arXiv subjects

A. Mastichiadis

Publications and source records attributed to A. Mastichiadis.

At least 19 recordsLinked to original sources

arXiv2024

LeHaMoC: a versatile time-dependent lepto-hadronic modeling code for high-energy astrophysical sources

Recent associations of high-energy neutrinos with active galactic nuclei (AGN) have revived the interest in leptohadronic models of radiation from astrophysical sources. The rapid increase in the amount of acquired multi-messenger data will require soon fast numerical models that may be applied to large source samples. We develop a time-dependent leptohadronic code, LeHaMoC, that offers several notable benefits compared to other existing codes, such as versatility and speed. LeHaMoC solves the Fokker-Planck equations of photons and relativistic particles (i.e. electrons, positrons, protons, and neutrinos) produced in a homogeneous magnetized source that may also be expanding. The code utilizes a fully implicit difference scheme that allows fast computation of steady-state and dynamically evolving physical problems. We first present test cases where we compare the numerical results obtained with LeHaMoC against exact analytical solutions and numerical results computed with ATHE$ν$A, a well-tested code of similar philosophy but different numerical implementation. We find a good agreement (within 10-30%) with the numerical results obtained with ATHE$ν$A without evidence of systematic differences. We then demonstrate the capabilities of the code through illustrative examples. First, we fit the spectral energy distribution from a jetted AGN in the context of a synchrotron-self Compton model and a proton-synchrotron model using Bayesian inference. Second, we compute the high-energy neutrino signal and the electromagnetic cascade induced by hadronic interactions in the corona of NGC 1068. LeHaMoC is easily customized to model a variety of high-energy astrophysical sources and has the potential to become a widely utilized tool in multi-messenger astrophysics.

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arXiv2024

TeV pion bumps in the gamma-ray spectra of flaring blazars

(Abridged) Very high-energy (VHE, $E>100$ GeV) observations of the blazar Mrk 501 with MAGIC in 2014 provided evidence for an unusual narrow spectral feature at about 3 TeV during an extreme X-ray flaring activity. The one-zone synchrotron-self Compton scenario, widely used in blazar broadband spectral modeling, fails to explain the narrow TeV component. Motivated by this rare observation, we propose an alternative model for the production of narrow features in the VHE spectra of flaring blazars. These spectral features may result from the decay of neutral pions ($π^0$ bumps) that are in turn produced via interactions of protons (of tens of TeV energy) with energetic photons, whose density increases during hard X-ray flares. We explored the conditions needed for the emergence of narrow $π^0$ bumps in VHE blazar spectra during X-ray flares reaching synchrotron energies $\sim 100$ keV using time-dependent radiative transfer calculations. We focused on high-synchrotron peaked (HSP) blazars, which comprise the majority of VHE-detected extragalactic sources. We find that synchrotron-dominated flares with peak energies $\gtrsim100$ keV can be ideal periods for the search of $π^0$ bumps in the VHE spectra of HSP blazars. The flaring region is optically thin to photopion production, its energy content is dominated by the relativistic proton population, and the inferred jet power is highly super-Eddington. Application of the model to the spectral energy distribution of Mrk 501 on MJD 56857.98 shows that the VHE spectrum of the flare is described well by the sum of a synchrotron self-Compton (SSC) component and a distinct $π^0$ bump centered at 3 TeV. Spectral fitting of simulated SSC+$π^0$ spectra for the Cherenkov Telescope Array (CTA) show that a $π^0$ bump could be detected at a 5$σ$ significance level with a 30-min exposure.

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arXiv2023

Application of neural networks to synchro-Compton blazar emission models

Jets from supermassive black holes in the centers of active galaxies are the most powerful persistent sources of electromagnetic radiation in the Universe. To infer the physical conditions in the otherwise out-of-reach regions of extragalactic jets we usually rely on fitting of their spectral energy distribution (SED). The calculation of radiative models for the jet non-thermal emission usually relies on numerical solvers of coupled partial differential equations. In this work machine learning is used to tackle the problem of high computational complexity in order to significantly reduce the SED model evaluation time, which is needed for SED fitting with Bayesian inference methods. We compute SEDs based on the synchrotron self-Compton model for blazar emission using the radiation code ATHE$ν$A, and use them to train Neural Networks exploring whether these can replace the original computational expensive code. We find that a Neural Network with Gated Recurrent Unit neurons can effectively replace the ATHE$ν$A leptonic code for this application, while it can be efficiently coupled with MCMC and nested sampling algorithms for fitting purposes. We demonstrate this through an application to simulated data sets and with an application to observational data. We offer this tool in the community through a public repository. We present a proof-of-concept application of neural networks to blazar science. This is the first step in a list of future applications involving hadronic processes and even larger parameter spaces.

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arXiv2022

A study of natural frequencies in a dynamic corona-disk system

Black-hole X-ray binaries (BHXRBs) in the hard and hard-intermediate spectral (and temporal) states exhibit in their power spectra characteristic frequencies called type-C quasi-periodic oscillations (QPOs). Various models that can explain them with various degrees of success have been proposed, but a definitive answer is still missing. The hot Comptonizing corona interacting with the cold accretion disk, both of which are central in understanding BHXRBs, is essentially a dynamical system. Our aim is to investigate if the radiative coupling between the two components can produce QPOs. We write and solve the time-dependent equations that describe energy conservation in the system corona - accretion disk. We examine both constant and variable mass accretion rates. By necessity, in this first investigation we use a simple model, but it contains all the essential ingredients. For a constant mass accretion rate and certain justifiable conditions, the dynamic corona - disk system exhibits oscillations, which die out after a few cycles. The characteristic frequencies of these oscillations are similar to the ones observed in the power spectra of BHXRBs. For most parameters, the natural frequencies persist even in the case of variable accretion rates. We argue that type-C QPOs in BHXRBs could, in principle, arise from the interaction of the hot Comptonizing corona with the much colder accretion disk. If this picture is correct, it has immediate implications for other systems that contain the above constituents, such as active galactic nuclei.

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arXiv2021

High-energy neutrinos from X-rays flares of blazars frequently observed by the Swift X-Ray Telescope

Blazar flares have been suggested as ideal candidates for enhanced neutrino production. While the neutrino signal of $γ$-ray flares has been widely discussed, the neutrino yield of X-ray flares has received less attention. Here, we compute the predicted neutrino signal from X-ray flares detected in 66 blazars observed more than 50 times with the X-ray Telescope (XRT) on board the Neil Gehrels Swift Observatory. We consider a scenario where X-ray flares are powered by synchrotron radiation of relativistic protons, and neutrinos are produced through photomeson interactions between protons with their own synchrotron X-ray photons. Using the 1 keV X-ray light curves for flare identification, the 0.5-10 keV fluence of each flare as a proxy for the all-flavour neutrino fluence, and the IceCube point-source effective area for different detector configurations, we calculate the number of muon and antimuon neutrinos above 100 TeV expected for IceCube from each flaring source. The bulk of the neutrino events from the sample originates from flares with durations $\sim 1-10$ d. Accounting for the X-ray flare duty cycle of the sources in the sample, which ranges between $\sim2$ and 24 per cent, we compute an average yearly neutrino rate for each source. The median of the distribution (in logarithm) is $\sim0.03$ yr$^{-1}$, with Mkn 421 having the highest predicted rate $1.2\pm 0.3$ yr$^{-1}$, followed by 3C 273 $(0.33\pm0.03$ yr$^{-1})$ and PG 1553+113 ($0.25\pm0.02$ yr$^{-1}$). Next-generation neutrino detectors together with regular X-ray monitoring of blazars could constrain the duty cycle of hadronic X-ray flares.

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arXiv2021

A numerical study of long-term multi-wavelength blazar variability

Decade-long monitoring of blazars at optical and infrared (OIR) wavelengths with the Small and Moderate Aperture Research Telescope System (SMARTS) in Chile and in $γ$-rays with the Fermi Large Area Telescope (LAT) has enabled the systematic study of their multi-wavelength long-term variability. In this work we investigate, from a theoretical perspective, the long-term variability properties of blazar emission by introducing an observationally motivated time-dependence to four main parameters of the one-zone leptonic model: injection luminosity of relativistic electrons, strength of magnetic field, Doppler factor, and external photon field luminosity. For the first time, we use both the probability density function and the power spectral density of the 10 year-long Fermi-LAT light curves to create variation patterns for the model parameters. Using as test beds two bright blazars from the SMARTS sample (PKS 2155-304 and 3C 273), we compute 10 year-long OIR, X-ray, and $γ$-ray model light curves for different varying parameters. We compare the findings of our theoretical investigation with multi-wavelength observations using various measures of variability. While no single-varying parameter simulation can explain all multi-wavelength variability properties, changes in the electron luminosity and external radiation field in PKS 2155-304 and 3C 273, respectively, can account for most of them. Our results motivate future time-dependent studies with coupling between two or more physical parameters to describe the multi-wavelength long-term blazar variability.

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arXiv2019

Multi-Physics of AGN Jets in the Multi-Messenger Era

Active galactic nuclei (AGN) with relativistic jets, powered by gas accretion onto their central supermassive black hole (SMBH), are unique laboratories for studying the physics of matter and elementary particles in extreme conditions that cannot be realized on Earth. For a long time since the discovery of AGN, photons were the only way to probe the underlying physical processes. The recent discovery of a very high energy neutrino, IceCube-170922A, coincident with a flaring blazar, TXS 0506+056, provides the first evidence that AGN jets are multi-messenger sources; they are capable of accelerating hadrons to very high energies, while producing non-thermal EM radiation and high-energy neutrinos. This new era of multi-messenger astronomy, which will mature in the next decade, offers us the unprecedented opportunity to combine more than one messenger to solve some long-standing puzzles of AGN jet physics: How do jets dissipate their energy to accelerate particles? What is the jet total kinetic power? Where and how do jets produce high-energy emission and neutrinos? What physical mechanisms drive the particle acceleration?

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arXiv2018

A Multimessenger Picture of the Flaring Blazar TXS 0506+056: implications for High-Energy Neutrino Emission and Cosmic Ray Acceleration

Detection of the IceCube-170922A neutrino coincident with the flaring blazar TXS 0506+056, the first and only 3-sigma high-energy neutrino source association to date, offers a potential breakthrough in our understanding of high-energy cosmic particles and blazar physics. We present a comprehensive analysis of TXS 0506+056 during its flaring state, using newly collected Swift, NuSTAR, and X-shooter data with Fermi observations and numerical models to constrain the blazar's particle acceleration processes and multimessenger (electromagnetic and high-energy neutrino) emissions. Accounting properly for electromagnetic cascades in the emission region, we find a physically-consistent picture only within a hybrid leptonic scenario, with gamma-rays produced by external inverse-Compton processes and high-energy neutrinos via a radiatively-subdominant hadronic component. We derive robust constraints on the blazar's neutrino and cosmic-ray emissions and demonstrate that, because of cascade effects, the 0.1-100keV emissions of TXS 0506+056 serve as a better probe of its hadronic acceleration and high-energy neutrino production processes than its GeV-TeV emissions. If the IceCube neutrino association holds, physical conditions in the TXS 0506+056 jet must be close to optimal for high-energy neutrino production, and are not favorable for ultra-high-energy cosmic-ray acceleration. Alternatively, the challenges we identify in generating a significant rate of IceCube neutrino detections from TXS 0506+056 may disfavor single-zone models. In concert with continued operations of the high-energy neutrino observatories, we advocate regular X-ray monitoring of TXS 0506+056 and other blazars in order to test single-zone blazar emission models, clarify the nature and extent of their hadronic acceleration processes, and carry out the most sensitive possible search for additional multimessenger sources.

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arXiv2016

A hadronic minute-scale GeV flare from quasar 3C 279?

The flat spectrum radio quasar 3C 279 is a known $γ$-ray variable source that has recently exhibited minute-scale variability at energies $>100$ MeV. One-zone leptonic models for blazar emission are severely constrained by the short timescale variability that implies a very compact emission region at a distance of hundreds of Schwarzschild radii from the central black hole. Here, we investigate a hadronic scenario where GeV $γ$-rays are produced via proton synchrotron radiation. We also take into account the effects of the hadronically initiated electromagnetic cascades (EMC). For a $γ$-ray emitting region in rough equipartition between particles and kG magnetic fields, located within the broad-line region (BLR), the development of EMC redistributes the $γ$-ray luminosity to softer energy bands and eventually leads to broad-band spectra that differ from the observed ones. Suppression of EMC and energy equipartition are still possible, if the $γ$-ray emitting region is located beyond the BLR, is fast moving with Doppler factor ($>70$), and contains strong magnetic fields ($>100$ G). Yet, these conditions cannot be easily met in parsec-scale jets, thus disfavouring a proton synchrotron origin of the Fermi-LAT flare.

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arXiv2015

The Statistics of the Prompt-to-Afterglow GRB Flux Ratios and the Supercritical Pile GRB Model

We present the statistics of the ratio, ${\mathrm R}$, between the prompt and afterglow "plateau" fluxes of GRB. This we define as the ratio between the mean prompt energy flux in the {\em Swift} BAT and the {\em Swift} XRT, immediately following the steep transition between these two states and the beginning of the afterglow stage referred to as the "plateau". Like the distribution of other GRB observables, the histogram of ${\mathrm R}$ is close to log-normal, with maximum at ${\mathrm R = R}_{\rm m} \simeq 2,000$, FWHM of about 2 decades and with the entire distribution spanning about 6 decades in the value of ${\mathrm R}$. We note that the peak of the distribution is close to the proton-to-electron mass ratio $({\mathrm R}_{\rm m} \simeq m_p/m_e = 1836)$, as proposed by us earlier, on the basis of a specific model for the conversion of the GRB blast wave kinetic energy into radiation, before any similar analysis were made. It therefore appears that, in addition to the values of the energy of peak luminosity ${E_{\rm pk}\sim m_{e} c^2}$, GRB present us with one more quantity with an apparently characteristic value. The fact that the values of both these quantities (i.e. $E_{\rm pk}$ and ${\mathrm R}$) comply with those implied by the same specific model devised to account for an altogether different issue, namely the efficient conversion of the GRB blast wave kinetic energy into radiation, argues favorably for its underlying assumptions.

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arXiv2014

Bethe-Heitler emission in BL Lacs: filling the gap between X-rays and $γ$-rays

We present the spectral signatures of the Bethe-Heitler pair production ($pe$) process on the spectral energy distribution (SED) of blazars, in scenarios where the hard $γ$-ray emission is of photohadronic origin. If relativistic protons interact with the synchrotron blazar photons producing $γ$ rays through photopion processes, we show that, besides the $2-20$ PeV neutrino emission, the typical blazar SED should have an emission feature due to the synchrotron emission of $pe$ secondaries that bridges the gap betweeen the low-and high-energy humps of the SED, namely in the energy range 40 keV-40 MeV. We first present analytical expressions for the photopion and $pe$ loss rates in terms of observable quantities of blazar emission. For the $pe$ loss rate in particular, we derive a new approximate analytical expression for the case of a power-law photon distribution, which has an excellent accuracy with the numerically calculated exact one, especially at energies above the threshold for pair production. We show that for typical blazar parameters, the photopair synchrotron emission emerges in the hard X-ray/soft $γ$-ray energy range with a characteristic spectral shape and non negligible flux, which may be even comparable to the hard $γ$-ray flux produced through photopion processes. We argue that the expected "$pe$ bumps" are a natural consequence of leptohadronic models, and as such, they may indicate that blazars with a three-hump SED are possible emitters of high-energy neutrinos.

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arXiv2012

The time-dependent one-zone hadronic model - First principles

We present a time-dependent approach to the one-zone hadronic model in the case where the photon spectrum is produced by ultrarelativistic protons interacting with soft photons that are produced from protons and low magnetic fields. Assuming that protons are injected at a certain rate in a homogeneous spherical volume containing a magnetic field, the evolution of the system can be described by five coupled kinetic equations, for protons, electrons, photons, neutrons, and neutrinos. Photopair and photopion interactions are modelled using the results of Monte-Carlo simulations and, in particular from the SOPHIA code for the latter. The coupling of energy losses and injection introduces a self-consistency in our approach and allows the study of the comparative relevancy of processes at various conditions, the efficiency of the conversion of proton luminosity to radiation, the resulting neutrino spectra, and the effects of time variability on proton injection, among other topics. We present some characteristic examples of the temporal behaviour of the system and show that this can be very different from the one exhibited by leptonic models. Furthermore, we argue that, contrary to the wide-held belief, there are parameter regimes where the hadronic models can become quite efficient. However, to keep the free parameters at a minimum and facilitate an in-depth study of the system, we have only concentrated on the case where protons are injected; i.e., we did not consider the effects of a co-accelerated leptonic component.

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arXiv2010

X-ray variability patterns in blazars

We study the expected variability patterns of blazars within the two-zone acceleration model putting special emphasis on flare shapes and spectral lags. We solve semi-analytically the kinetic equations which describe the particle evolution in the acceleration and radiation zone. We then perturb the solutions by introducing Lorentzian variations in its key parameters and examine the flaring behavior of the system. We apply the above to the X-ray observations of blazar 1ES 1218+304 which exhibited a hard lag behavior during a flaring episode and discuss possibilities of producing it within the context of our model. The steady-state radio to X-rays emission of 1ES 1218+304 can be reproduced with parameters which lie well within the ones generally accepted from blazar modeling. Additionally, we find that the best way to explain its flaring behavior is by varying the rate of particles injected in the acceleration zone.

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arXiv2009

On the multiwavelength emission from Gamma Ray Burst afterglows

Aims: Drawing an analogy with Active Galactic Nuclei, we investigate the one-zone SSC model of Gamma Ray Bursts afterglows in the presence of electron injection and cooling both by synchrotron and SSC losses. Methods: We solve the spatially averaged kinetic equations which describe the simultaneous evolution of particles and photons, obtaining the multi-wavelength spectrum as a function of time. We back up our numerical calculations with analytical solutions of the equations using various profiles of the magnetic field evolution under certain simplifying assumptions. Results: We apply the model to the afterglow evolution of GRBs in a uniform density environment and examine the impact various parameters have on the multiwavelength spectra. We find that in cases where the electron injection and/or the ambient density is high, the losses are dominated by SSC and the solutions depart significantly from the ones derived in the synchrotron standard cases.

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arXiv2009

The Supercritical Pile GRB Model: The Prompt to Afterglow Evolution

The "Supercritical Pile" is a very economical GRB model that provides for the efficient conversion of the energy stored in the protons of a Relativistic Blast Wave (RBW) into radiation and at the same time produces - in the prompt GRB phase, even in the absence of any particle acceleration - a spectral peak at energy $\sim 1$ MeV. We extend this model to include the evolution of the RBW Lorentz factor $Γ$ and thus follow its spectral and temporal features into the early GRB afterglow stage. One of the novel features of the present treatment is the inclusion of the feedback of the GRB produced radiation on the evolution of $Γ$ with radius. This feedback and the presence of kinematic and dynamic thresholds in the model are sources of potentially very rich time evolution which we have began to explore. In particular, one can this way obtain afterglow light curves with steep decays followed by the more conventional flatter afterglow slopes, while at the same time preserving the desirable features of the model, i.e. the well defined relativistic electron source and radiative processes that produce the proper peak in the $νF_ν$ spectra. In this note we present the results of a specific set of parameters of this model with emphasis on the multiwavelength prompt emission and transition to the early afterglow.

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arXiv2008

On the rapid TeV flaring activity of Markarian 501

Aims: We investigate the one-zone SSC model of TeV blazars in the presence of electron acceleration. In this picture electrons reach a maximum energy where acceleration saturates from a combination of synchrotron and inverse Compton scattering losses. Methods: We solve the spatially averaged kinetic equations which describe the simultaneous evolution of particles and photons, obtaining the multi-wavelength spectrum as a function of time. Results: We apply the model to the rapid flare of Mrk 501 of July 9, 2005 as this was observed by the MAGIC telescope and obtain the relevant parameters for the pre-flare quasi steady state and the ones during the flare. We show that a hard lag flare can be obtained with parameters which lie well within the range already accepted for this source. Especially the choice of a high value of the Doppler factor seems to be necessary.

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arXiv2008

Simulation of Cosmic Ray propagation in the Galactic Centre Ridge in Accordance with Observed VHE gamma-ray Emission

Diffuse VHE gamma radiation from the Galactic Centre ridge observed by the H.E.S.S. telescope has been convincingly linked with the propagation of recently accelerated cosmic rays that interact with molecular hydrogen clouds during their diffusion. Through a series of time-dependent simulations of that diffusion for different propagation parameters we have obtained the most probable values of the diffusion coefficient for the Galactic Centre region. Assuming that the diffusion coefficient is of the form kappa(E) = kappa_0*(E/E_0)^delta, then for different optimal combinations of kappa_0 and delta its value is obtained for cosmic rays originating from a central point (possibly Sgr A East) 10 kyr ago.

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arXiv2008

Two-dimensional PIC simulations of ion-beam instabilities in Supernova-driven plasma flows

Supernova remnant (SNR) blast shells can reach the flow speed $v_s = 0.1 c$ and shocks form at its front. Instabilities driven by shock-reflected ion beams heat the plasma in the foreshock, which may inject particles into diffusive acceleration. The ion beams can have the speed $v_b \approx v_s$. For $v_b \ll v_s$ the Buneman or upper-hybrid instabilities dominate, while for $v_b \gg v_s$ the filamentation and mixed modes grow faster. Here the relevant waves for $v_b \approx v_s$ are examined and how they interact nonlinearly with the particles. The collision of two plasma clouds at the speed $v_s$ is modelled with particle-in-cell (PIC) simulations, which convect with them magnetic fields oriented perpendicular to their flow velocity vector. One simulation models equally dense clouds and the other one uses a density ratio of 2. Both simulations show upper-hybrid waves that are planar over large spatial intervals and that accelerate electrons to $\sim$ 10 keV. The symmetric collision yields only short oscillatory wave pulses, while the asymmetric collision also produces large-scale electric fields, probably through a magnetic pressure gradient. The large-scale fields destroy the electron phase space holes and they accelerate the ions, which facilitates the formation of a precursor shock.

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