arXiv subjects

E. López

Publications and source records attributed to E. López.

7 recordsLinked to original sources

arXiv2022

Broad-line Region Clouds orbiting an AGN sample

We present a spectral and temporal analysis of XMM-Newton data from a sample of six galaxies (NGC 3783, Mrk 279, Mrk 766, NGC 3227, NGC 7314, and NGC 3516). Using the hardness-ratio curves, we identify time-intervals in which clouds are eclipsing the central X-ray source in five of the six sources. We detect three occultations in NGC 3227 and one occultation in NGC 3783, NGC 7314, and NGC 3516, together with the well-known occultations in Mrk 766. We estimate the physical properties of the eclipsing clouds. The derived physical size of the X-ray sources ($\sim$(3-28)$\times$10$^{13}$ cm) is less than that of the eclipsing clouds with column densities of $\sim$10$^{22}$-10$^{23}$ cm$^{-2}$, thus a single cloud may block the X-ray source, leading to notorious temporal variability of the X-ray flux. The eclipsing clouds in Mrk 766, NGC 3227, NGC 7314, and NGC 3516 with distances from the X-ray source of $\sim$(0.3-3.6)$\times$10$^4$ $R_g$ are moving at Keplerian velocities $>$1122 km s$^{-1}$, typical parameters of broad-line region clouds, while the eclipsing cloud in NGC 3783 is likely located in the dusty torus. We also find a good anti-correlation with a slope of -187$\pm$62 between the known masses of the supermassive black hole in the center of the galaxies with the equivalent width (EW) of the 6.4 keV Fe line for the five Seyfert 1 galaxies of our sample, while the Seyfert 2 galaxy NGC 7314 shows an average EW value of 100$\pm$11 eV inconsistent with the above anti-correlation.

astro-ph.GA↗

arXiv2020

Vassiliev Invariants for Flows Via Chern-Simons Perturbation Theory

The perturbative expansion of Chern-Simons gauge theory leads to invariants of knots and links, the finite type invariants or Vassiliev invariants. It has been proven that at any order in perturbation theory the resulting expression is an invariant of that order. Bott-Taubes integrals on configuration spaces are introduced in the present context to write Feynman diagrams at a given order in perturbation theory in a geometrical and topological setting. One of the consequences of the configuration space formalism is that the resulting amplitudes are given in cohomological terms. This cohomological structure can be used to translate Bott-Taubes integrals into Chern-Simons perturbative amplitudes and vice versa. In this article this program is performed up to third order in the coupling constant. This expands some work previously worked out by Thurston. Finally we take advantage of these results to incorporate in the formalism a smooth and divergenceless vector field on the $3$-manifold. The Bott-Taubes integrals obtained are used for constructing higher-order asymptotic Vassiliev invariants extending the work of Komendarczyk and Volić.

hep-th↗

arXiv2020

On the effects of UV photons/X-rays on the chemistry of the Sgr B2 cloud

The lines of HOC$^+$, HCO and CO$^+$ are considered good tracers of photon-dominated regions (PDRs) and X-ray dominated regions. We study these tracers towards regions of the Sgr B2 cloud selected to be affected by different heating mechanisms. We find the lowest values of the column density ratios of HCO$^+$ versus HOC$^+$, HCO and CO$^+$ in dense HII gas, where UV photons dominate the heating and chemistry of gas. HOC$^+$, HCO and CO$^+$ abundances and the above ratios are compared with those of chemical modeling, finding that high temperature chemistry, a cosmic-ray ionization rate of 10$^{-16}$ s$^{-1}$ and timescales $>$10$^{5.0}$ years explain well the HOC$^+$ abundances in quiescent Sgr B2 regions, while shocks are also needed to explain the highest HCO abundances derived for these regions. CO$^+$ is mainly formed in PDRs since the highest CO$^+$ abundances of $\sim$(6-10)$\times$10$^{-10}$ are found in HII regions with electron densities $>$540 cm$^{-3}$ and that CO$^+$ emission is undetected in quiescent gas. Between the ratios, the HCO$^+$/HCO ratio is sensitive to the electron density as it shows different values in dense and diffuse HII regions. We compare SiO J=2-1 emission maps of Sgr B2 with X-ray maps from 2004 and 2012. One known spot shown on the 2012 X-ray map is likely associated with molecular gas at velocities of 15-25 km s$^{-1}$. We also derive the X-ray ionization rate of $\sim$10$^{-19}$ s$^{-1}$ for Sgr B2 regions pervaded by X-rays in 2004, which is quite low to affect the chemistry of the molecular gas.

astro-ph.GA↗

arXiv2018

Study of diffuse HII regions potentially forming part of the gas streams around Sgr A*

We present a study of diffuse extended ionised gas toward three clouds located in the Galactic Centre (GC). One line of sight (LOS) is toward the 20 km s$^{-1}$ cloud (LOS$-$0.11) in the Sgr A region, another LOS is toward the 50 km s$^{-1}$ cloud (LOS$-$0.02), also in Sgr A, while the third is toward the Sgr B2 cloud (LOS+0.693). The emission from the ionised gas is detected from H$nα$ and H$mβ$ radio recombination lines (RRLs). He$nα$ and He$mβ$ RRL emission is detected with the same $n$ and $m$ as those from the hydrogen RRLs only toward LOS+0.693. RRLs probe gas with positive and negative velocities toward the two Sgr A sources. The H$mβ$ to H$nα$ ratios reveal that the ionised gas is emitted under local thermodynamic equilibrium conditions in these regions. We find a He to H mass fraction of 0.29$\pm$0.01 consistent with the typical GC value, supporting the idea that massive stars have increased the He abundance compared to its primordial value. Physical properties are derived for the studied sources. We propose that the negative velocity component of both Sgr A sources is part of gas streams considered previously to model the GC cloud kinematics. Associated massive stars with what are presumably the closest HII regions to LOS$-$0.11 (positive velocity gas), LOS$-$0.02 and LOS+0.693 could be the main sources of UV photons ionising the gas. The negative velocity components of both Sgr A sources might be ionised by the same massive stars, but only if they are in the same gas stream.

astro-ph.GA↗

arXiv2017

Analysis of Magnetic Field Variations Produced by Equatorial Electro-Jets

The Equatorial Electrojet (EEJ) is a narrow band of electrons flowing from east to west at daytime at low latitudes. The electron current produces a magnetic field variation that can be measured at different latitudes. In this work, we have used the data analysis in order to quantify the solar and lunar contributions to those variations and study the morphology of the EEJ current.

physics.space-ph↗

arXiv2013

The principle of stationary action in the calculus of variations

We review some techniques from non-linear analysis in order to investigate critical paths for the action functional in the calculus of variations applied to physics. Previous attempts to analyse when these are minima ex- ist, but mainly based on physical reasoning and only for a restricted class of models. Our main intention in this regard is to develop precise mathematical conditions for critical paths to be minimum solutions in a variety of situations. Our claim is that, with a few techniques, a systematic analysis (including the domain for which critical points are genuine minima) of non-trivial models is possible. We present specific models arising in modern physical theories in order to make clear the ideas here exposed.

math-ph↗

arXiv2008

Effects of epidemic threshold definition on disease spread statistics

We study the statistical properties of the SIR epidemics in heterogeneous networks, when an epidemic is defined as only those SIR propagations that reach or exceed a minimum size s_c. Using percolation theory to calculate the average fractional size of an epidemic, we find that the strength of the spanning link percolation cluster $P_{\infty}$ is an upper bound to . For small values of s_c, $P_{\infty}$ is no longer a good approximation, and the average fractional size has to be computed directly. The value of s_c for which $P_{\infty}$ is a good approximation is found to depend on the transmissibility T of the SIR. We also study Q, the probability that an SIR propagation reaches the epidemic mass s_c, and find that it is well characterized by percolation theory. We apply our results to real networks (DIMES and Tracerouter) to measure the consequences of the choice s_c on predictions of average outcome sizes of computer failure epidemics.

q-bio.PE↗