arXiv subjects

Krzysztof Nalewajko

Publications and source records attributed to Krzysztof Nalewajko.

At least 19 recordsLinked to original sources

arXiv2026

Distortion of a relativistic jet echoing a magnetic flux eruption

Magnetized accretion onto spinning black holes can accumulate a large magnetic flux across the event horizon and launch a pair of relativistic jets via the Blandford-Znajek mechanism. In the magnetically saturated (arrested) state, excess magnetic flux is ejected from the black hole in episodic magnetic flux eruptions, which result in a significant yet temporary reduction of jet power. We analyze results of a high-resolution 3D general-relativistic magneto-hydro-dynamic numerical simulation of geometrically thick magnetically saturated accretion onto a high-spin Kerr black hole for a single cycle of magnetic flux eruption and accumulation. We show that following an eruption, a weakened jet develops a strong helical distortion with distinct structure of magnetic fields - the poloidal field along the jet core is unaffected by the eruption; while toroidal field lines, ejected from the black hole during the eruption and later re-advected onto it, form poloidal `bypasses' along the inner jet sheath. Such a distortion may appear in sources fed by geometrically thick accretion flows as an asymmetric superluminal knot, strongly interacting with the jet sheath along an oblique working surface. The jet section re-powered by magnetic flux re-accumulated on the black hole is tilted by a few degrees, implying significant variations in radiation boost towards observers of BL Lac blazars. The intrinsic structure of the jet spine is consistent with axisymmetric semi-analytical models.

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arXiv2025

High-energy variability of the gravitationally lensed blazar PKS 1830-211

The production site and process responsible for the highly variable high-energy emission observed from blazar jets are still debated. Gravitational lenses can be used as microscopes to investigate the nature of such sources. We study the broad-band spectral properties and the high-energy variability of the gravitationally-lensed blazar PKS 1830-211, for which radio observations have revealed two images, to put constraints on the jet physics and the existence of a gravitationally-induced time delay and magnification ratio between the images. We utilize Swift/XRT, Nustar, and Fermi-LAT observations from 2016 and 2019 to compare periods of low activity and high activity in PKS 1830-211. Short-timescale variability is elucidated with an unbinned power spectrum analysis of time-tagged NuSTAR photon data. To study the gravitationally-induced time delay in the gamma-ray light curve observed with Fermi-LAT, we improve existing autocorrelation function based methods. Our modified auto-correlation method yields a delay of t_0=21.1 +/- 0.1 d and magnification factor a=0.13 +/- 0.01. These parameters remain time-invariant. In data from 2016 and 2019, the X-ray spectra remain remarkably stable, contrasting with extreme changes in gamma-rays. Both states can be fitted with a single component from Comptonisation of infrared emission from the dusty torus, with different gamma-ray states arising solely from a shift in the break of the electron energy distribution. The detection of a consistent lag throughout the whole light curve suggests that they originate from a persistent location in the jet.

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arXiv2025

Tension of toroidal magnetic field in reconnection plasmoids and relativistic jets

Toroidal magnetic field is a key ingredient of relativistic jets launched by certain accreting astrophysical black holes, and of plasmoids emerging from the tearing instability during magnetic reconnection, a candidate dissipation mechanism in jets. Tension of toroidal field is an anisotropic force that can compress local energy and momentum densities. We investigate this effect in plasmoids produced during relativistic reconnection initiated from a Harris layer by means of kinetic particle-in-cell (PIC) numerical simulations, varying the system size (including 3D cases), magnetisation, or guide field. We find that: (1) plasmoid cores are dominated by plasma energy density for guide fields up to B_z ~ B_0; (2) relaxed 'monster' plasmoids compress plasma energy density only modestly (by factor ~3 above the initial level for drifting particle population); (3) energy density compressions by factors >~10 are achieved during plasmoid mergers, especially with the emergence of secondary plasmoids. This kinetic-scale effect can be combined with a global focusing of the jet Poynting flux along the quasi-cylindrical bunched spine (a proposed jet layer adjacent to the cylindrical core) due to poloidal line bunching (a prolonged effect of tension of the jet toroidal field) to enhance the luminosity of rapid radiation flares from blazars. The case of M87 as a misaligned blazar is discussed.

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arXiv2024

Chaotic magnetic disconnections trigger flux eruptions in accretion flows channeled onto magnetically saturated Kerr black holes

Magnetized accretion flow onto a black hole (BH) may lead to accumulation of poloidal magnetic flux across its horizon, which for high BH spin can power far-reaching relativistic jets. The BH magnetic flux is subject to a saturation mechanism by means of magnetic flux eruptions involving relativistic magnetic reconnection. Such accretion flows have been described as magnetically arrested (MAD) or choked (MCAF). The main goal of this work is to describe the onset of relativistic reconnection and initial development of magnetic flux eruption in accretion flow onto magnetically saturated BH. We analyze the results of 3D ideal GRMHD numerical simulations in the Kerr metric, starting from weakly magnetized geometrically thick tori rotating either prograde or retrograde. We integrate large samples of magnetic field lines in order to probe magnetic connectivity with the BH horizon. The boundary between magnetically connected and disconnected domains coincides roughly with enthalpy equipartition. The geometrically constricted innermost part of the disconnected domain develops a rigid structure of magnetic field lines - rotating slowly and insensitive to the BH spin orientation. The typical shape of innermost disconnected lines is a double spiral converging to a sharp inner tip anchored at the single equatorial current layer. The footpoints of magnetic flux eruptions are found to zip around the BH along with other azimuthal patterns. Magnetic flux eruptions from magnetically saturated accreting BHs can be triggered by minor density gaps in the disconnected domain, resulting from chaotic disconnection of plasma-depleted magnetospheric lines. Accretion flow is effectively channeled along the disconnected lines towards the current layer, and further towards the BH by turbulent cross-field diffusion. Rotation of flux eruption footpoints may contribute to the variability of BH crescent images.

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arXiv2022

The Role of Strong Magnetic Fields in Stabilizing Highly Luminous, Thin Disks

We present a set of three-dimensional, global, general relativistic radiation magnetohydrodynamic simulations of thin, radiation-pressure-dominated accretion disks surrounding a non-rotating, stellar-mass black hole. The simulations are initialized using the Shakura-Sunyaev model with a mass accretion rate of $\dot{M} = 3 L_\mathrm{Edd}/c^2$ (corresponding to $L=0.17 L_\mathrm{Edd}$). Our previous work demonstrated that such disks are thermally unstable when accretion is driven by an $α$-viscosity. In the present work, we test the hypothesis that strong magnetic fields can both drive accretion through the magneto-rotational instability and restore stability to such disks. We test four initial magnetic field configurations: 1) a zero-net-flux case with a single, radially extended set of magnetic field loops (dipole); 2) a zero-net-flux case with two radially extended sets of magnetic field loops of opposite polarity stacked vertically (quadrupole); 3) a zero-net-flux case with multiple radially concentric rings of alternating polarity (multi-loop); and 4) a net-flux, vertical magnetic field configuration (vertical). In all cases, the fields are initially weak, with the gas-to-magnetic pressure ratio $\gtrsim 100$. Based on the results of these simulations, we find that the dipole and multi-loop configurations remain thermally unstable like their $α$-viscosity counterpart, in our case collapsing vertically on the local thermal timescale and never fully recovering. The vertical case, on the other hand, stabilizes and remains so for the duration of our tests (many thermal timescales). The quadrupole case is intermediate, showing signs of both stability and instability. The key stabilizing criteria is, $P_\mathrm{mag} \gtrsim 0.5P_\mathrm{tot}$ with strong toroidal fields near the disk midplane. We also report a comparison of our models to the standard Shakura-Sunyaev disk.

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arXiv2022

Rapid X-ray Variability in Mkn 421 during a Multiwavelength Campaign

The study of short-term variability properties in AGN jets has the potential to shed light on their particle acceleration and emission mechanisms. We report results from a four-day coordinated multi-wavelength campaign on the highly-peaked blazar (HBL) Mkn 421 in 2019 January. We obtained X-ray data from AstroSAT, BVRI photometry with the Whole Earth Blazar Telescope (WEBT), and TeV data from FACT to explore short-term multi-wavelength variability in this HBL. The X-ray continuum is rapidly variable on time-scales of tens of ks. Fractional variability amplitude increases with energy across the synchrotron hump, consistent with previous studies; we interpret this observation in the context of a model with multiple cells whose emission spectra contain cutoffs that follow a power-law distribution. We also performed time-averaged and time-resolved (time-scales of 6 ks) spectral fits; a broken power-law model fits all spectra well; time-resolved spectral fitting reveals the usual hardening when brightening behaviour. Intra-X-ray cross correlations yield evidence for the 0.6-0.8 keV band to likely lead the other bands by an average of 4.6 +- 2.6 ks, but only during the first half of the observation. The source displayed minimal night-to-night variability at all wavebands thus precluding significant interband correlations during our campaign. The broadband SED is modeled well with a standard one-zone leptonic model, yielding jet parameters consistent with those obtained from previous SEDs of this source.

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arXiv2022

A simple analytical model of magnetic jets

We propose a simple analytical jet model of magnetic jets, in which radially-averaged profiles of main physical quantities are obtained based on conservation laws and some results of published GRMHD jet simulations. We take into account conversion of the magnetic energy flux to bulk acceleration in jets formed around rotating black holes assuming the mass continuity equation and constant jet power, which leads to the Bernoulli equation. For assumed profiles of the bulk Lorentz factor and the radius, this gives us the profile of the toroidal magnetic field component along the jet. We then consider the case where the poloidal field component is connected to a rotating black hole surrounded by an accretion disc. Our formalism then recovers the standard formula for the power extracted from a rotating black hole. We find that the poloidal field strength dominates over the toroidal one in the comoving frame up to large distances, which means that jets should be more stable to current-driven kink modes. The resulting magnetic field profiles can then be used to calculate the jet synchrotron emission.

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arXiv2022

Kinetic Simulations of Instabilities and Particle Acceleration in Cylindrical Magnetized Relativistic Jets

Relativistic magnetized jets, such as those from AGN, GRBs and XRBs, are susceptible to current- and pressure-driven MHD instabilities that can lead to particle acceleration and non-thermal radiation. Here we investigate the development of these instabilities through 3D kinetic simulations of cylindrically symmetric equilibria involving toroidal magnetic fields with electron-positron pair plasma. Generalizing recent treatments by Alves et al. (2018) and Davelaar et al. (2020), we consider a range of initial structures in which the force due to toroidal magnetic field is balanced by a combination of forces due to axial magnetic field and gas pressure. We argue that the particle energy limit identified by Alves et al. (2018) is due to the finite duration of the fast magnetic dissipation phase. We find a rather minor role of electric fields parallel to the local magnetic fields in particle acceleration. In all investigated cases a kink mode arises in the central core region with a growth timescale consistent with the predictions of linearized MHD models. In the case of a gas-pressure-balanced (Z-pinch) profile, we identify a weak local pinch mode well outside the jet core. We argue that pressure-driven modes are important for relativistic jets, in regions where sufficient gas pressure is produced by other dissipation mechanisms.

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arXiv2022

Multiwavelength variability power spectrum analysis of the blazars 3C 279 and PKS 1510-089 on multiple timescales

We present the results of variability power spectral density (PSD) analysis using multiwavelength radio to GeV\,$γ$-ray light curves covering decades/years to days/minutes timescales for the blazars 3C 279 and PKS 1510-089. The PSDs are modeled as single power-laws, and the best-fit spectral shape is derived using the `power spectral response' method. With more than ten years of data obtained with weekly/daily sampling intervals, most of the PSDs cover ~2-4 decades in temporal frequency; moreover, in the optical band, the PSDs cover ~6 decades for 3C 279 due to the availability of intranight light curves. Our main results are the following: (1) on timescales ranging from decades to days, the synchrotron and the inverse Compton spectral components, in general, exhibit red-noise (slope ~2) and flicker-noise (slope ~1) type variability, respectively; (2) the slopes of $γ$-ray variability PSDs obtained using a 3-hr integration bin and a 3-weeks total duration exhibit a range between ~1.4 and ~2.0 (mean slope = 1.60$\pm$0.70), consistent within errors with the slope on longer timescales; (3) comparisons of fractional variability indicate more power on timescales $\leq$100\, days at $γ$-ray frequencies as compared to longer wavelengths, in general (except between $γ$-ray and optical frequencies for PKS 1510$-$089); (4) the normalization of intranight optical PSDs for 3C\,279 appears to be a simple extrapolation from longer timescales, indicating a continuous (single) process driving the variability at optical wavelengths; (5) the emission at optical/infrared wavelengths may involve a combination of disk and jet processes for PKS\,1510$-$089.

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arXiv2021

Rotation of the crescent image of M87* and polarization of its ESE hotspot

The first image of the black hole (BH) M87* obtained by the Event Horizon Telescope (EHT) has the shape of a crescent extending from the E to WSW position angles, with a tentative 'ESE hotspot'. Assuming that the BH spin vector is aligned with both the inner accretion axis and the projected direction of the kpc-scale relativistic jet, the position of the ESE hotspot is inconsistent with the axisymmetric accretion flow. Recent polarimetric EHT images of M87* show that the ESE hotspot is essentially unpolarized, which strongly supports its distinct origin. If the hotspot emission is due to the synchrotron radiation, its depolarization requires either isotropically tangled magnetic fields or an additional Faraday dispersion measure. The 6-day EHT observing campaign in April 2017 allowed in principle to detect orbital motions advancing by up to ~60 deg. The apparent rotation rate of the major axis of the EHT crescent image is consistent with the rotation rate of the Faraday-corrected polarization angle measured by the ALMA. However, the counterclockwise (CCW) sense of these rotations is opposite to the clockwise (CW) rotation of the plasma flows implied by the N-S brightness asymmetry, which might indicate that accretion in M87 is retrograde.

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arXiv2021

Scaling of Magnetic Dissipation and Particle Acceleration in ABC Fields

Using particle-in-cell (PIC) numerical simulations with electron-positron pair plasma, we study how the efficiencies of magnetic dissipation and particle acceleration scale with the initial coherence length $λ_0$ in relation to the system size $L$ of the two-dimensional (2D) `Arnold-Beltrami-Childress' (ABC) magnetic field configurations. Topological constraints on the distribution of magnetic helicity in 2D systems, identified earlier in relativistic force-free (FF) simulations, that prevent the high-$(L/λ_0)$ configurations from reaching the Taylor state, limit the magnetic dissipation efficiency to about $ε_{\rm diss} \simeq 60\%$. We find that the peak growth time scale of the electric energy $τ_{\rm E,peak}$ scales with the characteristic value of initial Alfven velocity $β_{\rm A,ini}$ like $τ_{\rm E,peak} \propto (λ_0/L)β_{\rm A,ini}^{-3}$. The particle energy change is decomposed into non-thermal and thermal parts, with non-thermal energy gain dominant only for high initial magnetisation. The most robust description of the non-thermal high-energy part of the particle distribution is that the power-law index is a linear function of the initial magnetic energy fraction.

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arXiv2021

Orientation of the crescent image of M 87*

The first image of the black hole (BH) M 87* obtained by the Event Horizon Telescope (EHT) has the shape of a crescent extending from the E to WSW position angles, while the observed direction of the large-scale jet is WNW. Images based on numerical simulations of BH accretion flows suggest that on average the projected BH spin axis should be oriented SSW. We explore highly simplified toy models for geometric distribution and kinematics of emitting regions in the Kerr metric, perform ray tracing to calculate the corresponding images, and simulate their observation by the EHT to calculate the corresponding visibilities and closure phases. We strictly assume that (1) the BH spin vector is fixed to the jet axis, (2) the emitting regions are stationary and symmetric with respect to the BH spin, and that (3) the emissivities are isotropic in the local rest frames. Emission from the crescent sector between SSE and WSW can be readily explained in terms of an equatorial ring with either circular or plunging geodesic flows, regardless of the value of BH spin. In the case of plane-symmetric polar caps with plunging geodesic flows, the dominant image of the cap located behind the BH is sensitive to the angular momentum of the emitter. Within the constraints of our model, we have not found a viable explanation for the observed brightness of the ESE sector. Most likely, the ESE "hotspot" has been produced by a non-stationary localised perturbation in the inner accretion flow. Alternatively, it could result from locally anisotropic synchrotron emissivities. Multi-epoch and polarimetric results from the EHT will be essential to verify the theoretically expected alignment of the BH spin with the large-scale jet.

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arXiv2020

On the significance of relativistically hot pairs in the jets of FR II radio galaxies

The energetic composition of radio lobes in the FR II galaxies $-$ estimated by comparing their radio luminosities with the powers required to inflate cavities in the external medium $-$ seems to exclude the possibility of their energetic domination by protons. Furthermore, if the jets were dominated by the kinetic energy of cold protons, it would be difficult to efficiently accelerate leptons in the jets' terminal shocks. Assuming that the relative energy contents of leptons, protons and magnetic fields are preserved across the shocks, the above implies that the large-scale jets should also be energetically dominated by leptons: $P_{\rm e,j} \gtrsim P_{\rm p,j}$. On the other hand, previous studies of small-scale jets in blazars and radio cores suggest a pair content (number of electrons and positrons per proton) of the order of $n_{\rm e}/n_{\rm p} \sim 20$. Assuming further that the particle composition of jets does not evolve beyond the blazar scales, we show that this implies an average random Lorentz factor of leptons in large-scale jets of $\barγ_{\rm e,j} \gtrsim 70(1+χ_{\rm p})(20n_{\rm p}/n_{\rm e})$, and that the protons should be mildly relativistic with $χ_{\rm p} \equiv (ε_{\rm p} + p_{\rm p})/ρ_{\rm p} c^2 \lesssim 2$, $p_{\rm p}$ the pressure of protons, $ε_{\rm p}$ the internal energy density of protons, and $ρ_{\rm p} c^2$ the rest-mass energy density of protons. We derive the necessary conditions for loading the inner jets by electron-positron pairs and proton-electron plasma, and provide arguments that heating of leptons in jets is dominated by magnetic reconnection.

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arXiv2020

Long-term optical spectroscopic variations in blazar 3C 454.3

Characterisation of the long-term variations in the broad line region in a luminous blazar, where Comptonisation of broad-line emission within a relativistic jet is the standard scenario for production of gamma-ray emission that dominates the spectral energy distribution. We analysed ten years of optical spectroscopic data from the Steward Observatory for the blazar 3C 454.3, as well as gamma-ray data from the Fermi Large Area Telescope (LAT). The optical spectra are dominated by a highly variable non-thermal synchrotron continuum with a prominent Mg II broad emission line. The line flux was obtained by spectral decomposition including significant contribution from the Fe II pseudo-continuum. Three methods were used to characterise variations in the line flux: (1) stacking of the continuum-subtracted spectra, (2) subtracting the running mean light curves calculated for different timescales, and (3) evaluating potential time delays via the discrete correlation function (DCF). Despite very large variations in the gamma-ray and optical continua, the line flux changes only moderately (< 0.1 dex). The data suggest that the line flux responds to a dramatic change in the blazar activity from a very high state in 2010 to a deep low state in 2012. Two interpretations are possible: either the line flux is anti-correlated with the continuum or the increase in the line luminosity is delayed by ~600 days. If this time delay results from the reverberation of poorly constrained accretion disc emission in both the broad-line region (BLR) and the synchrotron emitting blazar zone within a relativistic jet, we would obtain natural estimates for the BLR radius [R_{BLR,MgII} >~ 0.28 pc] and for the supermassive black hole mass [M_SMBH ~ 8.5x10^8 M_sun]. We did not identify additional examples of short-term 'flares' of the line flux, in addition to the previously reported case observed in 2010.

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arXiv2020

Radiative kinetic simulations of steady-state relativistic plasmoid magnetic reconnection

We present the results of 2D particle-in-cell (PIC) simulations of relativistic magnetic reconnection (RMR) in electron-positron plasma, including the dynamical influence of the synchrotron radiation process, and integrating the observable emission signatures. The simulations are initiated with a single Harris current layer with a central gap that triggers the RMR process. We achieve a steady-state reconnection with unrestricted outflows by means of open boundary conditions. The radiative cooling efficiency is regulated by the choice of initial plasma temperature Theta. We explore different values of Theta and of the background magnetisation sigma_0. Throughout the simulations, plasmoids are generated in the central region of the layer, and they evolve at different rates, achieving a wide range of sizes. The gaps between plasmoids are filled by smooth relativistic outflows called minijets, whose contribution to the observed radiation is very limited due to their low particle densities. Small-sized plasmoids are rapidly accelerated, however, they have lower contributions to the observed emission, despite stronger relativistic beaming. Large-sized plasmoids are slow, but produce most of the observed synchrotron emission, with major part of their radiation produced within the central cores, the density of which is enhanced by radiative cooling. Synchrotron lightcurves show rapid bright flares that can be identified as originating from mergers between small/fast plasmoids and large/slow targets moving in the same direction. In the high-magnetisation case, the accelerated particles form a broken power-law energy distribution with a soft tail produced by particles accelerated in the minijets.

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arXiv2019

Two-zone emission modeling of PKS 1510-089 during the high state of 2015

PKS 1510-089 is one of the most variable blazars in the third Fermi-LAT source catalog. During 2015, this source has shown four flares identified as flare A, B, C, and D in between three quiescent states Q1, Q2, and Q3. The multi-wavelength data from Fermi-LAT, Swift-XRT/UVOT, OVRO, and SMA observatory are used in our work to model these states. Different flux doubling times have been observed in different energy bands which indicate there could be multiple emission zones. The flux doubling time from the gamma-ray and X-ray light curves are found to be 10.6 hr, 2.5 days, and the average flux doubling time in the optical/UV band is 1 day. It is possible that the gamma-ray and optical/UV emission are produced in the same region whereas X-ray emission is coming from a different region along the jet axis. We have also estimated the discrete correlations functions (DCFs) among the light curves of different energy bands to infer about their emission regions. However, our DCF analysis does not show significant correlation in different energy bands though it shows peaks in some cases at small time lags. We perform a two-zone multi-wavelength time-dependent SED modeling with one emission zone located near the outer edge of the broad line region (BLR) and another further away in the dusty/molecular torus (DT/MT) region to study this high state.

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arXiv2018

Three-dimensional kinetic simulations of relativistic magnetostatic equilibria

We present the results of three-dimensional kinetic particle-in-cell (PIC) simulations of isotropic periodic relativistically magnetized pair-plasma equilibria known as the ABC fields. We performed several simulations for initial wavenumbers k_ini = 2 or k_ini = 4, different efficiencies of radiative cooling (including radiation reaction from synchrotron and inverse Compton processes), and different mean magnetization values. These equilibria evolve by means of ideal coalescence instability, the saturation of which generates ab initio localized kinetically-thin current layers -- sites of magnetic reconnection and non-thermal particle acceleration -- eventually relaxing to a state of lower magnetic energy at conserved total magnetic helicity. We demonstrate that magnetic relaxation involves in addition localized collapses of magnetic minima and bulk mergers of current layer pairs, which represents a novel scenario of spontaneous magnetic dissipation with application to the rapid gamma-ray flares of blazars and of the Crab Nebula. Particle acceleration under strong radiative losses leads to formation of power-law indices N(gamma) ~ gamma^(-p) up to p ~= -2.3 at mean hot magnetization values of ~ 6. Individual energetic particles can be accelerated within one light-crossing time by electric fields that are largely perpendicular to the local magnetic fields. The energetic particles are highly anisotropic due to the kinetic beaming effect, implying complex patterns of rapid variability. A significant fraction of the initial total energy can be radiated away in the overall process of magnetoluminescence.

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arXiv2018

Relativistic magnetic reconnection in application to gamma-ray astrophysics

Cosmic sources of gamma-ray radiation in the GeV range are often characterized by violent variability, in particular this concerns blazars, gamma-ray bursts, and the pulsar wind nebula Crab. Such gamma-ray emission requires a very efficient particle acceleration mechanism. If the environment, in which such emission is produced, is relativistically magnetized (i.e., that magnetic energy density dominates even the rest-mass energy density of matter), then the most natural mechanism of energy dissipation and particle acceleration is relativistic magnetic reconnection. Basic research into this mechanism is performed by means of kinetic numerical simulations of various configurations of collisionless relativistic plasma with the use of the particle-in-cell algorithm. Such technique allows to investigate the details of particle acceleration mechanism, including radiative energy losses, and to calculate the temporal, spatial, spectral and angular distributions of synchrotron and inverse Compton radiation. The results of these simulations indicate that the effective variability time scale of the observed radiation can be much shorter than the light-crossing time scale of the simulated domain.

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