Chandra B. Singh

SPATIAL GROWTH OF CURRENT-DRIVEN INSTABILITY IN RELATIVISTIC ROTATING JETS AND THE SEARCH FOR MAGNETIC RECONNECTION

Using the three-dimensional relativistic magnetohydrodynamic code RAISHIN, we investigated the influence of radial density profile on the spatial development of the current-driven kink instability along magnetized rotating, relativistic jets. For the purpose of our study, we used a non-periodic computational box, the jet flow is initially established across the computational grid, and a precessional perturbation at the inlet triggers the growth of the kink instability. We studied light as well as heavy jets with respect to the environment depending on the density profile. Different angular velocity amplitudes have been also tested. The results show the propagation of a helically kinked structure along the jet and relatively stable configuration for the lighter jets. The jets appear to be collimated by the magnetic field and the flow is accelerated due to conversion of electromagnetic into kinetic energy. We also identify regions of high current density in filamentary current sheets, indicative of magnetic reconnection, which are associated to the kink unstable regions and correlated to the decrease of the sigma parameter of the flow. We discuss the implications of our findings for Poynting-flux dominated jets in connection with magnetic reconnection process. We find that fast magnetic reconnection may be driven by the kink-instability turbulence and govern the transformation of magnetic into kinetic energy thus providing an efficient way to power and accelerate particles in AGN and gamma-ray-burst relativistic jets.

THE ROLE OF FAST MAGNETIC RECONNECTION ON THE RADIO AND GAMMA-RAY EMISSION FROM THE NUCLEAR REGIONS OF MICROQUASARS AND LOW LUMINOSITY AGNS

ABSTRACTFast magnetic reconnection events can be a very powerful mechanism operating in the core region,near the jet launching area, of microquasars and AGNs. In earlier work, it has been suggested that thepower released by fast reconnection events between the magnetic field lines lifting from the inner accre-tion disk region and the lines anchored into the central black hole could accelerate relativistic particlesand produce the observed radio emission from microquasars and low luminosity AGNs (LLAGNs).Moreover, it has been proposed that the observed correlation between the radio emission and the massof these sources, spanning 10 10 orders of magnitude in mass, might be related to this process. In thepresent work, we revisit this model comparing different fast magnetic reconnection mechanisms, andapply the scenario above to a much larger sample of sources (including also blazars, and gamma-raybursts - GRBs). We find that LLAGNs and microquasars confirm the trend above, i.e., their radioemission could be due to magnetic power released by turbulent driven fast reconnection which mayaccelerate particles to relativistic velocities in the core region of these sources. The correspondinggamma-ray emission, which is believed to be produced by interactions of the accelerated particleswith the surrounding photon and density fields, seems to be also produced in the same core regionin these sources. On the other hand, the emission from blazars and GRBs does not follow the sametrend as that of the LLAGNs and microquasars, suggesting that the radio and gamma-ray emissionin these cases is produced by another population of relativistic particles, accelerated further out inthe jet, as expected.Keywords: Accretion Disks — AGNs and Microquasars — Magnetic Reconnection — Radio andGamma Emission

On the Role of Fast Magnetic Reconnection in Accreting Black Hole Sources

We attempt to explain the observed radio and gamma-ray emission produced in the surrounds of black holes by employing a magnetically-dominated accretion flow (MDAF) model and fast magnetic reconnection triggered by turbulence. In earlier work, standard disk model was used and we refine the model by focussing on the sub-Eddington regime to address the fundamental plane of black hole activity. The results do not change substantially with regard to previous work ensuring that the details of the accretion physics are not relevant in the magnetic reconnection process occurring in the corona. Rather our work puts fast magnetic reconnection events as a powerful mechanism operating in the core region, near the jet base of black hole sources on more solid ground. For microquasars and low-luminosity active galactic nuclei (LLAGNs) the observed correlation between radio emission and mass of the sources can be explained by this process. The corresponding gamma-ray emission also seems to be produced in the same core region. On the other hand, the emission from blazars and gamma-ray bursts (GRBs) cannot be correlated to core emission based on fast reconnection.

The distribution and lifetime of powerful radio galaxies as a function of environment and redshift

Correlations between jet power and active time for z < 0.1 high excitation and low excitation radio galaxies are explored as well as evidence in favor of a specific, non-random distribution for these objects including mid-infrared emitting radio galaxies as a function of environment and redshift. In addition, so-called weak line radio galaxies with FRII jet morphology have been identified as a class of active galaxies in the process of shutting down. This paper identifies common features between these seemingly disparate phenomena described above for the population of radio galaxies, and strings them together by way of a simple phenomenological framework that has shed light on the radio loud/radio quiet dichotomy, the jet-disk connection, and the distribution of all active galaxies as a function of redshift.

Magnetic reconnection and Blandford-Znajek process around rotating black holes

We provide a semi-analytic comparison between the Blandford-Znajek (BZ) and the magnetic reconnection power for accreting black holes in the curved spacetime of a rotating black hole. Our main result is that for a realistic range of astrophysical parameters, the reconnection power may compete with the BZ power. The field lines anchored close to or on the black hole usually evolve to open field lines in general relativistic magnetohydrodynamic (GRMHD) simulations. The BZ power is dependent on the black hole spin while magnetic reconnection power is independent of it for the force-free magnetic configuration with open field lines adopted in our theoretical study. This has obvious consequences for the time evolution of such systems particularly in the context of black hole X-ray binary state transitions. Our results provide analytical justification of the results obtained in GRMHD simulations.

Magnetic Reconnection on Jet-Accretion disk Systems

Fast Magnetic Reconnection is currently regarded as an important process also beyond the solar system, specially in magnetically dominated regions of galactic and extragalactic sources like the surrounds of black holes and relativistic jets. In this lecture we discuss briefly the theory of fast magnetic reconnection, specially when driven by turbulence which is very frequent in Astrophysical flows, and its implications for relativistic particle acceleration. Then we discuss these processes in the context of the sources above, showing recent analytical and multidimensional numerical MHD studies that indicate that fast reconnection can be a powerful process to accelerate particles to relativistic velocities, produce the associated high energy non-thermal emission, and account for efficient conversion of magnetic into kinetic energy in these flows.

Scale-invariant jet suppression across the black hole mass scale

We provide a schematic framework for understanding observations of jet suppression in soft state black hole X-ray binaries based on the Blandford-Payne process and the net magnetic flux threading the black hole. Due to the geometrical thinness of soft state disks, mass-loading of field lines is ineffective compared to both geometrically thick disks as well as thin disks with greater black hole threading flux, a simple physical picture that allows us to understand the weakness of jets in radiatively efficient thin disks accreting in the prograde direction around high-spinning black holes. Despite a simplicity that forbids insights into the complexity of turbulent-driven evolution or the physics of the observed short-term time variability, we show how the breadth of this framework is such that it can serve as a coarse-grained foundation for understanding black hole accretion and jet formation across the mass scale.