skip to main content

This content will become publicly available on June 1, 2023

Title: Testing particle acceleration models for BL Lac jets with the Imaging X-ray Polarimetry Explorer
Mechanisms such as shock acceleration, magnetic reconnection in a kink unstable jet, and extreme turbulence in the jet flow are all expected to produce a distinctive time variability pattern of the X-ray polarization properties of high synchrotron peak blazars (HSP). To determine whether the recently launched Imaging X-ray Polarimetry Explorer (IXPE) can follow the polarization variations induced by different particle acceleration mechanisms in blazar jets, we simulated observations of an HSP blazar variable in terms of the polarization degree and angle according to theoretical predictions. We used the Monte Carlo tool ixpeobssim to create realistic IXPE data products for each model and for three values of flux (i.e., 1, 5, and 10 × 10 −10 erg s −1 cm −2 ). We generated simulated light curves of the polarization degree and angle by time-slicing the simulated data into arbitrary short time bins. We used an χ 2 test to assess the performance of the observations in detecting the time variability of the polarization properties. In all cases, even when the light curves are diluted in an individual time bin, some degree of polarization is still measurable with IXPE. A series of ~10 ks long observations permits IXPE to follow the more » time variability of the polarization degree in the case of the shock acceleration model. In the case of the magnetic reconnection model, the nominal injected model provides the best fit of the simulated IXPE data for time bins of ~5–10 ks, depending on the tested flux level. For the TEMZ model, shorter time slices of ~0.5 ks are needed for obtaining a formally good fit of the simulated IXPE data with the injected model. On the other hand, we find that a fit with a constant model provides a χ 2 lower than the fit with the nominal injected model when using time slices of ~20 ks, ~60/70 ks, and ~5 ks for the case of the shock acceleration, magnetic reconnection, and TEMZ model, respectively. In conclusion, provided that the statistics of the observation allows for the slicing of the data in adequately short time bins, IXPE observations of an HSP blazar at a typical flux level can detect the time variability predicted by popular models for particle acceleration in jets. IXPE observations of HSP blazars are a useful tool for addressing the issue of particle acceleration in blazar jets. « less
; ; ; ; ;
Award ID(s):
Publication Date:
Journal Name:
Astronomy & Astrophysics
Page Range or eLocation-ID:
Sponsoring Org:
National Science Foundation
More Like this
  1. Context. Standing and moving shocks in relativistic astrophysical jets are very promising sites for particle acceleration to large Lorentz factors and for the emission from the radio up to the γ -ray band. They are thought to be responsible for at least part of the observed variability in radio-loud active galactic nuclei. Aims. We aim to simulate the interactions of moving shock waves with standing recollimation shocks in structured and magnetized relativistic jets and to characterize the profiles of connected flares in the radio light curve. Methods. Using the relativistic magneto-hydrodynamic code MPI-AMRVAC and a radiative transfer code in post-processing, we explore the influence of the magnetic-field configuration and transverse stratification of an over-pressured jet on its morphology, on the moving shock dynamics, and on the emitted radio light curve. First, we investigate different large-scale magnetic fields with their effects on the standing shocks and on the stratified jet morphology. Secondly, we study the interaction of a moving shock wave with the standing shocks. We calculated the synthetic synchrotron maps and radio light curves and analyze the variability at two frequencies 1 and 15.3 GHz and for several observation angles. Finally, we compare the characteristics of our simulated light curvesmore »with radio flares observed from the blazar 3C 273 with the Owens Valley Radio Observatory and Very Long Baseline Array in the MOJAVE survey between 2008 and 2019. Results. We find that in a structured over-pressured relativistic jet, the presence of the large-scale magnetic field structure changes the properties of the standing shock waves and leads to an opening in the jet. The interaction between waves from inner and outer jet components can produce strong standing shocks. When crossing such standing shocks, moving shock waves accompanying overdensities injected in the base of the jet cause very luminous radio flares. The observation of the temporal structure of these flares under different viewing angles probes the jet at different optical depths. At 1 GHz and for small angles, the self-absorption caused by the moving shock wave becomes more important and leads to a drop in the observed flux after it interacts with the brightest standing knot. A weak asymmetry is seen in the shape of the simulated flares, resulting from the remnant emission of the shocked standing shocks. The characteristics of the simulated flares and the correlation of peaks in the light curve with the crossing of moving and standing shocks favor this scenario as an explanation of the observed radio flares of 3C 273.« less
  2. Abstract It is commonly believed that blazar jets are relativistic magnetized plasma outflows from supermassive black holes. One key question is how the jets dissipate magnetic energy to accelerate particles and drive powerful multiwavelength flares. Relativistic magnetic reconnection has been proposed as the primary plasma physical process in the blazar emission region. Recent numerical simulations have shown strong acceleration of nonthermal particles that may lead to multiwavelength flares. Nevertheless, previous works have not directly evaluated γ -ray signatures from first-principles simulations. In this paper, we employ combined particle-in-cell and polarized radiation transfer simulations to study multiwavelength radiation and optical polarization signatures under the leptonic scenario from relativistic magnetic reconnection. We find harder-when-brighter trends in optical and Fermi-LAT γ -ray bands as well as closely correlated optical and γ -ray flares. The swings in optical polarization angle are also accompanied by γ -ray flares with trivial time delays. Intriguingly, we find highly variable synchrotron self-Compton signatures due to inhomogeneous particle distributions during plasmoid mergers. This feature may result in fast γ -ray flares or orphan γ -ray flares under the leptonic scenario, complementary to the frequently considered minijet scenario. It may also imply neutrino emission with low secondary synchrotron flux undermore »the hadronic scenario, if plasmoid mergers can accelerate protons to very high energy.« less
  3. ABSTRACT The most extreme active galactic nuclei are the radio active ones whose relativistic jet propagates close to our line of sight. These objects were first classified according to their emission-line features into flat-spectrum radio quasars (FSRQs) and BL Lacertae objects (BL Lacs). More recently, observations revealed a trend between these objects known as the blazar sequence, along with an anticorrelation between the observed power and the frequency of the synchrotron peak. In this work, we propose a fairly simple idea that could account for the whole blazar population: all jets are launched with similar energy per baryon, independently of their power. In the case of FSRQs, the most powerful jets manage to accelerate to high-bulk Lorentz factors, as observed in the radio. As a result, they have a rather modest magnetization in the emission region, resulting in magnetic reconnection injecting a steep particle–energy distribution and, consequently, steep emission spectra in the γ-rays. For the weaker jets, namely BL Lacs, the opposite holds true; i.e. the jet does not achieve a very high bulk Lorentz factor, leading to more magnetic energy available for non-thermal particle acceleration, and harder emission spectra at frequencies ≳ GeV. In this scenario, we recover allmore »observable properties of blazars with our simulations, including the blazar sequence for models with mild baryon loading (50 ≲ μ ≲ 80). This interpretation of the blazar population therefore tightly constrains the energy per baryon of blazar jets regardless of their accretion rate.« less
  4. ABSTRACT High-energy astrophysical systems frequently contain collision-less relativistic plasmas that are heated by turbulent cascades and cooled by emission of radiation. Understanding the nature of this radiative turbulence is a frontier of extreme plasma astrophysics. In this paper, we use particle-in-cell simulations to study the effects of external inverse Compton radiation on turbulence driven in an optically thin, relativistic pair plasma. We focus on the statistical steady state (where injected energy is balanced by radiated energy) and perform a parameter scan spanning from low magnetization to high magnetization (0.04 ≲ σ ≲ 11). We demonstrate that the global particle energy distributions are quasi-thermal in all simulations, with only a modest population of non-thermal energetic particles (extending the tail by a factor of ∼2). This indicates that non-thermal particle acceleration (observed in similar non-radiative simulations) is quenched by strong radiative cooling. The quasi-thermal energy distributions are well fit by analytic models in which stochastic particle acceleration (due to, e.g. second-order Fermi mechanism or gyroresonant interactions) is balanced by the radiation reaction force. Despite the efficient thermalization of the plasma, non-thermal energetic particles do make a conspicuous appearance in the anisotropy of the global momentum distribution as highly variable, intermittent beams (formore »high magnetization cases). The beamed high-energy particles are spatially coincident with intermittent current sheets, suggesting that localized magnetic reconnection may be a mechanism for kinetic beaming. This beaming phenomenon may explain rapid flares observed in various astrophysical systems (such as blazar jets, the Crab nebula, and Sagittarius A*).« less
  5. ABSTRACT Relativistic jets from supermassive black holes are among the most powerful and luminous astrophysical systems in Universe. We propose that the open magnetic field lines through the black hole, which drive a strongly magnetized jet, may have their polarity reversing over time scales related to the growth of the magnetorotational dynamo in the disc, resulting in dissipative structures in the jet characterized by reversing toroidal field polarities, referred to as ‘stripes’. The magnetic reconnection between the stripes dissipates the magnetic energy and powers jet acceleration. The striped jet model can explain the jet acceleration, large-scale jet emission, and blazar emission signatures consistently in a unified physical picture. Specifically, we find that the jet accelerates to the bulk Lorentz factor Γ ≳ 10 within 1-parsec distance from the central engine. The acceleration slows down but continues at larger distances, with intrinsic acceleration rate $\dot{\Gamma }/\Gamma$ between $0.0005$ and $0.005~\rm {yr^{-1}}$ at tens of parsecs, which is in very good agreement with recent radio observations. Magnetic reconnection continuously accelerates non-thermal particles over large distances from the central engine, resulting in the core-shift effect and overall flat-to-inverted synchrotron spectrum. The large-scale spectral luminosity peak νpeak is antiproportional to the location of themore »peak of the dissipation, which is set by the minimal stripe width lmin. The blazar zone is approximately at the same location. At this distance, the jet is moderately magnetized, with the comoving magnetic field strength and dissipation power consistent with typical leptonic blazar model parameters.« less