arXiv subjects

M. V. del Valle

Publications and source records attributed to M. V. del Valle.

10 recordsLinked to original sources

arXiv2026

High-energy Processes in the Bubbles of Wolf-Rayet Stars: The case of WR 102

Supersonic winds from massive stars carry great amounts of kinetic power and modify the surrounding interstellar medium. Through this interaction a stellar bubble is formed. Theoretical studies and recent observations suggest that the winds of massive stars could be sources of Galactic cosmic rays. The first detection of synchrotron emission from the bubble of a single star was reported, indicating the presence of relativistic electrons. Studying the non-thermal emission from a single massive star can help to better understand the acceleration of particles taking place in massive star clusters. WR 102 is the perfect case of study. In this work, we present the first high-energy model for the bubble of WR 102: G2.4+1.4. We aim at fitting the radio data and predicting gamma-ray emission. We assume that both electrons and protons are accelerated at the wind shock. We applied a classical model for the stellar bubble and adopted a one-zone model for estimating the radiation produced by the relativistic particles near the acceleration region. Additionally, we computed the expected emission from the protons that diffuse to the outer regions of the bubble. Also, we estimated the leptonic and hadronic contributions expected from cosmic rays. We fitted the observations considering that 3% of the wind kinetic power goes into relativistic electrons, and a magnetic field of 250 $μ$G. The dominant component at high energies is produced by locally accelerated protons reaching the shell. Protons might reach PeV energies in the wind bubble, but the predicted gamma-ray flux is too low to be detectable.

astro-ph.HE↗

arXiv2025

Magnetic flux transport via reconnection diffusion in different sonic regimes of interstellar MHD turbulence

Turbulence and magnetic fields are components of the interstellar medium and are interconnected through plasma processes. In particular, the magnetic flux transport in the presence of magneto-hydrodynamic (MHD) turbulence is an essential factor for understanding star formation. The theory of Reconnection Diffusion (RD), based on statistics of Alfvénic turbulence, predicts a dependence of the diffusion coefficient of the magnetic field on the Alfvénic Mach number $M_A$. However, this theory does not consider the effects of compressibility which are important in the regime of supersonic MHD turbulence. In this work, we measure the diffusion coefficient of magnetic fields in sub-Alfvénic MHD turbulence, with different sonic Mach numbers $M_S$. We perform numerical simulations of forced turbulence in periodic domains from the incompressible limit to the supersonic regime. We introduce two methods to extract the diffusion coefficient, based on the analysis of tracer particles. Our results confirm the RD assumption regarding the correspondence between the diffusion of magnetic field and that of fluid Lagrangian particles. The measured diffusion rate provided by incompressible turbulence agrees with the suppression predicted by the RD theory in the presence of strong magnetic fields: $D \propto M_A^3$. Our simulations also indicate an increase in RD efficiency when the turbulence is compressible. The dependency on $M_A$ and $M_S$ from the simulations can be described by the relation $D \propto M_A^α$, where $α(M_S) \approx 3/(1 + M_S)$. This quantitative characterization of $D$ is critical for modeling star formation in turbulent molecular clouds and evaluating the efficiency of this transport compared to other mechanisms.

astro-ph.HE↗

arXiv2025

Radio polarization from runaway star bowshocks-I. The general case

High velocity stars move through the interstellar medium with V > 30 km/s. When the star has powerful winds, under the appropriate conditions, the interaction of the wind with the interstellar material produces a system of shocks. The outer shock, called the bowshock, perturbs the ambient medium, heating and compressing the gas. The dust in the compressed bowshock cools, producing infrared radiation. This emission appears as extended coma-shape structures. The discovery of radio nonthermal emission from two stellar bowshock nebulae indicates that these sources might be accelerating electrons up to relativistic energies. The produced nonthermal radio emission is most probably synchrotron which has a high degree of polarization. In this work we model the synchrotron emission of runaway massive star bowshocks aiming to produce synthetic radio emission and polarization maps for two frequencies: 1.40 and 4.86 GHz. We model the interacting plasmas in a steady-state regime by means of magnetohydrodynamics simulations and we compute the injection and transport of the relativistic electrons in the diffusion approximation. We include in the model the most important depolarization effects. Our main conclusions are i) the effects of Faraday rotation within the source are important at the lowest frequency considered, ii) inferring the local magnetic field direction from polarization measurements only can be misleading, iii) thermal radio emission produced by ionized plasma within the bowshock structure and surroundings can surpass the polarized one for the considered frequencies, and iv) the contribution from the background electrons is minor.

astro-ph.SR↗

arXiv2025

Multi-messenger emission from magnetic reconnection in blazar jets: the case of TXS 0506+056

Measurements from astroparticle experiments, such as the 2017 flare associated with the source TXS 0506+056, indicate that blazars act as multi-messenger (MM; radiation and neutrinos) factories. Theoretically, the particle acceleration mechanisms responsible for blazar emissions and the precise location within the jet where this occurs remain undetermined. This paper explores MM emission driven by magnetic reconnection in a blazar jet. Previous studies have shown that reconnection in the magnetically dominated regions of these relativistic jets can efficiently accelerate particles to very high energies (VHE). Assuming that turbulent-driven magnetic reconnection accelerates cosmic-ray protons and electrons by a Fermi process, we developed a lepto-hadronic radiation model without the influence of external soft-photons to explain the 2017 MM flare from TXS 0506+056. In the proposed scenario, the emission blob moves downstream in the jet from $\sim$2 to 4 pc from the central engine, which is a supermassive black hole (SMBH) of $3 \times 10^{8}$ M$_\odot$ launching a jet with $150L_\mathrm{Edd}$ power. As the blob moves, we observe a sequence of spectral energy distribution (SED) profiles that match the observed arrival of the high energy neutrino and electromagnetic emission from TXS 0506+056. This arrival coincides with the high state of intermediate energy $γ$-rays ($E \sim 1 $ GeV) detection, followed by the subsequent appearance of the VHE $γ$-ray signal and then no further significant neutrino detection. We obtain a time delay between the neutrino and VHE events $\simeq 6.4$ days, which is consistent with that observed in the 2017 MM flare.

astro-ph.HE↗

arXiv2022

ASTRI Mini-Array Core Science at the Observatorio del Teide

The ASTRI (Astrofisica con Specchi a Tecnologia Replicante Italiana) Project led by the Italian National Institute for Astrophysics (INAF) is developing and will deploy at the Observatorio del Teide a mini-array (ASTRI Mini-Array) composed of nine telescopes similar to the small-size dual-mirror Schwarzschild-Couder telescope (ASTRI-Horn) currently operating on the slopes of Mt. Etna in Sicily. The ASTRI Mini-Array will surpass the current Cherenkov telescope array differential sensitivity above a few tera-electronvolt (TeV), extending the energy band well above hundreds of TeV. This will allow us to explore a new window of the electromagnetic spectrum, by convolving the sensitivity performance with excellent angular and energy resolution figures. In this paper we describe the Core Science that we will address during the first four years of operation, providing examples of the breakthrough results that we will obtain when dealing with current open questions, such as the acceleration of cosmic rays, cosmology and fundamental physics and the new window, for the TeV energy band, of the time-domain astrophysics.

astro-ph.HE↗

arXiv2022

Adiabatic-radiative shock systems in YSO jets and novae outflows

The termination regions of non-relativistic jets in protostars and supersonic outflows in classical novae are nonthermal emitters. Given the high densities in these systems, radiative shocks are expected to form. However, in the presence of high velocities, the formation of adiabatic shocks is also possible. A case of interest is when the two types of shocks occur simultaneously. These dense jets/outflows are excellent candidates for laboratory experiments as demonstrated by MHD scaling. We aim at studying the combination of adiabatic and radiative shocks in these systems. We focus on determining the conditions under which this combination is feasible together with its physical implications. We perform an analytical study of the shocks in both types of sources for a set of parameters. The hydrodynamical evolution of a jet colliding with an ambient medium is studied with 2D numerical simulations confirming our initial theoretical estimates. We show that for a wide set of parameters the combination of an adiabatic and a radiative shock is possible at the working surface of the termination region in jets from young stars and novae outflows. We find that instabilities are developed at the contact discontinuity, mixing the shocked materials. Also, we explore the MHD parameter scaling required for studying protostellar jets and novae outflows using laboratory experiments on laser facilities. The coexistence of an adiabatic and a radiative shock is expected at the termination region of protostellar jets and novae outflows. This scenario is very promising for particle acceleration and gamma-ray emission. The parameters for scaled laboratory experiments are very much in line with plasma conditions achievable in currently operating high-power laser facilities. This opens the door to new means for studying novae outflows never considered before.

astro-ph.HE↗

arXiv2017

X-ray study of bow shocks in runaway stars

Massive runaway stars produce bow shocks through the interaction of their winds with the interstellar medium, with the prospect for particle acceleration by the shocks. These objects are consequently candidates for non-thermal emission. Our aim is to investigate the X-ray emission from these sources. We observed with XMM-Newton a sample of 5 bow shock runaways, which constitutes a significant improvement of the sample of bow shock runaways studied in X-rays so far. A careful analysis of the data did not reveal any X-ray emission related to the bow shocks. However, X-ray emission from the stars is detected, in agreement with the expected thermal emission from stellar winds. On the basis of background measurements we derive conservative upper limits between 0.3 and 10 keV on the bow shocks emission. Using a simple radiation model, these limits together with radio upper limits allow us to constrain some of the main physical quantities involved in the non-thermal emission processes, such as the magnetic field strength and the amount of incident infrared photons. The reasons likely responsible for the non-detection of non-thermal radiation are discussed. Finally, using energy budget arguments, we investigate the detectability of inverse Compton X-rays in a more extended sample of catalogued runaway star bow shocks. From our analysis we conclude that a clear identification of non-thermal X-rays from massive runaway bow shocks requires one order of magnitude (or higher) sensitivity improvement with respect to present observatories.

astro-ph.HE↗

arXiv2016

Properties of the First-order Fermi acceleration in fast magnetic reconnection driven by turbulence in collisional MHD flows

Fast magnetic reconnection may occur in different astrophysical sources, producing flare-like emission and particle acceleration. Currently, this process is being studied as an efficient mechanism to accelerate particles via a first-order Fermi process. In this work we analyse the acceleration rate and the energy distribution of test particles injected in three-dimensional magnetohydrodynamical (MHD) domains with large-scale current sheets where reconnection is made fast by the presence of turbulence. We study the dependence of the particle acceleration time with the relevant parameters of the embedded turbulence, i.e., the Alfvén speed $V_{\rm A}$, the injection power $P_{\rm inj}$ and scale $k_{\rm inj}$ ($k_{\rm inj} = 1/l_{\rm inj}$). We find that the acceleration time follows a power-law dependence with the particle kinetic energy: $t_{acc} \propto E^α$, with $0.2 < α< 0.6$ for a vast range of values of $c / V_{\rm A} \sim 20 - 1000$. The acceleration time decreases with the Alfvén speed (and therefore with the reconnection velocity) as expected, having an approximate dependence $t_{acc} \propto (V_{\rm A} / c)^{-κ}$, with $κ\sim 2.1- 2.4$ for particles reaching kinetic energies between $1 - 100 \, m_p c^2$, respectively. Furthermore, we find that the acceleration time is only weakly dependent on the $P_{\rm inj}$ and $l_{\rm inj}$ parameters of the turbulence. The particle spectrum develops a high-energy tail which can be fitted by a hard power-law already in the early times of the acceleration, in consistency with the results of kinetic studies of particle acceleration by magnetic reconnection in collisionless plasmas.

astro-ph.HE↗

arXiv2012

Is the bowshock of the runaway massive star HD 195592 a Fermi source?

HD 195592 is an O-type super-giant star, known as a well-established runaway. Recently, a Fermi gamma-ray source (2FGL J2030.7+4417) with a position compatible with that of HD 195592 has been reported. Our goal is to explore the scenario where HD 195592 is the counterpart of the Fermi gamma-ray source modeling the non-thermal emission produced in the bowshock of the runaway star. We calculate the spectral energy distribution of the radiation produced in the bowshock of HD 195592 and we compare it with Fermi observations of 2FGL J2030.7+4417. We present relativistic particle losses and the resulting radiation of the bowshock of HD 195592 and show that the latter is compatible with the detected gamma-ray emission. We conclude that the Fermi source 2FGL J2030.7+4417 might be produced, under some energetic assumptions, by inverse Compton up-scattering of photons from the heated dust in the bowshock of the runaway star. HD 195592 might therefore be the very first object detected belonging to the category of gamma-ray emitting runaway massive stars, whose existence has been recently predicted.

astro-ph.HE↗

arXiv2012

AE Aurigae: first detection of non-thermal X-ray emission from a bow shock produced by a runaway star

Runaway stars produce shocks when passing through interstellar medium at supersonic velocities. Bow shocks have been detected in the mid-infrared for several high-mass runaway stars and in radio waves for one star. Theoretical models predict the production of high-energy photons by non-thermal radiative processes in a number sufficiently large to be detected in X-rays. To date, no stellar bow shock has been detected at such energies. We present the first detection of X-ray emission from a bow shock produced by a runaway star. The star is AE Aur, which was likely expelled from its birthplace by the encounter of two massive binary systems and now is passing through the dense nebula IC 405. The X-ray emission from the bow shock is detected at 30" to the northeast of the star, coinciding with an enhancement in the density of the nebula. From the analysis of the observed X-ray spectrum of the source and our theoretical emission model, we confirm that the X-ray emission is produced mainly by inverse Compton upscattering of infrared photons from dust in the shock front.

astro-ph.HE↗