Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>We present broadband UV/X-ray spectral variability of the changing-look active galactic nucleus (AGN) NGC 1566, based on simultaneous near-ultraviolet and X-ray observations performed by the XMM-Newton, Swift, and NuSTAR satellites at five different epochs during the declining phase of the 2018 outburst. We found that the accretion disk, soft X-ray excess, and X-ray power-law components were extremely variable. Additionally, the X-ray power-law flux was correlated with both the soft excess plus disk and the pure disk fluxes. Our finding shows that at high-flux levels the soft X-ray excess and the disk emission both provided the seed photons for thermal Comptonization in the hot corona, whereas at low-flux levels, where the soft excess was absent, the pure disk emission alone provided the seed photons. The X-ray power-law photon index was only weakly variable (ΔΓ<jats:sub>hot</jats:sub> ≤ 0.06), and it was not well correlated with the X-ray flux over the declining timescale. On the other hand, we found that the electron temperature of the corona increased from ∼22 to ∼200 keV with the decreasing numbers of seed photons from 2018 June to 2019 August. At the same time, the optical depth of the corona decreased from <jats:italic>τ</jats:italic> <jats:sub>hot</jats:sub> ∼ 4 to ∼0.7, and the scattering fraction increased from ∼1% to ∼10%. These changes suggest structural changes in the hot corona, such as it was growing in size and becoming hotter with the decreasing accretion rate during the declining phase. The AGN is most likely evolving with a decreasing accretion rate toward a state similar to the low/hard state of black hole X-ray binaries.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Through its magnetic activity, the Sun governs the conditions in Earth’s vicinity, creating space weather events, which have drastic effects on our space- and ground-based technology. One of the most important solar magnetic features creating the space weather is the solar wind that originates from the coronal holes (CHs). The identification of the CHs on the Sun as one of the source regions of the solar wind is therefore crucial to achieve predictive capabilities. In this study, we used an unsupervised machine-learning method, <jats:italic>k</jats:italic>-means, to pixel-wise cluster the passband images of the Sun taken by the Atmospheric Imaging Assembly on the Solar Dynamics Observatory in 171, 193, and 211 Å in different combinations. Our results show that the pixel-wise <jats:italic>k</jats:italic>-means clustering together with systematic pre- and postprocessing steps provides compatible results with those from complex methods, such as convolutional neural networks. More importantly, our study shows that there is a need for a CH database where a consensus about the CH boundaries is reached by observers independently. This database then can be used as the “ground truth,” when using a supervised method or just to evaluate the goodness of the models.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In a substantial number of core-collapse supernovae (SNe), early-time interaction indicates a dense circumstellar medium (CSM) that may be produced by outbursts from the progenitor star. Wave-driven mass loss is a possible mechanism to produce these signatures, with previous work suggesting that this mechanism is most effective for low-mass (∼11 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>) SN progenitors. Using one-dimensional hydrodynamic simulations with MESA, we study the effects of this wave heating in SN progenitors of masses <jats:italic>M</jats:italic> <jats:sub>ZAMS</jats:sub> = 10–13 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. This range encompasses stars that experience semidegenerate central neon burning and more degenerate off-center neon ignition. We find that central Ne ignition at <jats:italic>M</jats:italic> <jats:sub>ZAMS</jats:sub> = 11 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> produces a burst of intense wave heating that transmits ∼10<jats:sup>47</jats:sup> erg of energy at 10 yr before core collapse, whereas other masses experience smaller levels of wave heating. Wave heating does not hydrodynamically drive mass loss in any of our models and is unlikely to produce a very massive CSM on its own. However, wave heating can cause large radial expansion (by more than an order of magnitude), photospheric cooling, and luminosity brightening by up to ∼10<jats:sup>6</jats:sup> <jats:italic>L</jats:italic> <jats:sub>⊙</jats:sub> in hydrogen-poor stripped star models. Some Type Ib/c progenitors could drastically change their appearance in the final years of their lives, with brightness in the visual bands increasing by nearly 3 mag. Moreover, interaction with a close binary companion could drive intense mass loss, with implications for Type Ibn and other interaction-powered SNe.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The proton–alpha drift instability is a possible mechanism of the alpha-particle deceleration and the resulting proton heating in the solar wind. We present hybrid numerical simulations of this instability with particle-in-cell ions and a quasi-neutralizing electron fluid for typical conditions at 1 au. For the parameters used in this paper, we find that fast magnetosonic unstable modes propagate only in the direction opposite to the alpha-particle drift and do not produce the perpendicular proton heating necessary to accelerate the solar wind. Alfvén modes propagate in both directions and heat the protons perpendicularly to the mean magnetic field. Despite being driven by the alpha temperature anisotropy, the Alfvén instability also extracts the energy from the bulk motion of the alpha particles. In the solar wind, the instabilities operate in a turbulent ambient medium. We show that the turbulence suppresses the Alfvén instability but the perpendicular proton heating persists. Unlike a static nonuniform background, the turbulence does not invert the sense of the proton heating associated with the fast magnetosonic instability and it remains preferentially parallel.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Stellar active regions, including spots and faculae, can create radial velocity (RV) signals that interfere with the detection and mass measurements of low-mass exoplanets. In doing so, these active regions affect each spectral line differently, but the origin of these differences is not fully understood. Here we explore how spectral line variability correlated with S-index (Ca H and K emission) is related to the atomic properties of each spectral line. Next, we develop a simple analytic stellar atmosphere model that can account for the largest sources of line variability with S-index. Then, we apply this model to HARPS spectra of <jats:italic>α</jats:italic> Cen B to explain Fe <jats:sc>i</jats:sc> line depth changes in terms of a disk-averaged temperature difference between active and quiet regions on the visible hemisphere of the star. This work helps establish a physical basis for understanding how stellar activity manifests differently in each spectral line and may help future work mitigating the impact of stellar activity on exoplanet RV surveys.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Extreme-mass-ratio inspirals (EMRIs) are important targets for future space-borne gravitational-wave (GW) detectors, such as the Laser Interferometer Space Antenna (LISA). Recent works suggest that EMRIs may reside in a population of newly discovered X-ray transients called “quasiperiodic eruptions” (QPEs). Here, we follow this scenario and investigate whether LISA could in the future detect the QPEs. We consider two specific models, in which the QPEs are made of either stellar-mass objects moving on circular orbits around massive black holes (MBHs) or white dwarfs (WDs) on eccentric orbits around MBHs. We find that in either case the five QPEs detected so far are too weak to be resolvable by LISA. However, if QPEs are made of eccentric WD–MBH binaries, they radiate GWs over a wide range of frequencies. The broad spectra overlap to form a background that peaks in the milli-Hertz band and has a signal-to-noise ratio of 9–17 even in the most pessimistic scenario. The presence of this GW background in the LISA band could impact future searches for seed black holes at high redshift as well as stellar-mass binary black holes in the local universe.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We explore whether assumptions about dust grain shape affect the resulting estimates of the composition and grain size distribution of the AU Microscopii (AU Mic) debris disk from scattered-light data collected by Lomax et al. The near edge-on orientation of the AU Mic debris disk makes it ideal for studying the effect of the scattering phase function on the measured flux ratios as a function of wavelength and projected distance. Previous efforts to model the AU Mic debris disk have invoked a variety of dust grain compositions and explored the effect of porosity, but did not undertake a systematic effort to explore a full range of size distributions and compositions to understand possible degeneracies in fitting the data. The degree to which modeling dust grains with more realistic shapes compounds these degeneracies has also not previously been explored. We find differences in the grain properties retrieved depending on the grain shape model used. We also present here our calculations of porous grains of size parameters <jats:italic>x</jats:italic> = 0.1 to 48 and complex refractive indices (<jats:italic>m</jats:italic> = <jats:italic>n</jats:italic> + <jats:italic>iκ</jats:italic>) ranging from <jats:italic>n</jats:italic> = 1.1 to 2.43 and <jats:italic>k</jats:italic> = 0 to 1.0, covering multiple compositions at visible and near-infrared wavelengths such as ice, silicates, amorphous carbon, and tholins.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Ultraviolet and X-rays from radiation sources disperse nearby gas clumps by driving winds due to heating associated with the photochemical processes. This dispersal process, photoevaporation, constrains the lifetimes of the parental bodies of stars and planets. To understand this process in a universal picture, we develop an analytical model that describes the fundamental physics in the vicinity of the wind-launching region. The model explicitly links the density and velocity of photoevaporative winds at the launch points to the radiation flux reaching the wind-launching base, using a jump condition. The model gives a natural boundary condition for the wind-emanating points. We compare the analytical model with the results of radiation hydrodynamic simulations, where a protoplanetary disk is irradiated by the stellar extreme-ultraviolet, and confirm good agreement of the base density and velocity, and radial profiles of mass-loss rates. We expect that our analytical model is applicable to other objects subject to photoevaporation not only by extreme-ultraviolet but by far-ultraviolet/X-rays with suitable modifications. Future self-consistent numerical studies can test the applicability.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>It has been suggested before that small-scale magnetic flux rope (SMFR) structures in the solar wind can temporarily trap energetic charged particles. We present the derivation of a new fractional Parker equation for energetic-particle interaction with SMFRs from our pitch-angle-dependent fractional diffusion-advection equation that can account for such trapping effects. The latter was derived previously in le Roux &amp; Zank from the first principles starting with the standard focused transport equation. The new equation features anomalous advection and diffusion terms. It suggests that energetic-particle parallel transport occurs with a decaying efficiency of advection effects as parallel superdiffusion becomes more dominant at late times. Parallel superdiffusion can be linked back to underlying anomalous pitch-angle transport, which might be subdiffusive during interaction with quasi-helical coherent SMFRs. We apply the new equation to time-dependent superdiffusive shock acceleration at a parallel shock. The results show that the superdiffusive-shock-acceleration timescale is fractional, the net fractional differential particle flux is conserved across the shock ignoring particle injection at the shock, and the accelerated particle spectrum at the shock converges to the familiar power-law spectrum predicted by standard steady-state diffusive-shock-acceleration theory at late times. Upstream, as parallel superdiffusion progressively dominates the advection of energetic particles, their spatial distributions decay on spatial scales that grow with time. Furthermore, superdiffusive parallel shock acceleration is found to be less efficient if parallel anomalous diffusion is more superdiffusive, while perpendicular particle escape from the shock, thought to be subdiffusive during SMFR interaction, is reduced when increasingly subdiffusive.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Lyman continuum (LyC) cannot be observed at the epoch of reionization (<jats:italic>z</jats:italic> ≳ 6) owing to intergalactic H <jats:sc>i</jats:sc> absorption. To identify LyC emitters (LCEs) and infer the fraction of escaping LyC, astronomers have developed various indirect diagnostics of LyC escape. Using measurements of the LyC from the Low-redshift Lyman Continuum Survey (LzLCS), we present the first statistical test of these diagnostics. While optical depth indicators based on Ly<jats:italic>α</jats:italic>, such as peak velocity separation and equivalent width, perform well, we also find that other diagnostics, such as the [O <jats:sc>iii</jats:sc>]/[O <jats:sc>ii</jats:sc>] flux ratio and star formation rate surface density, predict whether a galaxy is an LCE. The relationship between these galaxy properties and the fraction of escaping LyC flux suggests that LyC escape depends strongly on H <jats:sc>i</jats:sc> column density, ionization parameter, and stellar feedback. We find that LCEs occupy a range of stellar masses, metallicities, star formation histories, and ionization parameters, which may indicate episodic and/or different physical causes of LyC escape.</jats:p>