision cross-sections, and radical chemistry. In particular, we have investigated the influence of C2H2 on the gas phase polymeric growth and the elemental composition of the chemical products, and we have compared our findings to recently published solid phase analyses. The modeling results are consistent with experimental measurements of N2-CH4-C2H2 plasmas on COSmIC, showing the production of C6Hx intermediates and precursors of larger organics, as well as methanimine in small concentration. Our numerical results point to cationic pathways enabling efficient intermediate-sized and nitrogen-rich \ce{C2H2}-driven chemistry driving tholin production. Comparison of the modeled gas phase elemental composition with elemental composition of the solid phase samples produced in COSmIC reveal similar trends, with C/N increasing when C2H2 is present in the gas mixture. Finally, our results demonstrate the importance of such synergistic studies using low-temperature plasma chemistry experiments combined with modeling efforts to improve our understanding of cold planetary environments.

physics.plasm-ph↗

arXiv2024

First Very Long Baseline Interferometry Detections at 870{\mu}m

The first very long baseline interferometry (VLBI) detections at 870$\mu$m wavelength (345$\,$GHz frequency) are reported, achieving the highest diffraction-limited angular resolution yet obtained from the surface of the Earth, and the highest-frequency example of the VLBI technique to date. These include strong detections for multiple sources observed on inter-continental baselines between telescopes in Chile, Hawaii, and Spain, obtained during observations in October 2018. The longest-baseline detections approach 11$\,$G$\lambda$ corresponding to an angular resolution, or fringe spacing, of 19$\mu$as. The Allan deviation of the visibility phase at 870$\mu$m is comparable to that at 1.3$\,$mm on the relevant integration time scales between 2 and 100$\,$s. The detections confirm that the sensitivity and signal chain stability of stations in the Event Horizon Telescope (EHT) array are suitable for VLBI observations at 870$\mu$m. Operation at this short wavelength, combined with anticipated enhancements of the EHT, will lead to a unique high angular resolution instrument for black hole studies, capable of resolving the event horizons of supermassive black holes in both space and time.

astro-ph.IM↗

arXiv2023

A search for pulsars around Sgr A* in the first Event Horizon Telescope dataset

The Event Horizon Telescope (EHT) observed in 2017 the supermassive black hole at the center of the Milky Way, Sagittarius A* (Sgr A*), at a frequency of 228.1 GHz ($\lambda$=1.3 mm). The fundamental physics tests that even a single pulsar orbiting Sgr A* would enable motivate searching for pulsars in EHT datasets. The high observing frequency means that pulsars - which typically exhibit steep emission spectra - are expected to be very faint. However, it also negates pulse scattering, an effect that could hinder pulsar detections in the Galactic Center. Additionally, magnetars or a secondary inverse Compton emission could be stronger at millimeter wavelengths than at lower frequencies. We present a search for pulsars close to Sgr A* using the data from the three most-sensitive stations in the EHT 2017 campaign: the Atacama Large Millimeter/submillimeter Array, the Large Millimeter Telescope and the IRAM 30 m Telescope. We apply three detection methods based on Fourier-domain analysis, the Fast-Folding-Algorithm and single pulse search targeting both pulsars and burst-like transient emission; using the simultaneity of the observations to confirm potential candidates. No new pulsars or significant bursts were found. Being the first pulsar search ever carried out at such high radio frequencies, we detail our analysis methods and give a detailed estimation of the sensitivity of the search. We conclude that the EHT 2017 observations are only sensitive to a small fraction ($\lesssim$2.2%) of the pulsars that may exist close to Sgr A*, motivating further searches for fainter pulsars in the region.

astro-ph.HE↗

arXiv2023

Reference Array and Design Consideration for the next-generation Event Horizon Telescope

We describe the process to design, architect, and implement a transformative enhancement of the Event Horizon Telescope (ngEHT). This program - the next-generation Event Horizon Telescope (ngEHT) - will form a networked global array of radio dishes capable of making high-fidelity real-time movies of supermassive black holes (SMBH) and their emanating jets. This builds upon the EHT principally by deploying additional modest-diameter dishes to optimized geographic locations to enhance the current global mm/submm wavelength Very Long Baseline Interferometric (VLBI) array, which has, to date, utilized mostly pre-existing radio telescopes. The ngEHT program further focuses on observing at three frequencies simultaneously for increased sensitivity and Fourier spatial frequency coverage. Here, the concept, science goals, design considerations, station siting and instrument prototyping are discussed, and a preliminary reference array to be implemented in phases is described.

astro-ph.IM↗

arXiv2023

Enabling Transformational ngEHT Science via the Inclusion of 86 GHz Capabilities

We present a case for significantly enhancing the utility and efficiency of the ngEHT by incorporating an additional 86 GHz observing band. In contrast to 230 or 345 GHz, weather conditions at the ngEHT sites are reliably good enough for 86 GHz to enable year-round observations. Multi-frequency imaging that incorporates 86 GHz observations would sufficiently augment the ($u,v$) coverage at 230 and 345 GHz to permit detection of the M87 jet structure without requiring EHT stations to join the array. The general calibration and sensitivity of the ngEHT would also be enhanced by leveraging frequency phase transfer techniques, whereby simultaneous observations at 86 GHz and higher-frequency bands have the potential to increase the effective coherence times from a few seconds to tens of minutes. When observation at the higher frequencies is not possible, there are opportunities for standalone 86 GHz science, such as studies of black hole jets and spectral lines. Finally, the addition of 86 GHz capabilities to the ngEHT would enable it to integrate into a community of other VLBI facilities $-$ such as the GMVA and ngVLA $-$ that are expected to operate at 86 GHz but not at the higher ngEHT observing frequencies.

astro-ph.IM↗

arXiv2021

Evaluation of New Submillimeter VLBI Sites for the Event Horizon Telescope

The Event Horizon Telescope (EHT) is a very long baseline interferometer built to image supermassive black holes on event-horizon scales. In this paper, we investigate candidate sites for an expanded EHT array with improved imaging capabilities. We use historical meteorology and radiative transfer analysis to evaluate site performance. Most of the existing sites in the EHT array have median zenith opacity less than 0.2 at 230 GHz during the March/April observing season. Seven of the existing EHT sites have 345 GHz opacity less than 0.5 during observing months. Out of more than forty candidate new locations analyzed, approximately half have 230 GHz opacity comparable to the existing EHT sites, and at least seventeen of the candidate sites would be comparably good for 345 GHz observing. A group of new sites with favorable transmittance and geographic placement leads to greatly enhanced imaging and science on horizon scales.

astro-ph.IM↗

arXiv2019

Prospects for Wideband VLBI Correlation in the Cloud

This paper proposes a cloud architecture for the correlation of wide bandwidth VLBI data. Cloud correlation facilitates processing of entire experiments in parallel using flexibly allocated and practically unlimited compute resources. This approach offers a potential improvement over dedicated correlation clusters, which are constrained by a fixed number of installed processor nodes and playback units. Additionally, cloud storage offers an alternative to maintaining a fleet of hard-disk drives that might be utilized intermittently. We describe benchmarks of VLBI correlation using the DiFX-2.5.2 software on the Google Cloud Platform to assess cloud-based correlation performance. The number of virtual CPUs per Virtual Machine was varied to determine the optimum configuration of cloud resources. The number of stations was varied to determine the scaling of correlation time with VLBI arrays of different sizes. Data transfer rates from Google Cloud Storage to the Virtual Machines performing the correlation were also measured. We also present an example cloud correlation configuration. Current cloud service and equipment pricing data is used to compile cost estimates allowing an approximate economic comparison between cloud and cluster processing. The economic comparisons are based on cost figures which are a moving target, and are highly dependent on factors such as the utilization of cluster and media, which are a challenge to estimate. Our model suggests that shifting to the cloud is an alternative path for high data rate, low duty cycle wideband VLBI correlation that should continue to be explored. In the production phase of VLBI correlation, the cloud has the potential to significantly reduce data processing times and allow the processing of more science experiments in a given year for the petabyte-scale data sets increasingly common in both astronomy and geodesy VLBI applications.

astro-ph.IM↗