Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>A novel application of machine-learning (ML) based image processing algorithms is proposed to analyze an all-sky map (ASM) obtained using the Fermi Gamma-ray Space Telescope. An attempt was made to simulate a 1 yr ASM from a short-exposure ASM generated from 1-week observation by applying three ML-based image processing algorithms: dictionary learning, U-net, and Noise2Noise. Although the inference based on ML is less clear compared to standard likelihood analysis, the quality of the ASM was generally improved. In particular, the complicated diffuse emission associated with the galactic plane was successfully reproduced only from 1-week observation data to mimic a ground truth (GT) generated from a 1 yr observation. Such ML algorithms can be implemented relatively easily to provide sharper images without various assumptions of emission models. In contrast, large deviations between simulated ML maps and the GT map were found, which are attributed to the significant temporal variability of blazar-type active galactic nuclei (AGNs) over a year. Thus, the proposed ML methods are viable not only to improve the image quality of an ASM but also to detect variable sources, such as AGNs, algorithmically, i.e., without human bias. Moreover, we argue that this approach is widely applicable to ASMs obtained by various other missions; thus, it has the potential to examine giant structures and transient events, both of which are rarely found in pointing observations.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Analysis of energetic particle inner heliospheric spacecraft data increasingly suggests the existence of anomalous diffusion phenomena that should be addressed to achieve a better understanding of energetic particle transport and acceleration in the expanding solar wind medium. Related to this is fast-growing observational evidence supporting the long-standing prediction from magnetohydrodynamic (MHD) theory and simulations of the presence of an inner heliospheric, dominant quasi-two-dimensional MHD turbulence component that contains coherent contracting and merging (reconnecting) small-scale magnetic flux rope (SMFR) structures. This suggests that energetic particle trapping in SMFRs should play a role in anomalous diffusion in the solar wind that warrants further investigation. However, progress in studying such anomalous energetic particle transport phenomena in the solar wind is hampered by the lack of a fundamental derivation of a general fractional kinetic transport equation linking macroscopic energetic particle fractional transport to the microscopic physics of energetic particle interaction with SMFR structures. Here, we outline details of how one can derive a closed ensemble-averaged focused transport equation in the form of a general kinetic fractional diffusion-advection equation from first principles following the nonlinear Eulerian correlation function closure approach of Sanchez et al. With this equation one can model the anomalous diffusion of energetic particles in ordinary, momentum, and pitch-angle space in response to particle trapping in numerous SMFRs advected with the solar wind flow.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Measurement of magnetic field strengths in a molecular cloud is essential for determining the criticality of magnetic support against gravitational collapse. In this paper, as part of the JCMT BISTRO survey, we suggest a new application of the Davis–Chandrasekhar–Fermi (DCF) method to estimate the distribution of magnetic field strengths in the OMC-1 region. We use observations of dust polarization emission at 450 and 850 <jats:italic>μ</jats:italic>m, and C<jats:sup>18</jats:sup>O (3–2) spectral line data obtained with the JCMT. We estimate the volume density, the velocity dispersion, and the polarization angle dispersion in a box, 40″ × 40″ (5×5 pixels), which moves over the OMC-1 region. By substituting three quantities in each box with the DCF method, we get magnetic field strengths over the OMC-1 region. We note that there are very large uncertainties in the inferred field strengths, as discussed in detail in this paper. The field strengths vary from 0.8 to 26.4 mG, and their mean value is about 6 mG. Additionally, we obtain maps of the mass-to-flux ratio in units of a critical value and the Alfvén Mach number. The central parts of the BN–KL and South (S) clumps in the OMC-1 region are magnetically supercritical, so the magnetic field cannot support the clumps against gravitational collapse. However, the outer parts of the region are magnetically subcritical. The mean Alfvén Mach number is about 0.4 over the region, which implies that the magnetic pressure exceeds the turbulent pressure in the OMC-1 region.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>GRB 190114C extends the focus of gamma-ray burst (GRB) research to the high-energy regime, in which a prime question is “Do all long-duration GRBs emit GeV photons?” Based on the Fermi Large Area Telescope (LAT) 10 yr observations, 54 GRBs initially within the Fermi-LAT field of view and with known redshift are sampled. Within 26 of these GRBs at least one GeV photon has been detected with a probability of &gt;95%, while the other 28 GRBs have no confident GeV photon detection. We hypothesize that all the samples intrinsically emit GeV photons, and the lack of detection is due to the limited capacity of the satellite. We estimate the theoretical number of photons that LAT receives by considering the GRB energy, the distance, and the LAT effective area. Results show, within the uncertainty, that all 26 GRBs with GeV photon detection have a theoretical photon number of &gt;1, and 27 out of 28 GRBs without GeV photon detection have a theoretical photon number of &lt;1. This agreement tends to support our hypothesis and give an answer of “yes” to our initial question.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>A time–distance helioseismic technique, similar to the one used by Ilonidis et al., is applied to two independent numerical models of subsurface sound-speed perturbations to determine the spatial resolution and accuracy of phase travel time shift measurements. The technique is also used to examine pre-emergence signatures of several active regions observed by the Michelson Doppler Imager and the Helioseismic Magnetic Imager. In the context of similar measurements of quiet-Sun regions, three of the five studied active regions show strong phase travel time shifts several hours prior to emergence. These results form the basis of a discussion of noise in the derived phase travel time maps and possible criteria to distinguish between true and false-positive detection of emerging flux.</jats:p>