Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>The X-ray spectra of nonmagnetic cataclysmic variables (nmCVs) in the ∼0.3–15 keV energy band have been described by either one or several optically thin thermal plasma components or by cooling flow models. We tested whether the spectral continuum in nmCVs could be successfully described by Comptonization of soft photons off hot electrons presented in a cloud surrounding the source (the transition layer (TL)). We used public XMM-Newton EPIC-pn, Chandra HETG/ACIS and LETG/HRC, and RXTE PCA and HEXTE observations of four dwarf novae (U Gem, SS Cyg, VW Hyi, and SS Aur) observed in the quiescent and outburst states. In total, we analyzed 18 observations, including a simultaneous 0.4–150 keV Chandra/RXTE spectrum of SS Cyg in quiescence. We fitted the spectral continuum with up to two thermal Comptonization components (the <jats:sc>compTT</jats:sc> or <jats:sc>compTB</jats:sc> models in XSPEC) using only one thermal plasma temperature and one optical depth. In this framework, the two seed photon components are presumably coming from the innermost and outer parts of the TL (or innermost part of the disk). We obtained that the thermal Comptonization can successfully describe the spectral continuum of these nmCVs in the ∼0.4–150 keV energy band. Moreover, we present the first principal radiative transfer model that explains the quasi-constancy of the spectral photon index observed around 1.8, which strongly supports the Comptonization framework in nmCVs.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We analyze the detection capability of coronal mass ejections (CMEs) for all currently operating coronagraphs in space. We define as CMEs events that propagate beyond 10 solar radii with morphologies broadly consistent with a magnetic flux-rope presence. We take advantage of multi-viewpoint observations over five month-long intervals, corresponding to special orbital configurations of the coronagraphs aboard the Solar Terrestrial Relations Observatory (STEREO) and Solar and Heliospheric Observatory missions. This allows us to sort out CMEs from other outward-propagating features (e.g., waves or outflows), and thus to identify the total number of unique CMEs ejected during those periods. We determine the CME visibility functions (VFs) of the STEREO COR2-A/B and LASCO C2/C3 coronagraphs directly as the ratio of observed to unique CMEs. The VFs range from 0.71 to 0.92 for a 95% confidence interval. By comparing detections between coronagraphs on the same spacecraft and from multiple spacecraft, we assess the influence of field of view (FOV), instrument performance, and projection effects on the CME detection ability without resorting to proxies, such as flares or radio bursts. We find that no major CMEs are missed by any of the coronagraphs, that a few slow halo-like events may be missed in synoptic cadence movies, and that narrow FOV coronagraphs have difficulties discriminating between CMEs and other ejections, leading to “false” detection rates. We conclude that CME detection can only be validated with multi-viewpoint imaging—two coronagraphs in quadrature offer adequate detection capability. Finally, we apply the VFs to observed CME rates resulting in upward corrections of 40%.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>A major goal in the field of galaxy formation is to understand the formation of the Milky Way’s disk. The first step toward doing this is to empirically describe its present state. We use the new high-dimensional data set of 19 abundances from 27,135 red clump Apache Point Observatory Galactic Evolution Experiment stars to examine the distribution of clusters defined using abundances. We explore different dimension reduction techniques and implement a nonparametric agglomerate hierarchical clustering method. We see that groups defined using abundances are spatially separated, as a function of age. Furthermore, the abundance groups represent different distributions in the [Fe/H]–age plane. Ordering our clusters by age reveals patterns suggestive of the sequence of chemical enrichment in the disk over time. Our results indicate that a promising avenue to trace the details of the disk’s assembly is via a full interpretation of the empirical connections we report.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Coronal loops reveal crucial information about the nature of both coronal magnetic fields and coronal heating. The shape of the corresponding flux tube cross section and how it varies with position are especially important properties. They are a direct indication of the expansion of the field and of the cross-field spatial distribution of the heating. We have studied 20 loops using high spatial resolution observations from the first flight of the Hi-C rocket experiment, measuring the intensity and width as a function of position along the loop axis. We find that intensity and width tend to either be uncorrelated or to have a direct dependence, such that they increase or decrease together. This implies that the flux tube cross sections are approximately circular under the assumptions that the tubes have nonnegligible twist and that the plasma emissivity is approximately uniform along the magnetic field. The shape need not be a perfect circle and the emissivity need not be uniform within the cross section, but subresolution patches of emission must be distributed quasi-uniformly within an envelope that has an aspect ratio of order unity. This raises questions about the suggestion that flux tubes expand with height, but primarily in the line-of-sight direction so that the corresponding (relatively noticeable) loops appear to have roughly uniform width, a long-standing puzzle. It also casts doubt on the idea that most loops correspond to simple warped sheets, although we leave open the possibility of more complex manifold structures.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report on the discovery of drifting subpulses from the Vela pulsar, observed using the Yunnan 40 m radio telescope at 6800 MHz observing frequency. The single pulses show the presence of two distinct drifting patterns, where the central component is modulated in longitude and amplitude simultaneously with a period of 9 pulses, and the trailing component remains phase stationary within the pulse window but periodically changes in intensity with a period of 29 pulses. The leading component emission remains constant in intensity and pulse phase. Such periodic modulation in the Vela pulsar is a separate phenomenon from periodic nulling and traditional subpulse drifting, which may lead to a greater understanding of the pulse emission mechanism.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Observations with the Australia Telescope Compact Array revealed intrahour variations in the radio source PKS B1322-110. As part of an optical follow-up, we obtained Gemini H<jats:italic>α</jats:italic> and H<jats:italic>α</jats:italic> continuum (H<jats:italic>α</jats:italic>C) images of the PKS B1322-110 field. A robust 19<jats:italic>σ</jats:italic> detection of PKS B1322-110 in the H<jats:italic>α</jats:italic>−H<jats:italic>α</jats:italic>C image prompted us to obtain the first optical spectrum of PKS B1322-110. With the Gemini spectrum we determine that PKS B1322-110 is a flat-spectrum radio quasar at a redshift of <jats:italic>z</jats:italic> = 3.007 ± 0.002. The apparent flux detected in the H<jats:italic>α</jats:italic> filter is likely to originate from He <jats:sc>ii</jats:sc> emission redshifted precisely on the Galactic H<jats:italic>α</jats:italic> narrowband filter. We set upper limits on the emission measure of the Galactic plasma, for various possible cloud geometries.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Recently, a fast radio burst (FRB)-like event was found to be associated with a Galactic magnetar, SGR 1935+2154, accompanied by an X-ray burst. We find this radio burst challenges the typical emission mechanisms involving magnetars, which includes coherent curvature radiation from charged bunches, fast magnetosonic (FMS) waves, synchrotron maser from shocks, and the pulsar-like mechanism for low-twist magnetars. More specifically, we find that (1) the X-rays are most likely to be produced inside the magnetosphere. (2) For the coherent curvature radiation from the decay of the Alfvén wave, it will generally predict a duration (≲0.1 ms) smaller than observations, because of the strong twists of magnetic field lines and the internal damping of Alfvén waves. (3) The FMS wave model predicts a very low emission frequency <jats:italic>ν</jats:italic> <jats:sub>p</jats:sub> ∼ 0.03 MHz ≪ GHz, unless it is produced inside the magnetosphere. But the absorption effect of the magnetospheric FMS wave model remains to be studied. (4) The synchrotron maser model is challenged because observations show that the peaks in both X-ray and radio light curves are with the same temporal separation Δ<jats:italic>t</jats:italic> <jats:sub>FRB</jats:sub> = Δ<jats:italic>t</jats:italic> <jats:sub> <jats:italic>γ</jats:italic> </jats:sub> ≈ 0.03 s, while it would predict Δ<jats:italic>t</jats:italic> <jats:sub>FRB</jats:sub> ≪ Δ<jats:italic>t</jats:italic> <jats:sub> <jats:italic>γ</jats:italic> </jats:sub>. (5) It seems to be difficult to directly apply the low-twist pulsar-like mechanism to flaring magnetars, as magnetar activity can significantly deform the magnetosphere. (6) We suggested four possibilities to study the general properties of FRBs for future observations, especially the possibility of identifying quasi-periodic oscillations with period ∼1–10 ms in double/multiple-peaked FRBs.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The detection of dusty winds dominating the infrared emission of active galactic nuclei (AGNs) on parsec scales has revealed the limitations of traditional radiative transfer models based on a toroidal distribution of dusty gas. A new, more complex, dynamical structure is emerging and the physical origin of such dusty winds has to be critically assessed. We present a semi-analytical model to test the hypothesis of radiatively accelerated dusty winds launched by the AGN and by the heated dust itself. The model consists of an AGN and an infrared radiating dusty disk, the latter being the primary mass reservoir for the outflow. We calculate the trajectories of dusty gas clumps in this environment, accounting for both gravity and the AGN radiation as well as the re-radiation by the hot, dusty gas clouds themselves. We find that the morphology consists of a disk of material that orbits with sub-Keplerian velocities and a hyperboloid polar wind launched at the inner edge of the dusty disk. This is consistent with high-angular resolution infrared and sub-mm observations of some local Seyfert AGN. The strength of the wind and its orientation depend on the Eddington ratio and the column density of the dusty clumps, which is in agreement with proposed radiation regulated obscuration models developed for the X-ray obscuring material around AGNs.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present very early ground-based optical follow-up observations of gamma-ray burst (GRB) 140423A, which was discovered by the Swift/Burst Alert Telescope (BAT) and by the Fermi/Gamma-Ray Burst Monitor. Its broadband afterglow was monitored by the Swift/X-Ray Telescope and ground-based optical telescopes from <jats:italic>T</jats:italic> <jats:sub>0</jats:sub> + 70.96 s to 4.8 days after the Swift/BAT trigger. This is one more case of a prompt optical emission observation. The temporal and spectral joint fits of the multiwavelength light curves of GRB 140423A reveal that achromatic behavior is consistent with the external shock model, including a transition from a stellar wind to the interstellar medium (ISM) and energy injection. In terms of the optical light curves, there is an onset bump in the early afterglow with a rising index <jats:inline-formula> <jats:tex-math> <?CDATA ${\alpha }_{{\rm{O}},{\rm{I}}}=-0.59\pm 0.04$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaba757ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> (peaking at <jats:inline-formula> <jats:tex-math> <?CDATA ${t}_{\mathrm{peak}}-{T}_{0}\approx 206$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaba757ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> s). It then decays with a steep index <jats:inline-formula> <jats:tex-math> <?CDATA ${\alpha }_{{\rm{O}},\mathrm{II}}=1.78\pm 0.03$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaba757ieqn3.gif" xlink:type="simple" /> </jats:inline-formula>, and shows a steeper to flatter “transition” with <jats:inline-formula> <jats:tex-math> <?CDATA ${\alpha }_{{\rm{O}},\mathrm{III}}=1.13\pm 0.03$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaba757ieqn4.gif" xlink:type="simple" /> </jats:inline-formula> at around <jats:italic>T</jats:italic> <jats:sub>0</jats:sub> + 5000 s. The observed X-ray afterglow reflects achromatic behavior, as does the optical light curve. There is no obvious evolution of the spectral energy distribution between the X-ray and optical afterglows, with an average value of the photon index <jats:inline-formula> <jats:tex-math> <?CDATA ${\rm{\Gamma }}\approx 1.95$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaba757ieqn5.gif" xlink:type="simple" /> </jats:inline-formula>. This “transition” is consistent with an external shock model having the circumburst medium transition from a wind to the ISM, by introducing a long-lasting energy injection with a Lorentz factor stratification of the ejecta. The best parameters from Monte Carlo Markov Chain fitting are <jats:inline-formula> <jats:tex-math> <?CDATA ${E}_{{\rm{K}},\mathrm{iso}}\approx 2.14\times {10}^{55}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaba757ieqn6.gif" xlink:type="simple" /> </jats:inline-formula> erg, <jats:inline-formula> <jats:tex-math> <?CDATA ${{\rm{\Gamma }}}_{0}\approx 162$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaba757ieqn7.gif" xlink:type="simple" /> </jats:inline-formula>, <jats:inline-formula> <jats:tex-math> <?CDATA ${\varepsilon }_{e}\approx 0.02$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaba757ieqn8.gif" xlink:type="simple" /> </jats:inline-formula>, <jats:inline-formula> <jats:tex-math> <?CDATA ${\varepsilon }_{B}\approx 1.7\times {10}^{-6}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaba757ieqn9.gif" xlink:type="simple" /> </jats:inline-formula>, <jats:inline-formula> <jats:tex-math> <?CDATA ${A}_{* }\approx 1.0$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaba757ieqn10.gif" xlink:type="simple" /> </jats:inline-formula>, <jats:inline-formula> <jats:tex-math> <?CDATA ${R}_{t}\approx 4.1\times {10}^{17}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaba757ieqn11.gif" xlink:type="simple" /> </jats:inline-formula> cm, <jats:inline-formula> <jats:tex-math> <?CDATA $n\approx 11.0\,\,{\mathrm{cm}}^{-3}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaba757ieqn12.gif" xlink:type="simple" /> </jats:inline-formula>, <jats:inline-formula> <jats:tex-math> <?CDATA ${L}_{0}\approx 3.1\times {10}^{52}\,\mathrm{erg}\ {{\rm{s}}}^{-1}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaba757ieqn13.gif" xlink:type="simple" /> </jats:inline-formula>, <jats:inline-formula> <jats:tex-math> <?CDATA $k\approx 1.98$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaba757ieqn14.gif" xlink:type="simple" /> </jats:inline-formula>, <jats:inline-formula> <jats:tex-math> <?CDATA $s\,\approx 1.54$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaba757ieqn15.gif" xlink:type="simple" /> </jats:inline-formula>, and <jats:inline-formula> <jats:tex-math> <?CDATA ${\theta }_{j}\gt 0.3\,\mathrm{rad}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaba757ieqn16.gif" xlink:type="simple" /> </jats:inline-formula>.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Multicomponent modeling of galaxies is a valuable tool in the effort to quantitatively understand galaxy evolution, yet the use of the technique is plagued by issues of convergence, model selection, and parameter degeneracies. These issues limit its application over large samples to the simplest models, with complex models being applied only to very small samples. We attempt to resolve this dilemma of “quantity or quality” by developing a novel framework, built inside the Zooniverse citizen-science platform, to enable the crowdsourcing of model creation for Sloan Digital Sky Survey galaxies. We have applied the method, including a final algorithmic optimization step, on a test sample of 198 galaxies, and examine the robustness of this new method. We also compare it to automated fitting pipelines, demonstrating that it is possible to consistently recover accurate models that either show good agreement with, or improve on, prior work. We conclude that citizen science is a promising technique for modeling images of complex galaxies, and release our catalog of models.</jats:p>