Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>We study the high-energy properties of GRB 181123B, a short gamma-ray burst (sGRB) at redshift <jats:italic>z</jats:italic> ≈ 1.75. We show that, despite its nominal short duration with <jats:italic>T</jats:italic> <jats:sub>90</jats:sub> &lt; 2 s, this burst displays evidence of a temporally extended emission (EE) at high energies and that the same trend is observed in the majority of sGRBs at <jats:italic>z</jats:italic> ≳ 1. We discuss the impact of instrumental selection effects on the GRB classification, stressing that the measured <jats:italic>T</jats:italic> <jats:sub>90</jats:sub> is not an unambiguous indicator of the burst physical origin. By examining their environment (e.g., stellar mass, star formation, offset distribution), we find that these high-<jats:italic>z</jats:italic> sGRBs share many properties of long GRBs at a similar distance and are consistent with a short-lived progenitor system. If produced by compact binary mergers, these sGRBs with EE may be easier to localize at large distances and herald a larger population of sGRBs in the early universe.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Observations of plasma waves by the Fields Suite and of electrons by the Solar Wind Electrons Alphas and Protons Investigation on the Parker Solar Probe provide strong evidence for pitch angle scattering of strahl-energy electrons by narrowband whistler-mode waves at radial distances less than ∼0.3 au. We present two example intervals of a few hours each that include eight waveform captures with whistler-mode waves and 26 representative electron distributions that are examined in detail. Two were narrow, seventeen were clearly broadened, and eight were very broad. The two with narrow strahl occurred when there were either no whistlers or very intermittent low amplitude waves. Six of the eight broadest distributions were associated with intense, long duration waves. Approximately half of the observed electron distributions have features consistent with an energy-dependent scattering mechanism, as would be expected from interactions with narrowband waves. A comparison of the wave power in the whistler-mode frequency band to pitch angle width and a measure of anisotropy provides additional evidence for electron scattering by whistler-mode waves. We estimate the range of resonances based on the wave properties and energies over which broadening is observed. These observations provide strong evidence that the narrowband whistler-mode waves scatter strahl-energy electrons to produce the halo and to reduce the electron heat flux.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The precise characterization of terrestrial atmospheres with the James Webb Space Telescope (JWST) is one of the utmost goals of exoplanet astronomy in the next decade. With JWST’s impending launch, it is crucial that we are well prepared to understand the subtleties of terrestrial atmospheres—particularly ones that we may have not needed to consider before due to instrumentation limitations. In this work we show that patchy ice cloud variability is present in the upper atmospheres of M-dwarf terrestrial planets, particularly along the limbs. Here we test whether these variable clouds will introduce unexpected biases in the multi-epoch observations necessary to constrain atmospheric abundances. Using 3D <jats:monospace>ExoCAM</jats:monospace> general circulation models of TRAPPIST-1e, we simulate five different climates with varying pCO<jats:sub>2</jats:sub> to explore the strength of this variability. These models are post-processed using NASA Goddard’s Planetary Spectrum Generator and <jats:monospace>PandExo</jats:monospace> to generate simulated observations with JWST’s NIRSpec PRISM mode at 365 different temporal outputs from each climate. Assuming the need for 10 transits of TRAPPIST-1e to detect molecular features at great confidence, we then use <jats:monospace>CHIMERA</jats:monospace> to retrieve on several randomly selected weighted averages of our simulated observations to explore the effect of multi-epoch observations with variable cloud cover along the limb on retrieved abundances. We find that the variable spectra do not affect retrieved abundances at detectable levels for our sample of TRAPPIST-1e models.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Using multipoint Magnetospheric Multiscale (MMS) observations in an unusual string-of-pearls configuration, we examine in detail observations of the reformation of a fast magnetosonic shock observed on the upstream edge of a foreshock transient structure upstream of Earth's bow shock. The four MMS spacecraft were separated by several hundred kilometers, comparable to suprathermal ion gyroradius scales or several ion inertial lengths. At least half of the shock reformation cycle was observed, with a new shock ramp rising up out of the “foot” region of the original shock ramp. Using the multipoint observations, we convert the observed time-series data into distance along the shock normal in the shock's rest frame. That conversion allows for a unique study of the relative spatial scales of the shock's various features, including the shock's growth rate, and how they evolve during the reformation cycle. Analysis indicates that the growth rate increases during reformation, electron-scale physics play an important role in the shock reformation, and energy conversion processes also undergo the same cyclical periodicity as reformation. Strong, thin electron-kinetic-scale current sheets and large-amplitude electrostatic and electromagnetic waves are reported. Results highlight the critical cross-scale coupling between electron-kinetic- and ion-kinetic-scale processes and details of the nature of nonstationarity, shock-front reformation at collisionless, fast magnetosonic shocks.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Tidally dissolved globular clusters form thin stellar streams that preserve a historical record of their past evolution. We report a radial velocity dispersion of 2.1 ± 0.3 km s<jats:sup>−1</jats:sup> in the GD-1 stellar stream using a sample of 43 spectroscopically confirmed members. The GD-1 velocity dispersion is constant over the surveyed ≈15° span of the stream. We also measured velocity dispersion in the spur adjacent to the main GD-1 stream, and found a similar value at the tip of the spur. Surprisingly, the region of the spur closer to the stream appears dynamically colder than the main stream. An unperturbed model of the GD-1 stream has a velocity dispersion of ≈0.5 km s<jats:sup>−1</jats:sup>, indicating that GD-1 has undergone dynamical heating. Stellar streams arising from globular clusters, which prior to their arrival in the Milky Way, orbited a dwarf galaxy with a cored density profile are expected to have experienced the amount of heating required to match the velocity dispersion observed in GD-1. This suggests that GD-1 has been accreted and that imprints of its original host galaxy, including the inner slope of its dark matter halo, remain observable in the stream today.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Sky surveys are the largest data generators in astronomy, making automated tools for extracting meaningful scientific information an absolute necessity. We show that, without the need for labels, self-supervised learning recovers representations of sky survey images that are semantically useful for a variety of scientific tasks. These representations can be directly used as features, or fine-tuned, to outperform supervised methods trained only on labeled data. We apply a contrastive learning framework on multiband galaxy photometry from the Sloan Digital Sky Survey (SDSS), to learn image representations. We then use them for galaxy morphology classification and fine-tune them for photometric redshift estimation, using labels from the Galaxy Zoo 2 data set and SDSS spectroscopy. In both downstream tasks, using the same learned representations, we outperform the supervised state-of-the-art results, and we show that our approach can achieve the accuracy of supervised models while using 2–4 times fewer labels for training. The codes, trained models, and data can be found at <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="https://portal.nersc.gov/project/dasrepo/self-supervised-learning-sdss" xlink:type="simple">https://portal.nersc.gov/project/dasrepo/self-supervised-learning-sdss</jats:ext-link>.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The NANOGrav Collaboration reported strong Bayesian evidence for a common-spectrum stochastic process in its 12.5 yr pulsar timing array data set, with median characteristic strain amplitude at periods of a year of <jats:inline-formula> <jats:tex-math> <?CDATA ${A}_{\mathrm{yr}}={1.92}_{-0.55}^{+0.75}\times {10}^{-15}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabf2c9ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>. However, evidence for the quadrupolar Hellings &amp; Downs interpulsar correlations, which are characteristic of gravitational-wave signals, was not yet significant. We emulate and extend the NANOGrav data set, injecting a wide range of stochastic gravitational-wave background (GWB) signals that encompass a variety of amplitudes and spectral shapes, and quantify three key milestones. (I) Given the amplitude measured in the 12.5 yr analysis and assuming this signal is a GWB, we expect to accumulate robust evidence of an interpulsar-correlated GWB signal with 15–17 yr of data, i.e., an additional 2–5 yr from the 12.5 yr data set. (II) At the initial detection, we expect a fractional uncertainty of 40% on the power-law strain spectrum slope, which is sufficient to distinguish a GWB of supermassive black hole binary origin from some models predicting more exotic origins. (III) Similarly, the measured GWB amplitude will have an uncertainty of 44% upon initial detection, allowing us to arbitrate between some population models of supermassive black hole binaries. In addition, power-law models are distinguishable from those having low-frequency spectral turnovers once 20 yr of data are reached. Even though our study is based on the NANOGrav data, we also derive relations that allow for a generalization to other pulsar timing array data sets. Most notably, by combining the data of individual arrays into the International Pulsar Timing Array, all of these milestones can be reached significantly earlier.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We observed the episodically active asteroid (6478) Gault in 2020 with multiple telescopes in Asia and North America and found that it is no longer active after its recent outbursts at the end of 2018 and the start of 2019. The inactivity during this apparition allowed us to measure the absolute magnitude of Gault of <jats:italic>H</jats:italic> <jats:sub> <jats:italic>r</jats:italic> </jats:sub> = 14.63 ± 0.02, <jats:italic>G</jats:italic> <jats:sub> <jats:italic>r</jats:italic> </jats:sub> = 0.21 ± 0.02 from our secular phase-curve observations. In addition, we were able to constrain Gault’s rotation period using time-series photometric lightcurves taken over 17 hr on multiple days in 2020 August, September, and October. The photometric lightcurves have a repeating ≲0.05 mag feature suggesting that (6478) Gault has a rotation period of ∼2.5 hr and may have a semispherical or top-like shape, much like the near-Earth asteroids Ryugu and Bennu. The rotation period of ∼2.5 hr is near the expected critical rotation period for an asteroid with the physical properties of (6478) Gault, suggesting that its activity observed over multiple epochs is due to surface mass shedding from its fast rotation spin-up by the Yarkovsky–O’Keefe–Radzievskii–Paddack effect.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Direct sampling of interstellar neutral (ISN) atoms close to the Sun enables studies of the very local interstellar medium (VLISM) around the heliosphere. The primary population of ISN helium atoms has, until now, been assumed to reflect the pristine VLISM conditions at the heliopause. Consequently, the atoms observed at 1 au by the Interstellar Boundary Explorer (IBEX) were used to determine the VLISM temperature and velocity relative to the Sun, without accounting for elastic collisions with other species outside the heliopause. Here, we evaluate the effect of these collisions on the primary ISN helium population. We follow trajectories of helium atoms and track their collisions with slowed plasma and interstellar hydrogen atoms ahead of the heliopause. Atoms typically collide a few times in the outer heliosheath, and only ∼1.5% of the atoms are not scattered at all. We use calculated differential cross sections to randomly choose scattering angles in these collisions. We estimate that the resulting primary ISN helium atoms at the heliopause are slowed down by ∼0.45 km s<jats:sup>−1</jats:sup> and heated by ∼1100 K compared to the pristine VLISM. The resulting velocity distribution is asymmetric and shows an extended tail in the antisunward direction. Accounting for this change in the parameters derived from IBEX observations gives the Sun’s relative speed of 25.85 km s<jats:sup>−1</jats:sup> and temperature of 6400 K in the pristine VLISM. Finally, this Letter serves as a source of the differential cross sections for elastic collisions with helium atoms.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Magnetic fields grow quickly even at early cosmological times, suggesting the action of a small-scale dynamo (SSD) in the interstellar medium of galaxies. Many studies have focused on idealized turbulent driving of the SSD. Here we simulate more realistic supernova-driven turbulence to determine whether it can drive an SSD. Magnetic field growth occurring in our models appears inconsistent with simple tangling of magnetic fields, but consistent with SSD action, reproducing and confirming models by Balsara et al. that did not include physical resistivity <jats:italic>η</jats:italic>. We vary <jats:italic>η</jats:italic>, as well as the numerical resolution and supernova rate, <jats:inline-formula> <jats:tex-math> <?CDATA $\dot{\sigma }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabed59ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, to delineate the regime in which an SSD occurs. For a given <jats:inline-formula> <jats:tex-math> <?CDATA $\dot{\sigma }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabed59ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> we find convergence for SSD growth rate with resolution of a parsec. For <jats:inline-formula> <jats:tex-math> <?CDATA $\dot{\sigma }\simeq {\dot{\sigma }}_{\mathrm{sn}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabed59ieqn3.gif" xlink:type="simple" /> </jats:inline-formula>, with <jats:inline-formula> <jats:tex-math> <?CDATA ${\dot{\sigma }}_{\mathrm{sn}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabed59ieqn4.gif" xlink:type="simple" /> </jats:inline-formula> the solar neighborhood rate, the critical resistivity below which an SSD occurs is <jats:inline-formula> <jats:tex-math> <?CDATA $0.005\gt {\eta }_{\mathrm{crit}}\gt 0.001\,\mathrm{kpc}\,\mathrm{km}\,{{\rm{s}}}^{-1}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabed59ieqn5.gif" xlink:type="simple" /> </jats:inline-formula>, and this increases with the supernova rate. Across the modeled range of 0.5–4 pc resolution we find that for <jats:inline-formula> <jats:tex-math> <?CDATA $\eta \lt {\eta }_{\mathrm{crit}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabed59ieqn6.gif" xlink:type="simple" /> </jats:inline-formula>, the SSD saturates at about 5% of kinetic energy equipartition, independent of growth rate. In the range <jats:inline-formula> <jats:tex-math> <?CDATA $0.2\,{\dot{\sigma }}_{\mathrm{sn}}\leqslant \dot{\sigma }\leqslant 8\,{\dot{\sigma }}_{\mathrm{sn}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabed59ieqn7.gif" xlink:type="simple" /> </jats:inline-formula> growth rate increases with <jats:inline-formula> <jats:tex-math> <?CDATA $\dot{\sigma }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabed59ieqn8.gif" xlink:type="simple" /> </jats:inline-formula>. SSDs in the supernova-driven interstellar medium commonly exhibit erratic growth.</jats:p>