Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>Ion escape to space through the interaction of solar wind and Mars is an important factor influencing the evolution of the Martian atmosphere. The plasma clouds (explosive bulk plasma escape), considered an important ion escaping channel, have been recently identified by spacecraft observations. However, our knowledge about Martian plasma clouds is lacking. Based on the observations of the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, we study a sequence of periodic plasma clouds that occurred at low altitudes (∼600 km) on Mars. We find that the heavy ions in these clouds are energy-dispersed and have the same velocity, regardless of species. By tracing such energy-dispersed ions, we find the source of these clouds is located in a low-altitude ionosphere (∼120 km). The average tailward moving flux of ionospheric plasma carried by clouds is on the order of 10<jats:sup>7</jats:sup> cm<jats:sup>−2</jats:sup> s<jats:sup>−1</jats:sup>, which is one order higher than the average escaping flux for the magnetotail, suggesting explosive ion escape via clouds. Based on the characteristics of clouds, we suggest, similar to the outflow of Earth’s cusp, these clouds might be the product of heating due to solar wind precipitation along the open field lines, which were generated by magnetic reconnection between the interplanetary magnetic field and crustal fields that occurred above the source.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Recently an intriguing transient, AT 2018lqh, with only a day-scale duration and a high luminosity of 7 × 10<jats:sup>42</jats:sup> erg s<jats:sup>−1</jats:sup>, was discovered. While several possibilities are raised on its origin, the nature of this transient is yet to be unveiled. We propose that a black hole (BH) with ∼30 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> forming from a rotating blue supergiant can generate a transient like AT 2018lqh. We find that this scenario can consistently explain the optical/UV emission and the tentative late-time X-ray detection, as well as the radio upper limits. If super-Eddington accretion onto the nascent BH powers the X-ray emission, continued X-ray observations may be able to test the presence of an accretion disk around the BH.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Using a hybrid-kinetic particle-in-cell simulation, we study the evolution of an expanding, collisionless, magnetized plasma in which strong Alfvénic turbulence is persistently driven. Temperature anisotropy generated adiabatically by the plasma expansion (and consequent decrease in the mean magnetic-field strength) gradually reduces the effective elasticity of the field lines, causing reductions in the linear frequency and residual energy of the Alfvénic fluctuations. In response, these fluctuations modify their interactions and spatial anisotropy to maintain a scale-by-scale “critical balance” between their characteristic linear and nonlinear frequencies. Eventually the plasma becomes unstable to kinetic firehose instabilities, which excite rapidly growing magnetic fluctuations at ion-Larmor scales. The consequent pitch-angle scattering of particles maintains the temperature anisotropy near marginal stability, even as the turbulent plasma continues to expand. The resulting evolution of parallel and perpendicular temperatures does not satisfy double-adiabatic conservation laws, but is described accurately by a simple model that includes anomalous scattering. Our results have implications for understanding the complex interplay between macro- and microscale physics in various hot, dilute, astrophysical plasmas, and offer predictions concerning power spectra, residual energy, ion-Larmor-scale spectral breaks, and non-Maxwellian features in ion distribution functions that may be tested by measurements taken in high-beta regions of the solar wind.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In this Letter, we study the evolution of the autocovariance function of density-field fluctuations in star-forming clouds and thus of the correlation length <jats:italic>l</jats:italic> <jats:sub> <jats:italic>c</jats:italic> </jats:sub>(<jats:italic>ρ</jats:italic>) of these fluctuations, which can be identified as the average size of the most correlated structures within the cloud. Generalizing the transport equation derived by Chandrasekhar for static, homogeneous turbulence, we show that the mass contained within these structures is an invariant, i.e., that the average mass contained in the most correlated structures remains constant during the evolution of the cloud, whatever dominates the global dynamics (gravity or turbulence). We show that the growing impact of gravity on the turbulent flow yields an increase of the variance of the density fluctuations and thus a drastic decrease of the correlation length. Theoretical relations are successfully compared to numerical simulations. This picture brings a robust support to star formation paradigms where the mass concentration in turbulent star-forming clouds evolves from initially large, weakly correlated filamentary structures to smaller, denser, more correlated ones, and eventually to small, tightly correlated, prestellar cores. We stress that the present results rely on a pure statistical approach of density fluctuations and do not involve any specific condition for the formation of prestellar cores. Interestingly enough, we show that, under average conditions typical of Milky-Way molecular clouds, this invariant average mass is about a solar mass, providing an appealing explanation for the apparent universality of the IMF in such environments.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Type I Be/X-ray binary outbursts are driven by mass transfer from a Be star decretion disk to a neutron star companion during each orbital period. Treiber et al. recently observed nonperiodic type I outbursts in RX J0529.8–6556 that has unknown binary orbital properties. We show that nonperiodic type I outbursts may be temporarily driven in a low eccentricity binary with a disk that is inclined sufficiently to be mildly unstable to Kozai–Lidov oscillations. The inclined disk becomes eccentric and material is transferred to the neutron star at up to three locations in each orbit: when the neutron star passes the disk apastron or one of the two nodes of the disk. The timing and magnitude of each vary with the disk argument of periapsis and longitude of the ascending node that precess in opposite directions. Calculating the orbital period of the RX J0529.8–6556 system is nontrivial but we suggest it may be &gt;300 days, longer than previous estimates.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Two forms of ohmic heating of astrophysical secondaries have received particular attention: unipolar-generator heating with currents running between the primary and secondary, and magnetic induction heating due to the primary’s time-varying field. Neither appears to cause significant dissipation in the contemporary solar system. But these discussions have overlooked heating derived from the spatial variation of the primary’s field across the interior of the secondary. This leads to Lorentz-force-driven currents around paths entirely internal to the secondary, with resulting ohmic heating. We examine three ways to drive such currents, by the cross product of (1) the secondary’s azimuthal orbital velocity with the nonaxially symmetric field of the primary, (2) the radial velocity (due to nonzero eccentricity) of the secondary with the primary’s field, or (3) the out-of-plane velocity (due to nonzero inclination) with the primary’s field. The first of these operates even for a spin-locked secondary whose orbit has zero eccentricity, in strong contrast to tidal dissipation. We show that Jupiter’s moon Io today could dissipate about 600 GW (more than likely current radiogenic heating) in the outer 100 m of its metallic core by this mechanism. Had Io ever been at 3 Jovian radii instead of its current 5.9, it could have been dissipating 15,000 GW. Ohmic dissipation provides a mechanism that could operate in any solar system to drive inward migration of secondaries that then necessarily comes to a halt upon reaching a sufficiently close distance to the primary.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Direct-collapse black holes (DCBHs) forming at <jats:italic>z</jats:italic> ∼ 20 are currently the leading candidates for the seeds of the first quasars, over 200 of which have now been found at <jats:italic>z</jats:italic> &gt; 6. Recent studies suggest that DCBHs could be detected in the near-infrared by the James Webb Space Telescope, Euclid, and the Roman Space Telescope. However, new radio telescopes with unprecedented sensitivities such as the Square Kilometre Array (SKA) and the Next-Generation Very Large Array (ngVLA) may open another window on the properties of DCBHs in the coming decade. Here we estimate the radio flux from DCBHs at birth at <jats:italic>z</jats:italic> = 8–20 with several fundamental planes of black hole accretion. We find that they could be detected at <jats:italic>z</jats:italic> ∼ 8 by the SKA-FIN all-sky survey. Furthermore, SKA and ngVLA could discover 10<jats:sup>6</jats:sup>–10<jats:sup>7</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> BHs out to <jats:italic>z</jats:italic> ∼ 20, probing the formation pathways of the first quasars in the universe.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Determining the fraction of nearby dwarf galaxies hosting massive black holes (BHs) can inform our understanding of the origin of “seed” BHs at high redshift. Here we search for signatures of accreting massive BHs in a sample of dwarf galaxies (<jats:italic>M</jats:italic> <jats:sub>⋆</jats:sub> ≤ 3 × 10<jats:sup>9</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, <jats:italic>z</jats:italic> ≤ 0.15) selected from the NASA-Sloan Atlas (NSA) using X-ray observations from the eROSITA Final Equatorial Depth Survey (eFEDS). On average, our search is sensitive to active galactic nuclei (AGNs) in dwarf galaxies that are accreting at ≳1% of their Eddington luminosity. Of the ∼28,000 X-ray sources in eFEDS and the 495 dwarf galaxies in the NSA within the eFEDS footprint, we find six galaxies hosting possible active massive BHs. If the X-ray sources are indeed associated with the dwarf galaxies, the X-ray emission is above that expected from star formation, with X-ray source luminosities of <jats:italic>L</jats:italic> <jats:sub>0.5–8 keV</jats:sub> ∼ 10<jats:sup>39–40</jats:sup> erg s<jats:sup>−1</jats:sup>. Additionally, after accounting for chance alignments of background AGNs with dwarf galaxies, we estimate there are between zero and nine real associations between dwarf galaxies and X-ray sources in the eFEDS field at the 95% confidence level. From this we find an upper limit on the eFEDS-detected dwarf galaxy AGN fraction of ≤1.8%, which is broadly consistent with similar studies at other wavelengths. We extrapolate these findings from the eFEDS sky coverage to the planned eROSITA All-Sky Survey and estimate that upon completion, the all-sky survey could yield as many as ∼1350 AGN candidates in dwarf galaxies at low redshift.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Advanced Small Analyzer for Neutrals (ASAN) on board the Chang’E-4 Yutu-2 rover first detected energetic neutral atoms (ENAs) originating from the lunar surface at various lunar local times on the lunar farside. In this work, we examine the ENA energy spectra, obtained in the first 23 lunar days from 2019 January 11 to 2020 October 12, and find a higher ENA differential flux on the lunar dawnside than on the duskside. Combined with Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon’s Interaction with the Sun (ARTEMIS) data, we analyze the correlation between the ENA differential flux and solar wind parameters, such as flux, density, dynamic pressure, and velocity, for each ASAN energy channel on the dawnside and duskside. The results show that ENA differential flux is positively correlated with solar wind flux, density, and dynamic pressure and relatively lower on the duskside than on the dawnside. To determine the relationship between solar wind energy and ENA energy, we analyze the correlation between solar wind energy and ENA cutoff energy and temperature on the dawnside and duskside. The results show that the ENA cutoff energy and temperature are lower on the duskside than on the dawnside at the same solar wind energy. The difference between the ENA–solar wind observation on the dawnside and duskside is possibly caused by solar wind deflection and deceleration on the duskside, which can be attributed to the interaction between solar wind and the lunar magnetic anomalies located nearby in the northwestern direction of the Chang’E-4 landing site.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Nova Her 2021 (V1674 Her), which erupted on 2021 June 12, reached naked-eye brightness and has been detected from radio to <jats:italic>γ</jats:italic>-rays. An extremely fast optical decline of 2 magnitudes in 1.2 days and strong Ne lines imply a high-mass white dwarf. The optical pre-outburst detection of a 501.42 s oscillation suggests a magnetic white dwarf. This is the first time that an oscillation of this magnitude has been detected in a classical nova prior to outburst. We report X-ray outburst observations from Swift and Chandra that uniquely show (1) a very strong modulation of supersoft X-rays at a different period from reported optical periods, (2) strong pulse profile variations and the possible presence of period variations of the order of 0.1–0.3 s, and (3) rich grating spectra that vary with modulation phase and show P Cygni–type emission lines with two dominant blueshifted absorption components at ∼3000 and 9000 km s<jats:sup>−1</jats:sup> indicating expansion velocities up to 11,000 km s<jats:sup>−1</jats:sup>. X-ray oscillations most likely arise from inhomogeneous photospheric emission related to the magnetic field. Period differences between reported pre- and post-outburst optical observations, if not due to other period drift mechanisms, suggest a large ejected mass for such a fast nova, in the range 2 × 10<jats:sup>−5</jats:sup>–2 × 10<jats:sup>−4</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. A difference between the period found in the Chandra data and a reported contemporaneous post-outburst optical period, as well as the presence of period drifts, could be due to weakly nonrigid photospheric rotation.</jats:p>