Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>Growing evidence indicates that the synchrotron radiation mechanism may be responsible for the prompt emission of gamma-ray bursts (GRBs). In the synchrotron radiation scenario, the electron energy spectrum of the prompt emission is diverse in theoretical works and has not been estimated from observations in a general way (i.e., without specifying a certain physical model for the electron spectrum). In this paper, we creatively propose a method to directly estimate the electron spectrum for the prompt emission, without specifying a certain physical model for the electron spectrum in the synchrotron radiation scenario. In this method, an empirical function (i.e., a four-order Bézier curve jointed with a linear function at high energy) is applied to describe the electron spectrum in log–log coordinates. It is found that our empirical function can well mimic the electron spectra obtained in many numerical calculations or simulations. Then, our method can figure out the electron spectrum for the prompt emission without specifying a model. By employing our method on observations, taking GRB 180720B and GRB 160509A as examples, it is found that the obtained electron spectra are generally different from that in the standard fast-cooling scenario and even a broken power law. Moreover, the morphology of electron spectra in its low-energy regime varies with time in a burst and even in a pulse. Our proposed method provides a valuable way to confront the synchrotron radiation mechanism with observations.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We examine a sample of 21 gamma-ray burst (GRB) afterglow light curves at radio frequencies and compare them to the X-ray and/or optical properties of the afterglows and to the predictions of the standard jet/fireball model. Our sample includes every Swift GRB with an X-ray light curve indicating a jet break and with a published radio light curve, as well as several other targets with observed X-ray or and/optical jet breaks. We examine the late-time decline of each burst and attempt to fit an analytical model based on the standard GRB afterglow equations to each data set. We show that most of the events in our Swift GRB sample are incompatible with the radio light-curve behavior predicted by conventional afterglow theory. Many exhibit a late-time radio decline incompatible with the post-break X-ray or optical afterglow. Only one radio afterglow in this sample, at any time, shows the eventually expected decline of ∼<jats:italic>t</jats:italic> <jats:sup>−2</jats:sup>, although two others show it in their millimeter light curve. Several others remain consistent with the standard model if such a decline began after the observations. The radio behavior alone does not, however, indicate whether a GRB can be fit by our modeling code. Indeed, several of the well-fit GRBs may only appear so due to a lack of multiwavelength data. While a second source of emission can account for some of the anomalous radio behavior, our tests indicate this is often not the case unless the main jet component is simultaneously suppressed.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We deploy and demonstrate the capabilities of the magnetic field model developed by Ewertowski &amp; Basu by fitting observed polarimetry data of the prestellar core FeSt 1–457. The analytic hourglass magnetic field function derived directly from Maxwell’s equations yields a central-to-surface magnetic field strength ratio in the equatorial plane, as well as magnetic field directions with relative magnitudes throughout the core. This fit emerges from a comparison of a single plane of the model with the polarization map that results from the integrated properties of the magnetic field and dust throughout the core. Importantly, our fit is independent of any assumed density profile of the core. We check the robustness of the fit by using the POLARIS code to create synthetic polarization maps that result from the integrated scattering and emission properties of the dust grains and their radiative transfer, employing an observationally motivated density profile. We find that the synthetic polarization maps obtained from the model also provide a good fit to the observed polarimetry. Our model fits the striking feature of significant curvature of magnetic field lines in the outer part of FeSt 1–457. Combined with independent column density estimates, we infer that the core of size <jats:italic>R</jats:italic> <jats:sub>gas</jats:sub> has a mildly supercritical mass-to-flux ratio and may have formed through dynamical motions starting from a significantly larger radius <jats:italic>R</jats:italic>. A breakdown of flux freezing through neutral-ion slip (ambipolar diffusion) could be responsible for effecting such a transition from a large-scale magnetic field structure to a more compact gas structure.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report distance measurements for the Norma cluster based on the near-infrared <jats:italic>J</jats:italic>- and <jats:italic>K</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub>-band fundamental plane (FP) relations. Our simultaneous <jats:italic>J-</jats:italic> and <jats:italic>K</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub>-band photometry analyses were performed using 31 early-type galaxies in the nearby Norma cluster obtained using the 1.4 m InfraRed Survey Facility (IRSF) at the South African Astronomical Observatory. Our final <jats:italic>K</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub>-band FP sample consists of 41 early-type galaxies from the Norma cluster observed using the IRSF and the New Technology Telescope (NTT) at the European Southern Observatory. This is the largest cluster sample used for peculiar velocity studies in the Great Attractor region to date. From the <jats:italic>K</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub>-band FP, we find a distance to the Norma cluster of 4915 ± 121 km s<jats:sup>−1</jats:sup>. The implied peculiar velocity for Norma is 44 ± 151 km s<jats:sup>−1</jats:sup>, which further supports a small peculiar velocity for the Norma cluster.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>It is generally thought that FRII radio galaxies host thin optically thick disks, while FRIs are powered by advection-dominated accretion flows. Sources with an efficient engine are optically classified as high-excitation radio galaxies (HERGs) and those with an inefficient motor as low-excitation radio galaxies (LERGs). Recently, the study of radio galaxies down to mJy fluxes has cast serious doubts on the LERG-FRI and HERG-FRII correspondence, revealing that many LERGs show FRII radio morphologies. The FR catalogs recently compiled by Capetti et al. and Baldi et al. have allowed us to explore this issue in the local (<jats:italic>z</jats:italic> ≤ 0.15) mJy universe. Our statistical study shows that the majority of nearby mJy objects are in a late stage of their life. FRII-LERGs appear more similar to the old FRI-LERGs than to the young FRII-HERGs. FRII-LERGs may be aged HERGs that, having exhausted their cold fuel, have changed their accretion regime or are a separate LERG class particularly efficient in launching jets. Exploiting the empirical relations that convert <jats:italic>L</jats:italic> <jats:sub>[O <jats:sc>III</jats:sc>]</jats:sub> and <jats:italic>L</jats:italic> <jats:sub>1.4 GHz</jats:sub> into accretion power and jet kinetic power, respectively, we observed that LERGs with similar masses and accretion rates seem to expel jets of different powers. We speculate that intrinsic differences related to the black hole properties (spin and magnetic field at its horizon) can determine the observed spread in jet luminosity. In this view, FRII-LERGs should have the fastest spinning black holes and/or the most intense magnetic fluxes. On the contrary, compact LERGs (i.e., FR0s) should host extremely slow black holes and/or weak magnetic fields.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present a 5 Å–100 <jats:italic>μ</jats:italic>m spectral energy distribution (SED) of the ultracool dwarf star TRAPPIST-1, obtained as part of the Mega-MUSCLES Treasury Survey. The SED combines ultraviolet and blue-optical spectroscopy obtained with the Hubble Space Telescope, X-ray spectroscopy obtained with XMM-Newton, and models of the stellar photosphere, chromosphere, transition region, and corona. A new differential emission measure model of the unobserved extreme-ultraviolet spectrum is provided, improving on the Ly<jats:italic>α</jats:italic>–EUV relations often used to estimate the 100–911 Å flux from low-mass stars. We describe the observations and models used, as well as the recipe for combining them into an SED. We also provide a semiempirical, noise-free model of the stellar ultraviolet spectrum based on our observations for use in atmospheric modeling of the TRAPPIST-1 planets.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The advent of global millimeter-band very long baseline interferometry (VLBI) in recent years has finally revealed the morphology of the base of the two most prominent nearby, bright, extragalactic radio jets in M87 and 3C 84. The images are quite surprising considering the predictions of jet theory and current numerical modeling. The jet bases are extremely wide compared to expectations, and the nucleus of 3C 84 is very complicated. It appears as a double nucleus in 86 GHz observations with 50 <jats:italic>μ</jats:italic>as resolution and a triple nucleus with 30 <jats:italic>μ</jats:italic>as resolution with space-based VLBI by RadioAstron at 22 GHz. What is even odder is that the double and triple nuclei are arranged along an east–west line that is approximately orthogonal to the north–south large-scale jet on 150 <jats:italic>μ</jats:italic>as−4 mas scales. We explore the emergence of an (east–west) double nucleus in the lower-resolution 43 GHz Very Long Baseline Array (VLBA) imaging from 2018 August to 2020 April. The double nucleus is marginally resolved. We exploit the east–west resolution associated with the longest baselines, ∼0.08 mas, to track a predominantly east–west separation speed of ≈0.086 ± 0.008<jats:italic>c</jats:italic>. We estimate that the observed mildly relativistic speed persists over a deprojected distance of ∼1900–9800 times the central, supermassive black hole gravitational radius (∼0.3–1.5 lt-yr) from the point of origin.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We have investigated the prompt light curves of long gamma-ray bursts (GRBs) from the Swift/BAT catalog. We aimed to characterize their power spectral densities (PSDs), search for quasiperiodic oscillations (QPOs), and conduct novel analyses directly in the time domain. We analyzed the PSDs using Lomb–Scargle periodograms, and searched for QPOs using wavelet scalograms. We also attempted to classify the GRBs using the Hurst exponent, <jats:italic>H</jats:italic>, and the <jats:inline-formula> <jats:tex-math> <?CDATA ${ \mathcal A }\mbox{--}{ \mathcal T }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe5b1ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> plane. The PSDs fall into three categories: power law (PL; <jats:italic>P</jats:italic>(<jats:italic>f</jats:italic>) ∝ 1/<jats:italic>f</jats:italic> <jats:sup> <jats:italic>β</jats:italic> </jats:sup>) with index <jats:italic>β</jats:italic> ∈ (0, 2), PL with a non-negligible Poisson noise level (PLC) with <jats:italic>β</jats:italic> ∈ (1, 3), and a smoothly broken PL (with Poisson noise level) yielding high-frequency index <jats:italic>β</jats:italic> <jats:sub>2</jats:sub> ∈ (2, 6). The latter yields break timescales of the order of 1–100 s. The PL and PLC models are broadly consistent with fully developed turbulence, <jats:italic>β</jats:italic> = 5/3. For an overwhelming majority of GRBs (93%), <jats:italic>H</jats:italic> &gt; 0.5, implying ubiquity of the long-term memory. We find no convincing substructure in the <jats:inline-formula> <jats:tex-math> <?CDATA ${ \mathcal A }\mbox{--}{ \mathcal T }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe5b1ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> plane. Finally, we report on 34 new QPOs, with one or more constant leading periods, as well as several chirping signals. The presence of breaks and QPOs suggests the existence of characteristic timescales that in at least some GRBs might be related to the dynamical properties of plasma trajectories in the accretion disks powering the relativistic jets.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The role of the environment on the formation of S0 galaxies is still not well understood, specifically in the outskirts of galaxy clusters. We study eight low-redshift clusters, analyzing galaxy members up to cluster-centric distances of ∼2.5 <jats:italic>R</jats:italic> <jats:sub>200</jats:sub>. We perform 2D photometric bulge–disk decomposition in the <jats:italic>g</jats:italic>, <jats:italic>r</jats:italic>, and <jats:italic>i </jats:italic>bands from which we identify 469 double-component galaxies. We separately analyze the colors of the bulges and disks and their dependence on the projected cluster-centric distance and local galaxy density. For our sample of cluster S0 galaxies, we find that bulges are redder than their surrounding disks, show a significant color–magnitude trend, and have colors that do not correlate with environment metrics. On the other hand, the disks associated with our cluster S0s become significantly bluer with increasing cluster-centric radius but show no evidence for a color–magnitude relation. The disk color–radius relation is mainly driven by galaxies in the cluster core at 0 ≤ <jats:italic>R</jats:italic>/<jats:italic>R</jats:italic> <jats:sub>200</jats:sub> &lt; 0.5. No significant difference is found for the disk colors of backsplash and infalling galaxies in the projected phase space (PPS). Beyond <jats:italic>R</jats:italic> <jats:sub>200</jats:sub>, the disk colors do not change with the local galaxy density, indicating that the colors of double-component galaxies are not affected by preprocessing. A significant color–density relation is observed for single-component disk-dominated galaxies beyond <jats:italic>R</jats:italic> <jats:sub>200</jats:sub>. We conclude that the formation of cluster S0 galaxies is primarily driven by cluster core processes acting on the disks, while evidence of preprocessing is found for single-component disk-dominated galaxies. We publicly release the data from the bulge–disk decomposition.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The present work explores the origin of the formation of star clusters in our Galaxy and in the Small Magellanic Cloud (SMC) through simulated H-R diagrams and compare those with observed star clusters. The simulation study produces synthetic H-R diagrams through the Markov Chain Monte Carlo (MCMC) technique using the star formation history (SFH), luminosity function (LF), abundance of heavy metal (Z), and a big library of isochrones as basic inputs and compares them with observed H-R diagrams of various star clusters. The distance-based comparison between those two diagrams is carried out through two-dimensional matching of points in the color−magnitude diagram (CMD) after the optimal choice of bin size and appropriate distance function. It is found that in a poor medium of heavy elements (<jats:italic>Z</jats:italic> = 0.0004), the Gaia LF along with a mixture of multiple Gaussian distributions of the SFH may be the origin of formation of globular clusters (GCs). On the contrary, an enriched medium (<jats:italic>Z</jats:italic> = 0.019) is generally favored with the Gaia LF along with a double power law or Beta-type (i.e., unimodal) SFH, for the formation of globular clusters. For SMC clusters, the choice of an exponential LF and exponential SFH is the proper combination for a poor medium, whereas the Gaia LF with a Beta-type SFH is preferred for the formation of star clusters in an enriched medium.</jats:p>