Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>Gaia is undertaking a deep synoptic survey of the Galaxy, but photometry from individual epochs has, as of yet, only been released for a minimal number of sources. We show that it is possible to identify variable stars in Gaia Data Release 2 by selecting stars with unexpectedly large photometric uncertainties given their brightness and number of observations. By comparing our results to existing catalogs of variables, we show that information on the amplitude of variability is also implicitly present in the Gaia photometric uncertainties. We present a catalog of about 9.3 million candidate variable stars, and discuss its limitations and prospects for future tests and extensions.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Sunquakes are helioseismic power enhancements initiated by solar flares, but not all flares generate sunquakes. It is curious why some flares cause sunquakes while others do not. Here we propose a hypothesis to explain the disproportionate occurrence of sunquakes: during a flare’s impulsive phase when the flare’s impulse acts upon the photosphere, delivered by shock waves, energetic particles from higher atmosphere, or by downward Lorentz Force, a sunquake tends to occur if the background oscillation at the flare footpoint happens to oscillate downward in the same direction with the impulse from above. To verify this hypothesis, we select 60 strong flares in Solar Cycle 24, and examine the background oscillatory velocity at the sunquake sources during the flares’ impulsive phases. Since the Doppler velocity observations at sunquake sources are usually corrupted during the flares, we reconstruct the oscillatory velocity in the flare sites using helioseismic holography method with an observation-based Green’s function. A total of 24 flares are found to be sunquake active, giving a total of 41 sunquakes. It is also found that in 3–5 mHz frequency band, 25 out of 31 sunquakes show net downward oscillatory velocities during the flares’ impulsive phases, and in the 5–7 mHz frequency band, 33 out of 38 sunquakes show net downward velocities. These results support the hypothesis that a sunquake more likely occurs when a flare impacts a photospheric area with a downward background oscillation.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present a large sample of infrared-luminous candidate active galactic nuclei (AGNs) that lack X-ray detections in Chandra, XMM-Newton, and NuSTAR fields. We selected all optically detected SDSS sources with redshift measurements, combined additional broadband photometry from WISE, UKIDSS, 2MASS, and GALEX, and modeled the spectral energy distributions (SEDs) of our sample sources. We parameterize nuclear obscuration in our SEDs with <jats:inline-formula> <jats:tex-math> <?CDATA $E{\left(B-V\right)}_{\mathrm{AGN}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd185ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> and uncover thousands of powerful obscured AGNs that lack X-ray counterparts, many of which are identified as AGN candidates based on straightforward WISE photometric criteria. Using the observed luminosity correlation between rest-frame 2–10 keV (<jats:inline-formula> <jats:tex-math> <?CDATA ${L}_{{\rm{X}}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd185ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>) and rest-frame AGN <jats:inline-formula> <jats:tex-math> <?CDATA $6\,\mu {\rm{m}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd185ieqn3.gif" xlink:type="simple" /> </jats:inline-formula> (<jats:inline-formula> <jats:tex-math> <?CDATA ${L}_{\mathrm{MIR}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd185ieqn4.gif" xlink:type="simple" /> </jats:inline-formula>), we estimate the intrinsic X-ray luminosities of our sample sources and combine these data with flux limits from X-ray catalogs to determine lower limits on nuclear obscuration. Using the ratio of intrinsic-to-observed X-ray luminosity (<jats:inline-formula> <jats:tex-math> <?CDATA ${R}_{{L}_{{\rm{X}}}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd185ieqn5.gif" xlink:type="simple" /> </jats:inline-formula>), we find a significant fraction of sources with column densities approaching <jats:inline-formula> <jats:tex-math> <?CDATA ${N}_{{\rm{H}}}\gt {10}^{24}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd185ieqn6.gif" xlink:type="simple" /> </jats:inline-formula> cm<jats:sup>–2</jats:sup>, suggesting that multiwavelength observations are necessary to account for the population of heavily obscured AGNs. We simulate the underlying <jats:inline-formula> <jats:tex-math> <?CDATA ${N}_{{\rm{H}}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd185ieqn7.gif" xlink:type="simple" /> </jats:inline-formula> distribution for the X-ray non-detected sources in our sample through survival analysis, and confirm the presence of AGN activity via X-ray stacking. Our results point to a considerable population of extremely obscured AGNs undetected by current X-ray observatories.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Auroras observed at middle and low geographic latitudes are related to external inputs and varying geomagnetic fields. This work aims to exclude corotating interaction region (CIR) storms and identify strong coronal mass ejection (CME) storms according to historical auroral records when the geomagnetic field varies substantially. An existing catalog of the aurora records in Chinese history reported by Zeng &amp; Jin from 193 B.C. to 1911 A.D. is used. Archaeomagnetic field models are adopted to estimate the variation of the dipole field. According to the empirical relation between the equatorward boundary of the auroral oval, Dst index, and geomagnetic field intensity, the auroras caused by CIRs can be excluded, and those caused by strong CMEs are identified. After 1500 A.D., China’s magnetic latitude decreased substantially due to the pole shift. This shift provides a better opportunity to investigate the existence of great-level storms. These great-storm CMEs occurred in both solar maximum and minimum. The space weather modeling framework is used to calculate the cusp area and the downward ion flux through the cusp for varied geomagnetic field and solar wind. For the present solar wind condition and tilt angle &lt;15°, stronger geomagnetic field tends to generate a larger cusp area and higher ion flux through the cusp. For the weaker solar wind in the Maunder minimum, the ion flux is lower, but the cusp area is similar to that at present.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the first detailed dissection of the circumgalactic medium (CGM) of massive starburst galaxies at <jats:italic>z</jats:italic> &gt; 2. Our target is a submillimeter galaxy (SMG) at <jats:italic>z</jats:italic> = 2.674 that has a star formation rate of 1200 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> yr<jats:sup>−1</jats:sup> and a molecular gas reservoir of 1.3 × 10<jats:sup>11</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. We characterize its CGM with two background QSOs at impact parameters of 93 kpc and 176 kpc. We detect strong H <jats:sc>i</jats:sc> and metal-line absorption near the redshift of the SMG toward both QSOs, each consisting of three main subsystems spanning over 1500 km s<jats:sup>−1</jats:sup>. The absorbers show remarkable kinematic and metallicity coherence across a separation of ∼86 kpc. In particular, the cool gas in the CGM of the SMG exhibits high H <jats:sc>i</jats:sc> column densities (<jats:inline-formula> <jats:tex-math> <?CDATA ${\rm{log}}({N}_{{\rm{H}}{\rm\small{I}}}/{{\rm{cm}}}^{-2})=20.2,18.6$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabdb32ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>), a low metallicity ([M/H] ≈ −2.0), and nearly the same radial velocity (<jats:italic>δ</jats:italic> <jats:italic>v</jats:italic> ∼ −300 km s<jats:sup>−1</jats:sup>). While the H <jats:sc>i</jats:sc> column densities match previous results on the massive halos hosting QSOs, the metallicity is lower by more than an order of magnitude, making it an outlier in the line width−metallicity relation of damped Ly<jats:italic>α</jats:italic> absorbers. The large physical extent, the velocity coherence, the high surface densities, and the low metallicity are all consistent with the cool, inflowing, and near-pristine gas streams predicted to penetrate hot massive halos at <jats:italic>z</jats:italic> &gt; 1.5. We estimate a total gas accretion rate of ∼100 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> yr<jats:sup>−1</jats:sup> from three such streams. At this rate, it takes only a gigayear to acquire the molecular gas reservoir of the central starburst.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We developed a new selection method of halo stars in the phase-space distribution defined by the three integrals of motion in an axisymmetric Galactic potential (<jats:italic>E</jats:italic>, <jats:italic>L</jats:italic> <jats:sub> <jats:italic>z</jats:italic> </jats:sub>, <jats:italic>I</jats:italic> <jats:sub>3</jats:sub>), where <jats:italic>I</jats:italic> <jats:sub>3</jats:sub> is the third integral of motion. The method is used to explore the general chemodynamical structure of the halo based on stellar samples from SDSS-SEGUE DR7 and APOGEE DR16, matched with Gaia DR2. We found the following. (a) Halo stars can be separated from disk stars by selecting over (1) 0 &lt; <jats:italic>L</jats:italic> <jats:sub> <jats:italic>z</jats:italic> </jats:sub> &lt; 1500 kpc km s<jats:sup>−1</jats:sup>, <jats:inline-formula> <jats:tex-math> <?CDATA ${(2{I}_{3})}^{1/2}\gt 1000$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd7a4ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> kpc km s<jats:sup>−1</jats:sup> (orbital angle <jats:italic>θ</jats:italic> <jats:sub>orb</jats:sub> &gt; 15–20 deg), and <jats:italic>E</jats:italic> &lt; −1.5 × 10<jats:sup>5</jats:sup> km<jats:sup>2</jats:sup> s<jats:sup>−2</jats:sup>, and (2) <jats:italic>L</jats:italic> <jats:sub> <jats:italic>z</jats:italic> </jats:sub> &lt; 0 kpc km s<jats:sup>−1</jats:sup>. These selection criteria are free from kinematical biases introduced by the simple high-velocity cuts adopted in recent literature. (b) The averaged, or <jats:italic>coarse-grained</jats:italic>, halo phase-space distribution shows a monotonic, exponential decrease with increasing <jats:italic>E</jats:italic> and <jats:italic>I</jats:italic> <jats:sub>3</jats:sub> like the Michie–Bodenheimer models. (c) The inner stellar halo described in Carollo et al. is found to comprise a combination of Gaia Enceladus debris (GE), lowest-<jats:italic>E</jats:italic> stars (likely in situ stars), and metal-poor prograde stars missed by the high-velocity cut selection. (d) The very metal-poor outer halo, ([Fe/H] &lt; −2.2), exhibits both retrograde and prograde rotation, with an asymmetric <jats:italic>L</jats:italic> <jats:sub> <jats:italic>z</jats:italic> </jats:sub> distribution toward high retrograde motions and larger <jats:italic>θ</jats:italic> <jats:sub>orb</jats:sub> than those possessed by the GE-dominated inner halo. (e) The Sgr dSph galaxy could induce a long-range dynamical effect on local halo stars. Implications for the formation of the stellar halo are also discussed.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Particle acceleration in magnetized relativistic jets still puzzles theorists. In this work, we investigate the acceleration of particles injected into a three-dimensional relativistic magnetohydrodynamical jet subject to current-driven kink (CDK) instability. We find that, once turbulence driven by CDK fully develops, achieving a nearly stationary state, the amplitude of excited wiggles along the jet spine attains maximum growth, causing disruption of the magnetic field lines and the formation of several sites of fast reconnection. Low-energy protons injected into the jet at this state experience exponential acceleration, mostly in directions parallel to the local magnetic field, up to maximum energies <jats:inline-formula> <jats:tex-math> <?CDATA $E\sim {10}^{16}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd6c2ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> eV for <jats:inline-formula> <jats:tex-math> <?CDATA $B\sim 0.1$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd6c2ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> G and <jats:inline-formula> <jats:tex-math> <?CDATA $E\sim {10}^{18}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd6c2ieqn3.gif" xlink:type="simple" /> </jats:inline-formula> eV for <jats:inline-formula> <jats:tex-math> <?CDATA $B\sim 10$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd6c2ieqn4.gif" xlink:type="simple" /> </jats:inline-formula> G. The Larmor radius of the particles attaining these energies corresponds to the size of the acceleration region (∼the diameter of the perturbed jet). There is a clear association of the accelerated particles with regions of fast reconnection. In the early nonlinear growth stage of the CDK, when there are no sites of fast reconnection yet, injected particles with initially much larger energy are accelerated by magnetic curvature drift. We have also obtained the acceleration time due to reconnection with a dependence on the particles’ energy, <jats:inline-formula> <jats:tex-math> <?CDATA ${t}_{A}\propto {E}^{0.1}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd6c2ieqn5.gif" xlink:type="simple" /> </jats:inline-formula>. The energy spectrum of the accelerated particles develops a power-law index <jats:inline-formula> <jats:tex-math> <?CDATA $p$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd6c2ieqn6.gif" xlink:type="simple" /> </jats:inline-formula> ∼ −1.2 in the beginning, in agreement with earlier works. Our results provide a multidimensional framework for exploring this process in real systems and explain their emission patterns, especially at very high energies, and associated neutrino emission recently detected in some blazars.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The recently discovered gravitational wave sources GW190521 and GW190814 have shown evidence of BH mergers with masses and spins outside of the range expected from isolated stellar evolution. These merging objects could have undergone previous mergers. Such hierarchical mergers are predicted to be frequent in active galactic nuclei (AGNs) disks, where binaries form and evolve efficiently by dynamical interactions and gaseous dissipation. Here we compare the properties of these observed events to the theoretical models of mergers in AGN disks, which are obtained by performing one-dimensional <jats:italic>N</jats:italic>-body simulations combined with semi-analytical prescriptions. The high BH masses in GW190521 are consistent with mergers of high-generation (high-g) BHs where the initial progenitor stars had high metallicity, 2g BHs if the original progenitors were metal-poor, or 1g BHs that had gained mass via super-Eddington accretion. Other measured properties related to spin parameters in GW190521 are also consistent with mergers in AGN disks. Furthermore, mergers in the lower mass gap or those with low mass ratio as found in GW190814 and GW190412 are also reproduced by mergers of 2g–1g or 1g–1g objects with significant accretion in AGN disks. Finally, due to gas accretion, the massive neutron star merger reported in GW190425 can be produced in an AGN disk.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The compact object in the interacting binary AR Sco has widely been presumed to be a rapidly rotating, magnetized white dwarf (WD), but it has never been detected directly. Isolating its spectrum has proven difficult because the spin-down of the WD generates pulsed synchrotron radiation that far outshines the WD’s photosphere. As a result, a previous study of AR Sco was unable to detect the WD in the averaged far-ultraviolet spectrum from a Hubble Space Telescope (HST) observation. In an effort to unveil the WD’s spectrum, we reanalyze these HST observations by calculating the average spectrum in the troughs between synchrotron pulses. We identify weak spectral features from the previously unseen WD and estimate its surface temperature to be 11,500 ± 500 K. Additionally, during the synchrotron pulses, we detect broad Ly<jats:italic>α</jats:italic> absorption consistent with hot WD spectral models. We infer the presence of a pair of hotspots, with temperatures between 23,000 and 28,000 K, near the magnetic poles of the WD. As the WD is not expected to be accreting from its companion, we describe two possible mechanisms for heating the magnetic poles. The Ly<jats:italic>α</jats:italic> absorption of the hotspots appears relatively undistorted by Zeeman splitting, constraining the WD’s field strength to be ≲100 MG, but the data are insufficient to search for the subtle Zeeman splits expected at lower field strengths.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Tidal disruption events (TDEs) occurring in active galactic nuclei (AGNs) are a special class of sources with outstanding scientific significance. TDEs can generate ultrafast winds, which should almost inevitably collide with the preexisting AGN dusty tori. We perform analytical calculations and simulations on the wind–torus interactions and find such a process can generate considerable X-ray afterglow radiation several years or decades later after the TDE outburst. This provides a new origin for the years delayed X-rays in TDEs. The X-ray luminosity can reach 10<jats:sup>41−42</jats:sup> erg s<jats:sup>−1</jats:sup>, and the light curve characteristics depend on the parameters of winds and tori. We apply the model to two TDE candidates, and provide lower limits on the masses of the disrupted stars, as well as rigorous constraints on the gas densities of tori. Our results suggest that the observations of the time delay, spectral shape, luminosity, and the light curve of the X-ray afterglow can be used to constrain the physical parameters of both TDE winds and tori, including the wind velocity, wind density, cloud density, and cloud size.</jats:p>