Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>The Event Horizon Telescope (EHT) is taking the first images of black holes resolved at horizon scales to measure their shadows and probe accretion physics. A promising avenue for testing the hypothesis that astrophysical black holes are described by the Kerr solution to Einstein’s equations is to compare the size and shape of the shadow a black hole casts on the surrounding emission to the predictions of the Kerr metric. We develop here an efficient parametric framework to perform this test. We carry out ray-tracing simulations for several parameterized non-Kerr metrics to create a large data set of non-Kerr shadows that probe the allowed parameter space for the free parameters of each metric. We then perform principal components analysis (PCA) on this set of shadows and show that only a small number of components are needed to accurately reconstruct all shadows within the set. We further show that the amplitude of the PCA components are smoothly related to the free parameters in the metrics and, therefore, that these PCA components can be fit to EHT observations in order to place constraints on the free parameters of these metrics that will help quantify any potential deviations from the Kerr solution.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Current instruments and spectral analysis programs are now able to decompose the integrated spectrum of a stellar system into <jats:italic>distributions</jats:italic> of ages and metallicities. The reliability of these methods has rarely been tested on nearby systems with resolved stellar ages and metallicities. Here we derive the age–metallicity distribution of M54, the nucleus of the Sagittarius dwarf spheroidal galaxy, from its integrated Multi-Unit Spectroscopic Explorer (MUSE) spectrum. We find a dominant old (8–14 Gyr), metal-poor (−1.5 dex) component and a young (1 Gyr), metal-rich (+0.25 dex) component—consistent with the complex stellar populations measured from individual stars in the same MUSE data set. There is excellent agreement between the (mass-weighted) average age and metallicity of the resolved and integrated analyses. Differences are only 3% in age and 0.2 dex metallicity. By co-adding individual stars to create M54's integrated spectrum, we show that the recovered age–metallicity distribution is insensitive to the magnitude limit of the stars or the contribution of blue horizontal branch stars—even when including additional blue wavelength coverage from the WiFeS Atlas of Galactic Globular cluster Spectra survey. However, we find that the brightest stars can induce the spurious recovery of an old (&gt;8 Gyr), metal-rich (+0.25 dex) stellar population, which is otherwise not expected from our understanding of chemical enrichment in M54. The overall derived stellar mass-to-light ratio of M54 is <jats:italic>M/L</jats:italic> <jats:sub>V</jats:sub> = 1.46 with a scatter of 0.22 across the field of view, which we attribute to the stochastic contribution of a young, metal-rich component. These findings provide strong evidence that complex stellar population distributions can be reliably recovered from integrated spectra of extragalactic systems.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Dust polarization depends on mechanical properties of dust as well as on local environments. To understand how dust polarization varies with different properties, we model the wavelength-dependence polarization of starlight and polarized dust emission of aligned grains by simultaneously taking into account grain alignment and rotational disruption by radiative torques (RATs). We explore a wide range of the local radiation field and grain mechanical properties characterized by tensile strength (<jats:italic>S</jats:italic> <jats:sub>max</jats:sub>). We find that the peak wavelength shifts to shorter wavelengths as the radiation strength (<jats:italic>U</jats:italic>) increases due to the enhanced alignment of small grains. Grain rotational disruption by RATs tends to decrease the optical-NIR polarization but increase the UV polarization of starlight due to the conversion of large grains into smaller ones. In particular, we find that the polarization degree at 850 <jats:italic>μ</jats:italic>m (<jats:italic>P</jats:italic> <jats:sub>850</jats:sub>) does not increase monotonically with <jats:italic>U</jats:italic> or grain temperature (<jats:italic>T</jats:italic> <jats:sub> <jats:italic>d</jats:italic> </jats:sub>), but it depends on <jats:italic>S</jats:italic> <jats:sub>max</jats:sub> of the grains. Our model can be tested with observations toward star-forming regions or molecular clouds irradiated by a nearby star, which have higher radiation intensities than the that of the average interstellar radiation field. Finally, we compare our predictions of the <jats:italic>P</jats:italic> <jats:sub>850</jats:sub>–<jats:italic>T</jats:italic> <jats:sub> <jats:italic>d</jats:italic> </jats:sub> relationship with Planck data and find that the observed decrease of <jats:italic>P</jats:italic> <jats:sub>850</jats:sub> with <jats:italic>T</jats:italic> <jats:sub> <jats:italic>d</jats:italic> </jats:sub> can be explained when grain disruption by RATs is accounted for, suggesting that as interstellar grains are unlikely to have a compact structure, perhaps they have a composite one. The variation of the polarization degree with <jats:italic>U</jats:italic> (or <jats:italic>T</jats:italic> <jats:sub> <jats:italic>d</jats:italic> </jats:sub>) can provide a valuable constraint on the internal structure of cosmic dust.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We study the photospheric evolution of an exploding granule observed in the quiet Sun at high spatial (∼0.″3) and temporal (31.5 s) resolution by the imaging magnetograph <jats:sc>Sunrise</jats:sc>/IMaX in 2009 June. These observations show that the exploding granule is cospatial to a magnetic flux emergence event occurring at mesogranular scale (up to ∼12 Mm<jats:sup>2</jats:sup> area). Using a modified version of the SIR code for inverting the IMaX spectropolarimetric measurements, we obtain information about the magnetic configuration of this photospheric feature. In particular, we find evidence of highly inclined emerging fields in the structure, carrying a magnetic flux content up to ∼4 × 10<jats:sup>18</jats:sup> Mx. The balance between gas and magnetic pressure in the region of flux emergence, compared with a very quiet region of the Sun, indicates that the additional pressure carried by the emerging flux increases the total pressure by about 5% and appears to allow the granulation to be modified, as predicted by numerical simulations. The overall characteristics suggest that a multipolar structure emerges into the photosphere, resembling an almost horizontal flux sheet. This seems to be associated with exploding granules. Finally, we discuss the origin of such flux emergence events.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Electron jets, common transient phenomena for which the electron velocity is much larger than the ion velocity, play an important role in the energy dissipation and transport in the terrestrial and other planetary magnetospheres. Using the unprecedented high-resolution data provided by the Magnetospheric Multiscale mission from May to August in 2017, we perform one systematically statistical study on the electron jets in the terrestrial magnetotail for the first time. In total, 466 electron jet events are selected, which mainly concentrate in the region −25 &lt; <jats:italic>X</jats:italic> &lt; −17 <jats:italic>R</jats:italic> <jats:sub>E</jats:sub>, −14 &lt; <jats:italic>Y</jats:italic> &lt; 12 <jats:italic>R</jats:italic> <jats:sub>E</jats:sub>, and 0 &lt; <jats:italic>Z</jats:italic> &lt; 6 <jats:italic>R</jats:italic> <jats:sub>E</jats:sub> (<jats:italic>R</jats:italic> <jats:sub>E</jats:sub> is the Earth’s radius). The electron velocity within the electron jets is much larger than the local Alfvén speed, implying that these jets belong to super-Alfvénic flows. The average occurrence rate of electron jets is 0.217 events hr<jats:sup>−1</jats:sup> in the <jats:italic>X</jats:italic>–<jats:italic>Y</jats:italic> plane and 0.189 events/hour in the <jats:italic>Z</jats:italic>–<jats:italic>Y</jats:italic> plane. The durations of the electron jets are mostly within 6 seconds with the average value of 2.89 seconds, which are much shorter than the duration of bursty bulk flows (BBFs) having the typical duration of several to tens of minutes. Through further analysis of the magnetic field, we find that electron jets can occur at five different structures, including 224 events detected during the crossing of current sheet, 57 events observed during the crossing of PSBL, 83 events around DFs, 79 events around magnetic holes, and 23 events around flux ropes. In addition, the relative positions of the electron jets in different structures are also identified in the present study. Our statistical results reveal the comprehensive features of electron jets in the terrestrial magnetotail, which can also be applied in the other planetary magnetosphere.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In this paper we simultaneously fit the orbits and line-of-sight velocities of the ionized gas around the supermassive black hole, Sagittarius A* (Sgr A*), at the center of the Milky Way. The data we use are taken with the <jats:italic>K</jats:italic>-band Multi Object Spectrograph (KMOS), presented in Feldmeier-Krause et al., and cover the central ∼2 pc of the Milky Way. From the brightest gas emission line in the <jats:italic>K</jats:italic>-band, the Br<jats:italic>γ</jats:italic> line, we derive the spatial distribution and line-of-sight velocities of the gas in the minispiral. Using the flux distribution and the line-of-sight velocity information, we perform a fit to the three main gas streamers in the minispiral, the Northern Arm, Eastern Arm, and Western Arc, using a Bayesian modeling method, and are able to reconstruct the three-dimensional orbits of these gas streamers. With the best-fit orbital parameters and the measured line-of-sight velocities, we constrain the mass of Sgr A*. The orbit of the Eastern Arm is the one that is best constrained using our data. It gives a best-fit orbital period of <jats:inline-formula> <jats:tex-math> <?CDATA ${17.4}_{-11.6}^{+31.0}\times {10}^{3}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab8ea8ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> yr and results in an enclosed mass of <jats:inline-formula> <jats:tex-math> <?CDATA ${14.9}_{-10.4}^{+69.4}\times {10}^{6}{M}_{\odot }.$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab8ea8ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> </jats:p>

<jats:title>Abstract</jats:title> <jats:p>Gamma-ray bursts (GRBs) are known to be the most violent explosions in the universe, and a variety of correlations between observable GRB properties have been proposed in the literature, but none of these correlations are valid for both long and short GRBs. In this paper we report the discovery of a universal correlation that is suitable for both long and short GRBs using three prompt emission properties of GRBs, i.e., the isotropic peak luminosity <jats:italic>L</jats:italic> <jats:sub>iso</jats:sub>, the peak energy of the time-integrated prompt emission spectrum <jats:italic>E</jats:italic> <jats:sub>peak</jats:sub>, and the “high signal” timescale <jats:italic>T</jats:italic> <jats:sub>0.45</jats:sub>, <jats:inline-formula> <jats:tex-math> <?CDATA ${L}_{\mathrm{iso}}\propto {E}_{\mathrm{peak}}^{1.94}{T}_{0.45}^{0.37}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab8f9dieqn1.gif" xlink:type="simple" /> </jats:inline-formula>. This universal correlation involves properties of GRB prompt emission and does not require knowledge of the afterglow phase, which can be used as a relatively unbiased redshift estimator. Here we use this correlation to estimate the pseudoredshifts for short GRBs and then use the Lynden–Bell method to obtain a nonparametric estimate of their luminosity function and formation rate. The luminosity function is <jats:inline-formula> <jats:tex-math> <?CDATA $\psi ({L}_{0})\propto {L}_{0}^{-0.63\pm 0.07}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab8f9dieqn2.gif" xlink:type="simple" /> </jats:inline-formula> for dim SGRBs and <jats:inline-formula> <jats:tex-math> <?CDATA $\psi ({L}_{0})\propto {L}_{0}^{-1.96\pm 0.28}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab8f9dieqn3.gif" xlink:type="simple" /> </jats:inline-formula> for bright SGRBs, with the break point <jats:inline-formula> <jats:tex-math> <?CDATA ${6.95}_{-0.76}^{+0.84}\times {10}^{50}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab8f9dieqn4.gif" xlink:type="simple" /> </jats:inline-formula>. The local formation rate of SGRBs is about 15 events Gpc<jats:sup>−3</jats:sup> yr<jats:sup>−1</jats:sup> . This universal correlation may have important implications for GRB physics, implying that the long and short GRBs should share similar radiation processes.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>A galaxy–galaxy merger and the subsequent triggering of starburst activity are fundamental processes linked to the morphological transformation of galaxies and the evolution of star formation across the history of the universe. Both nuclear and disk-wide starbursts are assumed to occur during the merger process. However, quantifying both nuclear and disk-wide star formation activity is nontrivial because the nuclear starburst is dusty in the most active merging starburst galaxies. This paper presents a new approach to this problem: combining hydrogen recombination lines in optical, millimeter, and free–free emission. Using NGC 3256 as a case study, H<jats:italic>β</jats:italic>, H40<jats:italic>α</jats:italic>, and free–free emissions are investigated using the Multi Unit Spectroscopic Explorer at the Very Large Telescope of the European Southern Observatory (MUSE) and the Atacama Large Millimeter/submillimeter Array (ALMA). The H<jats:italic>β</jats:italic> image obtained by MUSE identifies star-forming regions outside the nuclear regions, suggesting a disk-wide starburst. In contrast, the H40<jats:italic>α</jats:italic> image obtained by ALMA identifies a nuclear starburst where optical lines are undetected due to dust extinction (<jats:italic>A</jats:italic> <jats:sub>V</jats:sub> ∼ 25). Combining both MUSE and ALMA observations, we conclude that the total star formation rate (SFR) is 49 ± 2 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> yr<jats:sup>−1</jats:sup> and the contributions from nuclear and disk-wide starbursts are ∼34% and ∼66%, respectively. This suggests the dominance of disk-wide star formation in NGC 3256. In addition, pixel-by-pixel analyses for disk-wide star-forming regions suggest that shock gas tracers (e.g., CH<jats:sub>3</jats:sub>OH) are enhanced where gas depletion time (<jats:italic>τ</jats:italic> <jats:sub>gas</jats:sub> = <jats:italic>M</jats:italic> <jats:sub>gas</jats:sub>/SFR) is long. This possibly means that merger-induced shocks regulate disk-wide star formation activities.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We investigate the effect of magnetic helicity on the stability of buoyant magnetic cavities as found in the intergalactic medium. In these cavities we insert helical magnetic fields and test whether or not helicity can increase their stability to shredding through the Kelvin–Helmholtz instability and, with that, their lifetime. This is compared to the case of an external vertical magnetic field that is known to reduce the growth rate of the Kelvin–Helmholtz instability. By comparing a low-helicity configuration with a high-helicity one with the same magnetic energy, we find that an internal helical magnetic field stabilizes the cavity. This effect increases as we increase the helicity content. Stabilizing the cavity with an external magnetic field requires instead a significantly stronger field at higher magnetic energy. We conclude that the presence of helical magnetic fields is a viable mechanism to explain the stability of intergalactic cavities on timescales longer than 100 Myr.</jats:p>