arXiv subjects

Despina Karavola

Publications and source records attributed to Despina Karavola.

6 recordsLinked to original sources

arXiv2026

Guide field effects on particle acceleration in three-dimensional relativistic magnetic reconnection

Relativistic magnetic reconnection is a key mechanism for particle acceleration in astrophysical plasmas. Recent 3D particle-in-cell simulations have shown that, for weak guide fields, particles can be accelerated at nearly the maximal rate by escaping the reconnected plasma and gaining energy from the large-scale upstream electric field. Here, we investigate whether this fast acceleration channel persists as the guide field becomes comparable to the reconnecting field. We perform 3D simulations of relativistic pair-plasma reconnection, complemented by corresponding 2D runs, using open boundaries along the reconnection outflows to establish a quasi-steady state. We find that increasing the guide field strength reduces the reconnection rate and causes energetic particles to become more closely aligned with the local magnetic field. In 3D, the fast acceleration channel---absent in 2D---persists across the full range of guide fields investigated, but involves a progressively smaller fraction of particles as the guide field increases. Consequently, stronger guide fields produce steeper particle energy spectra and lower maximum Lorentz factors.

astro-ph.HE↗

arXiv2026

The role of Bethe-Heitler pair production in reconnection-driven flares in M87*

Rapid TeV flares have been observed from the core of the active galaxy M87. These have been attributed to inverse Compton scattering of disk photons by electrons and positrons accelerated in transient reconnection layers formed in baryon-poor regions of the magnetosphere of the central black hole, M87*. It was previously shown that even a small number of protons accelerated in the same layers can lead to bright GeV proton-synchrotron flares, if protons receive $\gtrsim20\%$ of the dissipated power for reconnecting fields of $\sim$100 G. We aim to investigate the role of Bethe-Heitler pair production in the emission of reconnection-driven flares from M87* in this physical regime. We perform numerical calculations that incorporate inelastic collisions between relativistic protons and photons, as well as photon-photon pair production, and compute the non-thermal radiation from the layer. The numerical calculations are also supported by analytical estimates. We find that disk photons act as targets for Bethe-Heitler pair production. The resulting pairs emit very high-energy synchrotron photons ($\gtrsim$0.1 TeV), which are subsequently attenuated by the disk photon field, leading to further pair production. The synchrotron emission of these secondary pairs produces soft photons, as part of an electromagnetic cascade, enhancing pion production and photon-photon attenuation down to tens-of-GeV energies.

astro-ph.HE↗

arXiv2025

Diffuse neutrino flux from coronal magnetospheric current sheets of accreting black holes

Non-jetted AGN exhibit hard X-ray emission with a power law spectrum above $\sim$2 keV, which is thought to be produced through Comptonization of soft photons by electrons and positrons (pairs) in the vicinity of the black hole. The origin and composition of this plasma source, known as the corona, is a matter open for debate. Our study focuses on the role of relativistic protons accelerated in black-hole magnetospheric current sheets in the neutrino production of AGN coronae. We present a model that has two free parameters, namely the proton plasma magnetization $σ_{\rm p}$, which controls the peak energy of the neutrino spectrum, and the Eddington ratio $λ_{\rm Edd}$ (defined as the ratio between X-ray luminosity $L_{\rm X}$ and Eddington luminosity $L_{\rm Edd}$), which controls the amount of energy transferred to secondary particles. Furthermore, we combine our coronal model with an AGN population in order to provide a prediction for the diffuse neutrino flux measured on Earth. We compare our results with the observational data by IceCube and we find a satisfactory agreement on both the flux value and the slope of the neutrino distribution when we assume a $σ_{\rm p}$ value of $10^5$ for all the sources in our sample.

astro-ph.HE↗

arXiv2025

Accelerating SED Modeling of Astrophysical Objects Using Neural Networks

Interpreting the spectral energy distributions (SEDs) of astrophysical objects with physically motivated models is computationally expensive. These models require solving coupled differential equations in high-dimensional parameter spaces, making traditional fitting techniques such as Markov Chain Monte Carlo or nested sampling prohibitive. A key example is modeling non-thermal emission from blazar jets - relativistic outflows from supermassive black holes in Active Galactic Nuclei that are among the most powerful emitters in the Universe. To address this challenge, we employ machine learning to accelerate SED evaluations, enabling efficient Bayesian inference. We generate a large sample of lepto-hadronic blazar emission models and train a neural network (NN) to predict the photon spectrum with strongly reduced run time while preserving accuracy. As a proof of concept, we present an NN-based tool for blazar SED modeling, laying the groundwork for future extensions and for providing an open-access resource for the astrophysics community.

astro-ph.HE↗

arXiv2024

A closer look at the electromagnetic signatures of Bethe-Heitler pair production process in blazars

The ''twin birth'' of a positron and an electron by a photon in the presence of a nucleus, known as Bethe-Heitler pair production, is a key process in astroparticle physics. The Bethe-Heitler process offers a way of channeling energy stored in a population of relativistic protons (or nuclei) to relativistic pairs with extended distributions. Contrary to accelerated leptons, whose maximum energy is limited by radiative losses, the maximal energy of pairs is determined by the kinematics of the process and can be as high as the parent proton energy. We take a closer look at the features of the injected pair distribution, and provide a novel empirical function that describes the spectrum of pairs produced by interactions of single-energy protons with single-energy photons. The function is the kernel of the Bethe-Heitler pair production spectrum that replaces a double numerical integration involving the complex differential cross section of the process, and can be easily implemented in numerical codes. We further examine under which conditions Bethe-Heitler pairs produced in blazar jets can emit $γ$-ray photons via synchrotron radiation, thus providing an alternative to the inverse Compton scattering process for high-energy emission in jetted active galactic nuclei. After taking into consideration the broadband spectral characteristics of the source, the jet energetics, and the properties of radiation fields present in the blazar environment, we conclude that $γ$-rays in low- and intermediate-peaked blazars may arise from Bethe-Heitler pairs in regions of the jet with typical transverse size $\sim 10^{15}$ cm and co-moving magnetic field $5-500$ G.

astro-ph.HE↗

arXiv2023

Dust-scattering rings of GRB 221009A as seen by the Neil Gehrels Swift satellite: can we count them all?

We present the first results for the dust-scattering rings of GRB 221009A, coined as the GRB of the century, as observed by the Neil Gehrels Swift satellite. We perform analysis of both time resolved observations and stacked data. The former approach enable us to study the expansion of the most prominent rings, associate their origin with the prompt X-ray emission of the GRB and determine the location of the dust layers. The stacked radial profiles increase the signal-to-noise ratio of the data and allows detection of fainter and overlapping peaks in the angular profile. We find a total of 16 dust concentrations (with hints of even more) that span about 15 kpc in depth and could be responsible for the highly structured X-ray angular profiles. By comparing the relative scattered fluxes of the five most prominent rings we show that the layer with the largest amount of dust is located at about 0.44 kpc away from us. We finally compare the location of the dust layers with results from experiments that study the 3D structure of our Galaxy via extinction or CO radio observations, and highlight the complementarity of dust X-ray tomography to these approaches.

astro-ph.HE↗