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Register a new userA stratified jet model for AGN emission in the two-flow paradigm
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High-energy emission of extragalactic objects is known to take place in relativistic jets, but the nature, the location, and the emission processes of the emitting particles are still unknown. One of the models proposed to explain the formation of relativistic ejections and their associated non-thermal emission is the two-flow model, where the jets are supposed to be composed of two different flows, a mildly relativistic baryonic jet surrounding a fast, relativistically moving electron-positron plasma. Here we present the simulation of the emission of such a structure taking into account the main sources of photons that are present in active galactic nuclei (AGNs). We reproduce the broadband spectra of radio-loud AGNs with a detailed model of emission taking into account synchrotron and inverse-Compton emission by a relativistically moving beam of electron-positron, heated by a surrounding turbulent baryonic jet. We compute the density and energy distribution of a relativistic pair plasma all along a jet, taking into account the synchrotron and inverse-Compton process on the various photon sources present in the core of the AGN, as well as the pair creation and annihilation processes. We use semi-analytical approximations to quickly compute the inverse-Compton process on a thermal photon distribution with any anisotropic angular distribution. The anisotropy of the photon field is also responsible for the bulk acceleration of the pair plasma through the Compton rocket effect, thus imposing the plasma velocity along the jet. As an example, the simulated emerging spectrum is compared to the broadband emission of 3C273. In the case of 3C273, we obtain an excellent fit of the average broadband energy distribution by assuming physical parameters compatible with known estimates.
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The transverse stratification of active galactic nuclei (AGN) jets is suggested by observations and theoretical arguments, as a consequence of intrinsic properties of the central engine (accretion disc + black hole) and external medium. On the other hand, the one-component jet approaches are heavily challenged by the various observed properties of plasmoids in radio jets (knots), often associated with internal shocks. Given that such a transverse stratification plays an important role on the jets acceleration, stability, and interaction with the external medium, it should also induce internal shocks with various strengths and configurations, able to describe the observed knots behaviours. By establishing a relation between the transverse stratification of the jets, the internal shock properties, and the multiple observed AGN jet morphologies and behaviours, our aim is to provide a consistent global scheme of the various AGN jet structures. Working on a large sample of AGN radio jets monitored in very long baseline interferometry (VLBI) by the MOJAVE collaboration, we determined the consistency of a systematic association of the multiple knots with successive re-collimation shocks. We then investigated the re-collimation shock formation and the influence of different transverse stratified structures by parametrically exploring the two relativistic outflow components with the specific relativistic hydrodynamic (SRHD) code AMRVAC. We were able to link the different spectral classes of AGN with specific stratified jet characteristics, in good accordance with their VLBI radio properties and their accretion regimes.
The afterglow of GRB 170817A has been detected for more than three years, but the origin of the multi-band afterglow light curves remains under debate. A classical top-hat jet model is faced with difficulties in producing a shallow rise of the afterglow light curves as observed $(F_{ u} propto T^{0.8})$. Here we reconsider the model of stratified ejecta with energy profile of $E(>Gamma beta)=E_0(Gamma beta)^{-k}$ as the origin of the afterglow light curves of the burst, where $Gamma$ and $beta$ are the Lorentz factor and speed of the ejecta, respectively. $k$ is the power-law slope of the energy profile. We consider the ejecta are collimated into jets. Two kinds of jet evolutions are investigated, including a lateral-spreading jet and a non-lateral-spreading jet. We fit the multi-band afterglow light curves, including the X-ray data at one thousand days post-burst, and find that both the models of the spreading and non-spreading jets can fit the light curves well, but the observed angular size of the source and the apparent velocity of the flux centroid for the spreading jet model are beyond the observation limits, while the non-spreading jet model meets the observation limits. Some of the best-fit parameters for the non-spreading jet model, such as the number density of the circumburst medium $sim10^{-2}$ cm$^{-3}$ and the total jet kinetic energy $E sim 4.8times 10^{51}$ erg, also appear plausible. The best-fit slope of the jet energy profile is $k sim 7.1$. Our results suggest that the afterglow of GRB 170817A may arise from the stratified jet and that the lateral spreading of the jet is not significant.
High-resolution Very-Long-Baseline Interferometry observations of relativistic jets are essential to constrain fundamental parameters of jet formation models. At a distance of 249 Mpc, Cygnus A is a unique target for such studies, being the only Fanaroff-Riley Class II radio galaxy for which a detailed sub-parsec scale imaging of the base of both jet and counter-jet can be obtained. Observing at millimeter wavelengths unveils those regions which appear self-absorbed at longer wavelengths and enables an extremely sharp view towards the nucleus to be obtained. We performed 7 mm Global VLBI observations, achieving ultra-high resolution imaging on scales down to 90 $mu$as. This resolution corresponds to a linear scale of only ${sim}$400 Schwarzschild radii (for $M_{mathrm{BH}}=2.5 times 10^9 M_{odot}$). We studied the kinematic properties of the main emission features of the two-sided flow and probed its transverse structure through a pixel-based analysis. We suggest that a fast and a slow layer, with different acceleration gradients, exist in the flow. The extension of the acceleration region is large (${sim} 10^4 R_{mathrm{S}}$), indicating that the jet is magnetically-driven. The limb brightening of both jet and counter-jet and their large opening angles ($phi_mathrm{J}{sim} 10^{circ}$) strongly favor a spine-sheath structure. In the acceleration zone, the flow has a parabolic shape ($rpropto z^{0.55pm 0.07}$). The acceleration gradients and the collimation profile are consistent with the expectations for a jet in equilibrium (Lyubarsky 2009), achieved in the presence of a mild gradient of the external pressure ($ppropto z^{-k}, kleq2$).}
Outflows are common in many astrophysical systems. In the Two Component Advective Flow ({fontfamily{qcr}selectfont TCAF}) paradigm which is essentially a generalized Bondi flow including rotation, viscosity and cooling effects, the outflow is originated from the hot, puffed up, post-shock region at the inner edge of the accretion disk. We consider this region to be the base of the jet carrying away matter with high velocity. In this paper, we study the spectral properties of black holes using {fontfamily{qcr}selectfont TCAF} which includes also a jet ({fontfamily{qcr}selectfont JeTCAF}) in the vertical direction of the disk plane. Soft photons from the Keplerian disk are up-scattered by the post-shock region as well as by the base of the jet and are emitted as hard radiation. We also include the bulk motion Comptonization effect by the diverging flow of jet. Our self-consistent accretion-ejection solution shows how the spectrum from the base of the jet varies with accretion rates, geometry of the flow and the collimation factor of the jet. We apply the solution to a jetted candidate GS,1354-64 to estimate its mass outflow rate and the geometric configuration of the flow during 2015 outburst using {it NuSTAR} observation. The estimated mass outflow to mass inflow rate is $0.12^{+0.02}_{-0.03}$. From the model fitted accretion rates, shock compression ratio and the energy spectral index, we identify the presence of hard and intermediate spectral states of the outburst. Our model fitted jet collimation factor ($f_{rm col}$) is found to be $0.47^{+0.09}_{-0.09}$.
It is commonly believed that the optical/UV and X-ray emissions in luminous AGN are produced in an accretion disk and an embedded hot corona respectively. The inverse Compton scattering of disk photons by hot electrons in the corona can effectively cool the coronal gas if the mass supply is predominantly via a cool disk like flow as in BHXRBs. Thus, the application of such a model to AGNs fails to produce their observed X-ray emission. As a consequence, a fraction of disk accretion energy is usually assumed to be transferred to the corona. To avoid this assumption, we propose that gas in a vertically extended distribution is supplied to a supermassive black hole by the gravitational capture of interstellar medium or stellar wind material. In this picture, the gas partially condenses to an underlying cool disk as it flows toward the black hole, releasing accretion energy as X-ray emission and supplying mass for the disk accretion. Detailed numerical calculations reveal that the X-ray luminosity can reach a few tens of percent of the bolometric luminosity. The value of $alpha_{rm ox}$ varies from 0.9 to 1.2 for the mass supply rate ranging from 0.03 to 0.1 times the Eddington value. The corresponding photon index in the 2-10 keV energy band varies from 1.9 to 2.3. Such a picture provides a natural extension of the model for low luminosity AGN where condensation is absent at low mass accretion rates and no optically thick disk exists in the inner region.
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