adsabs/SciX_UAT_keywords huggingface dataset:
Energy Power Spectra Measured at an Interplanetary Shock by the New Horizon's SWAP Experiment: 1D Full Particle Simulations versus Observations

One-dimensional particle-in-cell (PIC) simulations are used to analyze the energy spectra measured by the New Horizons' Solar Wind Around Pluto (SWAP) instrument in the upstream region of an interplanetary shock observed at a distance of ∼34 au from the Sun. The use of individual populations simulating the different solar wind ion and pick-up ion (PUI) populations allows us to clearly identify the contribution of each population to the global energy spectra. The important role of shock front obliquity is stressed in the formation of PUIs streaming back along the magnetic field into the upstream region far from the front. Energy spectra measured by the SWAP experiment are well reproduced in the present simulations. A detailed analysis shows that (1) the highest-energy part of the spectrum is formed primarily by both backstreaming PUI-H+ and PUI-He+; (2) the middle-energy part of the energy spectrum is composed of both solar wind SW-H+ and SW-He2+ incoming ions that are superimposed on the PUI-H+ population; and (3) the low-energy range is composed of incoming PUI-H+. The agreement between experimental and simulation results is improved by using an initially filled-shell distribution for the PUI-H+ population (instead of a zero-thickness shell), as this affects the low-energy part of the spectrum strongly. This means that PUI-H+ ions have sufficient time to diffuse onto and fill out a shell distribution after their initial pick-up in the heliosphere, indicating that the subsequent cooling has an important impact on the global energy spectrum.

Keywords: heliosphere; interplanetary shocks; space plasmas; interplanetary particle acceleration
Characterization of Temporarily Captured Minimoon 2020 CD3 by Keck Time-resolved Spectrophotometry

We present time-resolved visible spectrophotometry of 2020 CD3, the second known minimoon. The spectrophotometry was taken with the Keck I/Low Resolution Imaging Spectrometer between wavelengths 434 and 912 nm in the B, g, V, R, I, and RG850 filters as it was leaving the Earth-Moon system on 2020 March 23 UTC. The spectrum of 2020 CD3 resembles V-type asteroids and some lunar rock samples with a 434-761 nm reddish slope of ∼18%/100 nm (g-r = 0.62 ± 0.08 and r-i = 0.21 ± 0.06) with an absorption band at ∼900 nm corresponding to i-z = -0.54 ± 0.10. Combining our measured H of 31.9 ± 0.1 with an albedo of 0.35 typical for V-type asteroids, we determine 2020 CD3's diameter to be ∼0.9 ± 0.1 m, making it the first minimoon and one of the smallest asteroids to be spectrally studied. We use our time-series photometry to detect significant periodic light-curve variations with a period of ∼573 s and amplitude of ∼1. In addition, we extend the observational arc of 2020 CD3 to 37 days, to 2020 March 23 UTC. From the improved orbital solution for 2020 CD3, we estimate the likely duration of its capture to be ∼2 yr and the nongravitational perturbation on its orbit due to radiation pressure with an area-to-mass ratio of (6.9 ± 2.4) × 10-4 m2 kg-1 implying a density of 2.3 ± 0.8 g cm-3, broadly compatible with other meter-scale asteroids and lunar rock. We searched for prediscovery detections of 2020 CD3 in the Zwicky Transient Facility archive as far back as 2018 October but were unable to locate any positive detections.

Keywords: asteroids; near-earth objects; meteoroids
Modeling the Daily Variations of the Coronal X-ray Spectral Irradiance with Two Temperatures and Two Emission Measures

The Miniature X-ray Solar Spectrometer (MinXSS-1) CubeSat observed solar X-rays between 0.5 and 10 keV. A two-temperature, two-emission-measure model is fit to each daily averaged spectrum. These daily average temperatures and emission measures are plotted against the corresponding daily solar 10.7 cm radio flux (F10.7) value and a linear correlation is found between each that we call the Schwab Woods Mason (SWM) model. The linear trends show that one can estimate the solar spectrum between 0.5 and 10 keV based on the F10.7 measurement alone. The cooler temperature component of this model represents the quiescent Sun contribution to the spectra and is essentially independent of solar activity, meaning the daily average quiescent Sun is accurately described by a single temperature (1.70 MK) regardless of solar intensity and only the emission measure corresponding to this temperature needs to be adjusted for higher or lower solar intensity. The warmer temperature component is shown to represent active region contributions to the spectra and varies between 5 and 6 MK. The Geostationary Operational Environmental Satellite (GOES) XRS-B data between 1 and 8 Å is used to validate this model and it is found that the ratio between the SWM model irradiance and the GOES XRS-B irradiance is close to unity on average. MinXSS-1 spectra during quiescent solar conditions have very low counts beyond around 3 keV. The SWM model can generate MinXSS-1 or Dual Aperture X-ray Solar Spectrometer spectra at very high spectral resolution and with extended energy ranges to fill in gaps between measurements and extend predictions back to 1947.

Keywords: solar x-ray emission; quiet sun; quiet solar corona; solar spectral irradiance; spectrometers; plasma physics; solar corona; solar atmosphere; active solar corona
Analytic Light Curve for Mutual Transits of Two Bodies Across a Limb-darkened Star

We present a solution for the light curve of two bodies mutually transiting a star with polynomial limb darkening. The term "mutual transit" in this work refers to a transit of the star during which overlap occurs between the two transiting bodies. These could be an exoplanet with an exomoon companion, two exoplanets, an eclipsing binary and a planet, or two stars eclipsing a third in a triple-star system. We include analytic derivatives of the light curve with respect to the positions and radii of both bodies. We provide code that implements a photodynamical model for a mutual transit. We include two dynamical models, one for hierarchical systems in which a secondary body orbits a larger primary (e.g., an exomoon system) and a second for confocal systems in which two bodies independently orbit a central mass (e.g., two planets in widely separated orbits). Our code is fast enough to enable inference with Markov Chain Monte Carlo algorithms, and the inclusion of derivatives allows for the use of gradient-based inference methods such as Hamiltonian Monte Carlo. While applicable to a variety of systems, this work was undertaken primarily with exomoons in mind. It is our hope that making this code publicly available will reduce barriers for the community to assess the detectability of exomoons, conduct searches for exomoons, and attempt to validate existing exomoon candidates. We also anticipate that our code will be useful for studies of planet-planet transits in exoplanetary systems, transits of circumbinary planets, and eclipses in triple-star systems.

Keywords: exoplanets; exoplanet detection methods; exoplanet systems; natural satellites (extrasolar); binary stars; multiple stars; trinary stars; exoplanet astronomy; exoplanet dynamics; planet hosting stars
Simulation of the Solar Energetic Particle Event on 2020 May 29 Observed by Parker Solar Probe

This paper presents a stochastic three-dimensional focused transport simulation of solar energetic particles (SEPs) produced by a data-driven coronal mass ejection (CME) shock propagating through a data-driven model of coronal and heliospheric magnetic fields. The injection of SEPs at the CME shock is treated using diffusive shock acceleration of post-shock suprathermal solar wind ions. A time-backward stochastic simulation is employed to solve the transport equation to obtain the SEP time-intensity profile at any location, energy, and pitch angle. The model is applied to a SEP event on 2020 May 29, observed by STEREO-A close to ~1 au and by Parker Solar Probe (PSP) when it was about 0.33 au away from the Sun. The SEP event was associated with a very slow CME with a plane-of-sky speed of 337 km s-1 at a height below 6 R S as reported in the SOHO/LASCO CME catalog. We compute the time profiles of particle flux at PSP and STEREO-A locations, and estimate both the spectral index of the proton energy spectrum for energies between ~2 and 16 MeV and the equivalent path length of the magnetic field lines experienced by the first arriving SEPs. We find that the simulation results are well correlated with observations. The SEP event could be explained by the acceleration of particles by a weak CME shock in the low solar corona that is not magnetically connected to the observers.

Keywords: solar energetic particles; solar particle emission; solar coronal mass ejection shocks
Origin-of-life Molecules in the Atmosphere after Big Impacts on the Early Earth

The origin of life on Earth would benefit from a prebiotic atmosphere that produced nitriles, like HCN, which enable ribonucleotide synthesis. However, geochemical evidence suggests that Hadean air was relatively oxidizing with negligible photochemical production of prebiotic molecules. These paradoxes are resolved by iron-rich asteroid impacts that transiently reduced the entire atmosphere, allowing nitriles to form in subsequent photochemistry. Here we investigate impact-generated reducing atmospheres using new time-dependent, coupled atmospheric chemistry and climate models that account for gas-phase reactions and surface catalysis. The resulting H2-, CH4-, and NH3-rich atmospheres persist for millions of years, until the hydrogen escapes to space. The HCN and HCCCN production and rainout to the surface can reach 109 molecules cm-2 s-1 in hazy atmospheres with a mole ratio of CH4/CO2 > 0.1. Smaller CH4/CO2 ratios produce HCN rainout rates of <105 molecules cm-2 s-1 and negligible HCCCN. The minimum impactor mass that creates atmospheric CH4/CO2 > 0.1 is 4 × 1020-5 × 1021 kg (570-1330 km diameter), depending on how efficiently iron reacts with a steam atmosphere, the extent of atmospheric equilibration with an impact-induced melt pond, and the surface area of nickel that catalyzes CH4 production. Alternatively, if steam permeates and deeply oxidizes the crust, impactors of ~1020 kg could be effective. Atmospheres with copious nitriles have >360 K surface temperatures, perhaps posing a challenge for RNA longevity, although cloud albedo can produce cooler climates. Regardless, postimpact cyanide can be stockpiled and used in prebiotic schemes after hydrogen has escaped to space.

Keywords: earth (planet); earth atmosphere; astrobiology; atmospheric composition; pre-biotic astrochemistry
X-Ray Emission from Jupiter's Galilean Moons: A Tool for Determining Their Surface Composition and Particle Environment

We analyze archival Chandra X-ray Observatory observations of Jupiter to search for emission from the Galilean moons. X-ray emission has previously been reported from Io and Europa using a subset of these data. We confirm this detection, and marginally detect X-ray emission from both Ganymede and Callisto as well. The X-ray spectrum of Europa is strongly peaked around the neutral oxygen fluorescence line (525 eV), while Io's has peaks at both oxygen and sulfur (2308 eV) plus a broad continuum between 350 and 5000 eV. Ganymede's spectrum is similar to Io's, but without the sulfur peak. A few events, mostly clustered around the oxygen line, are detected from Callisto. Using measurements by the Galileo mission of the specific intensity of ambient protons and electrons, we model the X-ray spectra and flux of the moons from two processes: particle-induced X-ray emission (PIXE) from the impact of energetic protons and X-ray emission from electron bremsstrahlung. With uncertainties of a factor of a few, the electron bremsstrahlung and PIXE models overestimate the X-ray flux from Europa, preventing us from making a definitive statement about the origin of the X-ray emission. The PIXE model of Io predicts emission lines at O and S similar to those observed, but underestimates their flux by nearly two orders of magnitude. Based on this discrepancy in the PIXE flux, combined with the detected broadband continuum in the spectrum, we conclude that the X-ray emission from Io is due to electron bremsstrahlung. Likewise, because of Ganymede's broad continuum, we tentatively conclude that its X-ray emission is also due to electron bremsstrahlung. Callisto is too faint in the X-rays to draw any conclusion. Obtaining in situ X-ray observations of the moons would provide a direct measurement of their elemental composition.

Keywords: chemical abundances; impact phenomena; spectroscopy; non-thermal radiation sources; galilean satellites; x-ray sources; jupiter
Spectral Characteristics and Formation Height of Off-limb Flare Ribbons

Flare ribbons are bright manifestations of flare energy dissipation in the lower solar atmosphere. For the first time, we report on high-resolution imaging spectroscopy observations of flare ribbons situated off limb in the Hβ and Ca II 8542 Å lines and make a detailed comparison with radiative hydrodynamic simulations. Observations of the X8.2 class solar flare SOL 2017-09-10T16:06 UT obtained with the Swedish Solar Telescope reveal bright horizontal emission layers in Hβ line-wing images located near the footpoints of the flare loops. The apparent separation between the ribbon observed in the Hβ wing and the nominal photospheric limb is about 300-500 km. The Ca II 8542 Å line-wing images show much fainter ribbon emissions located right on the edge of the limb, without clear separation from the limb. RADYN models are used to investigate synthetic spectral line profiles for the flaring atmosphere, and good agreement is found with the observations. The simulations show that, toward the limb, where the line of sight is substantially oblique with respect to the vertical direction, the flaring atmosphere model reproduces the high contrast of the off-limb Hβ ribbons and their significant elevation above the photosphere. The ribbons in the Ca II 8542 Å line-wing images are located deeper in the lower solar atmosphere with a lower contrast. A comparison of the height deposition of electron beam energy and the intensity contribution function shows that the Hβ line-wing intensities can be a useful tracer of flare energy deposition in the lower solar atmosphere.

Keywords: solar flares; solar flare spectra; radiative transfer simulations; solar chromosphere; optical flares; high resolution spectroscopy; spectroscopy
Hot Gas in Galaxy Halos Traced by Coronal Broad Lyα Absorbers

We explore the possibility to systematically study the extended, hot gaseous halos of low-redshift galaxies with coronal broad Lyα absorbers (CBLAs). These are weak, thermally broadenend H I absorption lines arising from the tiny fraction of neutral hydrogen that resides in the collisionally ionized, million-degree halo gas in these galaxies. Using a semi-analytic approach, we model the spatial density and temperature distribution of hot coronal gas to predict strength, spectral shape, and cross section of CBLAs as a function of galaxy-halo mass and line-of-sight impact parameter. For virial halo masses in the range log (M/M⊙)=10.6-12.6, the characteristic logarithmic CBLA H I column densities and Doppler parameters are log N(H I) = 12.4-13.4 and b(H I) = 70-200 km s-1, indicating that CBLAs represent weak, shallow spectral features that are difficult to detect. Yet, the expected number density of CBLAs per unit redshift in the above given mass range is dN/dz (CBLA) ≈ 3, implying that CBLAs have a substantial absorption cross section. We compare the model predictions with a combined set of UV absorption-line spectra from the Hubble Space Telescope (HST)/Cosmic Origins Spectrograph and HST/Space Telescope Imaging Spectrograph that trace the halos of four low-redshift galaxies. We demonstrate that CBLAs might already have been detected in these spectra, but the complex multi-component structure and the limited signal-to-noise ratio complicate the interpretation of these CBLA candidate systems. Our study suggests that CBLAs represent a very interesting absorber class that potentially will allow us to further explore the hot coronae of galaxies with UV spectral data.

Keywords: circumgalactic medium; quasar absorption line spectroscopy; galaxy evolution; astronomical models
HInet: Generating Neutral Hydrogen from Dark Matter with Neural Networks

Upcoming 21 cm surveys will map the spatial distribution of cosmic neutral hydrogen (HI) over very large cosmological volumes. In order to maximize the scientific return of these surveys, accurate theoretical predictions are needed. Hydrodynamic simulations currently are the most accurate tool to provide those predictions in the mildly to nonlinear regime. Unfortunately, their computational cost is very high: tens of millions of CPU hours. We use convolutional neural networks to find the mapping between the spatial distribution of matter from N-body simulations and HI from the state-of-the-art hydrodynamic simulation IllustrisTNG. Our model performs better than the widely used theoretical model: halo occupation distribution for all statistical properties up to the nonlinear scales k ≲ 1 hr Mpc-1. Our method allows the generation of 21 cm mocks over very big cosmological volumes with similar properties to hydrodynamic simulations.

Keywords: convolutional neural networks; neural networks; cosmology; large-scale structure of the universe; cosmological evolution; observational cosmology
Constraining the Binarity of Black Hole Candidates: A Proof-of-concept Study of Gaia BH1 and Gaia BH2

Nearly a hundred binary black holes (BBHs) have been discovered with gravitational-wave signals emitted at their merging events. Thus, it is quite natural to expect that significantly more abundant BBHs with wider separations remain undetected in the Universe or even in our Galaxy. We consider a possibility that star-BH binary candidates may indeed host an inner BBH instead of a single BH. We present a detailed feasibility study of constraining the binarity of the currently available two targets, Gaia BH1 and Gaia BH2. Specifically, we examine three types of radial velocity (RV) modulations of a tertiary star in star-BBH triple systems; short-term RV modulations induced by the inner BBH, long-term RV modulations induced by the nodal precession, and long-term RV modulations induced by the von Zeipel-Kozai-Lidov oscillations. Direct three-body simulations combined with approximate analytic models reveal that the Gaia BH1 system may exhibit observable signatures of the hidden inner BBH if it exists at all. The methodology that we examine here is quite generic and is expected to be readily applicable to future star-BH binary candidates in a straightforward manner.

Keywords: radial velocity; celestial mechanics; three-body problem; multiple stars; black holes; compact binary stars
Hard X-Ray Flares and Spectral Variability in NGC 4395 ULX1

We report the detection of flaring events in NGC 4395 ULX1, a nearby ultraluminous X-ray source (ULX), for the first time, using recent XMM-Newton observations. The flaring episodes are spectrally harder than the steady-emission intervals, resulting in higher fractional variability in the high-energy regime. A thin Keplerian and a slim accretion disk provide the best-fit continuum for XMM-Newton spectra. All observations show a broad hump-like feature around ~0.9 keV, which can be associated with a collection of blended emission lines, and suggests the presence of a wind or outflow in this ULX through comparison with other ULXs that show a similar feature. The flaring spectra correspond to higher slim-disk temperatures due to a higher mass accretion rate under an advection-dominated accretion scenario. The luminosity-temperature (L-T) values in different flux states show a positive trend. When characterized with a power-law relation, the L-T profile is broadly consistent with both L ∝ T 2 and L ∝ T 4 relations for the analyzed data. The empirical predictions for a slim accretion disk in the case of super-Eddington accretion onto a stellar-mass compact object is L ∝ T 2, which is a possible scenario in ULX1. The origin of the flaring events is understood as an intrinsic change in accretion rate or presence of variable clumpy wind in the inner region of the accretion disk.

Keywords: ultraluminous x-ray sources; x-ray binary stars
Extracting the Subhalo Mass Function from Strong Lens Images with Image Segmentation

Detecting substructure within strongly lensed images is a promising route to shed light on the nature of dark matter. However, it is a challenging task, which traditionally requires detailed lens modeling and source reconstruction, taking weeks to analyze each system. We use machine learning to circumvent the need for lens and source modeling and develop a neural network to both locate subhalos in an image as well as determine their mass using the technique of image segmentation. The network is trained on images with a single subhalo located near the Einstein ring across a wide range of apparent source magnitudes. The network is then able to resolve subhalos with masses m ≳ 108.5 M ⊙. Training in this way allows the network to learn the gravitational lensing of light, and, remarkably, it is then able to detect entire populations of substructure, even for locations further away from the Einstein ring than those used in training. Over a wide range of the apparent source magnitude, the false-positive rate is around three false subhalos per 100 images, coming mostly from the lightest detectable subhalo for that signal-to-noise ratio. With good accuracy and a low false-positive rate, counting the number of pixels assigned to each subhalo class over multiple images allows for a measurement of the subhalo mass function (SMF). When measured over three mass bins from 109-1010 M ⊙ the SMF slope is recovered with an error of 36% for 50 images, and this improves to 10% for 1000 images with Hubble Space Telescope-like noise.

Keywords: strong gravitational lensing; neural networks; dark matter; cosmology
TriOS Schwarzschild Orbit Modeling: Robustness of Parameter Inference for Masses and Shapes of Triaxial Galaxies with Supermassive Black Holes

Evidence for the majority of the supermassive black holes in the local Universe has been obtained dynamically from stellar motions with the Schwarzschild orbit superposition method. However, there have been only a handful of studies using simulated data to examine the ability of this method to reliably recover known input black hole masses M BH and other galaxy parameters. Here, we conduct a comprehensive assessment of the reliability of the triaxial Schwarzschild method at simultaneously determining M BH, stellar mass-to-light ratio M*/L, dark matter mass, and three intrinsic triaxial shape parameters of simulated galaxies. For each of 25 rounds of mock observations using simulated stellar kinematics and the TriOS code, we derive best-fitting parameters and confidence intervals after a full search in the 6D parameter space with our likelihood-based model inference scheme. The two key mass parameters, M BH and M*/L, are recovered within the 68% confidence interval, and other parameters are recovered between the 68% and 95% confidence intervals. The spatially varying velocity anisotropy of the stellar orbits is also well recovered. We explore whether the goodness-of-fit measure used for galaxy model selection in our pipeline is biased by variable complexity across the 6D parameter space. In our tests, adding a penalty term to the likelihood measure either makes little difference, or worsens the recovery in some cases.

Keywords: galaxy dynamics; galaxy masses; supermassive black holes; early-type galaxies; galaxies; galaxy dark matter halos; galaxy evolution; galaxy kinematics