Blazar Jets Research Articles

Electron Acceleration in Blazars: Application to the 3C 279 Flare on 2013 December 20

Abstract The broadband spectrum from the 2013 December 20 -ray flare from 3C 279 is analyzed with our previously developed one-zone blazar jet model. We are able to reproduce two spectral energy distributions (SEDs), a quiescent and flaring state, the latter of which had an unusual SED, with hard -ray spectrum, high Compton dominance, and short duration. Our model suggests that there is insufficient energy for a comparable X-ray flare to have occurred simultaneously, which is an important constraint given the lack of X-ray data. We show that first- and second-order Fermi acceleration are sufficient to explain the flare, and that magnetic reconnection is not needed. The model includes particle acceleration, escape, and adiabatic and radiative energy losses, including the full Compton cross section, and emission from the synchrotron, synchrotron self-Compton, and external Compton processes. We provide a simple analytic approximation to the electron distribution solution to the transport equation that may be useful for simplified modeling in the future.

High-energy neutrino flux from individual blazar flares

ABSTRACT Motivated by the recently reported evidence of an association between a high-energy neutrino and a γ-ray flare from the blazar TXS 0506+056, we calculate the expected high-energy neutrino signal from past, individual flares, from 12 blazars, selected in declinations favourable for detection with IceCube. To keep the number of free parameters to a minimum, we mainly focus on BL Lac objects and assume the synchrotron self-Compton mechanism produces the bulk of the high-energy emission. We consider a broad range of the allowed parameter space for the efficiency of proton acceleration, the proton content of BL Lac jets, and the presence of external photon fields. To model the expected neutrino fluence, we use simultaneous multiwavelength observations. We find that in the absence of external photon fields and with jet proton luminosity normalized to match the observed production rate of ultrahigh-energy cosmic rays, individual flaring sources produce a modest neutrino flux in IceCube, $N^{\mathrm{IC,10 \,yr}}_{\nu _{\mu },{\mathrm{\gt 100~TeV}}} \lesssim 10^{-3}$ muon neutrinos with energy exceeding 100 TeV, stacking 10 yr of flare periods selected in the >800 MeV Fermi energy range, from each source. Under optimistic assumptions about the jet proton luminosity and in the presence of external photon fields, we find that the two most powerful sources in our sample, AO 0235+164, and OJ 287, would produce, in total, $N^{\mathrm{IC \times 10,10 \,yr}}_{\nu _{\mu }, \rm all~flares, \gt 100~TeV} \approx 3$ muon neutrinos during Fermi flaring periods, in future neutrino detectors with total instrumented volume ∼10 times larger than IceCube, or otherwise, constrain the proton luminosity of blazar jets.

Physical inference from the γ-ray, X-ray, and optical time variability of a large sample of Fermi blazars

ABSTRACT We present cross-correlation studies of γ-ray (0.1–300 GeV), X-ray (0.2–10 keV), and optical (R band) variability of a sample of 26 blazars during 2008–2016. The light curves are from Fermi-LAT, Swift-XRT, and the Yale-SMARTS blazar monitoring program. We stack the discrete cross-correlation functions of the blazars such that the features that are consistently present in a large fraction of the sample become more prominent in the final result. We repeat the same analysis for two subgroups, namely, low synchrotron peaked (LSP) and high synchrotron peaked (HSP) blazars. We find that, on average, the variability at multiple bands is correlated, with a time lag consistent with zero in both subgroups. We describe this correlation with a leptonic model of non-thermal emission from blazar jets. By comparing the model results with those from the actual data, we find that the inter-band cross-correlations are consistent with an emission region of size 0.1 pc within the broad-line region for LSP blazars. We rule out large changes of magnetic field (>0.5 Gauss) across the emission region or small values of magnetic field (e.g., 0.2 Gauss) for this population. We also find that the observed variability of the HSP blazars can be explained if the emission region is much larger than the distance to the broad-line region from the central black hole.

PKS 2155-304: A Case Study of Blazar Variability Power Spectrum at the Highest Energies and on the Longest Timescales

We present the results of our Power Spectral Density (PSD) analysis for the BL Lac object PKS 2155-304, utilizing the nightly-binned long-term light curve from the decade-long monitoring, as well as the minute-binned intra-night light curve from the High Energy Stereoscopic Survey (H.E.S.S.; >200 GeV). The source is unique for exhibiting the shortest flux-doubling timescale at Very High Energy (VHE) among its class and thus provides a rare opportunity to study the particle acceleration on the smallest spatial scales in blazar jets. The light curves are modeled in terms of the Continuous-Time Auto-Regressive Moving Average (CARMA) process. The combined long-term and intra-night PSD extends up to ∼6 decades in the temporal frequency range; unprecedented at the TeV energies for a blazar source. Our systematic approach reveals that PKS 2155-304 shows, on average, a complex shape of variability power spectrum, with more variability power on longer timescales. The long-term variability is best modeled by the CARMA(2,1) process, while the intra-night variability is modeled by a CARMA(1,0) process. We note that the CARMA(1,0) process refers to an Ornstein–Uhlenbeck process where the power-law PSD slope (PSD varies as a function of variability frequency to the power of the negative slope) changes from two to zero, above a certain “characteristic/relaxation” timescale. Even though the derived power spectrum of the intra-night light curve did not reveal a flattening, we speculate such relaxation must occur on timescales longer than a few hours for the source.

Prospects for Detecting X-Ray Polarization in Blazar Jets

Abstract X-ray polarization should provide new probes of magnetic field geometry and acceleration physics near the base of blazar jets, but near-future missions will have limited sensitivity. We thus use existing lower energy data and X-ray variability measurements in the context of a basic synchro-Compton model to predict the X-ray polarization level and the probability of detection success for individual sources, listing the most attractive candidates for an Imaging X-ray Polarimetry Explorer campaign. We find that, as expected, several high-peak blazars such as Mrk 421 can be easily measured in 100 ks exposures. Most low-peak sources should only be accessible to triggered campaigns during bright flares. Surprisingly, a few intermediate peak sources can have anomalously high X-ray polarization and thus are attractive targets.

Extremely High energy peaked BL Lac nature of the TeV blazar Mrk 501

Extremely High energy peaked BL Lac (EHBL) objects are a special class of blazars with peculiar observational properties at X-ray and γ–ray energies. The observations of these sources indicate hard X-ray and γ–ray spectra and absence of rapid flux variations in the multi-wavelength light curves. These observational features challenge the leptonic models for blazars due to unusually hard particle spectrum in the emission region of the blazar jet and provide a strong motivation for exploring alternative scenarios to interpret the broad-band emission from blazars. At present, only few TeV blazars have been observed as EHBL objects in the extragalactic Universe. Due to their hard γ–ray spectra and long term variability, the observations of EHBL type of blazars at different redshifts help in probing the cosmic magnetic field and extragalactic background light in the Universe. Such objects also provide astrophysical sites to explore the particle acceleration mechanisms like magnetic reconnection and second order Fermi acceleration. Therefore, it has become important to identify more objects as EHBL using the observations available in the literature. Recent studies on the blazar Mrk 501 indicate that this source may exhibit an EHBL behaviour. In this paper, we use long term observations of Mrk 501 to explore its nature. Two sets of data, related to low and high/flaring activity states of Mrk 501, have been presented and compared with the observed features of a few well known EHBL type of blazars. We find that the spectral features of the blazar Mrk 501 indicate an EHBL nature of the source. Whereas, the temporal characteristics with fast variability during the high activity state of the source in X-ray and γ–ray energy bands are not compatible with the behaviour of EHBL type of blazars. However, Mrk 501 can be considered as an EHBL candidate in its low emission state. We also discuss the implications of identifying more EHBL objects using present and future ground-based γ-ray observatories.

Effects of Synchrotron Cooling and Pair Production on Collisionless Relativistic Reconnection

Abstract High-energy radiation from nonthermal particles accelerated in relativistic magnetic reconnection is thought to be important in many astrophysical systems, ranging from blazar jets and black hole accretion disk coronae to pulsars and magnetar flares. The presence of a substantial density of high-energy photons (>MeV) in these systems can make two-photon pair production (γγ → e − e +) an additional source of plasma particles and can affect the radiative properties of these objects. We present the results of novel particle-in-cell simulations that track both the radiated synchrotron photons and the created pairs, with which we study the evolution of a two-dimensional reconnecting current sheet in pair plasma. Synchrotron radiation from accelerated particles in the current sheet produces hot secondary pairs in the upstream, which are later advected into the current sheet where they are reaccelerated and produce more photons. In the optically thin regime, when most of the radiation is leaving the upstream unaffected, this process is self-regulating and depends only on the background magnetic field and the optical depth of photons to pair production. The extra plasma loading also affects the properties of reconnection. We study how the inflow of the secondary plasma, with multiplicities up to several hundred, reduces the effective magnetization of the plasma, suppressing the acceleration and thus decreasing the high-energy photon spectrum cutoff. This offers an explanation for the weak dependence of the observed gamma-ray cutoff in pulsars on the magnetic field at the light cylinder.

Describing correlated observations of neutrinos and gamma-ray flares from the blazar TXS 0506+056 with a proton blazar model

Recent detection of the neutrino event IceCube-170922A by the IceCube Observatory from the blazar TXS $0506+056$ in the state of enhanced gamma-ray emission indicates the acceleration of cosmic rays in the blazar jet. The nondetection of the broadline emission in the optical spectrum of TXS $0506+056$ and other BL Lac objects suggests that external photon emissions are weak, and hence, photo-meson ($p\ensuremath{\gamma}$) interaction may not be a favored mechanism for high-energy neutrino production. The lack of broadline signatures also creates doubt about the presence of a high density cloud in the vicinity of the supermassive black hole of TXS $0506+056$ and consequently raised question on hadronuclear ($pp$) interaction interpretation like relativistic jet meets with high density cloud. Here we demonstrate that nonrelativistic protons in the proton blazar model---those that come into existence under the charge neutrality condition of the blazar jet---offer sufficient target matter for $pp$ interaction with shock-accelerated protons, and consequently, the model can describe consistently the observed high-energy gamma rays and neutrino signal from the blazar TXS $0506+056$.

Imaging strong blazars with space VLBI

The RadioAstron mission has obtained a series of detailed multi-frequency images of the brightest blazars of the radio sky concentrated in three key science programs. We present here results of the program on powerful jets in blazars. In the first two years of the mission, observations of compact relativistic jets in 0836+710, 3C345, 3C273, and 4C+69.21 were made at λλ18, 6, and 1.3 cm. The resulting images have revealed compact emitting regions with brightness temperature in excess of 1013 K and a complex jet structure that can be explained by plasma instability developing in a relativistic outflow.We present here some highlights of these space-VLBI observations, designed to resolve the innermost regions in these powerful targets and address some of the still unanswered questions on their physical nature.

Probing the Emission Mechanism and Magnetic Field of Neutrino Blazars with Multiwavelength Polarization Signatures

Abstract The characteristic two-component blazar spectral energy distribution (SED) can be of either leptonic and/or hadronic origins. The potential association of the high-energy neutrino event IceCube-170922A with the flaring blazar TXS 0506+056 indicates that hadronic processes may operate in a blazar jet. Despite multiwavelength follow-ups of the event and extensive theoretical modelings, the radiation mechanisms and the underlying magnetic field strength and configuration remain poorly understood. In this paper, we consider generic leptonic and hadronic blazar spectral models with distinct magnetic field strengths and radiation mechanisms. We analytically reproduce the SEDs and neutrino flux of hadronic models, and predict their X-ray to γ-ray polarization degrees. Furthermore, by performing relativistic magnetohydrodynamic simulations taking into account the polarization-dependent radiation transfer, we study the time-dependent multiwavelength polarization variability of the proton synchrotron (PS) model under a shock scenario. Our results suggest that the high-energy polarization degree and the neutrino flux can be jointly used to pinpoint the leptonic and/or hadronic blazar radiation mechanisms in the X-ray and γ-ray bands, and to infer the magnetic field strength in the emission region. In addition, the temporal multiwavelength polarization signatures in the PS shed light on the jet energy composition and the dynamical importance of magnetic fields in the blazar emission region. Future multiwavelength polarimetry facilities such as the Imaging X-ray Polarimetry Explorer and All-sky Medium Energy Gamma-ray Observatory together with neutrino telescopes such as IceCube can provide unprecedented observational constraints to probe the blazar radiation mechanisms and jet dynamics.

Gamma rays from red giant wind bubbles entering the jets of elliptical host blazars

Context. Blazars in elliptical hosts have a population of red giants surrounding their jet. These stars can carry large wind-blown bubbles into the jets, leading to gamma-ray emission through bubble-jet interactions. Aims. We study the interaction dynamics and the gamma-ray emission produced when the bubbles formed by red giant winds penetrate the jet of a blazar in an elliptical galaxy. Methods. First, we characterized the masses and penetration rates of the red giant wind bubbles that enter the jet. Then, the dynamical evolution of these bubbles under the jet impact was analysed analytically and numerically, and the radiation losses of the particles accelerated in the interaction were characterised. Finally, the synchrotron and the inverse Compton contributions above ∼100 MeV were estimated under different jet magnetic fields, powers, and Lorentz factors. Results. We find that an analytical dynamical model is a reasonable approximation for the red giant wind bubble-jet interaction. The radiation produced by these wind bubbles interacting with a jet can have a duty cycle of up to ∼1. For realistic magnetic fields, gamma rays could be detectable from sources within the local universe, preferentially from those with high Lorentz factors (∼10), and this could be a relatively common phenomenon for these sources. For magnetic fields in equipartition with the jet power, and high acceleration rates, synchrotron gamma rays may be detectable even for modest Lorentz factors (∼3), but with a much lower duty cycle. Conclusions. Blazars in elliptical galaxies within the local universe can produce detectable transient or persistent gamma-ray emission from red giant wind bubbles entering their jets.

Is There a Blazar Nested in the Core of the Radio Galaxy 3C 411?

Abstract Previous spectral energy distribution modeling based on XMM-Newton X-ray observation of the classical double-lobed radio galaxy 3C 411 left the possibility open for the presence of a blazar-like core. We investigated this scenario by characterizing the radio brightness distribution in the inner ∼10 pc region of the source. We applied the very long baseline interferometry (VLBI) technique at four different frequencies from 1.7 to 7.6 GHz. We analyzed archival data from the Very Long Baseline Array taken in 2014, and data from the European VLBI Network obtained in 2017. The VLBI images reveal parsec-scale extended structure in the core of 3C 411 that can be modeled with multiple jet components. The measured core brightness temperatures indicate no Doppler enhancement that would be expected from a blazar jet pointing close to the line of sight. While there is no blazar-type core in 3C 411, we found an indication of flux density variability. The overall morphology of the source is consistent with a straight jet with ∼50° inclination angle at all scales from parsecs to kiloparsecs.

Particle Acceleration and Synchrotron Self-Compton Emission in Blazar Jets. I. An Application to Quiescent Emission

Abstract There are still some important unanswered questions about the detailed particle acceleration and escape occurring during quiescent epochs. As a result, the particle distribution that is adopted in the blazar quiescent spectral model has numerous unconstrained shapes. To help remedy this problem, we introduce an analytical particle transport model to reproduce the quiescent broadband spectral energy distribution of blazars. In this model, the exact electron distribution is solved from a generalized transport equation that contains the terms describing the first-order and second-order Fermi acceleration, the escape of particles due to both advection and spatial diffusion, and energy losses due to synchrotron emission and inverse-Compton scattering of an assumed soft photon field. We suggest that advection is a significant escape mechanism in blazar jets. We find that in our model, the advection process tends to harden the particle distribution, which enhances the high-energy components of the resulting synchrotron and synchrotron self-Compton spectra from the jets. Our model is able to roughly reproduce the observed spectra of the extreme BL Lac object 1ES 0414+009 with reasonable assumptions about the physical parameters.

Blazar jet evolution revealed by multi-epoch broad-band radio polarimetry

We investigate the previously proposed possibility that multi-epoch broadband polarimetry could act as a complement or limited proxy for VLBI observations of blazars, in that the number of polarised emission components in the jet, and some of their properties and those of the foreground environment, might be inferred from the object's time-varying 1D Faraday depth spectrum (FDS) alone. We report on a pilot-scale experiment designed to establish the basic plausibility and utility of this idea. We analyse temporal changes in the complex polarisation spectra of nine spatially unresolved (at arcsecond scales) blazars in two epochs separated by $\sim$5 years, using data taken with the Australia Telescope Compact Array. The data allow for precise modelling, and we demonstrate that all objects in our sample show changes in their polarisation spectrum that cannot be accounted for by uncertainties in calibration or observational effects. By associating polarised emission components across epochs, we infer changes in their number, intrinsic fractional polarisation, intrinsic polarisation angle, rotation measure, and depolarisation characteristics. We attribute these changes to evolution in the structure of the blazar jets, most likely located at distances of up to tens of parsecs from the central active galactic nuclei. Our results suggest that continued work in this area is warranted; in particular, it will be important to determine the frequency ranges and temporal cadence most useful for scientifically exploiting the effects.

The jet of S5 0716[formula omitted]71 at [formula omitted]as scales with RadioAstron

Ground-space interferometer RadioAstron provides unique opportunity to probe detail structure of the distant active galactic nuclei at μas scales. Here we report on RadioAstron observations of the BL Lac object S5 0716+71, performed in a framework of the AGN Polarization and Survey Key Science Programs at 22 GHz during 2012–2018. We obtained the highest angular resolution image of the source to date, at 57×24μas. It reveals complex structure of the blazar jet in the inner 100 μas, with emission regions that can be responsible for the blazar variability at timescales of a few days to week. Linear polarization is detected in the core and jet areas at the projected baselines up to about 5.6 Earth diameters. The observed core brightness temperature in the source frame of ⩾2.2×1013 K is in excess of theoretical limits, suggesting the physical conditions are far from the equipartition between relativistic particles and magnetic field.

Interpretation of the Diffuse Astrophysical Neutrino Flux in Terms of the Blazar Sequence

Abstract We study whether the diffuse astrophysical neutrino flux can come from blazar jets—a subclass of active galactic nuclei—while it, at the same time, respects the blazar stacking limit based on source catalogs and is consistent with the observation from TXS 0506+056. We compute the neutrino flux from resolved and unresolved sources using an averaged, empirical relationship between electromagnetic spectrum and luminosity, known as the blazar sequence, for two populations of blazars (BL Lacs and FSRQs). Using a source model with realistic neutrino flux computations, we demonstrate that blazars can indeed power the diffuse neutrino flux at the highest energies and obey the stacking limit at the same time, and we derive the conditions for the baryonic loading (proton versus γ-ray luminosity) evolving over the blazar sequence. Under the hypothesis that low-luminosity blazars power the diffuse astrophysical neutrino flux, we find that the dominant contribution of the diffuse flux up to PeV energies must come from unresolved BL Lacs with baryonic loadings larger than about 105—while only a very small contribution may come from resolved high-luminosity BL Lacs or FSRQs, which can be directly tested by the stacking limit. We find that the blazar TXS 0506+056 is on the verge of these populations in our baseline scenario, at a relatively high luminosity and redshift; as a consequence, we predict about 0.3 γ-ray-neutrino associations per year from the whole population, dominated by BL Lacs with and z ∼ 0.1.

GRB and blazar jets shining through their stripes

Black-hole driven relativistic astrophysical jets, such as blazars and gamma-ray bursts (GRBs), are powerful sources of electromagnetic radiation. Their emission is powered by some energy dissipation and particle acceleration mechanism operating in-situ at large distances from the black hole. We propose that the formation of the dissipative structures in the jet is controlled by the time variability of the accretion disc. We argue 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 magneto-rotational dynamo in the disc. Consequently, the jet is characterized by an alternating toroidal field polarity along its propagation axis, i.e., it is a "striped jet". Magnetic reconnection in the current sheets that form between the stripes dissipates the alternating-field energy and thus powers further jet acceleration. Here, we consider a jet with a broad distribution of stripe widths $l>l_{\rm min}$, above a dominant scale $l_{\rm min}$. We find that the bulk acceleration of the jet, driven by the annihilation of the stripes, is very gradual. The dissipation rate peaks at a distance $z_{\rm peak}\sim 10^6 R_g\, (\Gamma_\infty/30)^2\, (l_{\rm min}/1000R_g)$, where $R_{g}$ is the black-hole's gravititional radius and $\Gamma_{\infty}$ the jet's asymptotic Lorentz factor, and exhibits a very broad plateau extending by $\sim 4-5$ orders of magnitude in distance. The prolonged energy dissipation accounts for the flat-to-inverted long-wavelength spectra commonly observed in jets. The model can also account for the broad range of flaring timescales of blazars and the fact that their bulk acceleration appears to continue out to ~100 pc scales. In GRB jets, the model predicts comparable power for the photopsheric and Thomson-thin emission components.

Maser radiation from collisionless shocks: application to astrophysical jets

This paper describes a model of electron energization and cyclotron-maser emission applicable to astrophysical magnetized collisionless shocks. It is motivated by the work of Begelman, Ergun and Rees [Astrophys. J. 625, 51 (2005)] who argued that the cyclotron-maser instability occurs in localized magnetized collisionless shocks such as those expected in blazar jets. We report on recent research carried out to investigate electron acceleration at collisionless shocks and maser radiation associated with the accelerated electrons. We describe how electrons accelerated by lower-hybrid waves at collisionless shocks generate cyclotron-maser radiation when the accelerated electrons move into regions of stronger magnetic fields. The electrons are accelerated along the magnetic field and magnetically compressed leading to the formation of an electron velocity distribution having a horseshoe shape due to conservation of the electron magnetic moment. Under certain conditions the horseshoe electron velocity distribution function is unstable to the cyclotron-maser instability [Bingham and Cairns, Phys. Plasmas 7, 3089 (2000); Melrose, Rev. Mod. Plasma Phys. 1, 5 (2017)].

The Properties of Parsec-scale Blazar Jets

Abstract I show that, by assuming a standard Blandford–Königl jet, it is possible to determine the bulk Lorentz factor and angle to the line of sight of self-similar parsec-scale blazar jets by using five measured quantities: redshift, core radio flux, extended radio flux, the magnitude of the core shift between two frequencies, and apparent jet opening angle. From the bulk Lorentz factor and angle computed with this method, one can compute other jet properties such as the Doppler factor, magnetic field strength, and intrinsic jet opening angle. I use data taken from the literature and marginalize over nuisance parameters associated with the electron distribution and equipartition to compute these quantities, although the errors are large. Results are generally consistent with constraints from other methods. Primary sources of uncertainty are the errors on the core shift measurements and the uncertainty in the electron spectral index.

On the Connection of Radio and γ-Ray Emission in Blazars

Blazars are a sub-category of radio-loud active galactic nuclei with relativistic jets pointing towards to the observer. They are well-known for their non-thermal variable emission, which practically extends over the whole electromagnetic spectrum. Despite the plethora of multi-wavelength observations, the issue about the origin of the γ -ray and radio emission in blazar jets remains unsettled. Here, we construct a parametric leptonic model for studying the connection between the γ -ray and radio emission in both steady-state and flaring states of blazars. Assuming that relativistic electrons are injected continuously at a fixed distance from the black hole, we numerically study the evolution of their population as it propagates to larger distances while losing energy due to expansion and radiative cooling. In this framework, γ -ray photons are naturally produced at small distances (e.g., 10 − 3 pc) when the electrons are still very energetic, whereas the radio emission is produced at larger distances (e.g., 1 pc), after the electrons have cooled and the emitting region has become optically thin to synchrotron self-absorption due to expansion. We present preliminary results of our numerical investigation for the steady-state jet emission and the predicted time lags between γ -rays and radio during flares.