arXiv subjects

Jack O. Burns

Publications and source records attributed to Jack O. Burns.

At least 37 records · Page 2Linked to original sources

arXiv2020

Transformative Science from the Lunar Farside: Observations of the Dark Ages and Exoplanetary Systems at Low Radio Frequencies

The farside of the Moon is a pristine, quiet platform to conduct low radio frequency observations of the early Universe's Dark Ages, as well as space weather and magnetospheres associated with habitable exoplanets. In this paper, the astrophysics associated with NASA-funded concept studies will be described including a lunar-orbiting spacecraft, DAPPER, that will measure the 21 cm global spectrum at redshifts 40-80, and an array of low frequency dipoles on the lunar farside surface, FARSIDE, that would detect exoplanet magnetic fields. DAPPER observations (17-38 MHz), using a single cross-dipole antenna, will measure the amplitude of the 21 cm spectrum to the level required to distinguish the standard ΛCDM cosmological model from those of additional cooling models possibly produced by exotic physics such as dark matter interactions. FARSIDE has a notional architecture consisting of 128 dipole antennas deployed across a 10 km area by a rover. FARSIDE would image the entire sky each minute in 1400 channels over 0.1-40 MHz. This would enable monitoring of the nearest stellar systems for the radio signatures of coronal mass ejections and energetic particle events, and would also detect the magnetospheres of the nearest candidate habitable exoplanets. In addition, FARSIDE would determine the Dark Ages global 21 cm signal at yet lower frequencies and provide a pathfinder for power spectrum measurements.

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arXiv2020

Characterizing the Radio Quiet Region Behind the Lunar Farside for Low Radio Frequency Experiments

Low radio frequency experiments performed on Earth are contaminated by both ionospheric effects and radio frequency interference (RFI) from Earth-based sources. The lunar farside provides a unique environment above the ionosphere where RFI is heavily attenuated by the presence of the Moon. We present electrodynamics simulations of the propagation of radio waves around and through the Moon in order to characterize the level of attenuation on the farside. The simulations are performed for a range of frequencies up to 100 kHz, assuming a spherical lunar shape with an average, constant density. Additionally, we investigate the role of the topography and density profile of the Moon in the propagation of radio waves and find only small effects on the intensity of RFI. Due to the computational demands of performing simulations at higher frequencies, we propose a model for extrapolating the width of the quiet region above 100 kHz that also takes into account height above the lunar surface as well as the intensity threshold chosen to define the quiet region. This model, which we make publicly available through a Python package, allows the size of the radio quiet region to be easily calculated both in orbit or on the surface, making it directly applicable for lunar satellites as well as surface missions.

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arXiv2020

Formulating and critically examining the assumptions of global 21-cm signal analyses: How to avoid the false troughs that can appear in single spectrum fits

The assumptions inherent to global 21-cm signal analyses are rarely delineated. In this paper, we formulate a general list of suppositions underlying a given claimed detection of the global 21-cm signal. Then, we specify the form of these assumptions for two different analyses: 1) the one performed by the EDGES team showing an absorption trough in brightness temperature that they modeled separately from the sky foreground and 2) a new, so-called Minimum Assumption Analysis (MAA), that makes the most conservative assumptions possible for the signal. We show fits using the EDGES analysis on various beam-weighted foreground simulations from the EDGES latitude with no signal added. Depending on the beam used, these simulations produce large false troughs due to the invalidity of the foreground model to describe the combination of beam chromaticity and the shape of the Galactic plane in the sky, the residuals of which are captured by the ad hoc flattened Gaussian signal model. On the other hand, the MAA provides robust fits by including many spectra at different time bins and allowing any possible 21-cm spectrum to be modeled exactly. We present uncertainty levels and example signal reconstructions found with the MAA for different numbers of time bins. With enough time bins, one can determine the true 21-cm signal with the MAA to $<10$ times the noise level.

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arXiv2020

Global 21-cm signal extraction from foreground and instrumental effects III: Utilizing drift-scan time dependence and full Stokes measurements

When using valid foreground and signal models, the uncertainties on extracted signals in global 21-cm signal experiments depend principally on the overlap between signal and foreground models. In this paper, we investigate two strategies for decreasing this overlap: (i) utilizing time dependence by fitting multiple drift-scan spectra simultaneously and (ii) measuring all four Stokes parameters instead of only the total power, Stokes I. Although measuring polarization requires different instruments than are used in most existing experiments, all existing experiments can utilize drift-scan measurements merely by averaging their data differently. In order to evaluate the increase in constraining power from using these two techniques, we define a method for connecting Root-Mean-Square (RMS) uncertainties to probabilistic confidence levels. Employing simulations, we find that fitting only one total power spectrum leads to RMS uncertainties at the few K level, while fitting multiple time-binned, drift-scan spectra yields uncertainties at the $\lesssim 10$ mK level. This significant improvement only appears if the spectra are modeled with one set of basis vectors, instead of using multiple sets of basis vectors that independently model each spectrum. Assuming that they are simulated accurately, measuring all four Stokes parameters also leads to lower uncertainties. These two strategies can be employed simultaneously and fitting multiple time bins of all four Stokes parameters yields the best precision measurements of the 21-cm signal, approaching the noise level in the data.

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arXiv2020

The Case for Probe-class NASA Astrophysics Missions

Astrophysics spans an enormous range of questions on scales from individual planets to the entire cosmos. To address the richness of 21st century astrophysics requires a corresponding richness of telescopes spanning all bands and all messengers. Much scientific benefit comes from having the multi-wavelength capability available at the same time. Most of these bands,or measurement sensitivities, require space-based missions. Historically, NASA has addressed this need for breadth with a small number of flagship-class missions and a larger number of Explorer missions. While the Explorer program continues to flourish, there is a large gap between Explorers and strategic missions. A fortunate combination of new astrophysics technologies with new, high capacity, low dollar-per-kg to orbit launchers, and new satellite buses allow for cheaper missions with capabilities approaching strategic mission levels. NASA has recognized these developments by calling for Probe-class mission ideas for mission studies, spanning most of the electromagnetic spectrum from GeV gamma-rays to the far infrared, and the new messengers of neutrinos and ultra-high energy cosmic rays. The key insight from the Probes exercise is that order-of-magnitude advances in science performance metrics are possible across the board for initial total cost estimates in the range 500M-1B dollars.

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arXiv2020

Diffuse radio emission in the galaxy cluster SPT-CL J2031-4037: a steep spectrum intermediate radio halo?

The advent of sensitive low frequency radio observations has revealed a number of diffuse radio objects with peculiar properties that are challenging our understanding about the physics of the intracluster medium. Here, we report the discovery of a steep spectrum radio halo surrounding the central Brightest Cluster Galaxy (BCG) in the galaxy cluster SPT-CL J2031-4037. This cluster is morphologically disturbed yet has a weak cool core, an example of cool core/non-cool core transition system, which harbours a radio halo of $\sim 0.7$ Mpc in size. The halo emission detected at 1.7 GHz is less extended compared to that in the 325 MHz observation, and the spectral index of the part of the halo visible at 325 MHz to 1.7 GHz frequencies was found to be $-1.35 \pm 0.07$. Also, $P_{1.4\ \mathrm{GHz}}$ was found to be $0.77 \times 10^{24}$ W Hz$^{-1}$ which falls in the region where radio mini-halos, halo upper limits and ultra-steep spectrum (USS) halos are found in the $P_{1.4\ \mathrm{GHz}} - L_\mathrm{X}$ plane. Additionally, simulations presented in the paper provide support to the scenario of the steep spectrum. The diffuse radio emission found in this cluster may be a steep spectrum "intermediate" or "hybrid" radio halo which is transitioning into a mini-halo.

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arXiv2019

Probing the origin of diffuse radio emission in the cool-core of the Phoenix galaxy cluster

Cool core galaxy clusters are considered to be dynamically relaxed clusters with regular morphology and highly X-ray luminous central region. However, cool core clusters can also be sites for merging events that exhibit cold fronts in X-ray and mini-halos in radio. We present recent radio/X-ray observations of the Phoenix Cluster or SPT-CL J2344-4243 at the redshift of $z=0.596$. Using archival {\it Chandra} X-ray observations, we detect spiraling cool gas around the cluster core as well as discover two cold fronts near the core. It is perhaps the most distant galaxy cluster to date known to host cold fronts. Also, we present JVLA\footnote{Jansky Very Large Array\\ \url{https://science.nrao.edu/facilities/vla}} 1.52 GHz observations of the minihalo, previously discovered at 610 MHz with GMRT\footnote{Giant Metrewave Radio Telescope\\ \url{http://www.gmrt.ncra.tifr.res.in}} observations in the center of the Phoenix galaxy cluster. The minihalo flux density at 1.52 GHz is $9.65 \pm 0.97$ mJy with the spectral index between 610 MHz and 1.52 GHz being $-0.98 \pm 0.16$\footnote{$S_ν = ν^α$ where $S_ν$}. A possible origin of these radio sources is turbulence induced by sloshing of the gas in the cluster core.

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arXiv2019

Global 21-cm Signal Extraction from Foreground and Instrumental Effects II: Efficient and Self-Consistent Technique for Constraining Nonlinear Signal Models

We present the completion of a data analysis pipeline that self-consistently separates global 21-cm signals from large systematics using a pattern recognition technique. In the first paper of this series, we obtain optimal basis vectors from signal and foreground training sets to linearly fit both components with the minimal number of terms that best extracts the signal given its overlap with the foreground. In this second paper, we utilize the spectral constraints derived in the first paper to calculate the full posterior probability distribution of any signal parameter space of choice. The spectral fit provides the starting point for a Markov Chain Monte Carlo (MCMC) engine that samples the signal without traversing the foreground parameter space. At each MCMC step, we marginalize over the weights of all linear foreground modes and suppress those with unimportant variations by applying priors gleaned from the training set. This method drastically reduces the number of MCMC parameters, augmenting the efficiency of exploration, circumvents the need for selecting a minimal number of foreground modes, and allows the complexity of the foreground model to be greatly increased to simultaneously describe many observed spectra without requiring extra MCMC parameters. Using two nonlinear signal models, one based on EDGES observations and the other on phenomenological frequencies and temperatures of theoretically expected extrema, we demonstrate the success of this methodology by recovering the input parameters from multiple randomly simulated signals at low radio frequencies (10-200 MHz), while rigorously accounting for realistically modeled beam-weighted foregrounds.

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arXiv2019

NASA Probe Study Report: Farside Array for Radio Science Investigations of the Dark ages and Exoplanets (FARSIDE)

This is the final report submitted to NASA for a Probe-class concept study of the "Farside Array for Radio Science Investigations of the Dark ages and Exoplanets" (FARSIDE), a low radio frequency interferometric array on the farside of the Moon. The design study focused on the instrument, a deployment rover, the lander and base station, and delivered an architecture broadly consistent with the requirements for a Probe mission. This notional architecture consists of 128 dipole antennas deployed across a 10 km area by a rover, and tethered to a base station for central processing, power and data transmission to the Lunar Gateway, or an alternative relay satellite. FARSIDE would provide the capability to image the entire sky each minute in 1400 channels spanning frequencies from 150 kHz to 40 MHz, extending down two orders of magnitude below bands accessible to ground-based radio astronomy. The lunar farside can simultaneously provide isolation from terrestrial radio frequency interference, auroral kilometric radiation, and plasma noise from the solar wind. This would enable near-continuous monitoring of the nearest stellar systems in the search for the radio signatures of coronal mass ejections and energetic particle events, and would also detect the magnetospheres for the nearest candidate habitable exoplanets. Simultaneously, FARSIDE would be used to characterize similar activity in our own solar system, from the Sun to the outer planets. Through precision calibration via an orbiting beacon, and exquisite foreground characterization, FARSIDE would also measure the Dark Ages global 21-cm signal at redshifts from 50-100. It will also be a pathfinder for a larger 21-cm power spectrum instrument by carefully measuring the foreground with high dynamic range.

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arXiv2019

Assessment of the Projection-induced Polarimetry Technique for Constraining the Foreground Spectrum in Global 21 cm Cosmology

Detecting the cosmological sky-averaged (global) 21 cm signal as a function of observed frequency will provide a powerful tool to study the ionization and thermal history of the intergalactic medium (IGM) in the early Universe ($\sim$ 400 million years after the Big Bang). The greatest challenge in conventional total-power global 21 cm experiments is the removal of the foreground synchrotron emission ($\sim 10^3$-$10^4$ K) to uncover the weak cosmological signal (tens to hundreds of mK), especially since the intrinsic smoothness of the foreground spectrum is corrupted by instrumental effects. Although the EDGES team has recently reported an absorption profile at 78 MHz in the sky-averaged spectrum, it is necessary to confirm this detection with an independent approach. The projection effect from observing anisotropic foreground source emission with a wide-view antenna pointing at the North Celestial Pole (NCP) can induce a net polarization, referred as the Projection-Induced Polarization Effect (PIPE). Due to Earth's rotation, observation centered at the circumpolar region will impose a dynamic sky modulation on the net polarization's waveforms which is unique to the foreground component. In this study, we review the implementation practicality and underlying instrumental effects of this new polarimetry-based technique with detailed numerical simulation and a testbed instrument, the Cosmic Twilight Polarimeter (CTP). In addition, we explore an SVD-based analysis approach for separating the foreground and instrumental effects from the background global 21 cm signal using the sky-modulated PIPE.

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arXiv2019

The Galaxy Cluster 'Pypeline' for X-ray Temperature Maps: ClusterPyXT

ClusterPyXT is a new software pipeline to generate spectral temperature, X-ray surface brightness, pressure, and density maps from X-ray observations of galaxy clusters. These data products help elucidate the physics of processes occurring within clusters of galaxies, including turbulence, shock fronts, nonthermal phenomena, and the overall dynamics of cluster mergers. ClusterPyXT automates the creation of these data products with minimal user interaction, and allows for rapid analyses of archival data with user defined parameters and the ability to straightforwardly incorporate additional observations. In this paper, we describe in detail the use of this code and release it as an open source Python project on GitHub.

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arXiv2019

A Ground Plane Artifact that Induces an Absorption Profile in Averaged Spectra from Global 21-cm Measurements - with Possible Application to EDGES

Most of the current Global 21-cm experiments include ground screens that help moderate effects from the Earth. In this paper, we report on a possible systematic artifact within the ground plane that may produce broad absorption features in the spectra observed by these experiments. Using analytical approximations and numerical modeling, the origin of the artifact and its impact on the sky-averaged spectrum are described. The publicly released EDGES dataset, from which a 78 MHz absorption feature was recently suggested, is used to probe for the potential presence of ground plane resonances. While the lack of a noise level for the EDGES spectrum makes traditional goodness-of-fit statistics unattainable, the rms residual can be used to assess the relative goodness of fits performed under similar circumstances. The fit to the EDGES spectrum using a model with a simple 2-term foreground and three cavity-mode resonances is compared to a fit to the same spectrum with a model used by the EDGES team consisting of a 5-term foreground and a flattened Gaussian signal. The fits with the physically motivated resonance and empirical flattened Gaussian models have rms residuals of 20.8 mK (11 parameters) and 24.5 mK (9 parameters), respectively, allowing us to conclude that ground plane resonances constitute another plausible explanation for the EDGES data.

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arXiv2019

Dark Cosmology: Investigating Dark Matter & Exotic Physics in the Dark Ages using the Redshifted 21-cm Global Spectrum

The Dark Ages, probed by the redshifted 21-cm signal, is the ideal epoch for a new rigorous test of the standard LCDM cosmological model. Divergences from that model would indicate new physics, such as dark matter decay (heating) or baryonic cooling beyond that expected from adiabatic expansion of the Universe. In the early Universe, most of the baryonic matter was in the form of neutral hydrogen (HI), detectable via its ground state's spin-flip transition. A measurement of the redshifted 21-cm spectrum maps the history of the HI gas through the Dark Ages and Cosmic Dawn and up to the Epoch of Reionization (EoR). The Experiment to Detect the Global EoR Signature (EDGES) recently reported an absorption trough at 78 MHz (redshift z of 17), similar in frequency to expectations for Cosmic Dawn, but about 3 times deeper than was thought possible from standard cosmology and adiabatic cooling of HI. Interactions between baryons and slightly-charged dark matter particles with electron-like mass provide a potential explanation of this difference but other cooling mechanisms are also being investigated to explain these results. The Cosmic Dawn trough is affected by cosmology and the complex astrophysical history of the first luminous objects. Another trough is expected during the Dark Ages, prior to the formation of the first stars and thus determined entirely by cosmological phenomena (including dark matter). Observations on or in orbit above the Moon's farside can investigate this pristine epoch (15-40 MHz; z=100-35), which is inaccessible from Earth. A single cross-dipole antenna or compact array can measure the amplitude of the 21-cm spectrum to the level required to distinguish (at >5$σ$}) the standard cosmological model from that of additional cooling derived from current EDGES results. This observation constitutes a powerful, clean probe of exotic physics in the Dark Ages.

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arXiv2018

A new goodness-of-fit statistic and its application to 21-cm cosmology

The reduced chi-squared statistic is a commonly used goodness-of-fit measure, but it cannot easily detect features near the noise level, even when a large amount of data is available. In this paper, we introduce a new goodness-of-fit measure that we name the reduced psi-squared statistic. It probes the two-point correlations in the residuals of a fit, whereas chi-squared accounts for only the absolute values of each residual point, not considering the relationship between these points. The new statistic maintains sensitivity to individual outliers, but is superior to chi-squared in detecting wide, low level features in the presence of a large number of noisy data points. After presenting this new statistic, we show an instance of its use in the context of analyzing radio spectroscopic data for 21-cm cosmology experiments. We perform fits to simulated data with four components: foreground emission, the global 21-cm signal, an idealized instrument systematic, and noise. This example is particularly timely given the ongoing efforts to confirm the first observational result for this signal, where this work found its original motivation. In addition, we release a Python script dubbed $\texttt{psipy}$ which allows for quick, efficient calculation of the reduced psi-squared statistic on arbitrary data arrays, to be applied in any field of study.

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arXiv2018

Science on the Lunar Surface Facilitated by Low Latency Telerobotics from a Lunar Orbiting Platform-Gateway

NASA and ESA are preparing a series of human exploration missions using the four-person Orion crew vehicle, launched by NASA's Space Launch System, and a Lunar Orbiting Platform-Gateway (LOP-G) that enable long duration (>30 days) operations in cis-lunar space. This will provide an opportunity for science and exploration from the lunar surface facilitated by low latency surface telerobotics. We describe two precursor experiments, using the International Space Station (ISS) and a student-built teleoperated rover, which are laying the groundwork for remote operation of rovers on the Moon by astronauts aboard the LOP-G. Such missions will open the lunar far side, among other sites, for exploration and science. We describe examples of two high-priority, lunar science missions that can be conducted using low latency surface telerobotics including an astronaut-assisted far side sample return and the deployment/construction of a low frequency radio telescope array to observe the first stars and galaxies (Cosmic Dawn). The lessons learned from these lunar operations will feed-forward to future low latency telepresence missions on Mars.

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arXiv2018

Probing the Curious Case of a Galaxy Cluster Merger in Abell 115 with High Fidelity Chandra X-ray Temperature and Radio Maps

We present results from an X-ray and radio study of the merging galaxy cluster Abell 115. We use the full set of 5 Chandra observations taken of A115 to date (360 ks total integration) to construct high-fidelity temperature and surface brightness maps. We also examine radio data from the Very Large Array at 1.5 GHz and the Giant Metrewave Radio Telescope at 0.6 GHz. We propose that the high X-ray spectral temperature between the subclusters results from the interaction of the bow shocks driven into the intracluster medium by the motion of the subclusters relative to one another. We have identified morphologically similar scenarios in Enzo numerical N-body/hydrodynamic simulations of galaxy clusters in a cosmological context. In addition, the giant radio relic feature in A115, with an arc-like structure and a relatively flat spectral index, is likely consistent with other shock-associated giant radio relics seen in other massive galaxy clusters. We suggest a dynamical scenario that is consistent with the structure of the X-ray gas, the hot region between the clusters, and the radio relic feature.

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arXiv2018

The American Space Exploration Narrative from the Cold War through the Obama Administration

We document how the narrative and the policies of space exploration in the U.S. have changed over the past 50 years. We first examine the history of the U.S. space exploration program and also assess three current conditions of space exploration including: (1) the increasing role of the private sector, (2) the influence of global politics, and (3) the focus on a human mission to Mars. We identify five rhetorical themes: competition, prestige, collaboration, leadership, and a new paradigm. These themes are then used to analyze the content of forty documents from eight presidential administrations. The historical narrative and content analysis together suggest that space exploration has developed from a discourse about a bipolar world composed of the U.S. and U.S.S.R. into a complicated field that encompass many new players. We make three observations: (1) there is a disconnect between stated U.S. policy goals and the implementation of those goals, (2) the U.S. communicates mixed messages regarding its intent to be both the dominant leader in space exploration and also a committed participant in international collaborations, and (3) the U.S. cannot remain a true pioneer in space exploration if it does not embrace the realities of globalization and the changing dynamics within space exploration. We conclude with three suggestions: (1) the U.S. government and NASA should critically examine space exploration priorities and commit to implementing a program that will further realistic policy and goals, (2) the U.S. should re-examine its intention to play a dominant leadership role in space exploration and consider emphasizing a commitment toward active participation in international collaboration in space, and (3) the U.S. should fully embrace the new paradigm of space exploration by lowering barriers that hinder competitiveness.

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arXiv2018

Quantifying Operational Constraints of Low-Latency Telerobotics for Planetary Surface Operations

NASA's SLS and Orion crew vehicle will launch humans to cislunar space to begin the new era of space exploration. NASA plans to use the Orion crew vehicle to transport humans between Earth and cislunar space where there will be a stationed habitat known as the Deep Space Gateway (DSG). The proximity to the lunar surface allows for direct communication between the DSG and surface assets, which enables low-latency telerobotic exploration. The operational constraints for telerobotics must be fully explored on Earth before being utilized on space exploration missions. We identified two constraints on space exploration using low-latency surface telerobotics and attempts to quantify these constraints. A constraint associated with low-latency surface telerobotics is the bandwidth available between the orbiting command station and the ground assets. The bandwidth available will vary during operation. As a result, it is critical to quantify the operational video conditions required for effective exploration. We designed an experiment to quantify the threshold frame rate required for effective exploration. The experiment simulated geological exploration via low-latency surface telerobotics using a COTS rover in a lunar analog environment. The results from this experiment indicate that humans should operate above a threshold frame rate of 5 frames per second. In a separate, but similar experiment, we introduced a 2.6 second delay in the video system. This delay recreated the latency conditions present when operating rovers on the lunar farside from an Earth-based command station. This time delay was compared to low-latency conditions for teleoperation at the DSG ($\leq$0.4 seconds). The results from this experiment show a 150% increase in exploration time when the latency is increased to 2.6 seconds. This indicates that such a delay significantly complicates real-time exploration strategies.

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