Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>Using the first long-term photometry from the Transiting Exoplanet Survey Satellite, we have carried out a detailed time-resolved timing analysis of an intermediate polar TX Col. The power spectra of almost 52 days of continuous time-series data reveal the orbital period of 5.691 ± 0.006 hr, spin period of 1909.5 ± 0.2 s, and beat period of 2105.76 ± 0.25 s, which is consistent with the earlier results. We have also found the presence of quasi-periodic oscillations (QPOs) for a few days with a period of 5850–5950 s, which appears to be due to the beating of the Keplerian period with the spin period of the white dwarf. The continuous data allowed us to look thoroughly at the day-wise evolution of the system’s accretion geometry. We report here that the TX Col changes its accretion mechanism even on a timescale of one day, confirming its variable disk-overflow accretion nature. For the majority of the time, it was found to be disk-overflow system with stream-fed dominance; however, pure disk-fed and pure stream-fed accretions cannot be ruled out during the observations.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Observations proved that a distributed structure named a supra-arcade fan (SAF) exists above post-flare loops in solar eruptions. The locations of the SAF are spatially consistent with various emission sources. Termination shocks (TSs) that are often regarded as an efficient driver for particle acceleration possibly exist in the SAF. We performed the numerical simulations of solar flares based on the standard flare model to study the dynamical and thermal manifestations of the SAF, as well as the possibility of detecting TSs in extreme-ultraviolet (EUV) images. In the simulations, the SAF and TSs can be clearly identified. The motion history and temperature evolution of plasmas inside the SAF indicate that the mass of the SAF comes from the corona and the plasmas are heated in the current sheet. The height of the SAF decreases with the speed of about 64.6 km s<jats:sup>−1</jats:sup> when the rate of magnetic reconnection quickly increases, and then increases with a slightly lower velocity of about 50.5 km s<jats:sup>−1</jats:sup> after the peak of the rate of magnetic reconnection. The descent−ascent path of the SAF is due to the unbalance of the Lorentz force and the pressure force inside the magnetic loops. In synthetic EUV images, emission intensity variations in the area surrounding TSs are significant, indicating that, depending on the viewing angle, TSs could be identifiable in EUV observations. The results of numerical simulations are generally consistent with observations, helping us to better understand the characteristics of the SAF and the physical natures behind it.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>High-mass gravitational-wave events in the neutron-star mass range, such as GW190425, have recently started to be detected by the LIGO/Virgo detectors. If the masses of the two binary components fall in the neutron-star mass range, such a system is typically classified as a binary neutron-star system, although the detected gravitational-wave signal may be too noisy to clearly establish a neutron-star nature of the high-mass component in the binary and rule out a black hole–neutron star system for such an event. We show that high-mass binary neutron-star mergers with a very massive neutron-star primary close to the maximum-mass limit, <jats:italic>m</jats:italic> <jats:sub>1</jats:sub> ≳ 2.2 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, produce fast dynamical mass ejecta from the spin-up of the primary star at merger. By simulating the merger of black hole–neutron star systems of exactly the same masses and spins, we show that these fast ejecta are entirely absent if the primary is instead a black hole. In addition, we find that both systems leave almost identical amounts of baryon mass behind, which is not immediately accreted by the black hole. This implies that both systems will likely have comparable electromagnetic afterglow emission stemming from the remnant disk. Hence, fast ejecta at merger have the potential to distinguish neutron stars from black holes in high-mass mergers, although these ejecta may be challenging to detect observationally.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Solar and stellar flares are powerful events that produce intense radiation across the electromagnetic spectrum. Multiwavelength observations are highly important for understanding the nature of flares, because different flare-related processes reveal themselves in different spectral ranges. To study the correlation between thermal and nonthermal processes in stellar flares, we have searched the databases of Kepler (optical observations) and XMM-Newton (soft X-rays) for the flares observed simultaneously with both instruments; nine distinctive flares (with energies exceeding 10<jats:sup>33</jats:sup> erg) on three stars (of K-M spectral classes) have been found. We have analyzed and compared the flare parameters in the optical and X-ray spectral ranges; we have also compared the obtained results with similar observations of solar flares. Most of the studied stellar flares released more energy in the optical range than in X-rays. In one flare, X-ray emission strongly dominated, which could be caused either by a soft spectrum of energetic electrons or by a near-limb position of this flare. The X-ray flares were typically delayed with respect to and shorter than their optical counterparts, which is partially consistent with the Neupert effect. Using the scaling laws based on the magnetic reconnection theory, we have estimated the characteristic magnetic field strengths in the stellar active regions and the sizes of these active regions as about 25–70 G and 250,000–500,000 km, respectively. The observed stellar superflares appear to be scaled-up versions of solar flares, with a similar underlying mechanism and nearly the same characteristic magnetic field values, but with much larger active region sizes.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Collisionless shock heats ions very efficiently. Ion heating is a nonadiabatic process and the downstream temperature is not proportional to the upstream temperature. Ion distributions just behind the shock transition are gyrophase dependent and gradually gyrotropize. With the increase of the Mach number the contribution of reflected ions into ion heating gradually increases. Yet, directly transmitted ions may remain responsible for most of the downstream temperature. Direct transmission allows analytical treatment within the approximation of a narrow shock. We determine the gyrophase-dependent distribution just behind the magnetic jump, the gyrotropic distribution farther behind the shock, and establish the relation with the magnetic compression and the maximum overshoot magnetic field.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The random advection of passive additives in a turbulent fluid plays an important role in solar physics, astrophysics, and atmospheric sciences. We concern ourselves here with the case where the fluctuations are not statistically homogeneous in space, and, hence, where the transport coefficients vary with position. Using a numerical model in which the fluid turbulence is defined kinematically, we show that the evolution of the distribution of passive tracers in the fluid is not always governed by the ordinary diffusion equation. We find it is governed by a more general transport equation whose form depends on the nature of the turbulence, particularly on its compressibility, or divergence. The more general transport equation resembles the ordinary diffusion equation, but the transport coefficient appears in two places and is raised to a power that depends on the divergence of the fluid velocity. If the flow has zero divergence, the case for incompressible turbulence, the resulting transport equation is found to be the regular diffusion equation.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Solar wind protons undergo significant heating during the expansion of the solar wind and turbulence plays an important role in this process. It is believed that the energy is injected from the energy-containing range into the inertial range and then transferred to dissipate into heat eventually. However, the energy injection process in the heliosphere remains unclear. Here we analyze this process. We utilize Helios 2 and Ulysses measurements of the fast solar wind at different radial distances from 0.29 to 4.8 au. We obtain the perpendicular heating rate based on the gradient of the magnetic moment. We estimate for the first time the energy supply rate due to the sweeping of low-frequency break based on the identification of low-frequency break and the corresponding power spectra density profile. We find that the energy supply rate is comparable to the perpendicular heating rate of protons, which support the idea that low-frequency range becomes part of the inertial range as the solar wind turbulence ages. These results help us understand the energy supply process from the energy-containing range and the heating process of solar wind protons.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The measurements from the Venus Express spacecraft are analyzed for the basic properties of fast forward interplanetary shocks at Venus’s orbit (∼0.72 au). A total of 143 fast forward interplanetary shock candidates during 2006–2014 are identified. The shock angle Θ<jats:sub>Bn</jats:sub>, defined as the angle between the shock normal and the upstream magnetic field, and the magnetic compression ratio <jats:italic>r</jats:italic> <jats:sub>B</jats:sub>, defined as the ratio of the magnetic field strength downstream to that upstream, of these shocks are determined based on the magnetic coplanarity method. The shock occurrence at Venus shows a correlated variation with the solar activity level measured by the number of sunspots, while the shock angle and magnetic compression ratio do not show such a correspondence. The shock angle spreads almost uniformly between 10° and 80° with its mean value at about 45°, and the magnetic compression ratio shows a unimodal distribution between 1.0 and 4.5 with a mean value of 2.1. In addition, we also analyze the properties of fast forward shocks driven by interplanetary coronal mass ejections (ICMEs). We found that interplanetary shocks with and without detected ICMEs showed no significant differences in terms of the shock strength and the shock angle. Further comparison with previous observational results at 1 au shows that fast forward shocks at 1 au are generally weaker than those at 0.72 au, and the shock angle Θ<jats:sub>Bn</jats:sub> is more perpendicular at 1 au.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We analyze three prototypical black hole X-ray binaries, 4U 1630–472, GRO J1655–40, and H1743–322, in an effort to systematically understand the intrinsic state transition of the observed accretion disk winds between <jats:monospace>wind-on</jats:monospace> and <jats:monospace>wind-off</jats:monospace> states by utilizing state-of-the-art Chandra/HETGS archival data from multi-epoch observations. We apply our magnetically driven wind models in the context of magnetohydrodynamic (MHD) calculations to constrain (1) their global density slope (<jats:italic>p</jats:italic>), (2) their density (<jats:italic>n</jats:italic> <jats:sub>17</jats:sub>) at the foot point of the innermost launching radius, and (3) the abundances of heavier elements (<jats:italic>A</jats:italic> <jats:sub>Fe,S,Si</jats:sub>). Incorporating the MHD winds into <jats:monospace>xstar</jats:monospace> photoionization calculations in a self-consistent manner, we create a library of synthetic absorption spectra given the observed X-ray continua. Our analysis clearly indicates a characteristic bimodal transition of multi-ion X-ray winds; i.e., the wind density gradient is found to steepen (from <jats:italic>p</jats:italic> ∼ 1.2–1.4 to ∼1.4–1.5) while its density normalization declines as the source transitions from the <jats:monospace>wind-on</jats:monospace> to the <jats:monospace>wind-off</jats:monospace> state. The model implies that the ionized wind remains physically present even in the <jats:monospace>wind-off</jats:monospace> state, despite its apparent absence in the observed spectra. Supersolar abundances for heavier elements are also favored. Our global multi-ion wind models, taking into account soft X-ray ions as well as Fe K absorbers, show that the internal wind condition plays an important role in wind transitions besides photoionization changes. Simulated XRISM/Resolve and Athena/X-IFU spectra are presented to demonstrate a high fidelity of the multi-ion wind model for a better understanding of these powerful ionized winds in the coming decades.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The See Change survey was designed to make <jats:italic>z</jats:italic> &gt; 1 cosmological measurements by efficiently discovering high-redshift Type Ia supernovae (SNe Ia) and improving cluster mass measurements through weak lensing. This survey observed twelve galaxy clusters with the Hubble Space Telescope (HST) spanning the redshift range <jats:italic>z</jats:italic> = 1.13–1.75, discovering 57 likely transients and 27 likely SNe Ia at <jats:italic>z</jats:italic> ∼ 0.8–2.3. As in similar previous surveys, this proved to be a highly efficient use of HST for supernova observations; the See Change survey additionally tested the feasibility of maintaining, or further increasing, the efficiency at yet higher redshifts, where we have less detailed information on the expected cluster masses and star formation rates. We find that the resulting number of SNe Ia per orbit is a factor of ∼8 higher than for a field search, and 45% of our orbits contained an active SN Ia within 22 rest-frame days of peak, with one of the clusters by itself yielding 6 of the SNe Ia. We present the survey design, pipeline, and supernova discoveries. Novel features include fully blinded supernova searches, the first random forest candidate classifier for undersampled IR data (with a 50% detection threshold within 0.05 mag of human searchers), real-time forward-modeling photometry of candidates, and semi-automated photometric classifications and follow-up forecasts. We also describe the spectroscopic follow-up, instrumental in measuring host galaxy redshifts. The cosmology analysis of our sample will be presented in a companion paper.</jats:p>