Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>The Event Horizon Telescope (EHT) has recently delivered the first resolved images of M87*, the supermassive black hole in the center of the M87 galaxy. These images were produced using 230 GHz observations performed in 2017 April. Additional observations are required to investigate the persistence of the primary image feature—a ring with azimuthal brightness asymmetry—and to quantify the image variability on event horizon scales. To address this need, we analyze M87* data collected with prototype EHT arrays in 2009, 2011, 2012, and 2013. While these observations do not contain enough information to produce images, they are sufficient to constrain simple geometric models. We develop a modeling approach based on the framework utilized for the 2017 EHT data analysis and validate our procedures using synthetic data. Applying the same approach to the observational data sets, we find the M87* morphology in 2009–2017 to be consistent with a persistent asymmetric ring of ∼40 <jats:italic>μ</jats:italic>as diameter. The position angle of the peak intensity varies in time. In particular, we find a significant difference between the position angle measured in 2013 and 2017. These variations are in broad agreement with predictions of a subset of general relativistic magnetohydrodynamic simulations. We show that quantifying the variability across multiple observational epochs has the potential to constrain the physical properties of the source, such as the accretion state or the black hole spin.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We discuss the importance of Fe<jats:sup>23+</jats:sup> in determining the line intensities of the Fe <jats:sc>xxv</jats:sc> K<jats:italic>α</jats:italic> complex in an optically thick cloud, and investigate resonant auger destruction (RAD) with CLOUDY. Although initially motivated by the Perseus cluster, our calculations are extended to the wide range of column densities encountered in astronomy. A Fe <jats:sc>xxv</jats:sc> line photon can change/lose its identity upon absorption by three-electron iron as a result of “line interlocking.” This may lead to the autoionization of the absorbing ion, ultimately destroying the Fe <jats:sc>xxv</jats:sc> K<jats:italic>α</jats:italic> photon by RAD. Out of the four members in the Fe <jats:sc>xxv</jats:sc> K<jats:italic>α</jats:italic> complex, a significant fraction of the x line photons are absorbed by Fe<jats:sup>23+</jats:sup> and destroyed, causing the x line intensity to decrease. For example, at a hydrogen column density of 10<jats:sup>25</jats:sup> cm<jats:sup>−2</jats:sup>, ∼32% of x photons are destroyed due to RAD while w is mostly unaffected. The line intensity of y is slightly (≤2%) reduced. z is not directly affected by RAD, but the contrasting behavior between z and x line intensities points toward the possible conversion of a tiny fraction (∼2%) of x photons into z photons. The change in line intensities due to electron scattering escape off fast thermal electrons is also discussed.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Soft X-ray Spectrometer on board Hitomi, with the unprecedented resolving power of <jats:italic>R</jats:italic> ∼ 1250, allowed the detection of members of the Fe <jats:sc>xxv</jats:sc> K<jats:italic>α</jats:italic> complex emission spectra from the center of the Perseus Cluster. In this paper, we introduce a novel method of measuring the column density using the optically thin (Case A) to optically thick (Case B) transition for one- and two-electron systems. We compare the Fe <jats:sc>xxv</jats:sc> K <jats:italic>α</jats:italic> line ratios computed with CLOUDY with that from the Hitomi observations in the outer region of the Perseus core using collision strengths from different atomic data sets, and obtain good agreement. We also show the effect of turbulence on Fe <jats:sc>xxv</jats:sc> K<jats:italic>α</jats:italic> line ratios and interplay between column density and metallicity. Additionally, we discuss the atomic number dependence of transition probabilities for allowed and unallowed transitions, which causes highly charged He-like systems, such as Fe <jats:sc>xxv</jats:sc>, to behave fundamentally differently from He <jats:sc>i</jats:sc>.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We investigated the coronal properties of G-dwarf stars including the Sun over a wide range of X-ray luminosity <jats:italic>L</jats:italic> <jats:sub>X</jats:sub> (3 × 10<jats:sup>26</jats:sup> to 2 × 10<jats:sup>30</jats:sup> erg s<jats:sup>−1</jats:sup>). We analyzed the archival data of 10 X-ray bright (<jats:italic>L</jats:italic> <jats:sub>X</jats:sub> &gt; 10<jats:sup>28</jats:sup> erg s<jats:sup>−1</jats:sup>) G-dwarf stars to derive their emission measure (EM) and the coronal temperature (<jats:italic>T</jats:italic>) during the periods when no prominent stellar flares were observed. We attempted to explain the relation on the basis of our understanding of the present Sun: a steady corona model based on the so-called Rosner-Tucker-Vaiana (RTV) scaling laws and the observed power-law distribution function of surface magnetic features. We derived a theoretical scaling law of the EM–<jats:italic>T</jats:italic> relation for a star with multiple active regions, and applied it to the observations combined with data in the literature. We found that with the solar parameters, our scaling law seems to be consistent with the data of slowly rotating stars. However, more X-ray-bright stars are located well above the scaling law based on the solar parameter. The scaling law may explain the observations if those stars show a power-law distribution function of active regions with the same power-law index but a 10–100 times larger coefficient. This suggests that X-ray bright stars show more active regions for a given size than the Sun. Since our samples include rapidly rotating stars, we infer that the offset of the X-ray bright stars from the present Sun-based scaling law is due to the enhancement of the surface magnetic field generation by their rapid rotation.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Magnetic fields are expected to play an important role in accretion processes for circumstellar disks. However, measuring the magnetic field morphology is difficult, especially when given that polarimetric (sub)millimeter continuum observations may not trace the fields in most disks. The Goldreich–Kylafis effect suggests that line polarization is perpendicular or parallel to the magnetic field direction. We attempt to observe CO(2−1), <jats:sup>13</jats:sup>CO(2−1), and C<jats:sup>18</jats:sup>O(2−1) line polarization toward HD 142527 and IM Lup, which are large and bright protoplanetary disks. We use spatial averaging and spectral integration to search for signals in both disks, and we detect a potential CO(2−1) Stokes <jats:italic>Q</jats:italic> signal toward both disks. The total CO(2−1) polarization fractions are 1.57% ± 0.18% and 1.01% ± 0.10% for HD 142527 and IM Lup, respectively. Our Monte Carlo simulations indicate that these signals are marginal. We also stack Stokes parameters based on the Keplerian rotation, but no signal was found. Across the disk traced by dust of HD 142527, the 3<jats:italic>σ</jats:italic> upper limits for <jats:inline-formula> <jats:tex-math> <?CDATA ${P}_{\mathrm{frac}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabaef7ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> at 05 (∼80 au) resolution are typically less than 3% for CO(2−1) and <jats:sup>13</jats:sup>CO(2−1) and 4% for C<jats:sup>18</jats:sup>O(2−1). For IM Lup the 3<jats:italic>σ</jats:italic> upper limits for these three lines are typically less than 3%, 4%, and 12%, respectively. Upper limits based on our stacking technique are up to a factor of ∼10 lower, although stacking areas can potentially average out small-scale polarization structure. We also compare our continuum polarization at 1.3 mm to observations at 870 <jats:italic>μ</jats:italic>m from previous studies. The polarization in the northern dust trap of HD 142527 shows a significant change in morphology and an increase in <jats:inline-formula> <jats:tex-math> <?CDATA ${P}_{\mathrm{frac}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabaef7ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> as compared to 870 <jats:italic>μ</jats:italic>m. For IM Lup, the 1.3 mm polarization may be more azimuthal and has a higher <jats:inline-formula> <jats:tex-math> <?CDATA ${P}_{\mathrm{frac}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabaef7ieqn3.gif" xlink:type="simple" /> </jats:inline-formula> than at 870 <jats:italic>μ</jats:italic>m.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>High-resolution numerical simulations including feedback and aimed at calculating the escape fraction (<jats:italic>f</jats:italic> <jats:sub>esc</jats:sub>) of hydrogen-ionizing photons often assume stellar radiation based on single-stellar population synthesis models. However, strong evidence suggests the binary fraction of massive stars is ≳70%. Moreover, simulations so far have yielded values of <jats:italic>f</jats:italic> <jats:sub>esc</jats:sub> falling only on the lower end of the ∼10%–20% range, the amount presumed necessary to reionize the universe. Analyzing a high-resolution (4 pc) cosmological radiation-hydrodynamic simulation, we study how <jats:italic>f</jats:italic> <jats:sub>esc</jats:sub> changes when we include two different products of binary stellar evolution—stars stripped of their hydrogen envelopes and massive blue stragglers. Both produce significant amounts of ionizing photons 10–200 Myr after each starburst. We find the relative importance of these photons to be amplified with respect to escaped ionizing photons, because peaks in star formation rates (SFRs) and <jats:italic>f</jats:italic> <jats:sub>esc</jats:sub> are often out of phase by this 10–200 Myr. Additionally, low-mass, bursty galaxies emit Lyman continuum radiation primarily from binary products when SFRs are low. Observations of these galaxies by the James Webb Space Telescope could provide crucial information on the evolution of binary stars as a function of redshift. Overall, including stripped stars and massive blue stragglers increases our photon-weighted mean escape fraction (<jats:inline-formula> <jats:tex-math> <?CDATA $\langle {f}_{\mathrm{esc}}\rangle $?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabaefaieqn1.gif" xlink:type="simple" /> </jats:inline-formula>) by ∼13% and ∼10%, respectively, resulting in <jats:inline-formula> <jats:tex-math> <?CDATA $\langle {f}_{\mathrm{esc}}\rangle =17 \% $?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabaefaieqn2.gif" xlink:type="simple" /> </jats:inline-formula>. Our results emphasize that using updated stellar population synthesis models with binary stellar evolution provides a more sound physical basis for stellar reionization.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The origin of the gamma-ray burst (GRB) X-ray plateau, especially the internal plateau, is still unclear, but it could be related to a GRB’s magnetar central engine. It is generally believed that the spin-down power of the magnetar is injected into forward external shock; however, we propose here that most of the power will be dissipated behind the GRB jet through a larger amplitude electromagnetic wave. Based on this proposal, the relevant physical conditions and observational implications are analyzed and discussed, and various kinds of X-ray light curves could be reproduced. Although the chromatic multiband afterglow in the standard external afterglow fireball model is still a matter of debate, we can naturally explain this feature through this proposal, i.e., the electrons generating the X-ray plateau and emitting the optical afterglow are accelerated by different mechanisms. It is emphasized that both the GRB jet and the spin-down wind should have significant observational consequences in the magnetar scenario, and should be focused equally in GRB physics.</jats:p>