Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>GW170817 is the only gravitational-wave event for which a confirmed <jats:italic>γ</jats:italic>-ray counterpart, GRB 170817A, has been detected. Here, we present a method to search for another type of <jats:italic>γ</jats:italic>-ray signal, a <jats:italic>γ</jats:italic>-ray burst precursor, associated with a compact binary merger. If emitted shortly before the coalescence, a high-energy electromagnetic (EM) flash travels through a highly dynamical and relativistic environment, created by the two compact objects orbiting each other. Thus, the EM signal arriving at an Earth observer could present a somewhat predictable time-dependent modulation. We describe a targeted search method for light curves exhibiting such a modulation, parameterized by the observer-frame component masses and binary merger time, using Fermi-GBM data. The sensitivity of the method is assessed based on simulated signals added to GBM data. The method is then applied to a selection of potentially interesting compact binary mergers detected during the second (O2) and third (O3) observing runs of Advanced LIGO and Advanced Virgo. We find no significant modulated <jats:italic>γ</jats:italic>-ray precursor signal associated with any of the considered events.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We explore how the assumption of ionization equilibrium modulates the modeled intergalactic medium at the end of the hydrogen epoch of reionization using the cosmological radiation hydrodynamic <jats:sc>Technicolor Dawn</jats:sc> simulation. In neutral and partially ionized regions where the metagalactic ultraviolet background is weak, the ionization timescale <jats:italic>t</jats:italic> <jats:sub>ion</jats:sub> ≡ Γ<jats:sup>−1</jats:sup> exceeds the Hubble time. Assuming photoionization equilibrium in such regions artificially boosts the ionization rate, accelerating reionization. By contrast, the recombination time <jats:italic>t</jats:italic> <jats:sub>rec</jats:sub> &lt; <jats:italic>t</jats:italic> <jats:sub>ion</jats:sub> in photoionized regions, with the result that assuming photoionization equilibrium artificially increases the neutral hydrogen fraction. Using snapshots in the range 8 ≥ <jats:italic>z</jats:italic> ≥ 5, we compare the predicted Ly<jats:italic>α</jats:italic> forest (LAF) flux power spectrum with and without the assumption of ionization equilibrium. Small scales (<jats:italic>k</jats:italic> &gt; 0.1 rad s km<jats:sup>−1</jats:sup>) exhibit reduced power from 7 ≤ <jats:italic>z</jats:italic> ≤ 5.5 in the ionization equilibrium case, while larger scales are unaffected. This occurs for the same reasons: ionization equilibrium artificially suppresses the neutral fraction in self-shielded gas and boosts ionizations in voids, suppressing small-scale fluctuations in the ionization field. When the volume-averaged neutral fraction drops below 10<jats:sup>−4</jats:sup>, the signature of nonequilibrium ionizations on the LAF disappears. Comparing with recent observations indicates that these nonequilibrium effects are not yet observable in the LAF flux power spectrum.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Changing-look active galactic nuclei (CLAGNs) show the disappearance and reappearance of broad emission lines in a few years, which challenges the orientation-based AGN unification model. We reduce the X-ray data for five well-studied CLAGNs that show a strong change in broad emission lines in the past several decades. We find that the X-ray photon index, Γ, and the Eddington-scaled X-ray luminosity, <jats:italic>L</jats:italic> <jats:sub>2–10 keV</jats:sub>/<jats:italic>L</jats:italic> <jats:sub>Edd</jats:sub>, normally follow negative and positive correlations when the Eddington ratio is lower and higher than a critical value of ∼10<jats:sup>−3</jats:sup>. We find that the CLAGNs observed with broad H<jats:italic>β</jats:italic> emission lines stay in the positive part of the Γ–<jats:italic>L</jats:italic> <jats:sub>2–10 keV</jats:sub>/<jats:italic>L</jats:italic> <jats:sub>Edd</jats:sub> correlation, while the broad H<jats:italic>β</jats:italic> lines become weak or disappear in the anticorrelation part of the Γ–<jats:italic>L</jats:italic> <jats:sub>2–10 keV</jats:sub>/<jats:italic>L</jats:italic> <jats:sub>Edd</jats:sub> correlation, which suggests that the evolution of the broad lines should be correlated with the evolution of the underlying accretion process. We further find that the CLAGNs are consistent with the other different types of AGNs in the <jats:italic>L</jats:italic> <jats:sub>bol</jats:sub>–<jats:italic>L</jats:italic> <jats:sub>bol</jats:sub>/<jats:italic>L</jats:italic> <jats:sub>Edd</jats:sub> correlation. These results support that the CLAGNs belong to a special stage of AGNs with a bolometric Eddington ratio ∼1%, where the broad emission lines are easily affected by the strong variation in ionization luminosity that is caused by the transition of accretion modes.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>A low-lying resonance in FeCN<jats:sup>−</jats:sup> anion was identified through abrupt changes in the spectral dependence of the photoelectron angular distribution. Non-Franck–Condon transitions from the resonance to the neutral FeCN (<jats:sup>4</jats:sup>Δ), and the corresponding photoelectron angular distributions revealed that the resonance is a dipole scattering state. Significant thermionic electron emission was observed in the resonant photoelectron spectra, indicating a strong coupling of the resonance with the ground state of this triatomic anion and its competition over autodetachment. This low-lying resonance is identified to be an efficient pathway for the formation of FeCN<jats:sup>−</jats:sup> anion in the outer envelope of IRC+10216. The results in general reveal formation pathways in space for anions with low-lying resonances and large permanent dipole moment.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Extremely metal-poor (EMP) stars provide a valuable probe of early chemical enrichment in the Milky Way. Here we leverage a large sample of ∼600,000 high-resolution stellar spectra from the GALAH survey plus a machine-learning algorithm to find 54 candidates with estimated [Fe/H] ≤−3.0, six of which have [Fe/H] ≤−3.5. Our sample includes ∼20% main-sequence EMP candidates, unusually high for EMP star surveys. We find the magnitude-limited metallicity distribution function of our sample is consistent with previous work that used more complex selection criteria. The method we present has significant potential for application to the next generation of massive stellar spectroscopic surveys, which will expand the available spectroscopic data well into the millions of stars.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Direct-imaging spectra hold rich information about a planet’s atmosphere and surface, and several space-based missions aiming at such observations will become a reality in the near future. Previous spectral retrieval works have resulted in key atmospheric constraints under the assumption of a gray surface, but the effect of wavelength-dependent surface albedo on retrieval has not been shown. We explore the influence of the coupling effect of cloud and wavelength-dependent surface albedo on retrieval performance via modeling suites of Earth-like atmospheres with varying cloud and surface albedo parameterizations. Under the assumption of known cloud scattering properties, the surface spectral albedos can be reasonably recovered when the surface cover represents that of Earth-like vegetation or ocean, which may aid in characterizing the planet’s habitability. When the cloud scattering properties cannot be assumed, we show that the degeneracy between the cloud properties and wavelength-dependent surface albedo leads to biased results of atmospheric and cloud properties. The multiepoch visible-band observations offer limited improvement in disentangling this degeneracy. However, the constraints on atmospheric properties from the combination of the UV band (<jats:italic>R</jats:italic> ∼ 6) + visible band (<jats:italic>R</jats:italic> ∼ 140) are consistent with input values to within 1<jats:italic>σ</jats:italic>. If short-bandpass data are not available, an alternative solution to reduce the retrieval uncertainties would be to have the prior constraints on the planetary cloud fraction with less than 20% uncertainty.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We investigate properties of solar-wind-like plasma turbulence using direct numerical simulations. We analyze the transition from large, magnetohydrodynamic (MHD) scales to the ion characteristic ones using two-dimensional hybrid (fluid electrons and kinetic ions) simulations. To capture and quantify turbulence properties, we apply the Karman–Howarth–Monin (KHM) equation for compressible Hall–MHD (extended by considering the plasma pressure as a tensor quantity) to the numerical results. The KHM analysis indicates that the transition from MHD to ion scales (the so-called ion break in the power spectrum) results from a combination of an onset of Hall physics and an effective dissipation owing to the pressure–strain energy-exchange channel and resistivity. We discuss the simulation results in the context of the solar wind.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Laboratory measurements of O <jats:sc>vi</jats:sc> K-shell emission lines are presented that are situated near the O <jats:sc>viii</jats:sc> Ly<jats:italic>α</jats:italic> line at 19 Å. The data provide additional rest-frame references for velocity determinations based on absorption features in the spectra of warm absorbers in active galactic nuclei and other astrophysical objects. They also provide benchmarks for testing atomic structure calculations of energy levels with electrons in a high principal quantum number (<jats:italic>n</jats:italic> = 3, 4). Excellent agreement is found with our calculations using the many-body perturbation theory method, and we provide a complete listing of the O <jats:sc>vi</jats:sc> energy levels calculated with this approach.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present 870 <jats:italic>μ</jats:italic>m Atacama Large Millimeter/submillimeter Array polarization observations of thermal dust emission from the iconic, edge-on debris disk <jats:italic>β</jats:italic> Pic. While the spatially resolved map does not exhibit detectable polarized dust emission, we detect polarization at the ∼3<jats:italic>σ</jats:italic> level when averaging the emission across the entire disk. The corresponding polarization fraction is <jats:italic>P</jats:italic> <jats:sub>frac</jats:sub> = 0.51% ± 0.19%. The polarization position angle <jats:italic>χ</jats:italic> is aligned with the minor axis of the disk, as expected from models of dust grains aligned via radiative alignment torques (RAT) with respect to a toroidal magnetic field (<jats:italic>B</jats:italic>-RAT) or with respect to the anisotropy in the radiation field (<jats:italic>k</jats:italic>-RAT). When averaging the polarized emission across the outer versus inner thirds of the disk, we find that the polarization arises primarily from the SW third. We perform synthetic observations assuming grain alignment via both <jats:italic>k</jats:italic>-RAT and <jats:italic>B</jats:italic>-RAT. Both models produce polarization fractions close to our observed value when the emission is averaged across the entire disk. When we average the models in the inner versus outer thirds of the disk, we find that <jats:italic>k</jats:italic>-RAT is the likely mechanism producing the polarized emission in <jats:italic>β</jats:italic> Pic. A comparison of timescales relevant to grain alignment also yields the same conclusion. For dust grains with realistic aspect ratios (i.e., <jats:italic>s</jats:italic> &gt; 1.1), our models imply low grain-alignment efficiencies.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In this paper, we present two improved Amati correlations of gamma-ray burst (GRB) data via a powerful statistical tool called copula. After calibrating with the low-redshift GRB data, the improved Amati correlations based on a fiducial Λ cold dark matter (ΛCDM) model with Ω<jats:sub>m0</jats:sub> = 0.3 and <jats:italic>H</jats:italic> <jats:sub>0</jats:sub> = 70 km s<jats:sup>−1</jats:sup> Mpc<jats:sup>−1</jats:sup>, and extrapolating the results to the high-redshift GRB data, we obtain the Hubble diagram of GRB data points. Applying these GRB data to constrain the ΛCDM model, we find that the improved Amati correlation from copula can give a result well consistent with Ω<jats:sub>m0</jats:sub> = 0.3, while the standard Amati and extended Amati correlations do not. This results suggest that when the improved Amati correlation from copula is used in the low-redshift calibration method, the GRB data can be regarded as a viable cosmological explorer. However, the Bayesian information criterion indicates that the standard Amati correlation remains to be favored mildly since it has the least model parameters. Furthermore, once the simultaneous fitting method rather than the low-redshift calibration one is used, there is no apparent evidence that the improved Amati correlation is better than the standard one. Thus, more work needs to be done in the future in order to compare different Amati correlations.</jats:p>